Journal of Microelectronics, Electronic Components and Materials Vol. 47, No. 3(2017), September 2017 Revija za mikroelektroniko, elektronske sestavne dele in materiale letnik 47, številka 3(2017), September 2017 ISSN 0352-9045 144 UDK 621.3:(53+54+621+66)(05)(497.1)=00 ISSN 0352-9045 Informacije MIDEM 3-2017 Journal of Microelectronics, Electronic Components and Materials VOLUME 47, NO. 3(163), LJUBLJANA, SEPTEMBER 2017 | LETNIK 47, NO. 3(163), LJUBLJANA, SEPTEMBER 2017 Published quarterly (March, June, September, December) by Society for Microelectronics, Electronic Components and Materials - MIDEM. Copyright © 2016. All rights reserved. | Revija izhaja trimesečno (marec, junij, september, december). Izdaja Strokovno društvo za mikroelektroniko, elektronske sestavne dele in materiale – Društvo MIDEM. Copyright © 2016. Vse pravice pridržane. Editor in Chief | Glavni in odgovorni urednik Marko Topič, University of Ljubljana (UL), Faculty of Electrical Engineering, Slovenia Editor of Electronic Edition | Urednik elektronske izdaje Kristijan Brecl, UL, Faculty of Electrical Engineering, Slovenia Associate Editors | Odgovorni področni uredniki Vanja Ambrožič, UL, Faculty of Electrical Engineering, Slovenia Arpad Bürmen, UL, Faculty of Electrical Engineering, Slovenia Danjela Kuščer Hrovatin, Jožef Stefan Institute, Slovenia Matija Pirc, UL, Faculty of Electrical Engineering, Slovenia Matjaž Vidmar, UL, Faculty of Electrical Engineering, Slovenia Editorial Board | Uredniški odbor Mohamed Akil, ESIEE PARIS, France Giuseppe Buja, University of Padova, Italy Gian-Franco Dalla Betta, University of Trento, Italy Martyn Fice, University College London, United Kingdom Ciprian Iliescu, Institute of Bioengineering and Nanotechnology, A*STAR, Singapore Malgorzata Jakubowska, Warsaw University of Technology, Poland Marc Lethiecq, University of Tours, France Teresa Orlowska-Kowalska, Wroclaw University of Technology, Poland Luca Palmieri, University of Padova, Italy International Advisory Board | Časopisni svet Janez Trontelj, UL, Faculty of Electrical Engineering, Slovenia - Chairman Cor Claeys, IMEC, Leuven, Belgium Denis Đonlagić, University of Maribor, Faculty of Elec. Eng. and Computer Science, Slovenia Zvonko Fazarinc, CIS, Stanford University, Stanford, USA Leszek J. Golonka, Technical University Wroclaw, Wroclaw, Poland Jean-Marie Haussonne, EIC-LUSAC, Octeville, France Barbara Malič, Jožef Stefan Institute, Slovenia Miran Mozetič, Jožef Stefan Institute, Slovenia Stane Pejovnik, UL, Faculty of Chemistry and Chemical Technology, Slovenia Giorgio Pignatel, University of Perugia, Italy Giovanni Soncini, University of Trento, Trento, Italy Iztok Šorli, MIKROIKS d.o.o., Ljubljana, Slovenia Hong Wang, Xi´an Jiaotong University, China Headquarters | Naslov uredništva Uredništvo Informacije MIDEM MIDEM pri MIKROIKS Stegne 11, 1521 Ljubljana, Slovenia T. +386 (0)1 513 37 68 F. + 386 (0)1 513 37 71 E. info@midem-drustvo.si www.midem-drustvo.si Annual subscription rate is 160 EUR, separate issue is 40 EUR. MIDEM members and Society sponsors receive current issues for free. 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Prispevke iz revije zajema ISI® v naslednje svoje produkte: Sci Search®, Research Alert® in Materials Science Citation Index™. Design | Oblikovanje: Snežana Madić Lešnik; Printed by | tisk: Biro M, Ljubljana; Circulation | Naklada: 1000 issues | izvodov; Slovenia Taxe Percue | Poštnina plačana pri pošti 1102 Ljubljana 145 Content | Vsebina 147 155 165 171 179 187 Journal of Microelectronics, Electronic Components and Materials vol. 47, No. 3(2017) Izvirni znanstveni članki B. Pečar, M. Možek, D. Vrtačnik: Kovalentno spajanje termoplasta s PDMS polimerom za mikrofluidne aplikacije G. Matič, M. Jankovec, M. Topič: Razvoj in ovrednotenje sistema za merjenje kotne odvisnosti sončnih celic U. Prah, M. Wencka, Z. Kutnjak, M. Vrabelj, S. Drnovsek, B. Malic, H. Ursic: Multikalorčni pojav v polikristaliničnem Pb(Fe0.5Nb0.5)O3 S. Bernik, M. Podlogar, S. Rustja, M. Cergolj: Vpliv granulata in pritiska na zelene oblikovance in tokovno-napetostne karakteristike sintrane varistorske keramike na osnovi ZnO H. Mercier, B. Malič, H. Uršič, D. Kuscer, F. Levassort: Priprava in sintranje debelih plasti na osnovi natrijevega kalijevega niobata D. Berčan, A. Sešek, J. Trontelj: Načrtovanje operacijskega transkonduktančnega ojačevalnika s temperaturno kompenzacijo Naslovnica: Simulirana strukturna deformacija, hitrost fluida and porazdelitev tlaka v PDMS-termoplastični mikrocilindrski črpalki. (B. Pečar et al.) Original scientific paper B. Pečar, M. Možek, D. Vrtačnik: Thermoplastic - PDMS Polymer Covalent Bonding for Microfluidic Applicationsr G. Matič, M. Jankovec, M. Topič: Development and Evaluation of the Angular Response Measurement Setup for Solar Cells U. Prah, M. Wencka, Z. Kutnjak, M. Vrabelj, S. Drnovsek, B. Malic, H. Ursic: Multicaloric Effect in Polycrystalline Pb(Fe0.5Nb0.5)O3 S. Bernik, M. Podlogar, S. Rustja, M. Cergolj: Influence of Granulate and Pressure on Green Compacts and the Current-voltage Characteristics of Sintered ZnO-based Varistor Ceramics H. Mercier, B. Malič, H. Uršič, D. Kuscer, F. Levassort Processing and Sintering of Sodium-potasium Niobate–based Thick Films: D. Berčan, A. Sešek, J. Trontelj: Design of Operational Transconductance Amplifier with Temperature Compensation Front page: Simulated Structural Deformation, Fluid Velocity and Pressure Distribution in PDMS-thermoplastic Microcylinder Pump. (B. Pečar et al.) 146 147 Original scientific paper  MIDEM Society Thermoplastic - PDMS polymer covalent bonding for microfluidic applications Borut Pečar, Matej Možek and Danilo Vrtačnik University of Ljubljana, Faculty of Electrical Engineering, Laboratory of Microsensor Structures and Electronics, Ljubljana, Slovenia Abstract: Two room-temperature bonding processes for thermoplastic - PDMS polymer covalent bonding based on the organic substrate surface functionalization by means of organofunctional silanes APTES and amine-PDMS linker were developed and applied. The efficiency of covalent bonding was evaluated by measuring water contact angles on oxygen plasma pretreated surfaces and by measuring burst pressure on fabricated test devices. Developed amine-PDMS linker bonding process resulted in bond strength of 5 bar and 2 bar on continuous pressure of air and water respectively, while water initiated the hydrolysis of covalent bonds established via the modified APTES bonding process. Both bonding processes were applied on piezoelectric micropumps where glass substrate was replaced by thermoplastic substrate. Micropumps employing amine-PDMS linker exhibit no deterioration in their performance after eight weeks of continuous operation. Keywords: PDMS; WCA; APTES; thermoplastics; covalent bonding; micropump Kovalentno spajanje termoplasta s PDMS polimerom za mikrofluidne aplikacije Izvleček: Raziskali, razvili in vpeljali smo dva nizko temperaturna postopka kovalentnega spajanja termoplasta in PDMS polimera, ki temeljita na funkcionalizaciji organske površine preko organofunkcionalnega silana APTESa in amino-PDMS povezovalca. Učinkovitost površinske aktivacije, ki je ključna za učinkovit kovalenten spoj, smo ovrednotili z merjenjem omakalnih kotov vodnih kapelj na površini vzorcev pred in po aktivaciji površin v kisikovi plazmi. Za ovrednotenje kvalitete spoja smo na namensko izdelanih testnih čipih izvedli tlačne in porušitvene teste. Amino-PDMS povezovalec je zagotovil obstojnost spoja ob stiku z vodo, medtem ko so vezi vzpostavljene preko APTESa kljub dodatni toplotni obdelavi po nanosu in modifikaciji parametrov plazemske aktivacije površin razpadle. Razvita postopka spajanja smo vpeljali v proces izdelave piezoelektričnih mikročrpalk. Piezoelektrične mikročrpalke izdelane s postopkom amine-PDMS povezovalca po osmih tednih neprekinjenega delovanja ne izkazujejo upada pretočne zmogljivosti. Ključne besede: PDMS; WCA; APTES; termoplast; kovalentno spajanje; mikročrpalka * Corresponding Author’s e-mail: borut.pecar@fe.uni-lj.si Journal of Microelectronics, Electronic Components and Materials Vol. 47, No. 3(2017), 147 – 154 1 Introduction Plastics are indispensable in mass production of mi- crofluidic devices due to their robustness, light weight, optical transparency, simplicity of molding and cost efficiency [1]. Plastics rigidity enables a variety of reli- able external interface options, such as manifold inte- gration, direct barbed tubing connections, and gasket connectors [2]. In addition, thermoplastic (TP)–polydi- methylsiloxane (PDMS) assemblies have a number of advantages over homogeneous assemblies. The com- bined surface properties of the two materials, for ex- ample, could provide an optimal environment for con- ducting cell-based research necessitating precise fluid control for targeted cell or biomolecule immobilization [3]. Many strategies for plastic–PDMS bonding have been previously reported, such as sol–gel coating approach, chemical gluing approach and organofunctional si- lanes approach [4, 5]. First approach requires multiple coating procedures as well as complex technology. Sec- ond approach creates chemically robust amine–epoxy bonds at the interface at room temperature, however, two silane-coupling reagents are required and both 148 B. Pečar et al; Informacije Midem, Vol. 47, No. 3(2017), 147 – 154 surfaces had to be oxidized prior to chemical modifica- tion. Third approach requires only one coupling agent. In this approach, the most widely used organofunc- tional silane is 3-amino propyltriethoxysilane (APTES), aminosilane frequently employed in covalent bonding of organic films to metal oxides [6]. However, TP-PDMS covalent bonds established via or- ganofunctional silanes are prone to degradation over prolonged period in aqueous environment which might limit the use in specific microfluidic applications. Few studies attempted to increase hydrolytic stability of APTES by mixing it with complex agents such as BTI- SPA, BTMSPA, BTESE [2] or GPTMS [7]. Hydrolytic resist- ance of APTES mixed with BTISPA improved to hydro- lytically stable bonds over a range of 0 to 15 pH when BTISPA was prevailing in the mixture. Authors attribut- ed increased hydrolytic resistance to the greater cross- link density for bis-silanes. APTES mixed with GPTMS yielded higher bond strength as compared to APTES, but did not improved hydrolytic stability [8]. Being aware of reported limited hydrolytic resistance of APTES [2, 7], we tried to introduce additional post- deposition heat treatment and modification of post- deposition plasma treatment that might overcome this disadvantage and provide the bond strength sufficient for specific application e. g. in the range of 0.5-0.7 bar. The advantages of employing APTES linking agent in- clude easy availability of the product and well estab- lished bonding process due to substantial popularity in microfluidic community. In this work, approach of employing two organofunc- tional silanes, APTES and poly [dimethyl siloxane-co-(3- aminopropyl) methyl siloxane (amine-PDMS linker) for bonding PDMS elastomer to TP substrates via methanol aligning medium was developed and applied. Amine- PDMS linker incorporates an amine functionality at one terminal and a segment of low molecular weight PDMS at the other, which might provide better hydrolytic bond stability [3]. Surface properties of TPs and PDMS were analyzed by measuring the water contact angles (WCA). The bond strength and bond hydrolytic stabil- ity were evaluated by delamination and burst pressure tests. Both bonding processes were further improved and applied on piezoelectric micropumps, where sup- porting bottom glass was replaced by TP substrate. 2 Experimental 2.1 Materials In the presence of amine in organofunctional ami- nosilane bonding process, thermoplastics undergo aminolysis followed by chain scission of the carbonyl backbone, forming a strong urethane bond. Therefore, not all thermoplastics, but only thermoplastics that can undergo aminolysis are suitable for the purpose. For TP substrates, optically transparent 2 mm thick Polycar- bonate (PC), Acrylonitrile butadiene styrene (ABS) and Poly methyl methacrylate (PMMA) from INEOS Styrolu- tion Group GmbH were employed. In all experiments, a PDMS Sylagard® 184 two-part kit consisting of a pre-polymer (base) and a cross-linker (curing agent) from Dow Corning Corporation mixed at a ratio 10:1 was applied. For surface functionalization of plastic substrates, a commercial solution of APTES (Sigma Aldrich) and amine- PDMS linker (Sigma Al- drich) were used. In bonding processes, methanol and DI of technical purity were applied. 2.2 Surface wettability measurements As argued by Garbassi et al. [9], the oxidation of the surface layer increases the concentration of hydroxyl groups which leads to the formation of strong intermo- lecular bonds. As the silanol groups are polar in nature, they make the exposed surface highly hydrophilic and this can be observed by measuring WCAs [10]. Those WCAs were found in direct correlation with bond strength [11]. For WCA determination a method was developed which included photographing of droplets on inves- Figure 1: Micrograph (camera Nikkon e990) of water droplet on raw PDMS surface (a) and on oxygen plasma treated PDMS surface (b). 149 tigated surfaces. The images of the droplet were ana- lyzed by computer software “ImageJ” and “ContactAn- gle” plugin from points marked along the droplet-air interface to calculate the contact angle at the droplet- surface interface. An example of WCA measurement on raw PDMS and on PDMS treated in oxygen plasma (20 sec, 0.8 mbar, 40W) is shown in Fig. 1. For all plasma treatments, ATTO Low Pressure Plasma Systems Diener electronic GmbH was employed. 2.3 Bonding process In order to overcome reported limited hydrolytic resist- ance of APTES [2, 7], additional post-deposition heat treatment and modification of post-deposition plasma treatment were introduced in the bonding process. The process flow for TP-PDMS sandwich covalent bonding is schematically shown in Fig. 2. First, TP substrates were cleaned in ultrasonic bath, followed by silylation of the surfaces through the use of organofunctional silanes. In order to achieve good adhesion, TPs were pre-activated in oxygen plasma, resulting in the hydro- philization of the TP surfaces. TP sheets were then immersed in the APTES (5% per volume) or coated with amine-PDMS linker and addi- tionally heated to 60 °C for 20 min. Such an additional post-deposition heating step was expected to increase the number of established urethane bonds, especially between TP and APTES where bonds are prone to hy- drolytic decay. Unlinked organofunctional silanes were washed away with isopropyl alcohol in ultrasonic bath. Both PDMS and functionalized TP substrate surfaces were again activated in the oxygen plasma. Post-depo- sition plasma treatment was prolonged to 1 min at in- creased pressure of 2 mbars and increased power of 50 W in order to increase the formation of Si–OH groups on both surfaces (Fig. 2 b). For all further oxygen plas- ma treatments, modified parameters were employed. After plasma activation, the activated surfaces of the two substrates were brought into contact, using meth- anol as an aligning medium (Fig. 2 b). After methanol evaporation, covalent bonds were formed which were then additionally stabilized by curing at 80°C for 1 h in laboratory furnace. 2.4 Characterization of the bond strength The bond strength was evaluated by performing de- lamination and burst pressure tests. Fig. 3 shows PDMS residues on ABS substrate after the PDMS elastomer was delaminated. In this particular case, APTES was employed as a linking agent. It was presumed that the area of PDMS residues left on the TP surface was directly related to bond strength. No obvious correla- tion between TP type and bond strength was found by delamination tests. Figure 3: Photography of PDMS residues on ABS sub- strate after the PDMS elastomer was delaminated. Next, the bond strengths were measured using burst pressure test devices. Top view and lateral cross-sec- tion of designed burst pressure test device is shown in Fig. 4.Figure 2: Process flow for TP-PDMS sandwich covalent bonding. B. Pečar et al; Informacije Midem, Vol. 47, No. 3(2017), 147 – 154 150 Figure 4: Schematics illustration of burst pressure test device. Top view (a) and lateral cross section (b). Devices were fabricated by employing replica molding technique. Silicon mold for PDMS cast was fabricated by one-step photolithography and deep reactive ion etching (DRIE). Casted PDMS elastomer layers with mi- crostructures (square-shaped inlet chambers with mi- crochannels) were thermally cured at 60 °C for 1 hour and bonded to TP substrates applying bonding process described in Sect. 2.3. Pressure regulated air supply was connected to the inlet of the test device and the pressure at which the device failed was determined. Device always failed at the region where the square-shaped inlet chamber narrows into the channel indicated by arrow position in Fig. 6. Here, the structural stress caused by applied fluidic pressure was the largest. The region of failure was further confirmed by employ- ing 3-D numerical simulations in COMSOL Multiphysics software. Test device behavior can be explained consid- ering two different physics models coupled together. Fluid flow is described by the Navier-Stokes equation ( )( ) Tp t ρ ρ µ∂   + ⋅∇ = ∇ − + ∇ + ∇ +   ∂   v v v I v v F (1) where the left hand side represents contribution of the force acting on a differential volume of a fluid and the inertial force. v is the fluid velocity, ρ density, p pressure and µ dynamic viscosity. Equation (1), which is describ- ing conservation of momentum, needs to be solved together with equation of mass continuity which for incompressible fluid reads 0∇ ⋅ =v (2) Deformation of a structure is modeled by structural mechanics equation for displacement vector u 2 2t ρ ∂ = − ∇ ⋅ ∂ u u f σ (3) where fu is a force acting on a differential volume and σ is a stress tensor. Stress σ and strain ε tensors are re- lated through equation σ = cEε, where cE is the elastic- ity matrix (determined with Young modulus of elastic- ity and Poisson ratio). Equations (1) to (3) are solved for fluid velocities and structural deformations together with supporting relations described in the text. After the pressure boundary condition was applied on the inlet, stationary direct fully-coupled solver was em- ployed to solve structural deformation and stress in burst pressure test device. Outlet boundary condition velocity was set to zero, although model tolerates also non-zero outlet boundary condition, due to coupled fluidic module. Simulated magnitude of bond stress at applied inlet fluidic pressure of 50 kPa is shown in Fig. 5. Maximum stress was calculated at the region (pre- sented with dark red color in Fig. 5) where fabricated test devices failed first (pointed out with arrow in Fig. 6), which positively validated simulation model. Figure 5: Simulated magnitude of bond stress at el- evated fluidic pressure. Arrow indicates the point of maximum simulated bond stress. Figure 6: Micrograph of burst pressure test device. Ar- row shows the failure onset region. Simulation results show that the microfluidic channel design should avoid sharp edges throughout flow di- rection where layers delamination might be initiated. B. Pečar et al; Informacije Midem, Vol. 47, No. 3(2017), 147 – 154 151 3 Results Since surfaces wettability is an essential criterion for inspecting the degree of surface activation in covalent bonding procedures, WCAs were measured first on raw and with O2 plasma treated TP and PDMS surfaces. Re- sults are presented in Table 1 (see also Fig. 1). Table 1: WCAs for raw and oxygen plasma treated sam- ples. Polymer raw O2 plasma (1 min) O2 plasma (5 min) ABS 87 27 32 PC 74 38 37 PMMA 69 45 47 PDMS 87 4.5 / Next, APTES (5%v/v in DI) was employed for function- alization of TP surfaces. With respect to others [2, 12] an additional post-deposition heat treatment and modified post-deposition plasma treatment were in- troduced in order to improve hydrolytic stability of co- valent bonds. As expected, oxygen plasma treatment after modified APTES functionalization considerably improved surfaces wettability (see Table 2), which in- dicates high surface concentration of hydroxyl groups. However, wettability still declined considerably over time. In this study, no straightforward correlation be- tween wettability of TP surfaces and TP-PDMS bond strength was found after the TP surfaces were func- tionalized. Table 2: WCAs for APTES functionalized and oxygen plasma treated samples. Polymer APTES (5%v/v in DI) Bond strength [°] 0 min [°] after 30 min [°] after 24 h O2 plasma (20 sek) ABS 9.5 38.7 74 accept. PC 14.5 47 81 accept. PMMA 12.3 36.5 83 accept. In further investigation, 30 burst pressure tests em- ploying water and compressed air were performed on fabricated PDMS test devices where TPs were previ- ously functionalized with APTES or amine-PDMS linker. Again, no direct correlation between TP type (receiving equal surface preparation and bonding process) and bond strength was found. All test devices could sustain air pressures of 5 bars for at least 3 hours. However, when burst pressure tests were performed by pressurized water, APTES devices started to fail imme- diately after the fluidic pressure of 1 bar was applied with a channel edge delamination rate of 7.5µm min-1 in spite of additional post-deposition heat treatment and modification of post-deposition plasma treatment. Similar hydrolysis of covalent bonds established via conventionally treated APTES was reported by Aran K et al. [12]. Results from testing bond strength under applied air pressure showed that the thermoplastics - PDMS bond was able to withstand more than 227.8 kPa, which was the maximum limit of their measuring equipment, without any sign of delamination. In fur- ther tests, the channels of the microdevices were filled with water and stored at room temperature for 72 h or the devices were filled and completely immersed in water for 72 h. Surprisingly, the reported bonding strength for APTES coated membranes remained very strong (over 227.8 kPa) in devices stored at room temperature for 72 h with the device channels filled with water. However, complete immersion of the devices in water for an ex- tended period of time weakened the membrane bond- ing strength for all of their tested bonding methods. In another study, S. Kevin Lee et al. [2] reported bond failure and delamination of PDMS-APTES-TP sandwich structure after subjected to burst pressure test with water compressed above 15 psi. An overview of silane is needed in order to understand the mechanisms for hydrolysis-induced bond failure. An organofunctional silane is a molecule comprising a silicon atom with at least one bond to carbon to en- able organic functionality [6]. The inorganic side of the silane molecule consists of a silicon atom bound to alk- oxy groups through Si—O—C linkages [2]. Hydrolytic instability of these bound alkoxy groups allows silanes to hydrolyze in the presence of water, converting the bound alkoxy groups to hydroxyl groups while liber- ating alcohol molecules. Any contact with water after bond formation will result in Si—O—C bond hydro- lysis and ultimately bond failure [6, 13]. Furthermore, Si—O—C bonds have also been found to form directly between alkoxy groups such as methoxy and surface hydroxyl groups via alcoholysis [14]. Hydrolytic bond failure can occur at three locations in the bonding structure, at the thermoplastics-silane interface, at the PDMS-silane interface, and in the silane network itself [15]. While direct interface hydrolysis is unlikely due to the stability of the amide bond, any hydrophilic groups at the interface can act as nucleation sites for water condensation, allowing the silane network near the interface to be plasticized and weakened [16]. A simi- lar process can occur at the PDMS-silane interface but with the possibility of hydrolysis directly at the inter- B. Pečar et al; Informacije Midem, Vol. 47, No. 3(2017), 147 – 154 152 face in addition to the weakening of the silane network [17]. For the silane network itself, high crosslink density can provide a major increase in resistance. However, networks formed by typical silanes, containing three silanol groups, tend to be cyclic, decreasing their re- sistance to dissolution [18]. Addressing failure mecha- nisms in all three locations is necessary to ensure hy- drolytic stability [6,18]. In further investigation, burst pressure tests were per- formed on amine-PDMS linker test devices fabricated on PC, ABS and acrylic glass using pressurized water. Test devices withstood 2 bar water pressure for 3 hours without any delamination observed. Moreover, amine- PDMS linker devices were soaked in water for one week and successfully withstood all additional burst pressure tests. All tests confirmed hydrolytic stability of TP-PD- MS bonds established through amine-PDMS linker. It is speculated that the water-repelling nature of the PDMS component in amine-PDMS linker prevented penetra- tion of the aqueous solutions at the interface improv- ing bond hydrolytic resistance [2, 3, 6]. Finally, developed bonding processes employing AP- TES and amine-PDMS linker were applied in modified micropump fabrication process. Based on the poor re- sults of water tests on burst pressure devices, we fur- ther modified the APTES application by using multiple deposition steps (3 to 5 deposited layers), thus expect- ing the improvements of bonds hydrolytic resistance. Our previously developed piezoelectric microcylinder pump prototypes [19] comprise activated PDMS elasto- mer layer bonded on its bottom side to the supporting bottom glass. Supporting bottom glass includes im- provised fluidic connections and serves as a functional part of the micropump affecting micropump perfor- mance characteristics. Replacing supporting bottom glass with TP could pave the path toward micropump mass production in terms of enclosing the micropump in professional TP housing comprising professional flu- idic and electric connections. In the initial stage, supporting bottom glass was re- placed with flat TP substrate. Exploded view of a typical TP microcylinder pump structure is shown in Fig. 7. The TP microcylinder pump comprises PDMS elasto- mer layer with molded micropump chamber, fluidic mi- crochannel and rectifying elements (Fig. 7 c). Addition- ally, two through-holes are punched into an elastomer, one into the center of the micropump chamber and the other one at the end of the channel. PDMS elastomer layer (Fig. 7 c) and PDMS fluidic connections (Fig. 7 e) are covalently bonded to the supporting TP substrate (Fig. 7 d) by employing developed multiple deposition APTES or single deposition amine-PDMS linker bond- ing process. One inlet and one outlet fluid port is drilled through a supporting TP substrate that supply and drain the fluid into and out of the pump. The micropump cham- ber and the microchannel are sealed with a thin glass membrane (Fig. 7 b) by employing oxygen plasma PDMS-glass covalent bonding process. Piezoelectric actuator (Fig 7 a) is positioned in the axis of a micro- pump chamber, coupled rigidly to the micropump membrane through silver filled epoxy adhesive (EPO- TEK EE129-4). During excitation loosely attached glass membrane and PDMS elastomer layer deform in a controlled man- ner, which enables compression and expansion of the centrally placed inlet cylindrical port, micropump chamber and outlet throttle shaped port with a specific phase lag, contributing to efficient micropump opera- tion. Pumping test were performed by pumping air and DI water media. Both approaches passed air pumping tests, reaching maximum flow rate performance of 8 ml min-1 and maximum backpressure performance of 100 mbar at applied square excitation waveform Figure 7: Exploded view of a typical TP microcylinder pump structure (dimensions are not to scale). B. Pečar et al; Informacije Midem, Vol. 47, No. 3(2017), 147 – 154 153 with an amplitude of 140 V and a frequency of 300Hz. However, during DI water pumping tests and despite improved multiple layer deposition process of APTES, micropumps degraded after several minutes of opera- tion. This is depicted in Fig. 8 as APTES MP3 character- istics and further expanded on time scale in an in-set (see Fig. 8 central plot on the figure plane). Figure 8: Long term flowrate stability characteristics of three representative micropump devices and ambient temperature. In contrast, amine-PDMS linker composed of a PDMS backbone incorporating an amine side group estab- lished hydrolytically stable covalent bonds. This was confirmed by long-term flowrate stability measure- ments on two micropumps MP1 and MP2 with typical performances yielding initial maximum DI water flow- rate performance of 1.2 ml min-1 and 1.4 ml min-1 at ap- plied RC excitation waveform [20] with an amplitude of 140 V and a frequency of 300 Hz. Figure 8 also includes measurement setup ambient temperature (dotted line closest to the upper edge of the diagram). Due to par- tial correlation between flowrate performance char- acteristics of MP1 and MP2 and measurement setup ambient temperature it was concluded that transient deviations in flowrate characteristics are to be attrib- uted to temperature changes of the medium viscosity, the micropump and driving electronics throughout the measurement. Therefore, long term stability measure- ments should be improved by setting measuring setup in a temperature stabilized chamber. Micropumps em- ploying amine-PDMS linker bonding process exhibit no deterioration in their performance after eight weeks of continuous operation. 4 Conclusions Low temperature process for TP-PDMS irreversible covalent bonding was presented. Process is based on silylation of the TP surfaces through the use of orga- nofunctional silanes. As the silanol groups are polar in nature, they make the exposed surface highly hy- drophilic and this was observed by measuring WCAs. However in this study, no direct correlation between wettability of TP surfaces deposited with APTES and fi- nal bond strength was found. In further investigation, burst pressure tests were performed on designed and fabricated PDMS test devices employing TPs function- alized with APTES or amine-PDMS linker. All devices sustained air pressures of 5 bars for at least 3 hours, but only amine-PDMS linker test devices sustained continu- ous water pressure as high as 2 bars without delamina- tion. Bonds established via APTES and subjected to wa- ter decayed in spite of additional post-deposition heat treatment and modification of post-deposition plasma treatment. In further application oriented study, both bonding processes were applied on piezoelectric mi- cropumps where glass substrate was replaced by ther- moplastic substrate. Even implementation of multiple deposition steps of APTES was insufficient in prevent- ing hydrolysis of covalent bonds, resulting in micro- pumps performance deterioration. On the other hand, micropumps employing amine-PDMS linker exhibit no deterioration in their performance even after eight weeks of continuous operation. 5 Acknowledgments Authors would like to thank the Slovenian Research Agency/ARRS (P2-0105, P2-0244), Ministry of Educa- tion, Science and Sport and Kolektor Group d.d. for their support of this work. 6 References 1. Yetisen, A. K., Akram, M. S., & Lowe, C. R. (2013). Based microfluidic point-of-care diagnostic de- vices. Lab on a Chip, 13(12), 2210-2251. 2. Lee, S. K., Lee, H., & Ram, J. R. (2016). U.S. Patent No. 9,422,409. Washington, DC: U.S. Patent and Trademark Office. 3. Wu, J., & Lee, N. Y. (2014). One-step surface modifi- cation for irreversible bonding of various plastics with a poly (dimethylsiloxane) elastomer at room temperature. Lab on a Chip, 14(9), 1564-1571. 4. Suzuki, Y., Yamada, M., & Seki, M. (2010). Sol–gel based fabrication of hybrid microfluidic devices composed of PDMS and thermoplastic substrates. Sensors and Actuators B: Chemical, 148(1), 323- 329. 5. Tsao, C. W., & DeVoe, D. L. (2009). Bonding of ther- moplastic polymer microfluidics. Microfluidics and Nanofluidics, 6(1), 1-16. B. Pečar et al; Informacije Midem, Vol. 47, No. 3(2017), 147 – 154 154 6. Lee, K. S., & Ram, R. J. (2009). Plastic–PDMS bond- ing for high pressure hydrolytically stable active microfluidics. Lab on a Chip, 9(11), 1618-1624. 7. Tang, L., & Lee, N. Y. (2010). A facile route for ir- reversible bonding of plastic-PDMS hybrid mi- crodevices at room temperature. Lab on a Chip, 10(10), 1274-1280. 8. Karakoy, M., Gultepe, E., Pandey, S., Khashab, M. A., & Gracias, D. H. (2014). Silane surface modifi- cation for improved bioadhesion of esophageal stents. Applied surface science, 311, 684-689. 9. Garbassi, F., Morra, M., Occhiello, E., & Garbassi, F. (1998). Polymer surfaces: from physics to technol- ogy (pp. 169-200). Chichester: Wiley. 10. Hillborg, H., & Gedde, U. W. (1999). Hydrophobicity changes in silicone rubbers. IEEE Transactions on Dielectrics and Electrical insulation, 6(5), 703-717. 11. Bhattacharya, S., Datta, A., Berg, J. M., & Gango- padhyay, S. (2005). Studies on surface wettabil- ity of poly (dimethyl) siloxane (PDMS) and glass under oxygen-plasma treatment and correlation with bond strength. Journal of microelectrome- chanical systems, 14(3), 590-597. 12. Aran, K., Sasso, L. A., Kamdar, N., & Zahn, J. D. (2010). Irreversible, direct bonding of nanoporous polymer membranes to PDMS or glass microde- vices. Lab on a Chip, 10(5), 548-552. 13. Goebel, R. (2003). U.S. Patent No. 6,613,439. Wash- ington, DC: U.S. Patent and Trademark Office. 14. Pape, P. G., & Plueddemann, E. P. (1991). Methods for improving the performance of silane coupling agents. Journal of adhesion science and technol- ogy, 5(10), 831-842. 15. Plueddemann, E. P. (1991). Reminiscing on silane coupling agents. Journal of Adhesion Science and Technology, 5(4), 261-277. 16. Tesoro, G., & Wu, Y. (1991). Silane coupling agents: the role of the organofunctional group. Journal of adhesion science and technology, 5(10), 771-784. 17. Battjes, K. P., Barolo, A. M., & Dreyfuss, P. (1991). New evidence related to reactions of aminated silane coupling agents with carbon dioxide. Journal of Adhesion Science and Technology, 5(10), 785-799. 18. Plueddemann, E. P. (1991). Adhesion through si- lane coupling agents. In Fundamentals of adhe- sion (pp. 279-290). Springer US. 19. Dolžan, T., Pečar, B., Možek, M., Resnik, D., & Vrtačnik, D. (2015). Self-priming bubble tolerant microcylinder pump. Sensors and Actuators A: Physical, 233, 548-556. 20. http://www.electronics-tutorials.ws/rc/rc_3.html (17.10.2017) Arrived: 31. 08. 2017 Accepted: 26. 10. 2017 B. Pečar et al; Informacije Midem, Vol. 47, No. 3(2017), 147 – 154 155 Original scientific paper  MIDEM Society Development and Evaluation of the Angular Response Measurement Setup for Solar Cells Gašper Matič, Marko Jankovec, Marko Topič University of Ljubljana, Faculty of Electrical Engineering, Ljubljana, Slovenia Abstract: The paper presents the development and practical evaluation of a basic setup for measuring the angular dependence of solar cells. The main goal of this study is to verify whether it is possible to use off-the-shelf components to build a simple but reliable measurement setup which performs fast and efficient characterization of angular dependence and therefore enables quick evaluation of various design ideas, especially when it comes to evaluating reference solar cells. The proposed setup consists of a rotary stage, a source measure unit for measuring the short circuit current of a cell under test, a digital multimeter for measuring the irradiance power drift via a photodiode, a solar simulator and a computer which controls all of the instruments. The paper focuses on the mechanical construction of the setup and on the problems affecting the measurement precision, to which appropriate solutions are proposed. Keywords: solar cells, angular response, measurement setup development, mechanical construction, measurement precision issues Razvoj in ovrednotenje sistema za merjenje kotne odvisnosti sončnih celic Izvleček: Sledeči članek predstavlja razvoj in praktično ovrednotenje osnovnega sistema za merjenje kotne odvisnosti sončnih celic. Poglavitni namen študije je preveriti, ali je mogoče s pomočjo splošno dostopnih komponent izdelati enostaven vendar zanesljiv merilni sistem, ki nudi hitro in učinkovito karakterizacijo kotne odvisnosti ter tako omogoča nezamudno ovrednotenje različnih načrtovalskih idej, še posebej v primeru ovrednotenja referenčnih sončnih celic. Predlagani merilni sistem sestoji iz rotacijske enote, napajalno-merilne enote za merjenje kratkostičnega toka merjene sončne celice, digitalnega multimetra za merjenje lezenja jakosti osvetlitve s pomočjo fotodiode, simulatorja sončnega obsevanja in računalnika, ki nadzoruje vse inštrumente. Članek se osredotoča na izvedbo mehanske konstrukcije merilnega sistema ter na težave povezane z merilno točnostjo in natančnostjo, za katere predlagamo ustrezne rešitve. Ključne besede: sončne celice, kotna odvisnost, razvoj merilnega sistema, mehanska konstrukcija, merilna točnost in natančnost * Corresponding Author’s e-mail: gasper.matic@fe.uni-lj.si Journal of Microelectronics, Electronic Components and Materials Vol. 47, No. 3(2017), 155 – 163 1 Introduction Angular response of a photovoltaic module plays an important role since it is related to angle-dependent energy losses [1], which can become a crucial part of yearly performance losses due to nonstandard operat- ing conditions [2] or due to poor orientations and tilt angles [1]. An accurate knowledge of the angular de- pendence is also important in predicting the perfor- mance of a PV system [2] and crucial to the precision of photovoltaic sensors [3]. All of the above makes the angular response measurement setup an essential part of the PV characterization equipment. Although many studies and experiments have been published con- cerning the behavior of the angular response of solar cells (e.g. [2], [1], [3]), there is usually little specific detail on constructing the actual angular response measure- ment setup and on the problems related with perform- ing accurate measurements. In this paper we present the development and practi- cal evaluation of a basic setup for measuring the an- gular dependence of solar cells. The focus is put on the mechanical construction of the setup and on the problems that arise during measurements and affect the measurement precision. 156 G. Matič et al; Informacije Midem, Vol. 47, No. 3(2017), 155 – 163 2 Setup construction 2.1 Setup description The setup is basically comprised of a Keithley 238 SMU source measure unit which is used to provide 4-wire measurements of the short circuit current of the device under test (DUT); an Agilent A34401A digital multimeter used for measuring the irradiance power drift via a reference photodiode; a Newport Oriel Class A solar simulator 93194A; a precision rotary stage OWIS DMT65 used to set the DUT’s angle of incidence and a computer which controls all of the instruments via a La- bview routine through GPIB (General Purpose Interface Bus) and USB interfaces. At this point of development we are interested in using such a setup only to pre- cisely measure the shape of the angular response and to be able to make relative comparisons of responses from different solar cells, which is why the measured responses presented in the paper are typically normal- ized and represented by the abbreviation NR (i.e. nor- malized response). 2.2 Mechanical construction The supporting construction for the DMT65 rotary stage (Figure 1 and Figure 2) was built from a medium- density fiberboard (MDF) because this material is con- sistent in strength and size, has stable dimensions (in normal environmental conditions it does not expand or twist like wood, especially if painted) and is easy to shape. The rotary stage was mounted so that the axis of rotation becomes horizontal. The stage was placed on a spacer which allows a 20 cm long rail (Edmund optics) to be fixed onto it. The rail allows the distance between the DUT and the line of the rotation axis to be set. Two dovetail slide carriers were then attached onto the rail. These slide carriers hold the right angle metal bracket, onto which the optical filter holder (Edmund optics) is then fixed. The filter holder is used to hold the DUT in place during the rotation. Typically, devices under test were attached to a special Plexiglas adapter which was then inserted into the filter holder. The mounting of the rotary stage onto the supporting construction was reinforced in order to minimize the bending when the DUT is inserted into the holder. The rigidity and me- chanical stability of the whole setup is crucial for good repeatability and comparison of the measurements. Almost every construction part was spray painted with black matte paint in order to minimize light reflections. The rotary stage was mounted onto a tabletop that was placed on two supporting columns of modular spacers, which allow the distance between the DUT and the so- lar simulator lens to be varied (Figure 1). Figure 1: The rotary stage mounted onto the support- ing construction. A DUT is mounted into a Plexiglas adapter which is then attached to the rotary stage through the two-screw optical filter holder. 3 Measurement precision issues In the following, four groups of problems which, in our opinion, have important effect on measurement preci- sion are discussed. 3.1 Mechanical and geometrical problems 3.1.1 Mounting the DUT into the setup Firstly, it must be assured that the distance between the DUT’s photosensitive area and the imaginary line of the rotation axis is as small as possible. Secondly, this imaginary line must also run so that it splits the photo- sensitive area into two symmetrical parts. When these two conditions are met, the average distance between the photosensitive area and the source of (imperfectly) collimated light (i.e. the solar simulator collimating lens) does not change during the rotation. If it would, this would be a source of a systematic measurement error because at some point the photosensitive area would be in average closer to the source of light than at some other point. While the second problem of aligning the line of sym- metry of the photosensitive area with the line of the rotation axis is simply a matter of proper positioning of the DUT when inserting it into the holder, the first prob- lem of minimizing the distance between the photosen- sitive area and the line of the rotation axis requires ad- 157 ditional calibration. A simple solution is possible using a laser-cut Plexiglas calibration tool (Figure 2). The cali- bration procedure is as follows. First, additional slide carrier is attached onto the rail so that its top face lies exactly at the axis of rotation (Figure 3). This is how we get a zero-height reference point on the rail. Then an optional spacer is put on top of this carrier, which com- pensates for the material that covers the photosensi- tive area (in our example this is glass and ethylene-vinyl acetate (EVA) laminate). At the top of this spacer comes the bottom of the calibration tool. At the same time the vertical side of the calibration tool is pressed parallel to the rail. Now the two slide carriers that attach the DUT to the rail are made loose which allows the DUT to be slid upwards to meet the horizontal side of the calibration tool (Figure 2). Since the horizontal side of the calibration tool is perpendicular to the vertical side, the height relative to the rotation axis at the bottom of the tool is now the same as at the horizontal side that is touching the top of the DUT (i.e. the glass in our case – see Figure 2). In other words, the calibration tool sim- ply helps translate the level at its bottom over the me- chanical parts that hold the DUT to the location where the DUT actually lies. Figure 2: Calibrating the distance between the photo- sensitive area of the DUT and the imaginary line of the rotation axis. At the same time the tilt of the DUT rela- tive to the holder can be minimized. The height of the DUT on the rail is now set so that the photosensitive area that lies beneath the glass and EVA laminate is at the same height as the imaginary line of the rotation axis. The optional spacer assures that the DUT is moved slightly higher, compensating for the thickness of glass and EVA. As a matter of interest, some setups do not perform this compensation (e.g. [1]). Typically, it is desired that the plane of the photosensi- tive area is parallel to the line of the rotation axis. The calibration tool can also be used to minimize the tilt of the DUT. A narrow strip of elastic material is placed between a Plexiglas DUT adapter and the filter holder. If this strip is placed at the right location, then we can control the tilt of the DUT by increasing or decreasing the force provided by the two fastening screws in the filter holder. The elastic strip contracts under the pres- sure which causes the DUT adapter to tilt upwards or downwards, depending on the location of the strip relative to the screws. The tilt of the DUT is minimized when the face of the DUT is parallel to the horizontal side of the calibration tool (Figure 2). But in some situa- tions it is useful to provide a small amount of tilt as we will demonstrate later. Figure 3: Calibrating the distance between the photo- sensitive area of the DUT and the imaginary line of the rotation axis – a detail. 3.1.2 Initial position problem When the DUT is fixed into the setup, the measure- ments can begin. But in order to provide measure- ments with high repeatability which can be used in quality comparison analyses, the same initial position for all measurements must be defined. In order to dem- onstrate the importance in the precision and repeata- bility of the initial point, let us study the plot in Figure 4. The plot shows a comparison of a short circuit current response for two similar reference cells: one with white back sheet and the other with black back sheet. From the short circuit current ratio (solid line) one would conclude that the first cell has better response at high angles of incidence α than the second cell. Now, if the initial point of the measurement for the second DUT is offset by 1o relative to the original initial point (dashed line), a completely different conclusion is derived. The angular response measurements are very sensitive to errors in mechanical positioning, especially at high an- gles [4] due to the increased slope of the response. The initial position problem is not as simple as it seems at first sight. For instance, if we decide to use a fixed absolute position of our rotary stage as the initial point, then as soon as the setup (or DUT) is moved from its current location, the actual orientation of the setup G. Matič et al; Informacije Midem, Vol. 47, No. 3(2017), 155 – 163 158 (and DUT) relative to the beam of light emanating from the solar simulator may change. This means that the ir- radiance at the initial point becomes sensitive to the orientation and position of the setup (and DUT), which is a nuisance if precise measurements with high repeat- ability are required. Besides, the precision and repeat- ability of the absolute home reference point of the ro- tary stage may also be problematic, as it proved in our case. Figure 4: The effect of the precision of the initial point of a measurement. A relatively small offset in the initial point position can cause a completely different meas- urement conclusion when comparing short circuit re- sponses of two different solar cells. From the thought experiment above one can already sense the solution to this problem. The initial point must be defined relative to the beam of light provided by the solar simulator. If before each measurement the rotary stage is positioned so that the light hits the DUT at zero angle of incidence, i.e. α = 0o ≡ α0, then the re- peatability of the measurement results is very much improved and does not depend on the exactness of the position and orientation of the setup and DUT. The process of finding the position of the rotary stage where the zero angle of incidence occurs at DUT can be automated in our measurement setup. We devel- oped two algorithms to calibrate the α0 point. The first algorithm is based on a fact that at the zero angle of incidence a DUT provides the maximal response. The algorithm is therefore designed to iteratively search for the maximal response in a given range of rota- tional positions by sampling the DUT response with a specified resolution. At each next iteration, the range is narrowed and the resolution increased. The algo- rithm stops at the prescribed minimal resolution and the point of maximal response is declared the zero in- cidence angle α0 In order to decrease the effect of the measurement noise, the measurement samples can be smoothed by filtering and the algorithm then works with the smoothed samples. The problem with this approach is that the typical response of a DUT has the shape of a cosine function [1], which means that the maximum is very unpronounced, i.e. the small region around the peak is very flat, which makes the detection of the peak location difficult and at the same time the effect of measurement noise is increased. Neverthe- less, the algorithm has still proved useful in most cases, especially if averaging of the measurement samples is increased. The second algorithm we devised takes into account the problem of the unpronounced maximum and searches for the α0 point indirectly. The algorithm is based on a fact that in most cases the angular response of a solar cell is very much symmetrical [1] in a region near the α0 point. Therefore, the α0 point is determined on a criterion that for symmetrical angles ±α around the a0 point the measurement samples have the maxi- mal symmetry. The idea is demonstrated in Figure 5. Three measurement sets are made with samples 15o apart. Each next measurement set is offset by Dα from the previous set. The central measurement samples of each set lie around the actual zero angle of incidence point α = 0 = α0. Now for each of the sets the symmetry around the central sample is checked. For the first set we can observe that samples left of the central point (negative α) have smaller measured values than their symmetrical counterparts on the right side of the cen- tral point (positive α). Figure 5: Calibrating the zero angle of incidence point α0 by maximizing the symmetry of the samples around the α0 point. Obviously, there is an asymmetry to this set and we can conclude that the central point of the set does not lie in the zero angle of incidence point. Similar is true for the third measurement set, where the situation is turned around. In case of the second measurement set, at sym- metrical angles around the central point we get sym- metrical measurement samples, which indicates that the central point of the second set lies in the α0 point. The zero incidence angle is thus determined indirectly via the symmetry criterion. The algorithm proved to be much more sensitive with a much more pronounced maximum of the criterion function, which results in a high repeatability of the calibration results. The resolu- tion of this search algorithm is determined by the offset between the sequential measurement sets Dα. In our experience, the Dα = 0,2 o proved to be a practical value for the offset. -40 -30 -20 -10 0 10 20 30 40 0.80 0.85 0.90 0.95 1.00 G. Matič et al; Informacije Midem, Vol. 47, No. 3(2017), 155 – 163 159 We performed a repeatability test for the second algo- rithm in the following way: a reference solar cell was mounted into the setup and then the calibration algo- rithm was run 15 times. The absolute position of the rotary stage (i.e. the absolute offset of the stage) where the α0 point was detected was then recorded and used to plot a histogram in Figure 6. Obviously, the repeat- ability of the zero angle of incidence point is quite high, especially if the averaging of the measurement sam- ples is increased. a) b) Figure 6: A repeatability test for the calibration of the zero angle of incidence point α0. In case of b), the aver- aging of the measurement samples was increased. The resolution of the calibration was Dα = 0,2 o. The obvious drawback of the second algorithm is that it can be used only in cases where the expected response is fairly symmetrical. Luckily, this practically holds true in many cases. In cases of asymmetrical responses, the first algorithm can be used instead or both algorithms combined, using the second algorithm only on a small- er region that still displays fair symmetry. 3.2 Optical problems The first problem that can be considered as an optical one is the problem of stray light. By stray light we mean the light that emanates from the solar simulator and reaches the DUT indirectly due to reflections from the surrounding objects. Minimizing stray light is impor- tant since it can cause large relative measurement er- rors at higher angles of incidence. Namely, at very high angles there is expected that less and less direct exci- tation light reaches the photosensitive area (at 90o no light should hit the area), so the DUT response should approach zero value. But if the stray light is present and hits the DUT, this is not the case since the stray light cannot be distinguished from the direct excitation light and its effect compensated from the measurements. In order to minimize this effect, we developed a cascade system of masks that are placed in between the solar simulator lens and a DUT with intention to shape the light beam only to the DUT and the nearby surround- ing area (Figure 7). Also, the objects surrounding the setup were either moved far away or covered with a low reflectance mask (i.e. black painted plywood). The second problem also deals with indirect light hit- ting the photosensitive area, but in this case the light is not reflected from the surrounding objects but from the solar simulator optics itself. These reflections are named multiple- or also double-reflections and have been studied to some degree [5, 6]. Figure 7: A cascade system of masks that limit the light beam in order to minimize the stray light errors. Most references claim that the light is reflected back to the DUT from the collimating lens, but we have G. Matič et al; Informacije Midem, Vol. 47, No. 3(2017), 155 – 163 160 discovered that this is not a complete understanding of the problem. Namely, not just a lens, but every op- tical component that lies behind the lens inside the solar simulator also plays an important role in these double-reflections. These reflections cause a strong increase in DUT response near zero angle of incidence where a DUT reflects the incidence light directly back to the solar simulator. Figure 8 shows an example of this phenomenon in case of a WPVS reference cell from Fraunhofer ISE (see Figure 11a). The loss function is de- termined as a difference between the ideal normalized cosine response and the normalized measured value at a given angle α. To mitigate this problem, two different approaches were used. The first solution tilts the DUT in order to direct the light reflected from the DUT away from the central axis of the collimating lens (Figure 10a) thus redirecting these reflections away from the simulator optics. A tilt of e.g. β = 5o causes a decrease in response at zero angle of incidence α0 for a factor of cos(5 o) = 0,9962, which is about 5 per mills. This factor can be simply neglected if only normalized measurement response is required. Fig- ure 9 shows the mitigating effect of such a solution in case of the same WPVS reference cell. Figure 8: The effect of double-reflections in a measure- ment response of a WPVS reference cell. A relatively large spike occurs at zero angle of incidence. The second solution (Figure 10b) moves the DUT away from the central line of the lens where double-reflec- tions are most prominent and also increases the dis- tance from the lens, which decreases the solid angle taken by the area of DUT as seen from the center of lens and therefore decreases the power density of double- reflected light. Besides, increasing the distance is also beneficial because it mitigates the effects of light beam uniformity [1]. The second solution is as effective as the first one and the solutions can, of course, be combined if needed. To complete the picture about light hitting the DUT during measurements, we must explain how to deal with the effect of room ambient light. The procedure also applies to the case where a bias light is used to decrease the effect of non-linearity of a DUT at low illu- mination (as in e.g. [2]). Effect of this light can be easily compensated for in the following manner: at each an- gular position α the response of the DUT is first meas- ured before the DUT is exposed to the solar simulator light. In this way we get the information about the ambient light intensity. Then the electronic-controlled shutter is opened and another measurement is taken, combining the response to both ambient and solar simulator light. The final measurement result is simply the difference between the second and the first meas- urement, since it can be assumed that the DUT pro- vides a linear response [1]. In this case it is important that the instrument that is measuring the response has the resolution that is high enough, since we are sub- tracting two measurements that can be close to each other. This compensation is actually a simple variant of a lock-in measurement technique. Figure 9: The effect of double-reflections in the case of a WPVS reference cell is mitigated by tilting the DUT. The last problem concerning the optical circumstances is the problem of the solar simulator irradiance drift. This effect is compensated in the following way. At the same time when the DUT response is measured, the response of a reference PIN photodiode which meas- ures the solar simulator irradiance intensity is also re- corded. These reference irradiance measurements are then used to correct the DUT response measurements by scaling them all to the same irradiance intensity, re- lying on the proportionality of both DUT and PIN diode responses. 0.94 0.96 0.98 1.00 -15 -10 -5 0 5 10 15 0.000 0.005 0.010 0.015 0.96 0.97 0.98 0.99 1.00 -15 -10 -5 0 5 10 15 -2.0 0.0 2.0 4.0 10 -3 G. Matič et al; Informacije Midem, Vol. 47, No. 3(2017), 155 – 163 161 3.3 Temperature dependence The main problem here is the rise of the DUT tempera- ture due to the irradiation caused by the solar simula- tor, which affects the response of the DUT. Since our setup does not provide any kind of cooling mechanism as in [2] and since there are cases where the additional DUT temperature measurement for the temperature effect compensation is not available, the temperature problem can be addressed only by minimizing the temperature rise during the measurement. The average power absorbed by the DUT through irradiance P is ,ON ON OFF tP P t t = + (1) where P is the power being absorbed by the DUT when exposed to solar simulator light and the tON and tOFF are the durations of the solar simulator shutter being opened and closed, respectively. Obviously, there are two ways in decreasing the average absorbed power P during measurement and thus decreasing the tem- perature rise: the power of the solar simulator can be decreased, which decreases the P term, or the duty cy- cle of the shutter can be decreased by increasing the tOFF term. The first solution can be easily achieved by decreasing the power of the solar simulator lamp or increasing the distance between the DUT and the colli- mating lens, which at the same time helps mitigate the problem of double-reflections (see chapter 3.2). The second solution means that the time required to per- form the whole measurement gets increased, which is not that problematic. 3.4 Electrical measurements To increase the precision of the short circuit current measurements, a 4-wire measuring technique is used. As with any other precise electronic measurement, the important source of error is the measurement noise. Since in our case we are measuring a DC signal (i.e. a constant value response), the noise can be reduced by applying the averaging technique where greater num- ber of measurement samples is taken and then aver- aged. But this means that the time when the DUT is ex- posed to the solar simulator light tON is also increased, which causes the DUT to heat up. In order to prevent this, the duty cycle of the shutter must not be changed. Instead the number of measurement samples for a giv- en angle α is increased using the same duty cycle. In other words, instead of making one long measurement with large tON, we make several short measurements with small tON and unchanged shutter duty cycle, which means that the average absorbed power by the DUT is not increased. As an additional benefit of such an averaging several measurements for each angle α are recorded, which means that the measurement uncer- tainty can be evaluated for each angle. 4 A measurement example Angular responses of a reference cell that we devel- oped were measured at three different stages of pro- duction: before lamination (i.e. bare), after lamination (glass and EVA on top) and when in enclosure (Figure 11 b), c) and d), respectively). The results are shown in Figure 12. It can be clearly observed that the lamination reduces the losses at lower angles of incidence α, while the losses at higher α are increased, most probably due to the increased light reflection from the glass and due to different surface texturing [1]. The loss function of the laminated cell agrees well with the ones measured in [1]. The effect of the enclosure can also be observed, where at angles higher than about 50o the loss is in- creased due to the shading effect of the enclosure. Figure 10: Two practical solutions to the double-re- flections problem. lens DUT α β lens DUT α a) b) G. Matič et al; Informacije Midem, Vol. 47, No. 3(2017), 155 – 163 162 Figure 12: Normalized angular response of a mono-Si reference cell in three different stages of production. 5 Conclusion A development of a basic setup for measuring the an- gular dependence of solar cells was presented. We be- lieve that the concept of the proposed measurement setup proved successful as well as the solutions to the key problems that affect the measurement precision. The setup was used to perform a series of other meas- urement experiments which cannot be presented here and, to our belief, the setup proved precise with high level of repeatability being able to provide insightful results, despite its somewhat simplistic approach. In future we are planning to validate the precision of our setup by comparing our results to the results of a certi- fied measurement laboratory. 6 Acknowledgement We would like to thank Mizarstvo Jankovec for kindly providing the wooden building blocks required for the setup construction. 7 References 1. J. L. Balenzategui and F. Chenlo, “Measurement and analysis of angular response of bare and en- capsulated silicon solar cells,” Sol. Energy Mater. Sol. Cells, vol. 86, no. 1, pp. 53–83, 2005. 2. I. Geisemeyer et al., “Angle dependence of solar cells and modules: The role of cell texturization,” IEEE J. Photovoltaics, vol. 7, no. 1, pp. 19–24, 2017. 3. J. J. Michalsky, L. C. Harrison, and W. E. Berkheiser, “Cosine response characteristics of some radio- metric and photometric sensors,” Sol. energy, vol. 54, no. 6, pp. 397–402, 1995. 4. King, Boyson, Hansen, and Bower, “Improved Low-Cost Solar Irradiance Sensors,” Albuquerque, New Mexico, 1998. Figure 11: Devices under test: a) WPVS reference cell from Fraunhofer ISE and in-house developed mono-Si reference cells: b) non-laminated (bare), c) laminated (glass +EVA on top) and d) in-enclosure. 0.00 0.20 0.40 0.60 0.80 1.00 0 10 20 30 40 50 60 70 80 90 0.00 0.02 0.04 0.06 0.08 G. Matič et al; Informacije Midem, Vol. 47, No. 3(2017), 155 – 163 163 5. S. K. K. Morita, S. Kera, P. Sochor, “Solutions for Performance Measurement Error Due to Multi- ple-Reflections between Reference Solar Cell and Output Lens of Solar Simulator,” in 27th European Photovoltaic Solar Energy Conference and Exhibi- tion, 2012, pp. 3693–3696. 6. U. K. D. Romang, J. Meier, R. Adelhelm, “Reference Solar Cell Reflections in Solar Simulators,” in 26th European Photovoltaic Solar Energy Conference and Exhibition, 2011, pp. 2613–2617. Arrived: 31. 08. 2017 Accepted: 27. 10. 2017 G. Matič et al; Informacije Midem, Vol. 47, No. 3(2017), 155 – 163 164 165 Original scientific paper  MIDEM Society Multicaloric effect in polycrystalline Pb(Fe 0.5 Nb 0.5 )O 3 Uros Prah1,2, Magdalena Wencka3, Zdravko Kutnjak1, 2, Marko Vrabelj1, Silvo Drnovsek1, Barbara Malic1, 2 and Hana Ursic1,2 1Jožef Stefan Institute, Ljubljana, Slovenia 2Jožef Stefan International Postgraduate School, Ljubljana, Slovenia 3Institute of Molecular Physics, Polish Academy of Sciences, Poznań, Poland Abstract: In this work, magnetocaloric and electrocaloric properties of multiferroic Pb(Fe0.5Nb0.5)O3 ceramics have been investigated. Pb(Fe0.5Nb0.5)O3 was prepared by mechanochemical activation of constituent oxides, followed by sintering at 1273 K in oxygen atmosphere. Microstructure and X-ray powder-diffraction analysis revealed dense, homogeneous and uniform microstructure without the presence of undesired secondary phases. Magnetocaloric and electrocaloric effects were determined by the indirect methods - calculated from the changes of sample’s magnetization and polarization, respectively. The maximal magnetocaloric temperature change (0.16 K at 50 kOe) was obtained at 2 K coinciding with the observed anomaly in magnetization vs. temperature measurement. On the other hand, at room temperature the pronounced electrocaloric effect was determined, namely 0.81 K at 80 kV/cm, while the maximal value of electrocaloric temperature change 1.29 K was obtained near the paraelectric-ferroelectric phase transition i.e., at 373 K. Keywords: multiferroic; PFN; multicaloric; electrocaloric; magnetocaloric Multikalorčni pojav v polikristaliničnem Pb(Fe 0.5 Nb 0.5 )O 3 Izvleček: V članku smo proučevali magnetokalorični in elektrokalorični pojav v multiferoičnem Pb(Fe0.5Nb0.5)O3. Keramiko smo pripravili z mehanokemijsko aktivacijo kovinskih oksidov, ki ji je sledilo sintranje pri 1273 K v kisikovi atmosferi. Mikrostruktura keramike je bila gosta in homogena. Sekundarnih faz nismo opazili. Tako magnetokalorično kot tudi elektrokalorično temperaturno spremembo smo izračunali iz temperaturne spremembe magnetizacije oz. polarizacije vzorca pri različnih zunanjih poljih. Največjo magnetokalorično spremembo temperature (0,16 K pri 50 kOe) smo določili pri 2 K, kar je v skladu z opaženo anomalijo magnetizacije vzorca v odvisnosti od temperature. Izrazito elektrokalorično spremembo temperature smo opazili že pri sobni temperaturi (0,81 K pri 80 kV/cm), medtem ko je bila njena maksimalna vrednost 1,29 K opažena v bližini paraelektričnega-feroelektričnega faznega prehoda pri 373 K. Ključne besede: multiferoik; PFN; multikalorik; elektrokalorik; magnetokalorik * Corresponding Author’s e-mail: uros.prah@ijs.si Journal of Microelectronics, Electronic Components and Materials Vol. 47, No. 3(2017), 165 – 170 1 Introduction Nowadays the majority of commercially used refrigera- tion systems are still based on vapor-compression re- frigeration cycle. This technology was discovered at the beginning of the 19th century and has been developed and perfected through the years. Despite all the im- provements, the method shows a number of disadvan- tages. The major problems are low energy efficiency and the use of environmentally hazardous refrigerant media [1, 2]. These disadvantages has driven the devel- opment of more efficient and environmentally friendly cooling devices. Solid-state refrigeration technology represents a prom- ising alternative for the replacement of the conven- tional refrigeration systems. Most current activity in cooling research is looking at one of the caloric effects – magnetocaloric (MC), electrocaloric (EC) or mechano- caloric – where the material’s entropy changes under the application of external stimuli –magnetic, electric, or mechanical (stress) [3]. However, in bulk ceramic ma- terials the caloric effect is currently not large enough for commercial use. One idea how to overcome this problem is to prepare a material exhibiting more than one caloric effect, called multicaloric material, in which 166 U. Prah et al; Informacije Midem, Vol. 47, No. 3(2017), 165 – 170 the application of two or more stimuli can enhance the total caloric effect. Further, different caloric modes can be applied in different temperature regions extending the operating temperature range of the cooling device [4]. In 2012, the coexistence of the MC and EC effects in a single-phase material was theoretically introduced for the first time [5]. In the next years, many theoretical re- ports were followed [6-8]. In 2014, a multicaloric effect in Y2CoMnO6 was experimentally observed [9]. Howev- er, this material exhibit improper multiferroic proper- ties and therefore the conventional methods for deter- mining the EC effect are not appropriate. In 2016 the existence of multicaloric properties in 0.8Pb(Fe0.5Nb0.5) O3–0.2Pb(Mg0.5W0.5)O3 (PFN-20PMW) ceramic mate- rial was demonstrated, where the coexistence of MC and EC effects was unequivocally experimentally con- firmed [4]. While PFN-20PMW appears promising, it possesses very small MC and EC temperature changes (both ~0.25  K). Further, the largest caloric effects in this material are observed at low temperatures of 5  K (MC) and 220  K (EC), which is far too low for any practical applications. One of the more promising candidates is multiferroic Pb(Fe0.5Nb0.5)O3 (PFN) ceramic. It possesses a relatively high peak of the dielectric permittivity (several 10,000) at around 370  K, which is attributed to the paraelectric- ferroelectric phase transition [10]. Because the highest caloric effects are obtained near ferroic phase transi- tions [11, 12], PFN should possess high EC properties above the room temperature. On the other hand, in this material two anomalies are reported also in the temperature dependence of the magnetic susceptibil- ity. These two anomalies appear at 150  K and 10  K [13, 14] indicating a potential for enhanced MC effect close to these temperatures. One possibility of preparing complex oxides is the use of mechanochemical synthesis, where homogeneous powders can be prepared without thermal treatment. Later the powder compacts are sintered at elevate temperature (T > 1200  K) to obtain dense ceramics. It was shown that (K0.485Na0.485Li0.03)(Nb0.8Ta0.2)O3 and Pb(Sc0.5Nb0.5)O3 ceramics prepared from the mechano- chemically synthesized powders have exhibited supe- rior chemical homogeneity in comparison to the one prepared by the classical solid-state synthesis [15, 16]. PFN has already been prepared with a mechanochemi- cal synthesis [17-19] and in comparison to the solid- state synthesized samples, it possesses higher values of peak-permittivity, which can be presumably attrib- uted to the better chemical homogeneity of the former one [15]. In this work we prepared a single-phase PFN ceramic by mechanochemical synthesis and sintering aiming to study its dielectric, electrocaloric and mag- netocaloric properties. 2 Experimental For the synthesis of the PFN powder, Nb2O5 (99.9%, Sig- ma-Aldrich, 208515), Fe2O3 (99.9%, Alfa, 014680-Ven- tron) and PbO (99.9%, Sigma-Aldrich, 211907) were used. The homogenized stoichiometric mixture (200 g) was mechanochemically activated in a high-energy planetary ball mill (Retsch, Model PM 400) for 30 h at 300 rpm using a tungsten carbide milling vial (250 cm3) and 15 balls (2r = 20 mm). The synthesized powder was milled in an attrition mill with yttria-stabilized zirconia balls (2r = 3 mm) in isopropanol, for 4 h at 800 rpm. The powder was then uniaxially pressed (50 MPa) into pellets and further consolidated by isostatic pressing at 300  MPa. The powder compacts were sintered in double alumina crucibles in the presence of a packing powder with the same chemical composition, in order to avoid possible PbO losses. The compacts were sin- tered at 1273 K for 2 h in an oxygen atmosphere with the heating and cooling rates of 2 K/min. The density of the sintered pellets was determined with Archimedes’ method. For the calculation of rela- tive density, the theoretical density of 8.46 g/cm3 was used (PDF card no. 032-0522). The X-ray powder-diffraction (XRD) of the PFN powder after the mechanochemical treatment and crushed sin- tered pellet were recorded using a PANalytical X’Pert PRO (PANalytical, Almelo, Netherlands) diffractometer with Cu-Kα1 radiation. The XRD patterns were collected over the 2θ range 10–70°, with a step of 0.034° and 100 s per step. For the microstructural analysis the samples were frac- tured for fracture-surface examination, ground and polished using standard metallographic techniques for polish-surface examination and thermally etched at 1023 K for 20 min and fine polishing (by a colloidal silica suspension with 0.04  μm sized colloidal SiO2 particles for 1.5 h) for thermally etched-surface examination. The microstructure was studied with a field-emission scanning electron microscope (FE-SEM, JSM-7600F JEOL Ltd., Japan) at 15 kV with a working distance of 15 mm. The grain size and their distribution were deter- mined from the micrographs of the thermally etched samples, where more than 340 grains per sample were measured using the Image Tool Software [20]. The grain size is expressed as the Feret’s diameter [21]. 167 For the dielectric measurements, the pellets were cut and thinned to a thickness of about 200 µm and then the Cr/Au electrodes (2r  =  5  mm) were sputtered on samples’ surfaces. The dielectric permittivity (ε´) at dif- ferent temperatures was measured with a HP  4284  A Precision LCR impedance meter in the temperature range from 298 K to 473 K. The EC effect was determined by the indirect method; the EC temperature change (ΔTEC) was calculated from the polarization versus electric field (P–E) measured by Aixacct TF analyzer 2000 (Aixacct, Aachen, Germany) at 10 Hz in a temperature range between 298 and 363 K (step 5 K). For the calculation of ΔTEC, the equation giv- en in ref. [22] was used. The MC effect was determined by the indirect method; the MC temperature change (ΔTMC) was calculated from the magnetization (M) versus temperature measure- ments at different magnetic fields (1–50 kOe) using a Superconducting Quantum Interference Device mag- netometer in a temperature range from 2 to 350 K. The mass of the sample was 30 mg. For the calculation of ΔTMC, the equation given in ref. [23] was used. The specific heat capacity (Cp), which was needed for the calculation of ΔTEC and ΔTMC was measured on a 30  mg cube-shaped sample in a temperature range between 2 and 393 K using Physical Property Measure- ment System. 3 Results The XRD patterns of the PFN powder and crushed pel- lets are shown in Fig.  1. All the peaks correspond to the perovskite phase (PDF card no. 032-0522) and no Figure 1: XRD patterns of (a) the PFN powder and (b) crushed pellet. Figure 2: (a) Polished, (b) fractured and (c) thermally etched FE-SEM images. Inset: the grain size distribution with the cumulative curve. secondary phases were observed. Broader diffraction peaks and higher background in the case of powder can be attributed to smaller size of the crystallites and the presence of the amorphous phase, as suggested in [24] for Pb(Mg0.33Nb0.67)O3. The density of the ceramic was 8.1 g/cm3, which is equal to 95.7% of the theoreti- cal density. U. Prah et al; Informacije Midem, Vol. 47, No. 3(2017), 165 – 170 168 The FE-SEM micrographs of the polished, fractured and thermally etched surfaces (Figs.  2a-c) of PFN ceramic reveal dense, homogeneous and uniform microstruc- tures with the average grain size of (2.3 ± 1.2) µm and unimodal grain size distribution (inset on Fig.  2c). No secondary phases were detected with the FE-SEM anal- ysis in agreement with the XRD analysis. The temperature dependence of 𝜀′ is shown in Fig. 3. The value at room temperature (298  K) and 1  kHz is 3780. The maximal value of permittivity (𝜀′max) at 1 kHz is ~28200 and it just slightly decreases with increas- ing frequency (~27200 at 100 kHz). The measured 𝜀′max value is higher than previously reported one for solid- state synthesized PFN [25, 26] and comparable with the one for ceramics prepared by mechanochemical syn- thesis [19]. Figure 3: Temperature dependence of ε΄. As shown in Fig. 3 the peak-permittivity temperature (Tm) is at 371 K (for all frequencies), which is in agree- ment with the previously reported ones [26,  27]. Ac- cording to the literature, this anomaly of ε′ indicates a paraelectric-ferroelectric phase transition [28, 29]. P–E hysteresis loops at different temperatures are shown in Fig.  4a. At room temperature (298 K), typi- cal ferroelectric hysteresis loop is observed. The values of the remanent polarization (Pr), maximum polariza- tion (Pmax) and coercive field (Ec) at room temperature are ~22.5  µAs/cm2, ~41.6  µAs/cm2 and ~4.4  kV/cm, respectively. The Pr and Pmax values decreased with the increasing temperature (i.e., Pr  ~  9.1  µAs/cm 2 and Pmax ~ 35.1 µAs/cm 2 at 363 K). The temperature dependence of ΔTEC is shown in Fig. 4b. The ΔTEC at room temperature and 80 kV/cm is 0.81 K. The ΔTEC increases with the increasing tempera- ture and increasing applied electric field. The maximum ΔTEC of 1.29 K was obtained at 80 kV/cm and 373 K. The temperature dependence of M at different mag- netic fields is shown in Fig.  5a. The M increases with increasing magnetic field and decreasing temperature, therefore the highest value of 1.37  Am2/kg was ob- served at 2 K and 50 kOe. Two anomalies are observed in M (T,  H) curves in accordance with the literature [14,  15]. The one at ~150  K is attributed to the para- magnetic-antiferromagnetic phase transition, while the second one at ~10 K to antiferromagnetic-antifer- romagnetic phase transition. The temperature dependence of ΔTMC is shown in Fig. 5b. The ΔTMC at room temperature and 50 kOe is very low, i.e., ~2 mK. The ΔTMC increases with the decreasing tem- perature (in proportion to magnetization) and increas- ing applied magnetic field, therefore the maximum ΔTMC of 0.16 K was obtained at 50 kOe and 2 K. 4 Summary and conclusions In this work, we were able to prepare single-phase PFN ceramics showing both EC and MC properties. The ce- Figure 4: (a) P–E hysteresis loops and (b) ΔTEC versus temperature. U. Prah et al; Informacije Midem, Vol. 47, No. 3(2017), 165 – 170 169 ramic pellets were prepared by mechanochemical syn- thesis of the constituent oxides and thermal treatment at 1237 K. Microstructure analysis revealed dense, ho- mogeneous and uniform microstructures with the av- erage grain size of ~2.3 µm. At room temperature and 1 kHz the value of 𝜀′ was 3780. The highest value of magnetization (i.e., 1.37 Am2/kg) was measured at low temperature of 2 K. At this tem- perature also the maximum ΔTMC  ~  0.16 K was deter- mined. On the other hand, the ΔTEC reaches the value as high as 1.29 K at 373 K. But even at room temperature the ΔTEC was relatively high, i.e., 0.81 K. Further work is needed in order to confirm EC properties by direct EC temperature measurements, for example by high-reso- lution calorimetry. 5 Acknowledgments The authors would like to thank the Slovenian Research Agency (research core funding no. P2-0105 and project PR-07594) and joint research project between Polish and Slovenian Academy of Sciences “Multicaloric relax- or materials for new cooling technologies”. Figure 5: (a) M and (b) ΔTMC versus temperature. 6 References 1. M. Ozbolt, A. Kitanovski, J. Tusek, A. Poredos, “Electrocaloric vs. magnetocaloric energy conver- sion,” Int. J. Refrig. 37, pp. 16–27, 2014. 2. T. Correia, Q. Zhang, Eds., Electrocaloric materials: New Generation of Coolers, 1st ed. Berlin, Hei- delberg, Germany: Springer-Verlag Berlin Heidel- berg, 2014. 3. X. Moya, S. Kar-Narayan, N. D. 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Poredos, Magnetocaloric energy conver- tion, ISSN 1865-3529, Springer International Pub- lishing Switzerland, Switzerland, 2015. 24. D. Kuscer, J. Holc, M. Kosec, “Mechano-Synthesis of Lead-Magnesium-Niobate Ceramics,” J. Am. Ceram. Soc. 89, pp. 3081–3088, 2006. 25. R. Font, O. Raymond-Herrera, L. Mestres, J. Por- telles, J. Fuentes, J. M. Siqueiros, “Improvement of the dielectric and ferroelectric properties of multiferroic Pb(Fe1/2Nb1/2)O3 ceramics processed in oxygen atmosphere,” J. Mater. Sci. 51, pp. 6319– 6330, 2016. 26. C. C. Chiu, S. B. Desu, “Microstructure and proper- ties of lead ferroniobate ceramics (Pb(Fe0.5Nb0.5) O3),” Mater. Sci. Eng., B21, pp. 26–35, 1993. 27. R. Sun, W. Tan, B. Fang, “Perovskite phase forma- tion and electrical properties of Pb(Fe1/2Nb1/2)O3 ferroelectric ceramicse,” Phys. Status Solidi A, 206, pp. 326–331, 2009. 28. M. Yokosuka, “Electrical and Electromechanical Properties of Hot-Pressed Pb(Fe1/2Nb1/2)O3 Ferro- electric Ceramics,” Jpn. J. Appl. Phys. 32, pp. 1142– 1146, 1993. 29. X. S. Gao, X. Y. Chen, J. Yin, J. Wu, Z. G. Liu, “Ferro- electric and dielectric properties of ferroelectro- magnet Pb(Fe1/2Nb1/2)O3 ceramics and thin films,” J. Mater. Sci. 35, pp. 5421–425, 2000. Arrived: 31. 08. 2017 Accepted: 07. 11. 2017 U. Prah et al; Informacije Midem, Vol. 47, No. 3(2017), 165 – 170 171 Original scientific paper  MIDEM Society Influence of granulate and pressure on green compacts and the current-voltage characteristics of sintered ZnO-based varistor ceramics Slavko Bernik1, Matejka Podlogar1, Saša Rustja2, Mirjam Cergolj2 1Jožef Stefan Institute, Ljubljana, Slovenia 2VARSI d.o.o., Ljubljana, Slovenia Abstract: Granulates G1, G2 and G3, having the same varistor composition but different morphologies in terms of shape, size and size distribution, were characterized for their compactness and flow characteristics. They were also examined during the preparation of disc-shaped green pieces with a diameter of 20 mm by uniaxial pressing at pressures from 3.2MPa to 300MPa. The density and strength of the green samples showed similar pressure dependencies for all the granulates and for the same pressure the green densities were similar. The density of the varistor ceramics sintered at 1200 oC for 2 hours showed little dependence on the compression pressure and the used granulate due to sintering in the presence of a Bi2O3-rich liquid phase; they already had 93% of theoretical density when pressed at only 10MPa, while for higher pressures the density increased to 96%. The pressure applied during uniaxial pressing also had a small influence on the current-voltage (I-U) characteristics of the varistor ceramics. The varistor ceramics from granulate G1 with the preferred morphology of the granules showed a higher threshold voltage (UT) and better I-U nonlinearity (higher coefficient of nonlinearity a) than the ceramics produced from granulates G2 and G3. Keywords: ZnO; varistor ceramics; granulate morphology; compression pressure; strength; electrical properties Vpliv granulata in pritiska na zelene oblikovance in tokovno-napetostne karakteristike sintrane varistorske keramike na osnovi ZnO Izvleček: Analizirali smo kompaktnost in tečljivost granulatov G1, G2 in G3, ki imajo enako varistorsko sestavo in različno morfologijo granul glede oblike, velikosti in porazdelitve velikosti. Preverili smo vpliv granulata na pripravo zelenih kosov v obliki diska s premerom 20 mm pri enoosnem stiskanju s pritiski od le 3,2MPa do 300MPa. Pri vseh granulatih se je pokazala podobna odvisnost gostote in trdnosti zelenih oblikovancev od pritiska stiskanja in pri enakem pritisku so imeli podobno gostoto. Gostote varistorske keramike, sintrane pri 1200 °C 2 uri, so pokazale majhno odvisnost od pritiska stiskanja in granulata, kar je posledica sintranja v prisotnosti tekoče faze Bi2O3; keramika je imela 93% teoretično gostoto že pri stiskanju oblikovancev s pritiskom 10MPa, pri višjih pritiskih stiskanja pa je gostota narasla na 96%. Pritisk enoosnega stiskanja ima dokaj majhen vpliv tudi na tokovno-napetostne (I-U) lastnosti varistorske keramike. Varistorska keramika iz granulata G1 z želeno morfologijo granul je imela višjo prebojno napetost (UT) in boljšo nelinearnost I-U (višji koeficient nelinearnosti a) kot keramika iz granulatov G2 in G3. Ključne besede: ZnO; varistorska keramika; morfologija granulata; pritisk stiskanja; trdnost; električne lastnosti * Corresponding Author’s e-mail: slavko.bernik@ijs.si Journal of Microelectronics, Electronic Components and Materials Vol. 47, No. 3(2017), 171 – 177 1 Introduction An exceptional current-voltage (I-U) nonlinearity and a high energy-absorption capability are the reasons why ZnO-based varistors are widely used in the protection of electrical devices, electronic circuits and power sys- tems against impulse voltage transients over a broad range from a few volts up to several 100 kV. The unique characteristics of ZnO-based varistor ceramics are closely related to their microstructure and arise from the combined effects of the I-U nonlinearity of the 172 S. Bernik et al; Informacije Midem, Vol. 47, No. 3(2017), 171 – 177 grain boundaries and the high conductivity of the ZnO grains. The nonlinearity results from the electrostatic barriers, which ideally have a breakdown voltage of about 3.2V and are induced by the presence of a Bi2O3 layer at the grain boundaries. The high conductivity of the grains is obtained by doping the ZnO with oxides of Co, Mn and Ni. The size of the ZnO grains determines the number of grain boundaries at a certain thickness of the ceramic and hence its breakdown voltage, which is the sum of the breakdown voltages of all the non- linear grain boundaries.[1,2] Control of the grain size is therefore important when tailoring the breakdown voltage of the varistor ceramics; hence, either Sb2O3 or TiO2 are usually added to prepare fine-grained, high- voltage or coarse-grained, low-voltage ceramics, re- spectively.[3] Accordingly, the starting composition of the varistor powder mixture is rather complex, as typi- cally up to about 10% by weight of Bi, Sb, Co, Mn, Ni and Cr oxide is added to the ZnO powder. In the pro- cess of sintering at a temperature of about 1200 °C, a complex microstructure is developed, typically com- posed of the ZnO phase and the secondary phases, the Bi2O3-rich phase, the Zn7Sb2O12-type spinel phase and the Bi3Zn2Sb3O14-type pyrochlore phase, which has to support the I-U characteristics as required by a particu- lar application. However, sintering is only the final step in processing the varistor ceramics and in such a com- plex system regarding its chemical composition and the electrical properties, the entire previous history of preparation, especially the proper homogenization of the starting powder mixture for chemical (i.e., compo- sitional) homogeneity and compaction (i.e., pressing) of green molds, also has a strong influence on the final physical properties, much more than with the simpler systems. [4,5] In the processing of varistors, pieces of varistor ceramic with various dimensions are usually prepared using a standard powder metallurgy procedure with uniaxial pressing of the granulate. Under laboratory conditions, smaller pieces of ceramic can be shaped by pressing the powder or a mixture of powders without or, if nec- essary, with adding a binder to improve the compress- ibility and to prevent the formation of cracks or, in the extreme, layering due to internal stresses. The process of shaping must ensure the required repeatability in the production of green bodies without defects, which is requirement in order to obtain ceramics without defects also after sintering. Even with slightly larger samples and in large-scale production this cannot be ensured without the use of granulate, the characteris- tics of which become the determining factor. The use of a granulate with the appropriate mechanical prop- erties is therefore necessary for the optimization of the pressing process. Spray-drying of the suspension (i.e., the slurry) is typically used in the processing of granu- late; its characteristics (shape, size, size distribution, den- sity, packing, mechanical strength) are strongly affected by the characteristics of the suspension, which also has to ensure the compositional homogeneity. The granu- late must provide excellent compressibility, which means achieving the maximum green density using the lowest possible pressure and an adequate uniformity required for the production of green molds without defects. In order to provide unique compression properties, it is desirable that the granules are round and have a smooth surface. In order to understand what kind of granulate and what kind of granulate properties are required, the processes and mechanisms of granulate deformation during compres- sion need to be known.[6-17] The pressing of granules usually involves plastic and/or vis- coelastic deformations. In the plastic material, the bound- ary of elasticity, when the material returns to its original condition after load removal, is rapidly reached; with a further load there are irreversible changes. The viscoelas- ticity is reflected in the viscous and elastic properties. The viscous material resists the load and the strain in material is linearly increasing with the load time. The elastic mate- rial, however, deforms under load, but when the load is removed, it can return to its original condition. In the case of granules that exhibit plastic deformation and have dif- ferent mechanical strengths, some are crushed under the pressure, while those with greater mechanical strength re- main whole and cause the formation of a heterogeneous structure with defects. On the other hand, perfectly viscous granules would form perfectly close packing without an applied pressure. This implies that the viscoelastic behav- ior plays an important role in the compaction process of the granulate. In the process of pressing granulate, first, at low levels of compression pressure, rearrangement of the granules occurs, so that the gaps between them are filled. A further increase in the pressure level tends to increase the remaining tension due to fracture or deformation of the granules. From the viscoelastic deformation of gran- ules, the process of stress release results, which depends on the characteristics of the granules. Thus, the binder-free granules have a lower viscoelastic deformation than those having a binder, which indicates that the compressibility is highly dependent on the binder. With a further increase in the compression pressure, the relaxation tension of the granules increases, thereby increasing the packing den- sity.[6-17] The influence of the morphology of the granules on the compression quality is reflected in the bending strength of the pieces of ceramic. A higher bending strength comes from ceramics with higher density. A granulate of full granules with a spherical morphology, which allows the preparation of pieces with a higher green density, can con- tribute to the higher bending strength of ceramics. How- ever, the results showed that a higher density of granules 173 tends to reduce their compressibility. In the case of green bodies with a density virtually identical to the density of the granules, apparently there was no deformation of the granules in the process of compression; such pieces are non-homogeneous and contain defects that greatly im- pair the strength of sintered ceramics. The lower-density granules must break down in the compression process to form a homogeneous and compact body. The most appropriate is therefore granulate from spherical and full granules, which exhibit the highest relaxation stress as a result of the viscoelastic deformation; it is a granulate hav- ing a lower density of full granules, so that the difference in density between the granules and the green body is a maximum.[6-17] In this work granulates with different morphologies and same varistor composition were characterized for bulk and tapped density. Their flow characteristics were also deter- mined from the compressibility index and the Hausner ratio. Furthermore, their characteristics when uniaxially pressed were analyzed for pressures ranging from 3.2 MPa to 300 MPa by determining the density and strength of their green compacts. Finally, the influence of the granu- late and the compaction pressure on the density and I-U characteristics of ceramics sintered at 1200 °C for 2 hours was studied. 2 Experimental In the study the varistor granulates G1, G2 and G3 with different morphological characteristics were examined and used for the preparation of ZnO-based varistor ce- ramics. The granulates were prepared using the same spray-drying conditions from stable water slurries having the same composition and amount of varistor powder mixture (i.e., ZnO powder doped with oxides of Bi, Sb, Co, Mn, Ni and Cr) but a different amount of added binder. The bulk density (r0) of the granulates was obtained by adding a known mass (m) of granulate to a graduated cyl- inder to determine the unsettled apparent volume (bulk volume, V0). By mechanically tapping a graduated cylinder containing the sample until little further volume change is observed to obtain the final tapped volume (VF) the tapped density (rF) was obtained. The flow characteristics of the granulates were estimated from the compressibility index (i.e., the Carr index) and the Hausner ratio. The com- pressibility index was determined using the expression Ci = 100(V0-VF)/V0 (in %) and the Hausner ratio as HR = V0/VF. From the studied granulates disc-shaped green compacts having a mass of 3g and a diameter of 20 mm were uni- axially pressed in a stainless-steel die at different pressures of 3.2 MPa, 10 MPa, 50 MPa, 100 MPa, 150 MPa, 200 MPa and 300 MPa. Their green density and strength were deter- mined with respect to the compacting pressure to assess the influence of the granulate’s characteristics. The biaxial flexural strengths of the green samples were measured with a piston-on-three-balls set up, according to the ISO 6872 standard, on a universal testing machine (Quasar 50; Galdabini, Varese, Italy) at a loading rate of 1 mm/min. Microstructures of the fractured surfaces of green pieces shaped at different pressures from granulate G1 after a strength analysis were examined on a scanning electron microscope (SEM, JSM-5800, JEOL, Japan). Green com- pacts from all the studied granulates and pressed with different pressures were sintered in air at a temperature of 1200 °C for 2 hours. Their sintered densities (as well as their green densities) were measured using a den- sity-measurements system Densitec (Metar sa, Matran, Switzerland). For the DC current-voltage (I-U) charac- terization, silver electrodes were painted on both paral- lel surfaces of the discs and fired at 600 °C. The nominal varistor voltages (UN) at 1mA/cm 2 and 10mA/cm2 were measured using a Keithley 2410 Digital SourceMeter, and the threshold voltage UT (V/mm) and the non-line- ar coefficient α were determined. The leakage current (IL) was measured at 0.75UN (1mA/cm 2). Figure 1: SEM images of varistor granulates G1, G2 and G3 showing differences in their morphology. 3 Results and discussion The varistor granulates examined in this work and used for the preparation of the ZnO-based varistor ceramics are shown in Fig. 1. The differences in their morpholo- gies are clearly evident. The granulate G1 has nice spherical granules with quite a uniform size distribu- tion in the range from the smallest of about 10 µm to the largest of about 200 µm. In the granulate G2 the siz- es of the smallest and the largest granules are similar to G1; however, the share of smaller granules with sizes in the range from 10 µm to about 50 µm is evidently dom- inating. The granulate G3 has granules with similar sizes and size distribution as G1; however, the granules have imperfect shapes, often with a crater. While the smaller S. Bernik et al; Informacije Midem, Vol. 47, No. 3(2017), 171 – 177 174 granules are full, the larger granules can be either hollow or full in all three granulates, as can be seen in Fig. 2. Figure 2: SEM image showing that granules, especially larger ones could be either full or hollow. Table 1 lists the bulk (r0) and tapped (rF) densities of the granulates as well as their flow characteristics estimat- ed from the compressibility index (Ci) index and the Hausner ratio (HR) in accordance to the scale given in Table 2. [18] The granulate G1 with a perfect spherical morphology and uniform size distribution of the gran- ules has the highest bulk and tapped densities and an excellent flow characteristic. The granulate G2, which also has perfectly spherical granules, but non-uniform size distribution (i.e., the proportion of small granules predominates), has lower bulk and tapped densities than G1. Nevertheless, according to Ci and HR its flow characteristic is still excellent. However, the granu- late G3 with a deformed shape of the granules but a a uniform size distribution has similar bulk and higher tapped density than G2, and in regard to its higher Ci and HR values, one level poorer flow properties, classi- fied as good. Table 1: Bulk (r0) and tapped (rF) density of granulates, and their compressibility index (Ci) and Hausner ratio (HR) for an estimation of the flow characteristics with re- spect to the scale given in Table 2 Granu- late V0 (ml) VF (ml) ρ0 (g/ml) ρF (g/ml) Ci (%) HR Flow character G1 23.25 21.25 1.10 1.20 8.6 1.09 excellent G2 25.00 23.00 1.02 1.11 8.0 1.09 excellent G3 25.05 22.00 1.02 1.16 12.2 1.14 good Regardless of their morphological differences and the differences in their compactness and flow characteris- tics, all the granulates enable the uniaxial pressing of disc-shaped green compacts with a diameter of 20 mm across a broad range of pressures from the lowest of only 3.2 MPa to the highest of 300 MPa without any difficulties; even after pressing with the lowest pressures the green compacts from all the granulates were compact enough for subsequent handling without any problem and at the highest pressure no lamination of the discs was observed. The density of the green compacts from the granulate G1 with respect to the pressure of the uniaxial pressing is given in Fig. 3; the density rapidly increased with pres- sure up to 50 MPa to about 3 g/cm3, while with further increasing of pressure the density increased steadily to the value of about 3.6 to 3.7 g/cm3 at 300 MPa. Practically same green densities for the given pressure and the same dependence of the green density on the pressure were obtained also for the granulates G2 and G3. Table 2: Flow properties of solids with respect to their Carr index (i.e., compressibility index) and Hausner ratio.[18] Cerrs´ index (%) Flow character Hausner ratio 1-10 Excellent 1.00-1.11 1-15 Good 1.12-1.18 16-20 Fair 1.19-1.25 21-25 Passable 1.26-1.34 26-31 Poor 1.35-1.45 32-37 Very poor 1.46-1.59 >38 Very, very poor >1.60 The strengths of the green pieces prepared at differ- ent pressures of uniaxial pressing are given is Fig. 4. All the granulates show a similar dependence of the strength on the pressure. However, while the samples from granulates G2 and G3 have similar strengths at all pressures, it is higher for the samples from granulate G1 and the difference increases with increasing pres- sure. After pressing with 300 MPa the samples G1 have a strength of about 17 MPa and the samples G2 and G3, of about 13 MPa. Figure 3: Green-density (G1-gd) and density after sin- tering at 1200 °C (G1-sd) of samples from the granulate G1 with respect to the pressure of uniaxial pressing. S. Bernik et al; Informacije Midem, Vol. 47, No. 3(2017), 171 – 177 175 about 80% of theoretical density). However, for a pres- sure of just 10 MPa the density of the sintered ceramics increased to about 5.2 g/cm3 (93 % t.d.) and with further increasing of the pressure this only slightly increased to about 5.4 g/cm3 (96 % t.d.). The sintered ceramics from the granulates G2 and G3 had a similar density at a cer- tain pressure and a similar dependence of the density on the compression pressures as the ceramics form the granulate G1. Such results, which showed very little influ- ence of the granulate morphology and the pressure of uniaxial pressing on the density, could be explained by the sintering of the varistor ceramics, which takes place in the presence of the Bi2O3-rich liquid phase. The current-voltage (I-U) characteristics of the varis- tor ceramics from granulates G1, G2 and G3, uniaxially pressed at different pressures and sintered at 1200 °C for 2 hours are presented in Figs. 6 and 7. Surprisingly, very little influence of the compression pressure on the I-U characteristics was observed. Only in the ceramics pressed with 3.2 MPa were noticeably different values of the threshold voltage (UT) and the coefficient of nonlin- earity (a) observed, while already for a pressure of 50 MPa and higher they were similar for the ceramics from all three granulates. However, the ceramics form granulate G1 had a higher UT (about 125 V/mm) than the ceramics from the granulates G2 and G3 with values around 108 V/mm (Fig. 6). Also, the ceramics from granulate G1 had a much higher α of about 34 in comparison to values of about 20 for the ceramics from the granulates G2 and G3 (Fig. 7). Actually, in the ceramics from granulate G3 the value of α increases with a higher compression pressure from 19 at 10 MPa to 23 at 300 MPa. Interestingly, α in the ceramics from granulates G2 and G3 pressed with only 3.2 MPa is higher (25) than at higher pressures. In varis- tor ceramics from all the studied granulates the leak- age current (IL) is similar in the range from 4 to 8 µA, regardless of the pressure of the uniaxial pressing. Figure 6: Threshold voltage UT (V/mm) of the varistor ce- ramics from granulates G1, G2 and G3, uniaxially pressed at different pressures and sintered at 1200 °C for 2 hours. Figure 4: Strength of the green compacts from granu- lates G1, G2 and G3 with respect to the pressure of uni- axial pressing. Microstructures of a fractured surface of the green samples from the granulate G1, uniaxially pressed at different pressures are presented in Fig. 5. After a pres- sure of 3.2 MPa the granules remain whole, while be- ing slightly indented into each other so that porosity among them is still evident, which still ensures sufficient compactness for the green samples to be handled with- out difficulties. At a pressure of 10 MPa the shape of the individual granules is still clearly evident, the degree of indentation among the granules is already much larger so that that the remaining porosity among them is al- ready much lower and some granules are already partial- ly collapsed. However, already after a pressure of 50 MPa the microstructure of the fractured surface is perfectly homogeneous as all the granules are fully collapsed so that their shape was not distinct anymore. With a further increase of the pressure the microstructure of fractured surface remains the same. Figure 5: Microstructures of fractured surface of green samples pressed from granulate G1 at different pressures. The densities of the varistor ceramics from the granulate G1, uniaxial pressed at different pressures and sintered at 1200 °C for 2 hours, are graphically presented in Fig. 3. Interestingly, already the samples pressed with only 3.2 MPa had a sintered density of about 4.5 g/cm3 (i.e., S. Bernik et al; Informacije Midem, Vol. 47, No. 3(2017), 171 – 177 176 Figure 7: Coefficient of nonlinearity a of the varistor ceramics from granulates G1, G2 and G3, uniaxially pressed at different pressures and sintered at 1200 °C for 2 hours. 4 Conclusions In this work granulates G1, G2 and G3, having the same composition of the varistor powder mixture and differ- ent morphologies of the granules, were characterized for their compacting and flow characteristics. Granu- late G1 with a uniform size distribution of spherical granules had the highest bulk and tapped densities, and according to the Carr index (Ci) and Hausner ratio (HR) they also had excellent flow characteristics. The granulate G2 with the spherical granules but the non- uniform size distribution due to a dominant fraction of smaller granules also had excellent flow characteristics, while the granulate G3 with a uniform size distribution but deformed granules qualified for flow characteristic as good. All the granulates showed good properties in terms of the uniaxial pressing of green bodies for pres- sures in the range from only 3.2 MPa to 300 MPa; green pieces could be handled without difficulties and no lamination occurred. For all the granulates the density and strength of green samples showed similar depend- ence to the pressure and for the same pressure the green densities were similar. The samples from G1 with a preferred morphology of the granules had a higher strength. The density of varistor ceramics sintered at 1200 ˘C for 2 hours showed very little dependence on the compression pressure and the granulate used, and reached 93% of the theoretical density already when pressed with only 10MPa, while for higher pressures the density increased to 96%. This could be explained by the sintering of the varistor ceramics in the pres- ence of the Bi2O3-rich liquid phase. Accordingly, also a relatively low influence of compression pressure on the current-voltage (I-U) characteristics of the varistor ceramics was observed. However, the varistor ceram- ics from the granulate G1 showed a higher threshold voltage (UT) and a better I-U nonlinearity (higher co- efficient of nonlinearity a) than the ceramics from the granulates G2 and G3. The reasons can be found in their microstructure, which should be thoroughly ex- amined in the future. 5 References 1. T. K. Gupta, Application of zinc oxide varistors, J. Am. Ceram. Soc., vol. 73, no. 7, pp. 1817-1840, 1990. 2. D. R. Clarke, Varistor ceramics, J. Am. Ceram. Soc., vol. 82, no. 3, pp. 485-502, 1999. 3. S. Bernik, N. Daneu, A. Rečnik, Inversion bound- ary induced grain growth in TiO2 or Sb2O3 doped ZnO-based varistor ceramics, J. Eur. Ceram. Soc., vol. 24, pp. 3703–3708, 2004. 4. M. Inada, Formation Mechanism of nonohmic zinc oxide ceramics, Jpn. J. Appl. Phys., vol. 19, no. 3, pp. 409-419, 1980. 5. M. L. 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Bernik et al; Informacije Midem, Vol. 47, No. 3(2017), 171 – 177 178 179 Original scientific paper  MIDEM Society Processing and sintering of sodium-potasium niobate–based thick films Hugo Mercier1,2,3, Barbara Malič1,2, Hana Uršič1, Danjela Kuscer1, Franck Levassort3 1Jožef Stefan Institute, Electronic Ceramics Department, Ljubljana, Slovenia 2Jožef Stefan International Postgraduate School, Ljubljana, Slovenia 3GREMAN UMR-CNRS 7347, Université de Tours, INSA Centre Val de Loire, Tours, France Abstract: The electrophoretic deposition (EPD) and sintering of (K0.5Na0.5)0.99Sr0.005NbO3 (KNNSr) thick films on platinized alumina substrate is reported. We demonstrate that by a two-step deposition-sintering the KNNSr films thicker than 30 μm without defects can be prepared. The effect of the sintering time on structural, microstructural, dielectric and electromechanical characteristics of the KNNSr thick films is discussed. By increasing the sintering time from 2 to 4 hours, the density and the dielectric permittivity of the thick films increased. The unit cell parameters of the perovskite phase decreased which could be related to the formation of polyniobate and volatilization of akalies. Processed KNNSr exhibited promising electromechanical and piezoelectric properties, with a thickness coupling factor up to 35 % and piezoelectric coefficient d33 up to 80 pC/N. Keywords: sodium potassium niobate; electrophoretic deposition; thick films, piezoelectric properties; electromechanical properties Priprava in sintranje debelih plasti na osnovi natrijevega kalijevega niobata Izvleček: Študirali smo sintranje debelih plasti (K0.5Na0.5)0.99Sr0.005NbO3 (KNNSr), ki smo jih na metalizirani korundni podlagi pripravili z metodo elektroforetskega nanosa. Pokazali smo, da z dvostopenjskim nanosom in sintranjem lahko pripravimo plasti KNNSr debelejše od 30 µm in brez defektov. V prispevku poročamo o vplivu časa sintranja plasti KNNSr na njihovo strukturo, mikrostrukturo ter dielektrične in elektromehanske lastnosti. Gostota in dielektrična konstanta plasti KNNSr se povečata s podaljšanjem časa sintranja od 2 na 4 ure, parametri perovskitne osnovne celice pa se zmanjšajo, kar pripisujemo tvorbi sekundarne faze poliniobata in izhajanju alkalijskih oksidov med sintranjem. Plasti KNNSr imejo obetajoče elektromehanske in piezoelektrične lastnosti: povprečni sklopitveni faktor je 35 %, piezoelektrični koeficient d33 pa do 80 pC/N. Ključne besede: natrijev kalijev niobat; elektroforetski nanos; debele plasti; piezoelektrične lastnosti; elektromehanske lastnosti * Corresponding Author’s e-mail: hugo.mercier@univ-tours.fr Journal of Microelectronics, Electronic Components and Materials Vol. 47, No. 3(2017), 179 – 185 1 Introduction Piezoelectric energy harvester (PEH) allows the conver- sion of mechanical energy into electrical energy which can power wireless, self-powered microsystems and macroscale devices [1, 2]. A typical PEH structure is a bimorph cantilever with two layers of piezoelectric ma- terial on both sides of a flexible substrate. Piezoelectric layers of lead-based materials, typically lead zirconate titanate, have been mainly used due to their outstanding piezoelectric properties [3]. However due to lead toxicity and environmental problem there is a need to replace them with environment-benign material. Among the lead-free piezoelectric materials, potassium sodium niobate ((K0.5Na0.5)NbO3 - KNN) have been extensively studied. The processing of a dense single phase KNN ceramics is challenging, due to the hygroscopic nature of the raw material, the narrow temperature range of sintering, and the volatilization of alkaline species at the processing temperature [4]. Literature reports that addition of 0.5 % Sr, as A-site do- nor dopant, ((K0.5Na0.5)0.99Sr0.005NbO3 - KNNSr) improves the density and dielectric/piezoelectric properties of the material (d33 ~ 80 pC/N, kt ~ 0.4, dielectric permit- tivity ~ 500 and dielectric losses ~ 0.03) [4, 5]. 180 Bimorph cantilever PEH have been often prepared by thinning piezoelectric bulk ceramics from a few hun- dreds to tens of micrometers and consequent gluing of slices [7,8] onto a substrate. This process is complicated and time consuming. Another possibility is to process the thick piezoelectric films directly onto a conductive substrate using thick film technologies. Among them, electrophoretic deposition (EPD) [9] allows the depo- sition of piezoelectric thick films within a few minutes onto complex-shape substrates. EPD is suitable meth- od for processing a cantilever, since the deposition can be done simultaneously on the top and bottom side of the substrate. KNN-based thick films have been prepared by EPD us- ing ethanol-, acetone- and water-based suspensions [10–12]. In a previous work, the processing of KNNSr thick films prepared from ethanol-based suspension, and sintered at 1100 °C for 2 hours in air, was reported. The processed thick films were around 25 μm thick and they exhibited a relative density of 75 %, a dielectric constant of 294, dielectric losses of 0.05 at 1 MHz at 20 °C and a d33 of 60 pC/N [12]. The lower density of thick films compared to bulk ce- ramic can be related to the sintering of the thick film in constrained conditions. The clamping of the thick film on the substrate may result in a tensile stress which hindered the densification of the layers [13, 14]. The stresses relax through growth of defects and or/delam- ination of the thick film from the substrate [15]. In order to improve the density and functional prop- erties of KNN-based thick films, the effect of sintering conditions were studied. It was shown that density of KNN-based material increases with the sintering time [16], but larger time may lead to the formation of larger amount of secondary phases [17]. Sintering of KNN- based ceramics in oxygen atmosphere showed several advantages compared to sintering in air: increased density [18], limited amount of secondary phases [17], reduced oxygen vacancy concentration and improved electromechanical properties [19, 20]. The aim of this work was to process KNNSr thick films on platinized alumina substrate by EPD with final thick- ness greater than 30 μm. In this case the use of a two steps deposition-sintering process was developed. Moreover the effect of sintering time, i.e., 2 and 4 hours, on the structure, microstructure, dielectric and electro- mechanical properties of the thick films was studied. 2 Experimental (K0.5Na0.5)0.99Sr0.005NbO3 (KNNSr) powder was prepared by solid-state synthesis as described elsewhere [21]. Bulk KNNSr ceramics was prepared by compacting KNNSr powder with a uniaxial press into cylindrical samples, followed by cold isostatically pressing at 200 MPa and sintering at 1120 °C in air with heating and cooling rates of 5 K/min. The suspension was prepared by mixing 1 vol % of the KNNSr powder in absolute ethanol (C2H5OH, anhydrous, Carlo Ebra, Italy) with 50 μmol/g poly(acrylic acid-co- maleic acid) (Mw 3.000, 50 wt % water solution, Sigma Aldrich, Germany) and 100 μmol/g n-butylamine (Alfa Aesar, Germany), as detailed in [12]. The substrate acting as a working electrode was pre- pared by screen-printing platinum paste (Ferro 1192, USA) onto an alumina substrate (A493, Kyocera, Japan). The paste was deposited on a region of 8 x 8 mm and fired at 1200 °C for 1 hour in air. The counter electrode was a platinum disc (thickness of 0.1 mm and diameter of 8 mm). EPD were performed in a custom-made setup at a constant current density of 1.56 A/cm2 provided by a Keithley 2400 source meter. The layers were depos- ited either in a single step for 120 s (sample denoted KNNSr-R) or in two steps. In two steps deposition, the first step included the deposition of KNNSr for 60 s, and the presintering of as-deposited layers at 1000 °C for 2 h in oxygen with heating and cooling rate of 2K/min. Samples were denoted KNNSr-L. In the second step, second layers were deposited onto KNNSr-L for 60 s and afterward sintered at 1100 °C in oxygen for 2 hours (sample denoted KNNSr-2h) and 4 hours (sample de- noted KNNSr-4h). The X-ray powder-diffraction data (XRD) were collected in the 2θ range from 20° to 60° in steps of 0.034°, by PANalytical diffractometer (X’Pert PRO MPD, The Neth- erlands). The phases were identified using the soft- ware X-Pert High Score and the PDF-2 database [22]. The unit cell parameters were refined using JANA2006 software, by performing a full pattern matching, as- suming a P1m1 space group. A zero-error shift correc- tion was used to obtain the correct initial position, the background was calculated using a Legendre polyno- mial and the peak profiles were refined using a pseudo- Voigt function. The polished cross-sections of the sintered films were analyzed using scanning electron microscopes (SEM, JSM-5800 and JSM 7600F, both JEOL, Japan). H. Mercier et al; Informacije Midem, Vol. 47, No. 3(2017), 179 – 185 181 Gold electrodes, with a diameter of 1.5 mm and a thick- ness of ~100 nm, were sputtered (5 Pascal, Italy) on top of the sintered samples. The capacitance and dielectric losses (tan δ) were measured in the frequency range 10 kHz-1 MHz at room temperature with an impedance spectroscopy analyzer (4192A Hewlett Packard, USA). Samples were poled for 40 min at 120 °C with a DC- electric field of 3 kV/mm. The piezoelectric constants d33 were measured with a Berlincourt piezometer (Take Control PM10, Birmingham, UK). The complex electrical impedance, around the fun- damental thickness mode was measured using a vec- tor analyzer (HP4395) and its impedance test kit. A theoretical model, based on the Krimholtz-Leedom- Matthaei (KLM) equivalent electrical circuit [23], deliv- ers the theoretical complex impedance of the thick- ness mode as a function of the frequency. By fitting the experimental data with the theoretical model, the thickness mode parameters of the KNNSr sintered and poled thick films are deduced. The modeled structure was composed of 3 layers: the alumina substrate; the Pt bottom electrode and the thick piezoelectric film. The top gold electrode was neglected as it was too thin to have a significant effect. The parameters of the inert layers [24] and the thickness of the piezoelectric films are considered as fixed in the model. The following pa- rameters were deduced: the thickness coupling factor kt, the longitudinal wave velocity vl, the dielectric con- stant at constant strain ε33 S/ε0 and the tangent of the dielectric loss angle δe. 3 Results and discussion 3.1 Processing of KNNSr thick films Sample KNNSr-R, deposited in a single step, exhibited defects at its surface, resulting in shortcuts, preventing the electrical measurements, see Figure 1.a. The thick- ness of the as-deposited layer KNNSr-R was around 60 μm and around 40 μm after sintering. This thickness was presumably above the critical thickness thus de- fects occurred during the drying and/or the sintering of the thick film [25,26]. In order to process defect-free thick films with final thicknesses above 30 μm, a two steps process was used. A first layer was deposited and pre-sintered (KNNSr-L). KNNSr-L had a deposited yield of 3.5 ± 0.5 mg/cm2 and a sintered thickness of 18 ± 5 μm and did not exhibit large defects after its process- ing (Figure 1.b). The second layer was deposited on the top of the first one (60 s) and sintered at selected conditions given for KNNSr-2h and KNNSr-4h. The cumulative deposition time for the two layers was the same as for KNNSr-R, i.e., 120 s. The processed thick films prepared with the described procedure did not exhibit large defects at their surfaces (Figure 1.c and d). 3.2 Sintered KNNSr thick films The XRD patterns of the thick films sintered for 2 (KNNSr- 2h) and 4 hours (KNNSr-4h) are presented in Figure 2. Both samples consisted of a main monoclinic perovs- kite phase indexed as K0.65Na0.35NbO3 (PDF: 77-0038), and a polyniobate phase indexed as K5.75Nb10.85O30 (PDF: 38-0297). In addition the diffraction peaks of the top gold electrode and of the platinum bottom electrode were also observed on the diffraction pattern. The for- mation of the polyniobate phase, which was not identi- fied in the initial powder, may be related to the volatili- zation and segregation of alkaline species [4]. Figure 2: XRD patterns of KNNSr thick films sintered at 1100 °C in oxygen for 2 and 4 hours. x: K0.65Na0.35NbO3 (PDF: 77-0038), o: K5.75Nb10.85O30 (PDF: 38-0297), Δ: Plati- num (PDF:04-0802), □: Gold (PDF: 01-1172). H. Mercier et al; Informacije Midem, Vol. 47, No. 3(2017), 179 – 185 Figure 1: SEM image of the surface of sintered KNNSr thick films a) KNNSr-R b) KNNSr-L c) KNNSr-2h and d) KNNSr-4h. 182 The calculated cell parameters for the samples KNNSr- 2h and KNNSr-4h are presented in Table 1. The calcu- lated unit cell parameters exhibited lower values than the one reported in the literature for bulk ceramics with the same composition [27]. The reduction in the unit cell parameters may be attributed to two phenomena. First phenomenon was the loss of alkali oxides during the sintering. Literature reports that potassium vapor pressure above KNN is greater than sodium one [28,29]. Since potassium ions have a larger radius (K+, 0.164 nm) than sodium ions (Na+, 0.139 nm) the perovskite with K/Na ratio smaller than 1 resulted in a decrease of the unit cell parameters of the perovskite phase [29,30]. In the case of a thick film this phenomenon is expected to be enhanced due the increased area to volume ratio in comparison to bulk ceramic. The second phenom- enon was the formation of a polyniobate phase, clearly observed on the XRD spectra. The polyniobate phase had a potassium-rich composition, thus the perovskite phase could have lower potassium content and this again decreased the unit cell parameters of the per- ovskite phase. On XRD patterns peaks attributed to the polyniobate phase had higher intensity in the samples KNNSr-4h in comparison to KNNSr-2h which indicated the presence of a larger amount of polyniobate phase and may explain the further reduction in the unit cell parameters of KNNSr-4h. The polished cross-section SEM images of the sintered thick films KNNSr-2h and KNNSr-4h, are presented in Figure 3.a and b, respectively. The microstructure of the samples consisted of micrometers sized pores randomly distributed in a KNNSr particles matrix, see insets Figure 3. Phase composition by EDXS showed that the matrix is a perovskite with K/Na ratio of about 1. Larger grains (see black arrows on the SEM images) had a K/Na ratio great- er than 1 and were presumably polyniobate phase. The relative density, estimated from the SEM images were 77 ± 4 % and 87 ± 4 % for KNNSr-2h and KNNSr-4h, respec- tively. This showed that by increasing the sintering time from 2 to 4 hours, the density of the thick films increased by 10 %. The thickness of KNNSr-2h and KNNSr-4h were 33 and 45 μm, respectively. The discrepancy in the thick- ness was attributed to the variation in the as-deposited layers thickness, i.e. ± 5μm, for each deposited layer. Table 1: Refined unit cell parameters of the KNNSr thick films sintered at 1100 °C in oxygen for 2 h (KNNsr-2h) and 4 h (KNNSr-4h) and unit cell parameters reported for the KNNSr ceramics. Sample a b c β V [nm] [nm] [nm] [°] [nm3] KNNSr-2h 0.40028(9) 0.39443(6) 0.39961(9) 89.70(2) 0.06309(4) KNNSr-4h 0.40016(9) 0.39431(6) 0.39946(9) 89.69 0.06303(2) KNNSr bulk [27] 0.400375 0.394596 0.399938 90.3228 0.06318 Estimated error ± 0.0001 ± 0.0001 ± 0.0001 ± 0.001 ± 0.00002 3.3 Functional characterizations The dielectric measurements of the thick films are pre- sented in Figure 4. The permittivity and losses decreased with increasing frequency. Values of KNNSr-4h permit- tivity and losses are higher than those of KNNSr-2h in the same measured frequency range. The dielectric permit- tivity and losses at 100 kHz were 230 and 0.04, respec- Figure 3: Polished cross-section SEM images of KNNSr thick films sintered at 1100 °C in oxygen for a) 2 hours (KNNSr-2h) and b) 4 hours (KNNSr-4h). The insets show KNNSr thick films microstructures at larger magnifica- tion; the black arrows show examples of larger grains (polyniobate). H. Mercier et al; Informacije Midem, Vol. 47, No. 3(2017), 179 – 185 183 tively, for KNNSr-2h against 280 and 0.13, respectively for KNNSr-4h samples. The increased permittivity may be related to the increased density of the thick film. The increased losses at low frequencies are presumably re- lated to the increased electrical conductivity in the sam- ple. This may be due to the increased amount of second- ary phases and/or to the humidity during the dielectric measurement which can strongly impact the dielectric permittivity and losses at low frequencies (under 10 kHz) in strontium doped KNN ceramics [31]. The piezoelectric and electromechanical properties of KNNSr-2h and KNNSr-4h are presented in Table 2. The experimental and theoretical complex impedances of the KNNSr-2h sample are presented in Figure 5, show- ing the good agreement between the adjusted KLM scheme and the experimental data. For both KNNSr-2h and KNNSr-4h samples, high d33 co- efficients around 80 pC/N were obtained. Similar elec- tromechanical properties for KNNSr-2h and KNNSr-4h samples were measured with kt up to 35 %. Dielectric constant, kt and d33 of the thick films were lower than the one of the processed KNNSr bulk ceramics, which is related to the lower density of the thick films, their deviation from the stoichiometry KNN, and the polyni- obate phase. The thick films electromechanical prop- erties were similar to values reported in the literature [32]. This demonstrates that the thick films prepared by EPD were at the state of the art in terms of electrome- chanical properties. 4 Summary KNNSr thick films have been processed by EPD on plati- nized alumina substrates using a two steps deposition method followed by sintering at 1100 °C in oxygen for 2 and 4 hours. The structural, microstructural, dielectric and electromechanical properties have been reported. The increasing sintering time from 2 to 4 hours leads to higher density, and reduction in unit cell parameters which was related to the formation of a polyniobate phase and the volatilization of alkali oxides. Dielectric constant of the thick film sintered for 4 hours was 280 at 100 kHz and room temperature which is higher value compared to the samples sintered for 2 hours and in agreement with its increased density. The electrome- chanical performance stayed relatively stable with kt up to 35 % and d33 up to 80 pC/N. Table 2: Properties of thick piezoelectric films KNNSr-2h, KNNSr-4h, KNNSr bulk ceramic and KNN-based screen print- ed thick films. Sample Thickness kt ε33S/ ε0 δe d33 [μm] [%] [%] [pC/N] KNNSr-2h 33 35 155 3 80 KNNSr-4h 45 32 165 5 80 KNNSr-bulk ceramic 1120 °C, 2 h, air / 40 300 5 90 KNN-based screen printed thick films [32] 43 30 140 / / Figure 4: Dielectric permittivity (square) and dielectric losses (triangle) of the KNNSr thick films sintered at 1100 °C in oxygen for a) 2 hours (□,△, KNNSr-2h) and b) 4 hours (■,▲, KNNSr-4h). H. Mercier et al; Informacije Midem, Vol. 47, No. 3(2017), 179 – 185 Figure 5: Complex electrical impedance (Re(Z): real part, Im(Z): imaginary part) of KNNSr-2h thick film on platinized alumina substrate as a function of the fre- quency around fundamental resonance (■: experimen- tal; ̶̶̶̶̶ ̶̶ ̶ : theoretical). 184 5 Acknowledgment This work was supported by the Slovenian Research Agency (P2-0105), the French Research Agency (ANR 14-LAB5-004), the bilateral project BI-FR-16-17-PRO- TEUS-009/PHC PROTEUS 2016 (No. 35246NJ) and the Erasmus+ programme. The authors would like to thank the Center of Excellence NAMASTE for the use of equip- ment, Jena Cilenšek, Silvo Drnovšek, Maja Majcen and Hafsa Znibrat for their technical assistance. 6 Literature 1. R.M. Ferdous, A.W. Reza, M.F. Siddiqui, Renewable energy harvesting for wireless sensors using pas- sive RFID tag technology: A review, Renew. Sus- tain. Energy Rev. 58 (2016) 1114–1128. 2. H. Kim, Y. Tadesse, S. 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Kosec, Crystal structure and phase transitions of sodium potassium niobate perovskites, Solid State Sci. 11 (2009) 320–324. 31. V.Y. Shur, A.A. Esin, D.O. Alikin, A.P. Turygin, A.S. Abramov, J. Hre, A. Bencan, B. Malic, A.L. Kholkin, V.Y. Shur, Ceramics dielectric relaxation and charged domain walls in (K ,Na) NbO3 -based fer- roelectric ceramics, 74101 (2017). 32. F. Levassort, J.-M. Gregoire, M. Lethiecq, K. Asta- fiev, L. Nielsen, R. Lou-Moeller, W.W. Wolny, High frequency single element transducer based on pad-printed lead-free piezoelectric thick films, in: IEEE International Ultrasonics Symposium, ( 2011) 848–851. Arrived: 31. 08. 2017 Accepted: 22. 11. 2017 H. Mercier et al; Informacije Midem, Vol. 47, No. 3(2017), 179 – 185 186 187 Original scientific paper  MIDEM Society 1 Introduction In the paper, an integrated circuit including a symmet- rical Operational Transconductance Amplifier (OTA) with temperature compensation is described. The OTA can be presented as a voltage controlled current source. The ideal OTA has very high input and output impedances and a wide frequency bandwidth. The out- put current IOUT is proportional to the differential input voltage and is expressed as: dmOUT VgI = (1) where gm is the transconductance of the amplifier and Vd is the differential input voltage. Design of Operational Transconductance Amplifier with Temperature Compensation Damjan Berčan, Aleksander Sešek, Janez Trontelj University of Ljubljana, Faculty of Electrical Engineering, Laboratory for Microelectronics Abstract: In this paper an operational transconductance amplifier [1] with temperature compensation is presented. It is a voltage- controlled current source, which operates in an open loop configuration with a single output connected to a resistive load. The amplifier is internally compensated to keep the gain stable over the -40 °C to 125 °C temperature range. It features low input voltage noise and operates at supply voltages from 3 V to 5.5 V. Additionally, an internal 1.21 V bandgap reference is used to ensure a stable internal voltage reference point. The active area of the proposed integrated circuit designed with 0.18 µm Bipolar, CMOS, DMOS (BCD) technology is 750 µm x 260 µm. It consumes 423 µA of current and it has 8.87 nV/SHz of input noise at 500 kHz. The resulting simulated voltage gain is 40 dB and variations are less than ±0.3 dB over the temperature range of -40 °C to 125 °C. Keywords: Operational Transconductance Amplifier; Temperature compensated bias current; Temperature sensitivity optimization Načrtovanje operacijskega transkonduktančnega ojačevalnika s temperaturno kompenzacijo Izvleček: V članku je predstavljen transkonduktančni ojačevalnik s temperaturno kompenzacijo. Na izhod odprto-zančnega ojačevalnika oziroma napetostno krmiljenega tokovnega vira je priključeno uporovno breme. Notranja kompenzacija skrbi za stabilno ojačenje v temperaturnem območju od -40 °C do 125 °C. Poleg tega ojačevalnik izkazuje majhen vhodni šum in deluje v napetostnem območju od 3 V do 5.5 V. Vezje vsebuje napetostno referenco, ki poskrbi za stabilno referenčno točko. Integrirano vezje je bilo načrtano v 0.18 µm BCD (Bipolar, CMOS, DMOS) tehnologiji, njegova aktivna površina znaša 750 µm x 260 µm. Poraba ojačevalnika znaša 423 µA, njegova šumna gostota na vhodu je 8.87 nV/SHz pri frekvenci 500 kHz. Napetostno ojačenje znaša 40 dB in v temperaturnem območju od -40 °C do 125 °C odstopa za manj kot ±0.3 dB. Ključne besede: Transkonduktančni operacijski ojačevalnik; temperaturna kompenzacija delovnega toka; optimizacija temperaturne občutljivosti * Corresponding Author’s e-mail: damjan.bercan@fe.uni-lj.si Journal of Microelectronics, Electronic Components and Materials Vol. 47, No. 3(2017), 187 – 192 One of the major drawbacks of the OTA is its high tem- perature sensitivity, caused by inversely proportional temperature variations of gm [2, 3]. Transconductance of the Metal-Oxide-Semiconductor (MOS) transistor, using small signal model in saturation region is defined as follows: Doxnm IL WCg )(2µ= (2) where µn is carrier mobility, Cox is oxide capacitance, W is width of the device, L is length of the channel and ID is drain current [4]. 188 The root cause of temperature variations is the temper- ature dependent threshold voltage VT and the carrier mobility µn variations of the MOS transistor, according to equations (2) and (3): )()()( 00 TTTVTV VTTT −+= α (3) µα µµ     = 0 0 )()( T TTT nn (4) where T0 is the reference temperature (300 °K), αVT and αµ are negative values which vary with temperature [5]. As one of possible solutions, a circuit with output voltage Proportional To Absolute Temperature (PTAT) can be implemented to compensate the temperature variations of gm. The difference between the two base- emitter voltages in PTAT is expressed as: )/ln( 12 JJVV tBE =∆ (5) where Vt is thermal voltage and J1, J2 are different cur- rent densities of bipolar transistors. The PTAT circuit generates voltage which has a positive Temperature Coefficient (TC) [4]. The following section (Section 2) presents the OTA design method to overcome the mentioned problem. In section 3, simulation results for typical simulation conditions, pro- cess variations and Monte Carlo analyses are presented. 2 Ota circuit design The block diagram of the OTA with temperature com- pensation is shown in Fig.1. The main advantage of the proposed solution is the temperature compensated bias current. The compensation circuit consists of two resistors (R1 and R2) having different TC. The block dia- gram also includes internal a 1.21 V voltage reference (Vref), biasing current generator (Ibias generator) which compensate the temperature variations of gm, two Op- erational Amplifiers (OPA1 and OPA2) for voltage to current conversion and OTA circuit. Our objective was to keep the gain of the OTA stable over the -40 °C to Figure 1: Block diagram of integrated circuit. Figure 2: The schematic of integrated circuit. D. Berčan et al; Informacije Midem, Vol. 47, No. 3(2017), 187 – 192 189 125 °C temperature range, low input voltage noise and high Power Supply Rejection Ratio (PSRR). 2.1 Topology of the circuit The schematic of the balanced OTA with temperature compensation is shown in the Fig. 2. The voltage to currents converters are used to convert the reference bandgap voltage to the corresponding current. Both converters are designed as classic two stage amplifi- ers with a N type Metal-Oxide-Semiconductor (NMOS) input differential stage and a common source output stage including compensation capacitor and resistor. To ensure precise conversion, the temperature stable reference voltage is employed. The voltage reference - bandgap circuit, maintains a stable voltage over the temperature range and power supply voltage varia- tions. The OTA consists of NMOS input differential stage and three current mirrors. Transistors M20 – M23 form the first current mirror stage, M25 – M28 form the sec- ond current mirror stage and M13, M14, M18 and M19 form the third current mirror stage. To increase the out- put impedance the cascode current mirrors are used. The gate of transistors are connected and biased as low voltage cascode, which keeps the minimum drain source voltages of transistor M13, M22 and M25 and also insures transistor saturation operation. The Length (L) of transistor M10 is higher in order to compensate the body effect of cascode transistors M14 and M18 [6]. 2.4 Compensation circuit The compensation circuit consists of transistors M9 and M10 and two resistors (R1 and R2) with different TC, which is calculated using the box-method [7], -1618 ppm/ᵒC and 1322 ppm/ᵒC, respectively. The Fig. 3 pres- ents the currents IR1 and IR2, determined by resistors R1 and R2, and bias current Ibias, flowing through M10. The compensation circuit generates bias current Ibias which is stated as: Figure 3: Generated currents. 21 RRbias III +−= (6) The bias current Ibias increases with temperature com- pensating decreasing transconductance of OTA. 3 Simulation results 3.2 Results for typical simulation parameters The typical process parameters simulations at 25 °C were performed for the supply voltages 3.3 V and 5 V, respectively. The simulation results are presented in Table 1. Input noise density was measured at 500 kHz. Table 1: Simulation results at typical simulation condi- tions. Param. \ Suppl. 3.3 V 5 V Units Ivdda 423 540 µA G 40.2 40.4 dB BW 13.6 15.0 MHz Input noise density 8.87 8.56 nV/√Hz Input offset voltage 1.08 1.99 mV CMRR -132 -142 dB +SR 9.01 9.60 V/µs -SR 9.23 9.72 V/µs PSRR @ 100 Hz -73.5 -78.2 dB Transconductance 778 737 µA/V The input referred noise voltage can be further reduced by changing the channel W/L ratio of the M15 and M16 transistors or increasing the bias current Ibias. This ac- tion would lead to larger chip area (higher cost) and higher power consumption. To reduce the offset which effects the performance of the OTA, the auto-zeroing or chopping technique method could be used, adding complexity to the design. Precise matching strategies of the transistors and resistors devices must be used to minimize offsets and provide symmetry. Typical performance characteristics for a 3.3 V supply voltage are shown from Figures 4 to 7. The input re- ferred noise voltage was measured at 500 kHz, while PSRR was measured at 100 Hz. Fig. 4 shows the voltage gain vs. frequency at three temperatures (-40 °C, 25 °C, 125°C), which is 40 dB. The results show that the gain sensitivity to temperature variations are almost eliminated with the presented design. The bandwidth of the OTA varies from 14.1 MHz at -40 °C to 12.8 MHz at 125 °C. D. Berčan et al; Informacije Midem, Vol. 47, No. 3(2017), 187 – 192 190 Figure 4: Voltage Gain vs. Frequency Fig. 5 presents input referred noise voltage density as a function of frequency at temperatures (-40 °C, 25 °C, 125°C). The noise measured at 500 kHz increases with temperature from 8.16 nV/√Hz at -40 °C to 10.1 nV/√Hz at 125 °C. Figure 5: Input Noise Voltage vs. Frequency The Common Mode Rejection Ratio (CMRR) is meas- ured as the ratio of the common mode gain to differ- ential mode gain. At lower frequencies the CMRR varies from -131 dB at -40 °C to -121 dB at 125 °C. The results are shown in Fig. 6. Figure 6: CMRR vs. Frequency The PSRR is measured as the ratio of the OTA output variation vs. the supply voltage variation regardless the input signal. The PSRR varies from -78.2 dB at -40 °C to -62.9 dB at 125 °C, measured at 100 Hz. At higher fre- quencies, PSRR deteriorates. The results are shown in Fig. 7. 3.2.1 Corner analysis The process variations of MOSFETs and resistors influ- ence on performance of fabricated integrated circuit. Therefore, the OTA was simulated for different process corners – four corner models (FF, SS, FS and SF). The re- sults at 25 °C are gathered in Table 2. The input noise density was measured at 500 kHz. 3.2.2 Monte Carlo simulation In this section the results of the Monte Carlo (MC) anal- ysis are shown. The MC simulations were performed Table 2: Corner analysis results Parameters FF1 SS1 FS1 SF1 FF2 SS2 FS2 SF2 Units Ivdda 485 374 430 416 621 476 550 530 µA G 41.3 39.2 40.2 40.1 41.5 39.4 40.45 40.32 dB BW 14.5 12.6 13.8 13.3 16.0 14.0 15.2 14.7 MHz Input noise density 8.25 9.43 8.81 8.92 7.95 9.13 8.50 8.62 nV/SHz Offset 1.57 0.71 1.16 1.00 2.72 1.37 2.11 1.89 mV CMRR -132 -130 -134 -129 -140 -144 -141 -143 dB +SR 10.28 8.02 9.06 8.95 10.97 8.54 9.66 9.54 V/µs -SR 10.54 8.19 9.3 9.13 11.1 8.65 9.79 9.66 V/µs PSRR @ 100 Hz -68.9 -78.5 -75.3 -72.1 -73.8 -83.5 -81.3 -75.8 dB Transconductance 856 715 785 770 816 672 737 737 µA/V Note1: Supply voltage: 3.3 V Note2: Supply voltage: 5 V D. Berčan et al; Informacije Midem, Vol. 47, No. 3(2017), 187 – 192 191 including mismatch and process variations at typical conditions (25 °C and 3.3 V) comprising 512 MC runs. The simulation results are presented in the following histograms. Figure 7: PSRR vs. Frequency Fig. 8 shows the resulting histogram of input referred noise density at 500 kHz, which has a mean value of 8.873 nV/√Hz and the standard deviation of 151 pV/√Hz. The result is a bit higher than the mean value of previously shown corner analysis results, which is 8.853 nV/√Hz. Figure 8: MC test of Input Voltage Noise The voltage gain distribution is shown in the Fig. 9. The mean value from the corner analysis results is 40.20 dB and it is slightly higher than the mean value obtained by MC analysis, which is 40.13 dB with a standard devia- tion of 0.27 dB. The offset voltage mean value from MC runs is 1.079 mV with standard deviation of 103.7 uV. It is shown in Fig. 10. Figure 10: MC test of Offset Voltage The mean value of the voltage offset from corner analy- sis is 1.110 mV, which is higher than the MC results. The comparison between corner and MC analyses must be done carefully as the corner analysis comprises also temperature and supply voltage variations, which es- pecially influence voltage offset. 4 Layout of presented circuit The layout of the presented integrated circuit is shown on Fig. 11. Figure 11: Layout of the integrated circuit Figure 9: MC test of Voltage Gain D. Berčan et al; Informacije Midem, Vol. 47, No. 3(2017), 187 – 192 192 The active area of the circuit, without bonding pads and supply connection rings, is 750 µm x 260 µm. The integrated circuit has been designed with the 0.18 µm BCD technology as a part of the System on Chip (SoC). The BCD technology allows mixed - signal design using low and high voltage transistors (DMOS) on the same die (reduces cost, area and power consumption). The voltage to current converter main part is presented by resistors R1 – N+ poly without salicide and R2 – N+ dif- fusion without salicide, which means that resistors do not have an additional process mask of salicide, reduc- ing the sheet resistance (Ω/sq) of the resistors. For ASIC area reduction, N – well resistors could be used, but they have high nonlinearity and larger parasitic capaci- tance between N – well and substrate. The integrated circuit has been sent to the fabrication factory. 5 Conclusions and next steps The OTA was designed and analyzed using the 0.18 µm BCD technology. The temperature sensitivity of the OTA has been optimized and reduced, using the bias control technique and a stable internal voltage refer- ence. The proposed circuit shows voltage gain varia- tions lower than ±0.3 dB in the temperature range from -40 °C to 125 °C and lower than 0.5 dB when varying the supply voltage in the range from 3 V to 5.5 V. The equivalent input referred noise voltage is simulated to be lower than 10 nV/√Hz at 500 kHz and 25 °C, and volt- age offset lower than 2.8 mV. To overcome the problem of process variations, especially the resistors R1 and R2 which are used for compensation current generation and are one of the most critical elements in compen- sations circuit, the resistance of both R1 and R2 resis- tors will be precisely trimmed. The issue could be eas- ily solved by employing effective and uncomplicated trimming resistor stage for each of them separately and internal One-Time-Programmable (OTP) memory cells. 6 References 1. W. M. C. Sansen, Analog Design Essentials, let. 2006. Springer US. 2. W. Surakampontorn, V. Riewruja, K. Kumwachara, C. Surawatpunya, in K. Anuntahirunrat, „Tempera- ture-insensitive voltage-to-current converter and its applications“, IEEE Trans. Instrum. Meas., let. 48, št. 6, str. 1270–1277, dec. 1999. 3. T. Parveen, A textbook of operational transconduct- ance amplifier and analog integrated circuits. New Delhi: I. K. International Publishing House, 2012. 4. A. Pleteršek, Načrtovanje analognih integriranih vezij v tehnologijah CMOS in BiCMOS. Ljubljana: Fakulteta za elektrotehniko, 2006. 5. I. M. Filanovsky in A. Allam, „Mutual compensation of mobility and threshold voltage temperature effects with applications in CMOS circuits“, IEEE Trans. Circuits Syst. Fundam. Theory Appl., let. 48, št. 7, str. 876–884, jul. 2001. 6. T. C. Carusone, K. W. Martin, in D. Johns, Analog in- tegrated circuit design, 2nd edition. Hoboken, N.J.: John Wiley & Sons, 2011. 7. M. E. T. Instruments, „Voltage Reference Selection Basics“. Available at: http://www.ti.com/lit/wp/ slpy003/slpy003.pdf. Accessed: [6.8.2016] Arrived: 31. 08. 2017 Accepted: 11. 12. 2017 D. 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