MATERIALS FOR DIFFUSION-PATTERNING; THICK-FILM INTERCONNECTIONS TECHNOLOGY Marko Hrovat, Darko Belavič*, Marko Pavlin*, Janez Hole Jožef Stefan Institute, Ljubljana, Slovenia * HIPOT, Šentjernej, Slovenia INVITED PAPER MIDEM 2002 CONFERENCE 09.10.02 - 11.10.02, Hotel Klub, Lipica Key words; thick film, materials, diffusion patterning, multilayers, resistors Abstract: Diffusion patterning is a dielectric patterning tectinoiogy, which is used in the screen-printed thick-film technology for higher density multilayer circuits. This technology is suitable for producing lower cost multichip modules and requires a low additional investment in conventional thick-film technology production lines. Comparisons of via resolution capability of diffusion patterning versus conventional thick-film technology are described and discussed. Preliminary experimental results obtained with a test circuit showed that 200 (.im lines and 200 |.im vias could be achieved with acceptable yield and with minimal modification to standard production lines. A few results of an investigation of some thick-film materials, which comprise the "set" of pastes for diffusion patterning technology, are presented. The electronic circuit for the pressure sensor was designed with the advantages of semi-custom ASIC and realised with the verified technology as a low-cost ceramic multichip module. Materiali za difuzijsko oblikovanje; tehnologija za debeloplastne povezave Ključne besede: debeioplastna tehnologija, materiali, difuzijsko oblikovanje, večplastna vezja, upori Izvleček: Difuzijsko oblikovanje je debeioplastna tehnologija, ki z običajno tehniko tiskanja in žganja omogoča izdelavo večplastnih vezij z večjo gostoto komponent. Ta tehnologija je primerna za izdelavo cenejših keramičnih modulov z golimi silicijevimi tabletkami (MCM-multi-chip modules), ker zahteva samo minimalne dodatne investicije k obstoječim linijam za proizvodnjo debeloplastnih hibridnih vezij. V članku primerjamo in ocenimo "sposobnost" resolucije odprtin v dielektriku v večplastnih vezjih v primerjavi s "klasično" debeloplastno tehnologijo. Eksperimentalni rezultati so pokazali, da lahko ponovljivo izdelamo 200 um linije in 200 um odprtine z minimalnimi spremembami obstoječe tehnologije. Predstavljeni so izbrani rezultati testiranja materialov za difuzijsko oblikovanje. Elektronsko vezje za senzor pritiska, ki je bilo izdelano s to tehnologijo, je prikazano kot primer uporabe difuzijskega oblikovanja. 1. Introduction The density of electronic packaging is increasing due to the requirements for higher performance and smaller size in electronic systems. Since smaller size and lighter weight with lower cost are basic requirements, multi-chip module (MCM) technology is an essential technology to meet these demands. Typical MCMs are realised by using bare dies or dies in chip-scale packaging, because the absence of additional leads provides a shorter interconnection length and higher density. Multilayer interconnections contribute significantly to the reduction of overall dimensions. There are several technologies and materials which enable the realisation of interconnections for multichip modules. The general types of MCM are: high-density glass-epoxy laminated printed-circuit board (MCM-L), thick-film on ceramic substrate (MCM-C), and thin film on ceramic or silicon substrate (MCM-D). Each combination of these technologies and materials offers a different level of performance/cost ratio/1-7/, A ceramic MCM-C can be realised using LTCC (low-temperature co-fired ceramic), HTCC (high-temperature co-fired ceramic) and thick-film technology, including also photo-patternable and diffusion-patternable technology. An additional contribution to the smaller size and higher density of MCM-C is the ability to integrate screen-printed resistors or sometimes capacitors and inductors. These screen-printed components can be placed either beneath the discrete components on the surface of the multilayer dielectric or be buried (sandwiched) within the multilayer structure /8-11/. 2. Feasibility study The market for pressure sensors is one of the largest of all sensor technologies, as a consequence the technology and applications of pressure sensors have developed rapidly. The market niche for small and medium enterprises (SMEs) is to develop and produce application-specific sensors integrated in miniature electronic (sensor) modules. The technology foresight of the HIPOT-HYB Company (which is an SME) is based on a strategic orientation to research, design, develop and produce pressure sensors and hybrid circuits. The competitive advantage of the HIPOT-HYB Company is the use of thick-film technology in sensor applications. This technology is used in two ways, to produce the sensor elements themselves and/or the electronic circuits for signal processing. However, these days the design of new pressure sensors is faced with strict requirements: device size is reducing, functions and performances are expanding; while at the same time the cost of the sensor is restricted. In this respect the developments in sensor technology for small- and medium-volume production have two directions. The first direction is to Integrate all or most of the electronic functions into an application-specific integrated circuit (ASIC), the second is to use one of the lower cost high-density interconnection technologies to integrate the sensing element, ASICs and passive components in a sensor module. A multichip module (MCM) is an essential technology to meet these demands. The special requirements for the mechanical (pressure) sensor application which must be considered: Analog and mixed analog-digital functions; A low-frequency range from DC up to 10MHz clock in digital applications; Low power consumption; A small number of electronic components; The maximum number of conductive layers is 3 or in some cases 4; Restricted external dimensions; Mechanical and thermo-mechanical properties suitable for use in mechanical sensor applications; Electomagnetic compatibility (EMC) aspect; Ecological aspects. Due to above-listed requirements, and in particular the mechanical and thermo-mechanical properties, the ceramic multichip module (MCM-C) is an essential or at least a "good" technology to meet these demands /12-14/. A ceramic is probably the most common substrate material for pressure sensors, with a silicon die as the sensor element. The reason lies in its physical properties, which include: high compressive strength and hardness; thermal expansion similar to the silicon die and dimensional stability. In some applications high resistance to chemical attack is also important. The interconnection performance (number of layers, via size and line pitch) of ceramic multichip module technologies for photo-patternable, screen-printed, diffusion-pat-ternable and LTCC technologies is shown in Fig 1 /15/. 3. Diffusion-patterning technology Thick-film multilayers are made by printing and firing alternate layers of conductors and dielectrics. The dielectric Number of layers 14 Volume production / ' Diffusion / -^tCrnab le ä Screen printing Photo-patterr 1 0 50 100 150 200 250 300 350 400 450 500 Line pitch (via size) pm Fig. 1: Interconnection performance of ceramic muitichip module technologies /14/. layer covers the whole area of the substrate. Conductor layers are connected through openings - vias - in the dielectric film. Thick-film pastes are tixotropic; this means that the paste should flow easily when the squeegee pushes it through the screen mesh during screen printing and then "freeze up" in the desired shape on the substrate. However, in the case of multilayer dielectrics, which cover large areas, some compromises are needed. If the multilayer dielectric paste is "stiff", small and well-defined vias can be made. However, small undesirable pinholes could also appear in the layer resulting in short circuits between upper and lower conductor. Therefore the viscosity of the paste should be low enough so that it flows a little after screen printing to "heal", i.e. close up any pinholes. However, this means that vias should be large enough so that they will stay open. This is shown schematically on the left side of Fig. 2. In hybrid circuits production, this limits the dimensions of vias to something like 400x400 um^. Diffusion patterning (Diffusion Patterning is a trademark of the Du Pont company) is a technology, which enables the production of smaller vias with standard thick-film technology /16,17/. For diffusion patterning, a layer of dielectric paste Q-42-DP(DP-diffusion patterning) is screen printed overthe whole circuit without vias for connecting lower and upper conductor layers. The relatively low viscosity of the dielectric paste, partly due to a lower inorganic content, results in a smooth film with few or no pinholes. After drying of the dielectric layer, the droplets of diffusing (or image) paste are screen printed on to the dielectric layer. Image paste consists of an organic material and an inert alumina filler. At elevated temperatures this organic material diffuses down into the dried and polymerised organic vehicle of the dielectric. Diffused parts are then washed out with warm water (around 80°C) enabling the "creation" of small round vias. This is shown schematically on the right side of Fig. 2. All further production steps are the same as with standard thick-film materials. Unlike standard vias construction with screen-printing, diffusion patterning vias do not need any extra substrate space. The dimensions of vias are similar to the width of the conductor. It is estimated that complex hybrids can therefore be built on 20% to 40% smaller substrates. n......r "Stiff' dielectric paste Print blank dielectric layer Delectric paste witli iower viscosity Print dots with patterning paste Diffusion patterning dielectric paste Diffuse solubllizer into dielectric layer ¥ Wasfi solubilized areas out with spray or ultrasonic energy Fig. 2: Vias, realised with standard muitilayer dielectric (on the left) and with dielectric for diffusion patterning (on the right) - schematically The main difference between conventional thick-film multi-layertechnology and diffusion patterning is an organic part of the multilayer dielectric paste. It is based on a hydrogen bonded acidic acrylic polymer. The active phase in the image paste, which is coloured black for better screen-print-ed resolution, contains an alkaline organic. During diffusion base and acid materials react and break hydrogen bonds in the acrylic polymer. This results in a reduced green strength of the dielectric layer and enables washing out of the weakened material. The schematics of the diffusion mechanism are shown in Fig. 3 (after Needes et al. /18/). Image paste -^ Low green strength Fig. 3: The schematics of the diffusion mechanism/18/ The diffusion patterning technology is based on high-qual-ity multilayer dielectric material compatible with silver conductors and resistor materials for printing and firing on or under dielectrics. Inorganic material in the Q 42 DP dielectric is the same as in the multilayer dielectric QM42 and is based on the mixture of crystallizable glass and ceramic filler /19/. The silver conductors are used for the inner conductor layers and the Ag/Pd and/or Au are used for the top conductor layer only. Discrete components are added by chip-and-wiring technology and/or with one of the SMT technologies (SMD, Flip-chip,...). Thick-film materials include two resistor series, QM 80 and QM 90, for making resistors on the top of the dielectric layer. Resistors from the QM 80 series are designed for Pd/Ag termi- nation, while those from the QM 90 series are terminated with silver. /20,21/. Some features of the diffusion patterning process are: Substrate: 96%Al203 Multilayer dielectric for diffusion pattering Q42DP Image paste Q95IP Conductors: Ag (Au, Ag/Pd, Ag/Pt) Resistors: 1-^10 Mohm, on dielectric Minimum tracks width: 200 |am (150 jim) Minimum tracks separation: 200 jim (150 |im) Minimum via diameter: 200 iim (150 |i,m) Minimum crossover area/pitch: 400 |am (300 |am) Number of conductor layers: 4 Size reduction factor (compared to the standard multilayer process): 0.6 0.8 4. Diffusion-patterning - materials In this part of the paper the results of an investigation of some thick-film materials which comprise the "set" of pastes for diffusion-patterning technology will be presented. For microstructural investigation the thick-film materials, printed and fired on alumina ceramics, were mounted in epoxy in cross-sectional orientation and then cut and polished using standard metallographic techniques. A JEOL JSM 5800 scanning electron microscope (SEM) equipped with an energy dispersive X-ray analyser (EDS) was used for overall microstructural and compositional analysis. Priorto analysis in the SEM, the samples were coated with carbon to provide electrical conductivity and to avoid charging effects. The conductive phase in the resistors and the "nature" of ceramic filler in the Q42-DP multilayer dielectric were determined by X-ray powder diffraction analysis (XRD) with a Philips PW 1710 X-ray diffractometer using Cu Ka radiation. X-ray spectra were measured from 2 6 = 20° to 20 = 70° insteps of 0.04°. 4.1. Silver-based conductors QM 14 is a silver conductor for inner-layer interconnections and QM 34 is a via-fill conductor for buried vias and connections to Ag or Pd/Ag upper conducting layers in a multilayer structure. In Figs. 4.a and 4.b microstructures of QM-14 and QM-34 conductors are shown, respectively. Both materials were fired at 850°C. EDS microanalysis showed that both conductors are based on pure silver. The microstructrure of the QM 14 conductor is densely sintered. The diameter of the grains is from a few micrometers to more than ten micrometers. Exaggerated grain growth is due to the firing temperature, which is close to the melting point of silver at 960°C. On the other hand, the microstructure of QM 34 is porous with small grains of approximately one micron in diameter. On the boundaries of the silver grains small particles of secondary phase with sub-micrometer dimensions are seen. In Fig. 4.b the particles are denoted with arrows. EDS semiquantitative analysis showed the presence of aluminium, silicon and oxygen. This alumosilicate secondary phase is added to inhibit the grain growth and densification during firing. For the via-fill paste it is important that the volume of dried and fired material is similar. In that way the vias stay filled with the conductor "cylinder" and no cracks, due to shrinkage, appear either between the via-fill conductor and the dielectric or the via-fill conductor and the upper and lower conducting layers. Fig. 4.a: The microstructure of silver-based conductor QM-14, fired 10 min. at 850°C. trie, are Si, AI, Zn, Ba and Zr. A small amount of cobalt, presumably added for blue colouring, was also detected. The black grains imbedded in the dielectric matrix are alumina particles, added as the ceramic filler. XRD analysis confirmed that the ceramic filler in the Q42-DP dielectric is alumina. The X-ray spectrum of Q42-DP is shown in Fig. 6. AI2O3 peaks are denoted "A". Peaks of another crystalline phase, presumably SiZr04 (JCPDS file 83-1383), are denoted by an asterisk. Fig. 5: The microstructure of the interface between the resistor QM-93 and the Q42-DP dielectric. The dielectric is on the right. Fig. 4.b: The microstructure of silver-based via-fill conductor QM-34, fired 10 min. at 850°C. 4.2. QM-42 DP dielectric The inorganic material in the Q 42 DP dielectric is based on a mixture of crystalizable glass and ceramic filler /19/. Fig. 5 shows the cross-section of a thick-film resistor (QM-93), fired on the top of the prefired dielectric layer. The microstructure of the dielectric is dense, with a few small, closed pores. The dielectric is densely sintered. The main elements, detected by EDS microanalysis in the dielec- trt s; 60 Q42-DP lA. 40 50 2 theta (deg.) Fig. 6: X-ray spectrum of Q42-DP dielectric. AI2O3 and (presumably) SiZr04 peaks are denoted by "A" and by asterisk, respectively. 4.3. QM-80 and QM-90 resistors As mentioned before, the Du Pont resistor series QM-80 and QM-90 are designed for firing on a prefired multilayer dielectric layer instead of on the surface of alumina substrates /19,21 /. The resistors, made with QM 80 and QM 90 series, are intended for termination with palladium-sil-ver and silver conductors, respectively. X-ray spectra of 1 and 10 kohm/sq. members of both series, fired at 850°C, are shown in Fig. 7.a (QM-83 and QM-93) and Fig. 7.b (QM-84 and QM-94), respectively. The conductive phase in both 1 kohm/sq. resistors is a mixture of Ru02 and ruth-enate. For resistors v\/ith higher sheet resistivities, from 10 kohm/sq. up, only ruthenate w/as detected by X-ray analysis. Energy-dispersive X-ray quantitative analysis (EDX) indicated the presence of bismuth together w/ith ruthenium. Therefore it is presumed that the ruthenate phase is Bi2RU207 Or(Bii-xPbx)Ru07-y. 250 40 50 2 theta (deg.) Fig. 7.a: X-ray spectra of 1 kohm/sq. QM 83 and QM 93 resistors. Ru02 is denoted R and ruthenate phase is denoted BR. 250 200 C u C 100 a» cn Q/ 50 0 1 X QM 84 AAju. 20 30 40 2 theta (deg.) 50 60 Fig. 7.b: X-ray spectra of 10 l