J. Slunečko, J. Hole, M. Kosec, D. Kolar "J. Stefan" Institute, Ljubljana, Slovenija
LATE PAPER
24"^ International Conference on Microelectronics, MIEL'96
32"'^ Symposium on Devices and Materials, SD'96 September 25. - September 27., 1996, Nova Gorica, Slovenia
Key words: humidity sensor, thin-film sensors, RH, Relative Humidity, TiOs porous titanium dioxide, electrical properties, sol-gel method, morphology
Abstract: The effect of introducing pores in undoped and potassium doped TiOg thin films on their humidity sensing characteristics was studied. Films were prepared by the sol-gel technique. Different potassium compounds were used to introduce potassium ions into the sols that were used for preparation of samples. The influence of these compounds and of the film firing temperature on sample morphology, and on the electrical properties of Ti02 thin films were also studied. Introduction of the porosity in thin films had a positive effect on the humidity sensing characteristics of the samples. Sol-gel processed, porous, and 10 at% potassium doped TiOa thin films heated to 450''C exhibited an outstanding humidity sensitivity over the entire RH range.
Ključne besede: senzorji viage, senzorji tankoplastni, RH vlaga relativna, TiOg titanov dioksid porozni, lastnosti električne, postopki soi-gel, morfologija
Povzetek: V članku so predstavljeni senzorski elementi za zaznavanje relativne vlažnosti zraka na osnovi s kalijem dopiranih in nedopiranih Ti02 tankih filmov. Predstavljen je tudi način, kako v tanek film lahko vpeljemo poroznost in kako le ta vpliva na njegove senzorske lastnosti. Filmi so bili pripravljeni s pomočjo sol-gel procesa. Različne kalijeve spojine so bile uporabljene kot dodatek k solom za pripravo tankih filmov. Študirali smo vpliv teh spojin in temperature žganja na morfologijo in električne lastnosti Ti02 tankih filmov. S kalijem dopirani Ti02 tanki filmi, ki so bili žgani pri 450°C in ki so bili pripravljeni s pomočjo sol-gel postopka, so imeli odlično občutljivost na spremembo relativne zračne vlage. Rezultati so pokazali, da poroznost pozitivno vpliva na občutljivost teh vzorcev.
Introduction
The trend towards automated control systems for environmental monitoring and miniaturisation requirements for on chip integration of sensors is the driving force for development of integrated humidity sensors /1, 2/. The final target is the development of intelligent system that will be easy to operate and have microprocessor-compatible read-outs. However, not only the electronics but also the materials technology is fundamentally important in improving research and development on humidity sensors. A complete understanding of the material's characteristics and of its sensing mechanism, using prototypes, is needed for the further development of advanced devices. In the field of humidity sensors, several attempts have been made to integrate a humidity sensor on silicon microchips, along with all electronics needed for a practical device. Both polymeric /3/ and ceramic films /4/ have been investigated as humidity sensitive elements.
Today, most commercially available humidity sensors are based on polymeric films /5/, in spite of the fact that ceramic humidity sensors exhibit better chemical resistance and mechanical strength than polymeric sensors
/ 6/. This situation is a consequence of the high costs that are incurred during the production of ceramic humidity sensors based on porous sintered oxides /1, 7/, because of the use of conventional ceramic technology. For that reason less costly manufacturing technology for miniature ceramic sensing elements Is needed. Recently, films prepared by sol-gel methods were studied /8, 9/ as humidity sensing devices. This chemical technique offers a very promising feature, namely, the possibility of powder-free processing of ceramics in their final shape (films or fibres), which can be used as active elements in sensing devices /10/.
That is probably one of the reasons that several authors studied the suitability of sol-gel processed Ti02-based thin films for humidity sensors /8, 9,11,12/. Titania was used as a sensing material because very interesting results have been reported for sintered porous compacts and thick films of titania and doped titania-based humidity sensors /13, 14, 15, 16, 17/. In the literature it was considered that a large pore volume and control of the pore size distribution are necessary for high humidity sensitivity of ceramic materials /7,18/. For this reason it is interesting that an outstanding humidity sensitivity over the entire RH (relative humidity) range has been
reported in dense, pore-free thin filnns /12/. Such a high response of sol-gei processed K-doped TiOa films has also rarely been observed for porous sintered ceramics. It is known that addition of alkali ions is effective in increasing the RH sensitivity of several ceramic oxides by affecting the sinterability of the material in pellet form /15/, decreasing the intrinsic resistance of the material /19/, or increasing the number of adsorption sites /20/. However, measurements performed by the authors /8, 12/ indicated a humidity sensing mechanism that is different from that generally accepted for porous ceramics This mechanism involves the direct participation of alkali ions in conduction during the exposure of the sensing material to a humid environment / 8/.
In this study the influence of the introduction of pores in undoped and potassium doped Ti02 thin film on their sensing characteristics is studied. Different potassium compounds were used for introduction of potassium ions to the sols that were used for preparation of samples. The influence of these compounds on the morphology and on the electrical properties of Ti02 thin films were also studied.
Experimental
Doped and undoped Ti02 thin films were prepared by the sol-gel technique. Precursor solutions for production of porous Ti02 coatings were prepared by a slightly modified method reported by Kato et al. /21, 22/. Titanium tetraisopropoxide Ti(0Pr')4, diethanolamine, and ethanol were mixed and stirred in a nitrogen glove box to prepare a homogenous solution. For preparation of doped Ti02 films 10 at % of K was added to the solution. Three different compounds were used for doping of the sols: potassium nitrate (KNO3), potassium acetate (KOOCCH3), or potassium ethoxide (KOC2H5). Afterwards water diluted with ethanol was mixed into the solution. The water/alkoxide molar ratio was 1.
Polyethylene glycol (PEG), molecular weight 1800-2200, was used as a pore former /21/ in several samples. In order to evaluate the influence of the concentration of the pore former on the porosity, the amount of PEG was varied between 2 and 4 wt. %. A summary of the samples prepared is presented in Table 1.
Silicon wafers and alumina plates were used as support substrates. For electrical measurements, prototype sensors were prepared by depositing Ti02-based films on alumina substrates with comb-type Au electrodes. The films were deposited from the solutions by the spin coating technique, using a rotation speed of 3000 RPM. Gel coatings were dried at 100°G and fired at 650°C for 1 hour /21 /. Samples were also fired at 450°C for 2 hours in order to evaluate the influence of firing temperature on the sensing characteristics of Ti02 thin films. The thickness of the Ti02 coating was increased by repeating the cycle from spinning to firing.
The morphology of the coatings was examined by scanning electron microscopy (SEM, JEOL JXA-840A). The humidity sensitive electrical properties of the thin films were evaluated using an impedance analyser (HP 4192A LF). An environment varying relative humidity (RH), ranging from 15 to 95 % at 25°C was obtained using an environmental chamber (Weiss SB1 160).
Table 1: Summary of prepared samples
	Pore former	
potassium dopant	PEG added	No PEG added
KNO3	KN/Px.x*	KN
KOOCCH3	KA/Px.x*	KA
KOG2H5	KE/Px.x*	KE
No K'" added	NK/Px.x*	NK
x.x represents the wt. % of PEG added. Example: KN/P2.4, sample with KNO3, and 2.4 wt. % of PEG added to the sol.
Results and discussion
Figs 1 and 2 show scanning electron micrographs of the undoped, and potassium doped Ti02 coatings prepared from the precursor solutions.
The Ti02 coating prepared from the solution without addition of polyethylene glycol (NK/PO.O, Fig.lA) had almost no visible texture. Addition of 2.4 wt. % of PEG to undoped titanium tetraisopropoxide (TTIP) sols yielded thin films (NK/P2.4) with a porous microstruc-ture (Fig. IB). Pores were from 100 to approximately 270 nm in diameter. The sample morphology was slightly different from the morphology of thin films reported in the literature /21/ but this difference can probably be ascribed to the different coating technique used.
The influence of the addition of potassium compounds to the TTIP sol on the microstructure of the Ti02 thin film is presented in Fig. 2. The amount of potassium compound (KOOCCH3, KNO3) added was calculated to yield 10 at. % of potassium ions with respect to titanium ions.
Fig 2A show the morphology of a KA/P2.4 sample (potassium acetate and 2.4% of PEG added to the TTIP sol) fired at 650°C for 1 hour. There was no difference in morphology between samples without addition of PEG (KA/PO.O) and samples with 2.4 wt. % of PEG added (KA/P2.4). Both samples had microstructures almost identical to the fine granular microstructure of the sample KN/PO.O (KNO3 added, no PEG added) that were fired at 650°C for 1 hour. The samples had a grain size in the range from -20 nm to -100 nm (Fig 2A).
An increased concentration of pore former in the KNO3 doped precursors (sample KN/P2.4) yielded a micro-structure that is presented in Fig 2B. The sample had a grain size around 100 nm and pores in the range from -20 nm to -200 nm. Large cracks from 0.5 to 2 |im were also present in these samples. During the preparation of the KN/Px.x samples separation of small crystallites was observed. This was probably a consequence of the low solubility of KNO3 in TTIP sols.
Fig. 1: Morphology of undoped TiO?. thin films fired at 650"C for 1 h. A: NK/PO.O, B. NK/P2.4
Fig. 2: Morphology of potassium doped TiOp, thin films fired at 650°C for 1 h. Samples were prepared from KOOCCH3 and KNO3 doped sols.
A: KA/P2.4, B. KN/P2.4
Fig 3 show the morphology of K doped TiOa thin films fired at 650°C for 1 hour. Potassium ethoxide was used as precursor. The sample without PF2G added (KE/PO.O) had a granular microstructure with -100 nm grains and 10-50 nm pores (Fig 3A). There was no visible difference between the morphology of KE/PO.O and the morphology of the KE/P2.4 sample. Both samples had some areas with larger grains and different contrast (Fig 3A). EDX analysis revealed that concentration of K ions is higher in areas of different contrast (DC) than in the rest of the coating.
Areas with different contrast could also be observed in samples where the PEG concentration was increased to 4.0 wt. % (sample KE/P4.0 at Fig 3B), but their visibility was worse. It is interesting that in spite of the fact that the PEG concentration in KE/P4.0 samples was higher than in potassium undoped (NK/P2.4) samples (Fig 1B), no pores with higher diameter developed. The micro-structure was still similar to the microstructure of the KE/PO.O sample in Fig 3A.
It can be seen from these results that introduction of potassium compounds into the TTIP sols had a profound effect on the sample morphology. First, the formation of the pores in the samples with added potassium precursors was hindered with respect to the samples without K added. Second, the potassium doped samples fired at 650°C always exhibited the granular morphology whereas the undoped samples fired at same temperature (Fig. 1) had almost no granular texture.
Samples prepared from TTIP sois based on potassium ethoxide precursor, were selected to evaluate the influence of firing on the morphology and electrical properties of the thin films. Samples KE/PO.O and KE/P4.0 were fired at 450°C for 2 hours (Fig 4). TiOa thin films without added pore former (KE/PO.O) had no visible texture (Fig 4A). TiOa coatings prepared from the precursor solution with PEG added (sample KE/P4.0) developed a different microstructure. As can be seen from the micrograph in Fig 40, there were fine pores in the coating. The pores

4 '





Fig. 3: Morphology of potassium doped Ti02 thin fiims fired at 650 °C for 1 h. Samples were prepared from KOC2H5 doped sols. A. KE/PO.O, B. KE/P4.0
Fig. 4: Morphology of KE/PO. 0 and KE/P4.0 samples fired at 450°C for 2 h. A. KE/PO.O, B. KE/P4.0
were -30 to -100 nanometres in diameter. No areas of different contrast could be observed in these samples.
Electrical measurements proved that potassium nitrate was unsuitable for sample doping due to the low solubility of KNO3 in the TTIP sols. For this reason the potassium concentration in the TiOg thin films doped with this compound was under 10 at. % (with respect to Ti), and as a consequence/12, 15, 19, 20/KN/PO.O and KN/P2.4 samples were poorly sensitive to changes of relative humidity (RH).
Fig 5 show the dependence of the capacitance of potassium doped Ti02 thin films at 200 Hz and 25°C. Samples KE/PO.O and KE/P4.0, fired at 650°C for 1 hour, exhibit a change of capacitance only at high RH. The sensitivity of the samples to change of RH increased with addition of pore former (sample KE/P4.0) only for a relative humidity over 70 %.
The capacitance change for samples KE/PO.O and KE/P4.0 fired at 450°C was 4 orders of magnitude be-
tween 15 and 90 % RH. Sample prepared from precursors with an increased concentration of PEG (KE/P4.0) had higher sensitivity in the range from 15 to 45 % RH than the KE/PO.O sample. From 45 % RH, sensitivity of the KE/P4.0 sample decreased, and in the range from 50 to 95 % RH followed the sensitivity of the KE/PO.O sample at higher capacitance level.
On the one hand, the dramatic increase of sensitivity with decreased firing temperature could be explained by a coarsening of the microstructure at 650°C. At that temperature grains of Ti02 are already well formed. Grains are also connected with necks (Fig 3), in contrast to the sample films fired at 450°C (Fig 4) which show no grain morphology in SEM observations. It has been shown that formation of grain boundaries and ordering of the crystalline structure is able to block ionic conduction within the material / 8, 9/. The authors /8/ showed that the free movement of alkali ions is necessary for exceptional conductivity of K doped Ti02 thin films. The decrease of sensitivity could probably be also connected with the potassium segregation that can be
observed in the samples fired at 650°C (Fig 3). Such segregation is not yet weil understood, and further investigations are needed to explain it.
On the other hand, samples fired at 450°C show an exceptional change of capacitance in the entire RH range. This change could also be related to the increased number of water adsorption sites, due to the formation of a higher number of surface defect sites /1, 20/.
The presence of small pores in the KE/P4.0 sample fired at 450°C is probably responsible for the higher sensitivity of the thin film at lower relative humidity (below 45% RH) compared to the KE/PO.O sample which was fired at the same temperature, and that has no visible porosity. Formation of grains and further coarsening of the sample microstructure (KE/P4.0) at 650°C probably resulted in the disappearance of the porosity that can be observed in Fig 4B. In any case, the porosity of the KE/P4.0 sample sintered at 650°C was still higher than the porosity of the KE/0.0 sample sintered at same temperature, and that probably resulted in higher sample sensitivity at a relative humidity over 70% RH.
1,E+06
1.E+04
1.E+03
g: Ä Ü
1,E+02
1,E+01
-■O--KE/P4.0 650 - ■ o ■ • KE/PO.0-650
□ o. •-■•-■a
10 20
40 50 RH [%]
Fig. 5:
The RH dependence of the capacitance of K doped thin films at 25°C. Sensors fired at 650°C for 1 h and at 450°C for 2 h. The frequency of the appiied fiied was 200 i-iz.
Conclusions
Porous potassium doped and undoped Ti02 thin films were prepared from alkoxide solutions by the spinning technique. The results showed that the formation of pores in the samples prepared from TTIP sols with potassium precursors added was hindered with respect to the samples prepared from TTIP sols without added potassium precursors.
Introduction of porosity in thin films had a positive effect on the humidity sensing characteristics of the potassium doped samples. Sol-gel processed, porous, and 10 at % potassium doped Ti02 thin films heated to 450°C show an outstanding humidity sensitivity over the entire RH range. The change of capacitance was 4 orders of magnitude in the range from 15 to 95% RH.
Acknowledgements
The authors express their thanks to Dr Enrico Traversa for providing substrates with sputtered Au electrodes. Financial support from the Ministry of Science and Technology of Slovenia is gratefully acknowledged.
References
/1/E. Traversa, "Ceramic sensors for humidity detection: the state-of -the-art and future developments", Sensors and Actuators B 23, 135-156 (1995).
/2/ S.C. Chang, W.H. Ko, "Sensors: A Comprehensive Sun/ey", Ed. by W. Gophel, J. Hesse and J.N, Zemel, VCH, Weinheim (1989) pp, 169-94
13/ M,C, Glenn, J,A, Shutz, An IC compatible poymer humidity sensor, Tech, Digest, 3rd Int, Conf: Solid-State Sensors and Actuators (Transducers '85), Philadelphia, PA, USA, June 11-14 (1985) pp, 217-220
/4/ M, Parameswaran, H,P. Baltes, M,J, Brett, D,E, Eraser, A.M. Robinson, "A capacitive humidity sensor based on CMOS technology with adsorbing film", Sensors and Actuators 15, 325-335 (1988)
/5/L.I. Belič, K, Požun, "Capacitive thin film relative humidity sensor", Soc, Microelectron, Electron, Components & Mater,-MIDEM, Inf, MIDEM (Slovenia) 26 (1),14-17 (1996),
/6/ B,M, Kulwicky, "Humidity Sensors", J. Amer, Ceram, Soc, 74 (4), 697-708 (1991),
/7/ N, Yamazoe, "Humidity sensors: Principles and Applications", Sensors and Actuators 10, 379-398 (1986),
/8/ E, Traversa, "Ceramic thin films by sol-gel processing as novel materials for integrated humidity sensors", Sensors and Actuators B 31 (1-2), 59-70 (1996),
/9/ G. Gusmano, G. Montespirelli, P. Nunziante, E. Traversa, A. Montenero, M. Braghini, G. Mattogno, A. Berazotti, "Humidity-Sensitive Properties of Titania Films Prepared Using the Sol-Gel Process", J. Ceram, Soc. Jap. 101 (10), 1095-1100 (1993).
/10/ J.C. Brinker, G,W, Scherer, Sol-Gel Science, Academic press, San Diego (1990) pp, 839- 880
/11/S. Ito, S, Tomotsune, N, Koura, "Preparation of vanadium doped rutiie humidity sensor by dipping-pyrolysis process", Denki Kagaku 60 (6), 474-9 (1992).
/12/ G. Montespirelli, A. Pumo, E. Traversa, G. Gusmano, A. Berazotti, A. Montenero, G. Gnappi, "Sol-gel processed Ti02-based thin films as innovative humidity sensors". Sensors and Actuators B 24-25, 705-709 ( 1995).
/13/Y.C. Yeh, T.Y, Tseng, D.A. Chang, "Electrical Properties of Porous Titania Ceramic Humidity Sensors", J. Am. Ceram. Soc. 72 (8), 1472-75 ( 1989).
/14/Y.C. Yeh, T.Y. Tseng, D.A. Chang, "Electrical Properties of Ti02-K2Ti60i3 Porous Ceramic Humidity Sensors", J. Am. Ceram. Soc. 73 (7),1992-98 (1990).
/15/K. Katayama, K. Hasegava, Y. Takahashi, T. Akiba, H. Yanagida, "Humidity Sensitivity of NbgOs-doped Ti02 Ceramics", Sensors and Actuators A 24, 55-60 (1990).
/16/K. Katayama, K. Hasegava, Y, Takahashi, T. Akiba, H. Yanagida, "Effect of Alkaline Oxide Addition on the Humidity Sensitivity of NbaOg-doped Ti02". Sensors and Actuators B 2,143-149 (1990).
/17/ J. Slunečko, MSc Thesis, University of Ljubljana (1993)
/18/ T. Seiyama, N. Yannazoe, H. Arai, "Cerannic Humidity Sensors" Sensors and Actuators 4, 85-96 (1983).
/19/ Y. Sadaoka, Y. Sakai, S. Mitsui, "Hunnidity sensor using zirconium phosphates and silicates. Improvements of humidity sensitivity". Sensors and Actuators, 13, 147- 157 (1988)
/20/ F. Uchicawa, K. Shimammoto, "Time Variability of Surface Ionic Conduction on Humidity-Sensitive SIO2 Films", Am. Cera. Soc. Bull. 64,1137-1141 (1985)
/21/ K. Kato, A. Tsuzuki, Y. Torii, H. Taoda, "Morphology of thin anatase coatings prepared from alkoxide solution containing organic polymer, affecting the photocatalytic decomposition of aqueous acetic acid", J. Mat. Scl. 30, 837-841 (1995).
/22/ K. Kato, A. Tsuge, K. Niihara, "Mlcrostructure and Crystal-lographlc Orientation of Anatase Coatings Produced from Chemically Modified Titanium Tetralsopropoxide", J. Amer. Ceram. Soc. 79 (6),1483-88 (1996).
J. Slunečko, J. Hole, M. Kosec, D. Kolar "J. Stefan" Institute, Jamova 39, 1000 Ljubljana, Slovenija tel.: +386 61 1773 368 fax: +386 61 1261 029
Prispelo (Arrived): 18.11.1996 Sprejeto (Accepted): 19.11.1996