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Humidity sensitivity of sputtered TiO2 thin films
Senmrs and Actuators B, 18-19 (1994) 525-528
525
Humidity sensitivity of sputtered Ti02 thin films Andrea Bearzotti Inshkte of Solid
StateElectronics (IESS), National Research Council of Italy (CNR), via Cineto Romano 42, 00156 Rome (Italy)
Alessandra
Bianco, Giampiero
Montesperelli
and Enrico Traversa
Depaninent of Chemical Sciences and Technologies, Univets@ of Rome ‘Tar Vergata’, via della Ricaa (It+)
Scientifica, 00133 Rome
Abstract The humidity sensitivity of sputtered Ti02 thin films was studied by d.c. analysis techniques. Three different sets of specimens were prepared by sputtering in different conditions. The films prepared by reactive sputtering from a Ti target showed a current versus relative humidity (r.h.) sensitivity of 4 orders of magnitude in the r.h. range
from 2 to 90%. For these films the current-voltage (1-V) characteristics were linear, while films prepared by sputtering from a Ti02 target and those prepared by the thermal oxidation of sputtered Ti layers showed a back-to-back diode behaviour. The formation of Schottky barriers at the metal/oxide interfaces indicates that these films are semiconductors. Using the charging and discharging processes it was determined that for films prepared by reactive sputtering intrinsic electronic conduction was negligible, while for the other tilms at low r.h. the conduction carriers were mainly electrons, and protons and electrons at high r.h., protons being the dominant carriers. The films prepared by reactive sputtering showed a slow response time during water adsorption, and a faster response time during water desorption.
Introduction Silicon micromachining and on-chip integration technology are the most promising technologies in sensor production [l]. The miniaturization of sensors is one of the chief goals in the development of chemical sensors [2]. The most important recent developments in chemical sensor technology is the introduction of microfabrication and micromachining technologies, which need the production of sensing elements in thin-film form [3]. In the field of humidity sensors, with these technologies it is possible to carry out the integration of humiditysensing elements on silicon microchips, along with linearization, temperature compensation and signal-conditioning circuitry needed for a practical device [4]. Both polymeric and ceramic films have been investigated as humidity-sensitive elements, although polymer films have received wider attention [5]. Porous ceramic moisture-sensing elements have shown advantagesin terms
of thermal and chemical stability, and mechanical strength [6, 71. The recent progress in ceramic deposition technologies (chemical and physical) provides new possibilities for the miniaturization of ceramic humidity-sensing elements and on-chip integration. Several ceramic oxides have recently been studied in thin-film form, prepared
0954OW94/$07.00 0 1994 Elsevier Sequoia. All rights reserved SSDI 0925-4005(93)01072-C
either by the sol-gel method [g-10] or by sputtering [ll-151. TiO, films prepared by sol-gel showed a fast response time, but had a high intrinsic resistance, and drifted after exposure to humid environments [lo]. This paper deals with the study of TiOz thin films prepared by sputtering in different conditions. Chemical and electrical analyses were performed in order to study the use of these films as sensing elements in integrated devices. Experimental Three sets of TiO, specimens were prepared using different sputtering conditions, which are reported in Table 1. Set A specimens were obtained by reactive sputtering from a Ti target, set B by sputtering from a TiO, target in inert atmosphere, and set C by the thermal oxidation of sputtered Ti films. The TiO, target, 5 cm in diameter and 3 mm thick, was sintered at 14UO “C for 1 h, using a commercial powder (99.9% purity grade, Aldrich Chem. Co., Inc.). The films were deposited on alumina substrates. An interdigitated-comb electrode 7 mm square, with 17 fingers separated by gaps the same width as the fingers, was previously deposited on the substrates and defined by a standard photolithographic process.
526 TABLE! 1. Conditions of preparation
Target Pre-sputtering pressure (Pa) Atmosphere Sputtering pressure (Pa) r.f. direct power (W) Deposition time (min) Thermal treatment
of sputtered TiOz thin lilms
Set A Set B
Set C
Ti 0.67 02 0.67 500 120
Ti 1.3x10-5 Ar 0.67 200 10 6 h at 550 “C in wet O2
TiOl 1.3x10-J Ar 0.67 300 30
The chemical composition of the fihn’s surface was analyzed using X-ray photoelectron spectroscopy (XPS) (VG, model Escalab). The experimental procedure used for the XPS measurements was the same as that reported elsewhere for spine1 thin Iilms [13]. The electrical response of the thin films was analyzed by taking d.c. measurements using a Keithley quasistatic CV-meter 595, at different relative humidity (r.h.) values, in a test cell developed by us where both temperature and humidity were monitored. Relative humidity monitoring was carried out using a commercial sensor (Physthem, model PCRC-ll), which gave results accurate to within f 2%. Current-voltage (Z-v) characteristics were recorded at room temperature. The r.h. dependence of resistance was measured by recording the current upon application of a constant bias, increasing r.h. gradually from 2 to 90%, at a constant rate of 1% r.h. per min. Stability and response time tests were performed by measuring the current for a constant d.c. voltage, during r.h. cycling. The charging current upon application of a d.c. voltage to the thin films and the discharging current upon removal of the applied voltage were also measured until the current reached steadystate values, at room temperature for various r.h. values. Results and discussion The Z-V characteristics of the specimens in set A, recorded at various r.h. values, were nearly linear, showing ohmic behaviour. At room temperature, the resistivity of the specimens in nearly dry conditions was evaluated at about 10” Sz, while it was about 150 Gfl at 90% r.h. Back-to-back diode behaviour was observed for the specimens of sets B and C over the entire r.h. range. This behaviour indicates the presence of Schottky barriers at the oxide/electrode interfaces. In dry conditions and at room temperature, for the specimens in set B a current of 5 FA was recorded upon application of 10 V, while at the same voltage it was only of about 10 pA for the specimens in set C. It was found that the ratio between the current at a given r.h. and the current in dry conditions for these specimens was
independent of the bias applied. Given the preparation conditions, the different electrical behaviour may be explained in terms of a different stoichiometry in the titania films. The specimens in set A were prepared by reactive sputtering in the presence of oxygen. Therefore, it is likely that the content of oxygen in the films was close to the stoichiometric content of TiOz, and the limited number of oxygen defects made the films behave as insulators. On the other band the set B specimens, prepared by sputtering from a TiO, target, and the C specimens, prepared by the oxidation of sputtered Ti films, were n-type semiconductors. Their interface with the Au electrodes, which have a high work function, led to the formation of a Schottky barrier [16], explaining their Z-V characteristics. The set B specimens probably had more defects and, thus, higher conductivity. Considering the results of the XPS analysis, the Ti 2p lines showed for the films in set A a binding energy value of 458.6 eV, which is typical of the element in its Ti4’ oxidation state. A complex band in the Ti 2p region, centred at 457.9 eV, was observed for the set C films, showing that Ti was present also in a less oxidated state (metallic or sub-oxide). For the set B films, the binding energy was slightly shifted towards values lower than that measured for films in set A. This is probably due to an oxidation state slightly lower than 4+ for Ti. Given that these preliminary results were obtained on the outermost surface, the quantitative results were not entirely reliable because of the presence of oxygen-containing surface contaminants. The intrinsic resistance of the films was also related to their humidity sensitivity. Figure 1 shows the r.h. dependence of the current for the set A specimens. Their current versus r.h. variation was of 4 orders of magnitude in the r.h. range from 2 to 90%. Repeated tests showed that the sensors exhibited good reproducibility. The r.h. dependence of the current fits the sum of two exponentials well: in the range from 2 to 60% r.h. the d.c. variation was of 1 order of magnitude,
0
20
40
60 80 100 rh (%) Fig. 1. The r.h. dependence of the current for the set A films, measured with 20 V d.c. at room temperature.
527
while at r.h. > 60% the r.h. sensitivity was higher. This is similar to the case of sputtered MgAl,O, thin films [15]. On the other hand, the results obtained for the set B films were anomalous, since the r.h. increases did not bring about equivalent current increases. Nor were these results reproducible. This may be due to the low intrinsic resistance of these films and to the large number of defects. Figure 2 shows the r.h. dependence of the current for the set C specimens. These films had an intermediate intrinsic resistance, and their current versus r.h. sensitivity was of about 1 order of magnitude. Also in this case there was less r.h. sensitivity at lower r.h. than at higher r.h. The humidity sensitivity is to be ascribed to ionictype surface conduction in multilayered adsorbed water, because capillary condensation is impossible in thin films which are pore-free, and no electrolytic conduction can thus take place [15]. In order to confirm this hypothesis and to determine the conduction carriers for the thin films, current decay measurements were performed at various r.h. values. Figure 3 shows the time dependence of charging and discharging d.c. for the A films. At r.h. lower than 70% charging and discharging currents were not de-
tectable even upon application of 20 V. At higher r.h., a charging current, which decayed in a finite time to a steady-state value, and a discharging current, which went to zero in a finite time, were measured. These results indicate that the intrinsic electronic conduction of the set A films was negligible, and that the films are, as already stated, insulators. The conduction was due to ionic current at all r.h. values. On the other hand, for the set C films the dominant conduction carriers at low r.h. were electrons. As shown in Fig. 4, after an initial spike and a fast decay, which was probably due to residual chemisorbed water molecules or other contaminants present on the’ surface of the films even in nearly dry conditions, the steady-state current values depended on the applied voltage, clearly showing that electrons were the main carriers. With increasing r.h., the contribution of protons to charge transport became increasingly important, as shown by the increasing time needed to reach the steady-state value after the application of the external voltage, and to reach zero after discharging. These results also confirmed that the set C films were semiconductors. Figure 5 shows the response time of the set A films to 20 V d.c. The results were on the whole reproducible. 6 10”
-11 I
0% rh
4 lo-”
SO%rh
5v
80% rh
5V
5v
2 lo-” 2
0
-2 lo-” 4 lo-”
0
20
40
80
60
100
-6 IO” 0
500
rh (%) Fig. 2. The r.h. dependence of the current for the set C films, measured with 2 V d.c. at room temperature.
z -
0
r lov
-I 1o-‘O
0
1500
2000
Fig. 4. The time dependence of the charging current (upon application of 2 or 5 V dc.) and discharging current (upon removal of the applied potential) for the set C films, at various r.h. values at room temperature.
wet
I-
ov
iv
100
1OQO time (see)
200
300
I
4ccl
dry
500
time (SIX)
Fig. 3. The time dependence of the charging current (upon application of 20 V d.c.) and discharging current (upon removal of the applied potential) for the set A films, at various r.h. values at room temperature.
0
1000
zoo0 time (set)
3000
4000
Fig. 5. Current response of the set A films to cyclic r.h. variations at 20 V d.c. at room temperature.
528
The response time was slower with increasing r.h., but it was faster with decreasing r.h. The volume of the test chamber was 150 cm3 and the total flow was 200 scm3/min. The slow response may be explained in terms of a limited number of water adsorption sites. The absence of porosity makes water desorption easy and the process is extremely rapid, thereby resulting in a very short response time.
References 1 S.C. Chang and W.H. Ko, in W. G6pe1, J. Hesse and J.N.
2
3
Conclusions The humidity-sensitive electrical properties of titania thin films largely depend on their sputtering conditions. The best results were obtained with thin films prepared by reactive sputtering. In this case, highly resistive films were obtained, without oxygen defects. Their intrinsic resistance is however too high for practical applications. This problem may be overcome by optimizing the materials and the geometry of the electrodes, and by doping the TiOa tis to decrease their resist@. The production of TiOz films without complete control of the preparation parameters leads to the production of defective materials, which have lower resistivity, but which also have decreased sensitivity and give unreliable, non-reproducible results.
7
8
9
10
11
Acknowledgements The authors thank Dr E. Verona (IDAC, C.N.R., Rome) and Mr G. Petrocco (IESS, C.N.R., Rome) for assistance in the preparation of thin films, and Dr G. Righini (ESCA Laboratory, Research Area of Rome, C.N.R.) for technical assistance in providing XPS spectra. This work was supported by the National Research Council of Italy (C.N.R.), under the auspices of the Targeted Project ‘Special Materials for Advanced Technologies’.
12
Zemei (eds.), Sen.rora: A Comp&en.sive Survey, Vol. 1, VCH, Weinheim, 1989, Ch. 6, pp. 169-194. T. Seiyama, Chemical sensors - Current state and future outlook, in T. Seiyama (ed.), Chemical Sensor Tecgnofogy, Vol. 1, Kodansha, Tokyo, and Elsevier, Amsterdam, 1988, pp. I-13. Q. Wu, KM. Lee and C.C. Liu, Development of chemical sensors using microfabrication and micromachining techniques, Sensors and Actuators B, 13 (1993) l-6. B.M. Kuiwicki, Humidity sensors, L Am Cemm. Sot., 74 (1991) 697-708. N. Yamazoe and Y. Shim& Humidity sensors: principles and applications, Sensors and Actuator, 10 (1986) 379-398. J.G. Fagan and V.R.W. Amarakoon, Reliability and reproducrbiiity of ceramic sensors: part III, humidity sensors, Am &mm. Sot. Bull., 72 (1993) 119-130. H. Yagi, Humidity sensors using ceramic materials, in M. Butler, A. Ricco and N. Yamaxoe (eds.), Efoc. Sump. Chemical Sensors ll, Vol. 93-7, The Electrochemical Sot., Pennington, NJ, 1993, pp. 498-509. N. Yosbimura, S. Sato, M. Itoi and H. Taguchi, Electrical and humidity sensitive characteristics of TiO, and TiOrSnO, films prepared by sol-gel method, Sozai Burseigaku kassh[ 3 (1990) 47-56. J. Lin, M. Heurich and E. Obermeier, Manufacture and examination of various spin-on glass films with respect to their humidity-sensitive properties, Sensors and Acfuarors B, 23 (1993) 104-106. G. Gusmano, G. Montespereiii, P. Nundante, E. Traversa, A. Montenero, M. Braghini, G. Mattogno and A. Bearzotti, Humidity-sensitive properties of titania films prepared using the sol-gel process, J. Ceram. Sot. Jpn., 101 (1993) 1095-1100. S. Tsurumi, K. Mogi and J. Noda, Humidity sensors of PdZnO diodes, Digest Tech. Papers, 4th Int. Con$ Solid-State Sensors and Actuators, Tmnsducers ‘87, Tokyo, Japan, June 7-1q 1987, pp. 661664. G. Gusmano, G. Montesperebi, E. Traversa, A. Bearzotti, G. Petrocco, A. D’Amico and C. Di Natale. Magnesium aiuminium spine1 thin film as a humidity sensor, Sensors and Actuators B, 7 (1992) 460-+63. G. Gusmano, G: Montespereili, E. Traversa and G. Mattogno, Microstructure and electrical properties of MgAJrO, thin films for humidity sensing, 3. Am. Cemm. Sot., 76 (1993) 743-750. M. Fukazawa, H. Matuxaki and K. Hara, Humidity- and gassensing properties with an Fe,O, film sputtered on a porous Alx03 film, Sensors and Achutors B, I4 (1993) 521-522. G. Gusmano, G. Montespereili, E. Traversa and A. Bearxotti, Humidity-sensitive elect&al properties of MgAI,O, thin films, Sensors and Achrators B, 14 (19931 525-527. B.M. Kuiwicki, Ceramic sensors Hnd transducers, I. Phys. them. Solid.&45 (1984) 1015-1031.
525
Humidity sensitivity of sputtered Ti02 thin films Andrea Bearzotti Inshkte of Solid
StateElectronics (IESS), National Research Council of Italy (CNR), via Cineto Romano 42, 00156 Rome (Italy)
Alessandra
Bianco, Giampiero
Montesperelli
and Enrico Traversa
Depaninent of Chemical Sciences and Technologies, Univets@ of Rome ‘Tar Vergata’, via della Ricaa (It+)
Scientifica, 00133 Rome
Abstract The humidity sensitivity of sputtered Ti02 thin films was studied by d.c. analysis techniques. Three different sets of specimens were prepared by sputtering in different conditions. The films prepared by reactive sputtering from a Ti target showed a current versus relative humidity (r.h.) sensitivity of 4 orders of magnitude in the r.h. range
from 2 to 90%. For these films the current-voltage (1-V) characteristics were linear, while films prepared by sputtering from a Ti02 target and those prepared by the thermal oxidation of sputtered Ti layers showed a back-to-back diode behaviour. The formation of Schottky barriers at the metal/oxide interfaces indicates that these films are semiconductors. Using the charging and discharging processes it was determined that for films prepared by reactive sputtering intrinsic electronic conduction was negligible, while for the other tilms at low r.h. the conduction carriers were mainly electrons, and protons and electrons at high r.h., protons being the dominant carriers. The films prepared by reactive sputtering showed a slow response time during water adsorption, and a faster response time during water desorption.
Introduction Silicon micromachining and on-chip integration technology are the most promising technologies in sensor production [l]. The miniaturization of sensors is one of the chief goals in the development of chemical sensors [2]. The most important recent developments in chemical sensor technology is the introduction of microfabrication and micromachining technologies, which need the production of sensing elements in thin-film form [3]. In the field of humidity sensors, with these technologies it is possible to carry out the integration of humiditysensing elements on silicon microchips, along with linearization, temperature compensation and signal-conditioning circuitry needed for a practical device [4]. Both polymeric and ceramic films have been investigated as humidity-sensitive elements, although polymer films have received wider attention [5]. Porous ceramic moisture-sensing elements have shown advantagesin terms
of thermal and chemical stability, and mechanical strength [6, 71. The recent progress in ceramic deposition technologies (chemical and physical) provides new possibilities for the miniaturization of ceramic humidity-sensing elements and on-chip integration. Several ceramic oxides have recently been studied in thin-film form, prepared
0954OW94/$07.00 0 1994 Elsevier Sequoia. All rights reserved SSDI 0925-4005(93)01072-C
either by the sol-gel method [g-10] or by sputtering [ll-151. TiO, films prepared by sol-gel showed a fast response time, but had a high intrinsic resistance, and drifted after exposure to humid environments [lo]. This paper deals with the study of TiOz thin films prepared by sputtering in different conditions. Chemical and electrical analyses were performed in order to study the use of these films as sensing elements in integrated devices. Experimental Three sets of TiO, specimens were prepared using different sputtering conditions, which are reported in Table 1. Set A specimens were obtained by reactive sputtering from a Ti target, set B by sputtering from a TiO, target in inert atmosphere, and set C by the thermal oxidation of sputtered Ti films. The TiO, target, 5 cm in diameter and 3 mm thick, was sintered at 14UO “C for 1 h, using a commercial powder (99.9% purity grade, Aldrich Chem. Co., Inc.). The films were deposited on alumina substrates. An interdigitated-comb electrode 7 mm square, with 17 fingers separated by gaps the same width as the fingers, was previously deposited on the substrates and defined by a standard photolithographic process.
526 TABLE! 1. Conditions of preparation
Target Pre-sputtering pressure (Pa) Atmosphere Sputtering pressure (Pa) r.f. direct power (W) Deposition time (min) Thermal treatment
of sputtered TiOz thin lilms
Set A Set B
Set C
Ti 0.67 02 0.67 500 120
Ti 1.3x10-5 Ar 0.67 200 10 6 h at 550 “C in wet O2
TiOl 1.3x10-J Ar 0.67 300 30
The chemical composition of the fihn’s surface was analyzed using X-ray photoelectron spectroscopy (XPS) (VG, model Escalab). The experimental procedure used for the XPS measurements was the same as that reported elsewhere for spine1 thin Iilms [13]. The electrical response of the thin films was analyzed by taking d.c. measurements using a Keithley quasistatic CV-meter 595, at different relative humidity (r.h.) values, in a test cell developed by us where both temperature and humidity were monitored. Relative humidity monitoring was carried out using a commercial sensor (Physthem, model PCRC-ll), which gave results accurate to within f 2%. Current-voltage (Z-v) characteristics were recorded at room temperature. The r.h. dependence of resistance was measured by recording the current upon application of a constant bias, increasing r.h. gradually from 2 to 90%, at a constant rate of 1% r.h. per min. Stability and response time tests were performed by measuring the current for a constant d.c. voltage, during r.h. cycling. The charging current upon application of a d.c. voltage to the thin films and the discharging current upon removal of the applied voltage were also measured until the current reached steadystate values, at room temperature for various r.h. values. Results and discussion The Z-V characteristics of the specimens in set A, recorded at various r.h. values, were nearly linear, showing ohmic behaviour. At room temperature, the resistivity of the specimens in nearly dry conditions was evaluated at about 10” Sz, while it was about 150 Gfl at 90% r.h. Back-to-back diode behaviour was observed for the specimens of sets B and C over the entire r.h. range. This behaviour indicates the presence of Schottky barriers at the oxide/electrode interfaces. In dry conditions and at room temperature, for the specimens in set B a current of 5 FA was recorded upon application of 10 V, while at the same voltage it was only of about 10 pA for the specimens in set C. It was found that the ratio between the current at a given r.h. and the current in dry conditions for these specimens was
independent of the bias applied. Given the preparation conditions, the different electrical behaviour may be explained in terms of a different stoichiometry in the titania films. The specimens in set A were prepared by reactive sputtering in the presence of oxygen. Therefore, it is likely that the content of oxygen in the films was close to the stoichiometric content of TiOz, and the limited number of oxygen defects made the films behave as insulators. On the other band the set B specimens, prepared by sputtering from a TiO, target, and the C specimens, prepared by the oxidation of sputtered Ti films, were n-type semiconductors. Their interface with the Au electrodes, which have a high work function, led to the formation of a Schottky barrier [16], explaining their Z-V characteristics. The set B specimens probably had more defects and, thus, higher conductivity. Considering the results of the XPS analysis, the Ti 2p lines showed for the films in set A a binding energy value of 458.6 eV, which is typical of the element in its Ti4’ oxidation state. A complex band in the Ti 2p region, centred at 457.9 eV, was observed for the set C films, showing that Ti was present also in a less oxidated state (metallic or sub-oxide). For the set B films, the binding energy was slightly shifted towards values lower than that measured for films in set A. This is probably due to an oxidation state slightly lower than 4+ for Ti. Given that these preliminary results were obtained on the outermost surface, the quantitative results were not entirely reliable because of the presence of oxygen-containing surface contaminants. The intrinsic resistance of the films was also related to their humidity sensitivity. Figure 1 shows the r.h. dependence of the current for the set A specimens. Their current versus r.h. variation was of 4 orders of magnitude in the r.h. range from 2 to 90%. Repeated tests showed that the sensors exhibited good reproducibility. The r.h. dependence of the current fits the sum of two exponentials well: in the range from 2 to 60% r.h. the d.c. variation was of 1 order of magnitude,
0
20
40
60 80 100 rh (%) Fig. 1. The r.h. dependence of the current for the set A films, measured with 20 V d.c. at room temperature.
527
while at r.h. > 60% the r.h. sensitivity was higher. This is similar to the case of sputtered MgAl,O, thin films [15]. On the other hand, the results obtained for the set B films were anomalous, since the r.h. increases did not bring about equivalent current increases. Nor were these results reproducible. This may be due to the low intrinsic resistance of these films and to the large number of defects. Figure 2 shows the r.h. dependence of the current for the set C specimens. These films had an intermediate intrinsic resistance, and their current versus r.h. sensitivity was of about 1 order of magnitude. Also in this case there was less r.h. sensitivity at lower r.h. than at higher r.h. The humidity sensitivity is to be ascribed to ionictype surface conduction in multilayered adsorbed water, because capillary condensation is impossible in thin films which are pore-free, and no electrolytic conduction can thus take place [15]. In order to confirm this hypothesis and to determine the conduction carriers for the thin films, current decay measurements were performed at various r.h. values. Figure 3 shows the time dependence of charging and discharging d.c. for the A films. At r.h. lower than 70% charging and discharging currents were not de-
tectable even upon application of 20 V. At higher r.h., a charging current, which decayed in a finite time to a steady-state value, and a discharging current, which went to zero in a finite time, were measured. These results indicate that the intrinsic electronic conduction of the set A films was negligible, and that the films are, as already stated, insulators. The conduction was due to ionic current at all r.h. values. On the other hand, for the set C films the dominant conduction carriers at low r.h. were electrons. As shown in Fig. 4, after an initial spike and a fast decay, which was probably due to residual chemisorbed water molecules or other contaminants present on the’ surface of the films even in nearly dry conditions, the steady-state current values depended on the applied voltage, clearly showing that electrons were the main carriers. With increasing r.h., the contribution of protons to charge transport became increasingly important, as shown by the increasing time needed to reach the steady-state value after the application of the external voltage, and to reach zero after discharging. These results also confirmed that the set C films were semiconductors. Figure 5 shows the response time of the set A films to 20 V d.c. The results were on the whole reproducible. 6 10”
-11 I
0% rh
4 lo-”
SO%rh
5v
80% rh
5V
5v
2 lo-” 2
0
-2 lo-” 4 lo-”
0
20
40
80
60
100
-6 IO” 0
500
rh (%) Fig. 2. The r.h. dependence of the current for the set C films, measured with 2 V d.c. at room temperature.
z -
0
r lov
-I 1o-‘O
0
1500
2000
Fig. 4. The time dependence of the charging current (upon application of 2 or 5 V dc.) and discharging current (upon removal of the applied potential) for the set C films, at various r.h. values at room temperature.
wet
I-
ov
iv
100
1OQO time (see)
200
300
I
4ccl
dry
500
time (SIX)
Fig. 3. The time dependence of the charging current (upon application of 20 V d.c.) and discharging current (upon removal of the applied potential) for the set A films, at various r.h. values at room temperature.
0
1000
zoo0 time (set)
3000
4000
Fig. 5. Current response of the set A films to cyclic r.h. variations at 20 V d.c. at room temperature.
528
The response time was slower with increasing r.h., but it was faster with decreasing r.h. The volume of the test chamber was 150 cm3 and the total flow was 200 scm3/min. The slow response may be explained in terms of a limited number of water adsorption sites. The absence of porosity makes water desorption easy and the process is extremely rapid, thereby resulting in a very short response time.
References 1 S.C. Chang and W.H. Ko, in W. G6pe1, J. Hesse and J.N.
2
3
Conclusions The humidity-sensitive electrical properties of titania thin films largely depend on their sputtering conditions. The best results were obtained with thin films prepared by reactive sputtering. In this case, highly resistive films were obtained, without oxygen defects. Their intrinsic resistance is however too high for practical applications. This problem may be overcome by optimizing the materials and the geometry of the electrodes, and by doping the TiOa tis to decrease their resist@. The production of TiOz films without complete control of the preparation parameters leads to the production of defective materials, which have lower resistivity, but which also have decreased sensitivity and give unreliable, non-reproducible results.
7
8
9
10
11
Acknowledgements The authors thank Dr E. Verona (IDAC, C.N.R., Rome) and Mr G. Petrocco (IESS, C.N.R., Rome) for assistance in the preparation of thin films, and Dr G. Righini (ESCA Laboratory, Research Area of Rome, C.N.R.) for technical assistance in providing XPS spectra. This work was supported by the National Research Council of Italy (C.N.R.), under the auspices of the Targeted Project ‘Special Materials for Advanced Technologies’.
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