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Oxygen flow effect on gas sensitivity properties of tin oxide film prepared by r.f. sputtering
Sensors and Actuators B 55 (1999) 55 – 59
Oxygen flow effect on gas sensitivity properties of tin oxide film prepared by r.f. sputtering Vladimir V. Kissine a,*, Sergei A. Voroshilov b, Victor V. Sysoev a a
Department of Physics, Sarato6 State Technical Uni6ersity, ul.Polytechnicheskaya 77, 410054 Sarato6, Russian Federation Semiconductor Physics Department, Sarato6 State Uni6ersity, ul.B.Kazachija 112 -a, 410026 Sarato6, Russian Federation
b
Received 15 April 1998; received in revised form 6 November 1998; accepted 9 November 1998
Abstract SnO2 thin film gas sensors were prepared by r.f. magnetron sputtering. It was shown that the gas-sensitive properties of SnO2 thin films are managed substantially by the conditions of preparation which determine the oxygen content in the films. Oxygen-deficient films were fabricated in two manners: (1) deposition in Ar atmosphere followed by annealing in an oxygen atmosphere and; (2) deposition in Ar/O2 mixture with a variable oxygen concentration. Both methods facilitated with the same success the fabrication of polycrystalline films having a high sensitivity to reducing gases. A quantitative model was developed in order to describe the sputtering process of thin metal oxide films. It has been shown how the stoichiometry of as-deposited films is managed by an oxygen concentration in a reactive chamber. The calculations performed could be matched with the experimental results. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Tin oxide; Gas sensor; Thin film; R.f. sputtering
1. Introduction Metal oxides such as SnO2, TiO2, ZnO, etc., are well known as materials to develop resistive type gas sensors (for reviews see, for example [1 – 5]. The appearance of gas sensitivity in pure metal oxides is mostly caused by the presence of oxygen vacancies in the samples [6]. Therefore, non-stoichiometric films are appropriate for sensors. Among a number of preparation techniques [7], magnetron sputtering has been usefully employed to grow uniform metal oxide thin films with easy control of oxygen deficiencies (for example, [7 – 15]). There are two approaches to obtaining gas-sensitive films: (1) the deposition in an Ar/O2 mixture where the oxygen concentration is a variable parameter [8 – 12] and; (2) deposition in a pure Ar atmosphere followed by annealing in an oxygen atmosphere [13,15]. Both approaches allow the production of films with good gas-sensitivity for the sensor development. * Corresponding author. Tel.: +7-8452-506873; fax: + 7-8452507563. E-mail address: [email protected] (V.V. Kissine)
However, there have been only qualitative approaches to understanding the empirical correlations between the film properties and the oxygen concentration in the vacuum chamber. Moreover, the possibility of identifying a gas has been recently shown assuming a linear dependence between the oxygen flow and the oxygen content in the film [14]. Therefore, the question of interest is how the oxygen flow can quantitatively affect the oxygen content in a metal oxide film under a reactive sputtering. We compared the gas sensitivity of films prepared in a range of oxygen concentrations and made the attempt to propose a quantitative model of reactive sputtering which accounts for the parameters of the deposition system. Such a model allows one to predict the oxygen content in the sputtered film and hence to optimize the film gas-sensitivity properties
2. Experimental details SnO2 thin films were fabricated using a r.f. planar magnetron sputtering system. SnO2 powder of 99.99% purity was used to prepare the target. The powder was pressurized and sintered in an oxygen ambient before
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V.V. Kissine et al. / Sensors and Actuators B 55 (1999) 55–59
placement into the system. The distance between substrate and target was about 6 cm. High-purity argon and oxygen were introduced into the sputtering chamber through flow-meters: the fluxes were adjusted to obtain the desired composition of the mixture. The total gas pressure was approximately 3 mtorr and the r.f. discharge power was about 100 W cm − 2. The substrate temperature was kept at around 200°C to avoid the crystallization of a tin oxide into the structure other than the rutile type on account of the SnO disproportionating into Sn and SnO2 [16]. The deposition rate was to be about 24 nm min − 1. The sputter time equal to 40 min was employed yielding the film thickness equal to about 1000 nm. The unoriented substrates such as polycrystalline alumina, amorphous quartz and Si:SiO2 have been used in this study. Because of the low substrate temperature and the high deposition rate the growing of the film was affected mainly by the sputtering conditions and the substrate effect was minimized compared with other studies [11,12]. Some of the samples were subjected to post-deposition annealing in an oxygen atmosphere at 300– 700°C for 2 h. Metallic contacts for electrical measurements were made on the surface of the films by a vacuum evaporation of Cr through a mask as the final fabrication step. The crystalline structure was examined by X-ray diffraction technique (XRD) with a CuKa radiation source and the reflection high energy electron diffraction (RHEED) method. The surface morphology was observed by a transmission electron microscope (TEM). Conductance measurements were performed at 400°C (hereafter, the operating temperature) in air and testing mixture (generated mixture of synthetic air and ethanol vapors). The gas sensitivity was estimated as (Gg −Ga)/ Ga, where Gg is the conductance of film exposed to the testing mixture and Gg is the conductance of film exposed to air. The experimental set-up has been described previously [17,18].
Fig. 1. The XRD pattern of SnO2 film deposited in an Ar atmosphere: (a) as-deposited films, (b) films annealed at 600°C.
lattice planes (Fig. 1b). The ratio of peak intensities does not correspond to the standard data [19]. It can be attributed to destroyed packing because of the presence of oxygen vacancies in some lattice planes [20]. So, one can conclude that the treatment by annealing in an oxygen atmosphere at temperatures up to 550°C led to the film crystallization into the SnO2 structure with a presence of oxygen vacancies in the lattice. Annealing at higher temperatures allowed the obtaining of SnO2 films with a higher resistance without a change in the XRD pattern. In the latter case, the films have much less response to ethanol vapors (Fig. 3). As one can see, the best gas sensitivity is found in films annealed at 400°C and a loss of sensitivity appears at temperatures higher than 550°C.
3. Experimental results
3.1. SnO2 films deposited in a pure Ar atmosphere The XRD pattern of films deposited in a pure argon atmosphere shows an amorphous structure (Fig. 1a). As-deposited films were found to be not sensitive to ethanol vapors and were subjected to the following treatment. The annealing in an oxygen ambient resulted in a dramatic change in film conductance at 450 – 550°C (Fig. 2). It is apparent that such behavior of the film conductance has to be coupled with a recrystallization of the samples. The typical XRD pattern of the film annealed at 600°C shows the presence of SnO2 peaks corresponding mainly to (110), (101), (211) and (220)
Fig. 2. The conductance of SnO2 films deposited in an Ar atmosphere versus the annealing temperature.
V.V. Kissine et al. / Sensors and Actuators B 55 (1999) 55–59
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Fig. 3. The sensitivity of SnO2 films deposited in an Ar atmosphere to 1500 ppm of ethanol vapors versus the annealing temperature.
Fig. 5. The sensitivity of SnO2 films deposited in Ar/O2 mixture to 1500 ppm of ethanol vapor versus oxygen content in the mixture.
3.2. SnO2 films deposited in Ar/O2 mixture
4. Model of the reactive sputtering
Films as-deposited in Ar/O2 mixture were polycrystalline ones without a necessity to treat them by the post-deposition annealing. The typical XRD pattern of the film deposited in 3:1 Ar/O2 mixture presented SnO2 peaks corresponding to (110), (101), (211), (200), (220) and (002) lattice planes (Fig. 4a). The influence of the post-deposition annealing was negligible in affecting the film structure (Fig. 4b). RHEED studies allowed to find the film texture in [110] direction. The average grain size was estimated from TEM investigations to be about 100 nm in diameter.The as-deposited films were sensitive to ethanol vapors. The maximum sensitivity to 1500 ppm of ethanol vapors was monitored in films deposited in an atmosphere containing about 35% oxygen (Fig. 5).
Growing metal oxide films by sputtering in a reactive mode might be described as follows. When the target consists of a metal oxide the oxygen partial pressure is originated from internal (target) and external sources. In steady-state conditions the flux of oxygen atoms f is equal to the flux of metal atoms because of the target homogeneity. In this case the target supplies molecules into the sputtering chamber with the rate fAt where At is the target surface. The external source of oxygen gives a gain to the oxygen partial pressure in the chamber with the rate Naq0/V0 where Na is Avogadro’s number, V0 is one mole volume under normal conditions, q0 is the oxygen flow. The oxygen concentration inside the chamber is decreased because of the oxygen deposition on both the chamber walls and the substrate (receiving surface). In terms of molecular kinetic theory the flux of oxygen due to its partial pressure p is expressed as F= 2p/(2pmkT)1/2 where m is the mass of the oxygen molecule, k is Boltzmann’s constant and T is the absolute temperature. Also the oxygen will pass through the system and will be removed from the reactive chamber by the system pump at the rate pSNa/P0V0 where P0 is an atmosphere pressure, S is the pumping speed. Thus, the steady-state condition of oxygen partial pressure in the vacuum chamber will be balanced as follows q= C(s+ a+ hb)
Fig. 4. The XRD pattern of SnO2 film deposited in 3:1 Ar/O2 mixture: (a) as-deposited film, (b) film annealed at 700°C.
where a and b are the parts of the receiving surface covered by Sn and SnO phases respectively, h is a coefficient which shows the relative adsorption of oxy-
V.V. Kissine et al. / Sensors and Actuators B 55 (1999) 55–59
58
gen atoms onto the SnO phase comparative to their adsorption onto the Sn phase, q =1 + q0Na/V0fAt is the oxygen flow in respect to the flow from the target, s= SNa/(2p0/(2pmkT)1/2)V0As is the pumping efficiency, C= FAs/fAt, As is the surface of the film deposited on the substrate. Two suggestions have to be introduced to complete the equation set. First, the surface occupied by the Sn phase (a) is increased when Sn is deposited onto the SnO and SnO2 phases and is decreased because of the oxidation process. Secondly, the surface occupied by the SnO2 phase is increased when SnO is oxidized and is decreased because of deposition of Sn. Thus, in the equilibrium Ca= b
hCb = g
where g is the part of the receiving surface covered by SnO2 phase. The results of the calculations performed are drawn in Fig. 6. When the oxygen flow from an external source is rather small, i.e. q value is in 1}10 range, the O:Sn ratio is equal to about 1:1.Therefore, in this case an amorphous structure should be expected under the experimental conditions described above where the crystallization of SnO structure is avoided. When the oxygen flow is increased, i.e. q increases, the SnO2 structure is the most favorable one to appear. However, the crystal perfection and the presence of oxygen vacancies depends on the q value. As mentioned above, we found the best gas-sensitive properties in films deposited in an Ar/O2 mixture equal to 3:1. This situation corresponds to q = 103 or SnOx films where x $1.9 for our experimental conditions. The oxygen deficit agrees with data on abnormal intensities of SnO2 peaks in the XRD patterns of the films.
5. Conclusions The oxygen content in gas-sensitive SnO2 thin films was the subject of change by the oxygen annealing of films deposited in a pure Ar atmosphere or by variation of Ar/O2 ratio in the gas mixture during the deposition. The appearance of gas-sensitive properties in the samples was dependent on the amount of oxygen introduced into the film by both methods. The maximum of gas-sensitivity determined as the relative change of film conductance under exposure to a reducing gas may be reached by choosing the annealing temperature (first approach) or the Ar/O2 ratio (second approach). Calculation of stoichiometry of tin oxide film prepared under r.f. sputtering in Ar/O2 gas mixture can be performed on the basis of the proposed model. If the sputtering process is carried out under conditions when the SnO phase formation is avoided then SnO2 films with a variable oxygen deficiency are fabricated. The degree of oxygen deficit is managed by an oxygen flow into the reactive chamber. The XRD and electrical measurements of prepared films could be matched with calculation results. From comparing the theoretical and experimental results it has been obtained that the best gas-sensitivity might be monitored in films prepared by sputtering when the relative oxygen flux (parameter q) is equal to about 103.
Acknowledgements The authors would like to thank Dr. S.H. Finkelshtein for the useful discussion on X-ray and electron diffraction studies.
References
Fig. 6. Results of simulation of the SnOx film content versus the oxygen flow: 1,2,3 are Sn, SnO and the SnO2 fractions of the film, respectively; 4 is the O:Sn ratio.
[1] P.T. Moseley, B.C. Tofield (Eds.), Solid State Gas Sensors, Adam Hinger, Bristol, 1987. [2] W. Gopel, T.A. Jones, M. Kleitz, I. Lundstrom, T. Seijama (Eds.), Chemical and biochemical sensors, Pts. 1, 2, in: W. Gopel, J. Hesse, J.N. Zemel (Eds.), Sensors: a Comprehensive Survey, vol. 3, VCH, Weinheim, 1992. [3] S. Yamauchi (Ed.), Chemical Sensor Technology, vol. 4, Elsevier, Amsterdam, 1992. [4] G. Sberveglieri, Recent developments in semiconducting thin film gas sensor, Sensors and Actuators B 23 (1995) 103–109. [5] L.Yu. Kupriyanov (Ed.), Semiconductor Sensors in PhysicoChemical Studies, Elsevier, Amsterdam, 1996. [6] G. Heiland, D. Kohl, in: T. Seijama (Ed.), Chemical Sensor Technology, vol. 1, Elsevier, Amsterdam, 1988, pp.15 –38. [7] V. Demarne, R. Sanjines, in: G. Sberveglieri (Ed.), Gas Sensors: Principles, Operation and Developments, Kluwer, Dordrecht, 1992, (Chapter 3). [8] I. Sayago, J. Gutierrez, L. Ares, J.I. Robla, M.C. Horrillo, J. Getino, J. Rino, J.A. Agapito, The effect of additives in tin oxide on the sensitivity and selectivity to NOx, Sensors and Actuators B 26 – 27 (1995) 19 – 23.
V.V. Kissine et al. / Sensors and Actuators B 55 (1999) 55–59 [9] S. Kaciulis, G. Mattogno, A. Galdikas, A. Mironas, A. Setkus, Influence of surface oxygen on chemoresistance of tin oxide film, J. Vac. Sci. Technol. A 14 (1996) 3164–3168. [10] M. Di Giulio, A. Serra, A. Tepore, R. Rella, P. Siciliano, L. Mirenghi, Influence of the deposition parameters on the physical properties of tin oxide thin films, Mater. Sci. Forum 203 (1996) 143 – 148. [11] J. Vetrone, Y.W. Chung, R. Cavicchi, S. Semancik, Role of initial conductance and gas pressure on the conductance response of single-crystal SnO2 thin films to H2, O2 and CO, J. Appl. Phys. 73 (1993) 8371–8376. [12] R.E. Cavicchi, J.S. Suehle, K.G. Kreider, B.L. Shomaker, J.A. Small, M. Gaitan, P. Chaparala, Growth of SnO2 films on micromachined hotplates, Appl. Phys. Lett. 66 (1995) 812 – 814. [13] G. Williams, G.S.C. Coles, NOx response of tin dioxide based gas sensors, Sensors and Actuators B 15–16 (1993) 349– 353. [14] D.S. Vlachos, C.A. Papadopoulos, J.N. Avaritsiotis, The effect of film oxygen content on SnOx gas sensor selectivity, Sensors and Actuators B 24 –25 (1995) 883–885. [15] G. Williams, G.S.C. Coles, The influence of deposition parameters on the performance of tin dioxide NO2 sensors prepared by radio-frequency magnetron sputtering, Sensors and Actuators B 24 – 25 (1995) 469 – 473. [16] P. Kofstad, Nonstoichiometry, Diffusion and Electrical Conductivity in Binary Metal Oxides, Mir, Moscow, 1975 (transl. from Wiley, NY, 1972). [17] V.V. Kisin, V.V. Sysoev, V.V. Simakov, S.A. Voroshilov, Thin film gas sensor: nature of sensitivity, Proc. of 10th Europ. Conf. on Solid-State Transducers ‘Eurosensors X’, vol. 3, Leuven, Belgium, September 8–11, 1996, pp. 977–980. [18] V.V. Kisin, S.A. Voroshilov, V.V. Sysoev, V.V. Simakov, Threeelectrode gas sensor, Instrum. Exp. Technol. 38 (1995) 683 – 687. [19] Powder diffraction File. Data Cards. Inorganic Section. JCPDS, Swarthmore, Pennsylvania, USA, 1987, 21–1250. [20] K.N. Yu, Y. Xiong, Y. Lin, G. Xiong, Microstructural change of nano-SnO2 grain assemblages with the annealing temperature, Phys. Rev. B 55 (1997) 2666–2671.
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Biographies Vladimir V. Kissine was born in 1952. He received the M.Sc. degree in semiconductor physics from Saratov State University (SSU) in 1974 and Ph.D. degree, also in semiconductor physics, from Gorkyi State University in 1979. From 1980 to 1994 he worked in Institute for Mechanics and Physics at SSU as a senior researcher. In 1994 he joined the Saratov State Technical University as an associate professor. His current interests include thin-film microelectronic technologies and solid-state gas sensors. Sergei A. Voroshilo6 was born in 1957. In 1983 he received the M.Sc. degree in semiconductor physics from Saratov State University (SSU). From 1983 to 1996 he worked in Institute for Mechanics and Physics at SSU. Since 1996 he is a chief of laboratory in Semiconductor Chair of Physics Department at SSU. He has recently completed his Ph.D. in the field of solid-state physics. His present fields of interests are microelectronic technologies and solid-state gas sensors. Victor V. Sysoe6 was born in 1970. After studying in Leningrad Mechanical Institute and Saratov State University (SSU) he graduated in 1994 from SSU with M.Sc. degree in semiconductor physics. Since 1994 he is a Ph.D. student at the Saratov State Technical University. In 1996–1997 academic year he was a visiting researcher in National Microelectronics Research Centre (Cork, Ireland). His scientific interests are chemical sensors and multisensor systems.
Oxygen flow effect on gas sensitivity properties of tin oxide film prepared by r.f. sputtering Vladimir V. Kissine a,*, Sergei A. Voroshilov b, Victor V. Sysoev a a
Department of Physics, Sarato6 State Technical Uni6ersity, ul.Polytechnicheskaya 77, 410054 Sarato6, Russian Federation Semiconductor Physics Department, Sarato6 State Uni6ersity, ul.B.Kazachija 112 -a, 410026 Sarato6, Russian Federation
b
Received 15 April 1998; received in revised form 6 November 1998; accepted 9 November 1998
Abstract SnO2 thin film gas sensors were prepared by r.f. magnetron sputtering. It was shown that the gas-sensitive properties of SnO2 thin films are managed substantially by the conditions of preparation which determine the oxygen content in the films. Oxygen-deficient films were fabricated in two manners: (1) deposition in Ar atmosphere followed by annealing in an oxygen atmosphere and; (2) deposition in Ar/O2 mixture with a variable oxygen concentration. Both methods facilitated with the same success the fabrication of polycrystalline films having a high sensitivity to reducing gases. A quantitative model was developed in order to describe the sputtering process of thin metal oxide films. It has been shown how the stoichiometry of as-deposited films is managed by an oxygen concentration in a reactive chamber. The calculations performed could be matched with the experimental results. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Tin oxide; Gas sensor; Thin film; R.f. sputtering
1. Introduction Metal oxides such as SnO2, TiO2, ZnO, etc., are well known as materials to develop resistive type gas sensors (for reviews see, for example [1 – 5]. The appearance of gas sensitivity in pure metal oxides is mostly caused by the presence of oxygen vacancies in the samples [6]. Therefore, non-stoichiometric films are appropriate for sensors. Among a number of preparation techniques [7], magnetron sputtering has been usefully employed to grow uniform metal oxide thin films with easy control of oxygen deficiencies (for example, [7 – 15]). There are two approaches to obtaining gas-sensitive films: (1) the deposition in an Ar/O2 mixture where the oxygen concentration is a variable parameter [8 – 12] and; (2) deposition in a pure Ar atmosphere followed by annealing in an oxygen atmosphere [13,15]. Both approaches allow the production of films with good gas-sensitivity for the sensor development. * Corresponding author. Tel.: +7-8452-506873; fax: + 7-8452507563. E-mail address: [email protected] (V.V. Kissine)
However, there have been only qualitative approaches to understanding the empirical correlations between the film properties and the oxygen concentration in the vacuum chamber. Moreover, the possibility of identifying a gas has been recently shown assuming a linear dependence between the oxygen flow and the oxygen content in the film [14]. Therefore, the question of interest is how the oxygen flow can quantitatively affect the oxygen content in a metal oxide film under a reactive sputtering. We compared the gas sensitivity of films prepared in a range of oxygen concentrations and made the attempt to propose a quantitative model of reactive sputtering which accounts for the parameters of the deposition system. Such a model allows one to predict the oxygen content in the sputtered film and hence to optimize the film gas-sensitivity properties
2. Experimental details SnO2 thin films were fabricated using a r.f. planar magnetron sputtering system. SnO2 powder of 99.99% purity was used to prepare the target. The powder was pressurized and sintered in an oxygen ambient before
0925-4005/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 9 9 ) 0 0 0 2 2 - 2
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V.V. Kissine et al. / Sensors and Actuators B 55 (1999) 55–59
placement into the system. The distance between substrate and target was about 6 cm. High-purity argon and oxygen were introduced into the sputtering chamber through flow-meters: the fluxes were adjusted to obtain the desired composition of the mixture. The total gas pressure was approximately 3 mtorr and the r.f. discharge power was about 100 W cm − 2. The substrate temperature was kept at around 200°C to avoid the crystallization of a tin oxide into the structure other than the rutile type on account of the SnO disproportionating into Sn and SnO2 [16]. The deposition rate was to be about 24 nm min − 1. The sputter time equal to 40 min was employed yielding the film thickness equal to about 1000 nm. The unoriented substrates such as polycrystalline alumina, amorphous quartz and Si:SiO2 have been used in this study. Because of the low substrate temperature and the high deposition rate the growing of the film was affected mainly by the sputtering conditions and the substrate effect was minimized compared with other studies [11,12]. Some of the samples were subjected to post-deposition annealing in an oxygen atmosphere at 300– 700°C for 2 h. Metallic contacts for electrical measurements were made on the surface of the films by a vacuum evaporation of Cr through a mask as the final fabrication step. The crystalline structure was examined by X-ray diffraction technique (XRD) with a CuKa radiation source and the reflection high energy electron diffraction (RHEED) method. The surface morphology was observed by a transmission electron microscope (TEM). Conductance measurements were performed at 400°C (hereafter, the operating temperature) in air and testing mixture (generated mixture of synthetic air and ethanol vapors). The gas sensitivity was estimated as (Gg −Ga)/ Ga, where Gg is the conductance of film exposed to the testing mixture and Gg is the conductance of film exposed to air. The experimental set-up has been described previously [17,18].
Fig. 1. The XRD pattern of SnO2 film deposited in an Ar atmosphere: (a) as-deposited films, (b) films annealed at 600°C.
lattice planes (Fig. 1b). The ratio of peak intensities does not correspond to the standard data [19]. It can be attributed to destroyed packing because of the presence of oxygen vacancies in some lattice planes [20]. So, one can conclude that the treatment by annealing in an oxygen atmosphere at temperatures up to 550°C led to the film crystallization into the SnO2 structure with a presence of oxygen vacancies in the lattice. Annealing at higher temperatures allowed the obtaining of SnO2 films with a higher resistance without a change in the XRD pattern. In the latter case, the films have much less response to ethanol vapors (Fig. 3). As one can see, the best gas sensitivity is found in films annealed at 400°C and a loss of sensitivity appears at temperatures higher than 550°C.
3. Experimental results
3.1. SnO2 films deposited in a pure Ar atmosphere The XRD pattern of films deposited in a pure argon atmosphere shows an amorphous structure (Fig. 1a). As-deposited films were found to be not sensitive to ethanol vapors and were subjected to the following treatment. The annealing in an oxygen ambient resulted in a dramatic change in film conductance at 450 – 550°C (Fig. 2). It is apparent that such behavior of the film conductance has to be coupled with a recrystallization of the samples. The typical XRD pattern of the film annealed at 600°C shows the presence of SnO2 peaks corresponding mainly to (110), (101), (211) and (220)
Fig. 2. The conductance of SnO2 films deposited in an Ar atmosphere versus the annealing temperature.
V.V. Kissine et al. / Sensors and Actuators B 55 (1999) 55–59
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Fig. 3. The sensitivity of SnO2 films deposited in an Ar atmosphere to 1500 ppm of ethanol vapors versus the annealing temperature.
Fig. 5. The sensitivity of SnO2 films deposited in Ar/O2 mixture to 1500 ppm of ethanol vapor versus oxygen content in the mixture.
3.2. SnO2 films deposited in Ar/O2 mixture
4. Model of the reactive sputtering
Films as-deposited in Ar/O2 mixture were polycrystalline ones without a necessity to treat them by the post-deposition annealing. The typical XRD pattern of the film deposited in 3:1 Ar/O2 mixture presented SnO2 peaks corresponding to (110), (101), (211), (200), (220) and (002) lattice planes (Fig. 4a). The influence of the post-deposition annealing was negligible in affecting the film structure (Fig. 4b). RHEED studies allowed to find the film texture in [110] direction. The average grain size was estimated from TEM investigations to be about 100 nm in diameter.The as-deposited films were sensitive to ethanol vapors. The maximum sensitivity to 1500 ppm of ethanol vapors was monitored in films deposited in an atmosphere containing about 35% oxygen (Fig. 5).
Growing metal oxide films by sputtering in a reactive mode might be described as follows. When the target consists of a metal oxide the oxygen partial pressure is originated from internal (target) and external sources. In steady-state conditions the flux of oxygen atoms f is equal to the flux of metal atoms because of the target homogeneity. In this case the target supplies molecules into the sputtering chamber with the rate fAt where At is the target surface. The external source of oxygen gives a gain to the oxygen partial pressure in the chamber with the rate Naq0/V0 where Na is Avogadro’s number, V0 is one mole volume under normal conditions, q0 is the oxygen flow. The oxygen concentration inside the chamber is decreased because of the oxygen deposition on both the chamber walls and the substrate (receiving surface). In terms of molecular kinetic theory the flux of oxygen due to its partial pressure p is expressed as F= 2p/(2pmkT)1/2 where m is the mass of the oxygen molecule, k is Boltzmann’s constant and T is the absolute temperature. Also the oxygen will pass through the system and will be removed from the reactive chamber by the system pump at the rate pSNa/P0V0 where P0 is an atmosphere pressure, S is the pumping speed. Thus, the steady-state condition of oxygen partial pressure in the vacuum chamber will be balanced as follows q= C(s+ a+ hb)
Fig. 4. The XRD pattern of SnO2 film deposited in 3:1 Ar/O2 mixture: (a) as-deposited film, (b) film annealed at 700°C.
where a and b are the parts of the receiving surface covered by Sn and SnO phases respectively, h is a coefficient which shows the relative adsorption of oxy-
V.V. Kissine et al. / Sensors and Actuators B 55 (1999) 55–59
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gen atoms onto the SnO phase comparative to their adsorption onto the Sn phase, q =1 + q0Na/V0fAt is the oxygen flow in respect to the flow from the target, s= SNa/(2p0/(2pmkT)1/2)V0As is the pumping efficiency, C= FAs/fAt, As is the surface of the film deposited on the substrate. Two suggestions have to be introduced to complete the equation set. First, the surface occupied by the Sn phase (a) is increased when Sn is deposited onto the SnO and SnO2 phases and is decreased because of the oxidation process. Secondly, the surface occupied by the SnO2 phase is increased when SnO is oxidized and is decreased because of deposition of Sn. Thus, in the equilibrium Ca= b
hCb = g
where g is the part of the receiving surface covered by SnO2 phase. The results of the calculations performed are drawn in Fig. 6. When the oxygen flow from an external source is rather small, i.e. q value is in 1}10 range, the O:Sn ratio is equal to about 1:1.Therefore, in this case an amorphous structure should be expected under the experimental conditions described above where the crystallization of SnO structure is avoided. When the oxygen flow is increased, i.e. q increases, the SnO2 structure is the most favorable one to appear. However, the crystal perfection and the presence of oxygen vacancies depends on the q value. As mentioned above, we found the best gas-sensitive properties in films deposited in an Ar/O2 mixture equal to 3:1. This situation corresponds to q = 103 or SnOx films where x $1.9 for our experimental conditions. The oxygen deficit agrees with data on abnormal intensities of SnO2 peaks in the XRD patterns of the films.
5. Conclusions The oxygen content in gas-sensitive SnO2 thin films was the subject of change by the oxygen annealing of films deposited in a pure Ar atmosphere or by variation of Ar/O2 ratio in the gas mixture during the deposition. The appearance of gas-sensitive properties in the samples was dependent on the amount of oxygen introduced into the film by both methods. The maximum of gas-sensitivity determined as the relative change of film conductance under exposure to a reducing gas may be reached by choosing the annealing temperature (first approach) or the Ar/O2 ratio (second approach). Calculation of stoichiometry of tin oxide film prepared under r.f. sputtering in Ar/O2 gas mixture can be performed on the basis of the proposed model. If the sputtering process is carried out under conditions when the SnO phase formation is avoided then SnO2 films with a variable oxygen deficiency are fabricated. The degree of oxygen deficit is managed by an oxygen flow into the reactive chamber. The XRD and electrical measurements of prepared films could be matched with calculation results. From comparing the theoretical and experimental results it has been obtained that the best gas-sensitivity might be monitored in films prepared by sputtering when the relative oxygen flux (parameter q) is equal to about 103.
Acknowledgements The authors would like to thank Dr. S.H. Finkelshtein for the useful discussion on X-ray and electron diffraction studies.
References
Fig. 6. Results of simulation of the SnOx film content versus the oxygen flow: 1,2,3 are Sn, SnO and the SnO2 fractions of the film, respectively; 4 is the O:Sn ratio.
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Biographies Vladimir V. Kissine was born in 1952. He received the M.Sc. degree in semiconductor physics from Saratov State University (SSU) in 1974 and Ph.D. degree, also in semiconductor physics, from Gorkyi State University in 1979. From 1980 to 1994 he worked in Institute for Mechanics and Physics at SSU as a senior researcher. In 1994 he joined the Saratov State Technical University as an associate professor. His current interests include thin-film microelectronic technologies and solid-state gas sensors. Sergei A. Voroshilo6 was born in 1957. In 1983 he received the M.Sc. degree in semiconductor physics from Saratov State University (SSU). From 1983 to 1996 he worked in Institute for Mechanics and Physics at SSU. Since 1996 he is a chief of laboratory in Semiconductor Chair of Physics Department at SSU. He has recently completed his Ph.D. in the field of solid-state physics. His present fields of interests are microelectronic technologies and solid-state gas sensors. Victor V. Sysoe6 was born in 1970. After studying in Leningrad Mechanical Institute and Saratov State University (SSU) he graduated in 1994 from SSU with M.Sc. degree in semiconductor physics. Since 1994 he is a Ph.D. student at the Saratov State Technical University. In 1996–1997 academic year he was a visiting researcher in National Microelectronics Research Centre (Cork, Ireland). His scientific interests are chemical sensors and multisensor systems.