Peculiarities of SnO 2 thin ®lm deposition by spray pyrolysis for gas sensor application G. Korotcenkov b,* , V. Brinzari b , J. Schwank a , M. DiBattista a , A. Vasiliev b a Department of Chemical Engineering, University of Michigan, H. H. Dow Building, 2300 Hayward, Ann Arbor, MI 48109-2136, USA b Department of Microelectronics, Technical University of Moldova, Bld. Stefan cel Mare, 168 Chisinau, MD 2004, Moldova, USA Abstract The analysis of the in¯uence of deposition modes on the structure, electrical and gas sensitivity characteristics of SnO 2 ®lms, deposited by spray pyrolysis, is presented in this report. X-ray diffraction XRD), scanning electron microscopy SEM) and laser ellipsometry were used for this purpose. It was determined that nano-size polycrystalline SnO 2 ®lms with thickness >30 nm can have different structure in dependence on pyrolysis temperature and ®lm thickness. Ultra-thin SnO 2 ®lms with thickness d < 20 nm are discontinuous. The relation between structure of the ®lms and gas sensitivity properties is discussed. The explanation of observed effects was made. # 2001 Elsevier Science B.V. All rights reserved. Keywords: SnO 2 ; Films; Spray pyrolysis; Structure; Gas sensitivity 1. Introduction At present time SnO 2 ®lms are a basic material for thin ®lm gas sensor GS) design [1,2]. Various methods can be used for their deposition, such as: CVD; MOCVD; RGTO rheotaxial growth and thermal oxidation); high frequency sputtering; vacuum evaporation; etc. [1±3]. However, many researchers ®nd that the complexity and high cost of these traditional techniques for ®lm deposition somewhat limits the ability to systematically experiment with modi®cation of thin ®lm for GS applications. Such a situation limits the possibilities to study the nature of metal oxide ®lm's gas sensitivity, especially thin ®lms, also. The use of another deposition method, deposition from aerosol phase spray pyrolysis) [4±6], was proposed for overcoming the above mentioned technological problems. Application of aerosol particles allows making the ®lm deposition process more simple. The traditional multi- source CVD process can be converted to a single-source deposition process. Thanks to the constant concentration of precursors in the solution during the entire deposition process and the small dimensions of aerosol droplets, we can consider these aerosol particles as a single component precursor. Using relatively simple ¯uid ¯ow and reaction engineering models, we can create conditions, where the oxide ®lm formation will take place just on the substrate surface without decomposition of precursors in the vapor phase. For aerosol deposition it is possible to use much simpler precursors, which can have low volatility and low saturated vapor pressure. Therefore, we can use less expen- sive and less toxic precursors. The required equipment for aerosol spray) deposition becomes simple, safe and more versatile. The deposition process is straightforward, rapid, reliable and inexpensive. We can carry out the deposition of ®lms at air atmospheric pressure, in a wide range of processing conditions with concentration of precursors, temperature and deposition rates as controlled variables. Research of the last years, carried out in various labora- tories [4,7±14] and in our group [15±17], has proven the ef®ciency of this approach to the deposition of SnO 2 and another metal oxide ®lms with gas sensitive properties. Different modi®cations of the indicated method, such as: active spray [5]; aerosol ¯ow mode [18]; electrostatic spray [19]; ultrasonic spraying [8]; atomic layer epitaxy [14], and so on, can be used for this purpose. However, it is necessary to declare that the results, obtained in this ®eld of design, so far have a preliminary character. In most of the articles we do not ®nd interrelation between parameters of the thin ®lms and modes of deposition. We also do not ®nd a detailed analysis of the way of technological process optimization for achievement of maximum gas sensitivity of thin ®lms. These criticisms are applied to our previous works also [15±17]. In this report we tried to change this tradition. Therefore, in this manuscript we presented the results of detailed analysis of Sensors and Actuators B 77 2001) 244±252 * Corresponding author. E-mail address: koro@ch.moldpac.md G. Korotcenkov). 0925-4005/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0925-400501)00741-9