1. Introduction
In recent times, the development of tin dioxide (SnO2) thin films as gas sensors has attracted considerable interest [1]. Thin film SnO2 gas sensors have been fabricated by a number of techniques including spray pyrolysis [2], chemical vapor deposition [3], thermal evaporation [4] and sputtering [5]. The main problem encountered in all these techniques has been the lack of consistency in the film properties. The gas sensing properties of the films were found to depend strongly on mode of preparation. Atomic layer deposition (ALD) is one of the most promising techniques to get ultra thin uniform films. The usefulness of ALD, which is also referred to atomic layer epitaxy (ALE), consists in establishing an internal control over the growth by saturating partial self-limiting surface reactions from the division of overall CVD reaction. A careful selection of the reactant, source and deposition temperatures and the pulse and purge durations are necessary to grow a high grade ultra thin film of mono- or sub-mono layers. Moreover, the thickness of the mono layers can be determined by simply counting the number of ALD cycles and the layer growth is well controlled and stable overall of the deposition process.
In general agreement, the morphology and surface state of SnO2 thin film has a strong influence on gas sensing. The increased number of available sites with porosity of thin film for oxidation reaction of reducing gases would lead to increase in sensitivity. However, since the gas sensitivity depends on the electrical properties of the film, a precise control of the SnO2 microstructure is needed. Our basic idea of this experiment is to control the stoichiometry of the thin film by monitoring the thickness and other parameters of plasma enhanced-atomic layer deposition (PE-ALD) process for optimum use of gas sensing. And also the requirement of miniaturization in electronic components and low power consumption has led to thin film type SnO2 sensor.
The chemical composition and crystalline structure of SnO2 thin films used as gas sensors by RF sputtering has been reported by Serrini et al. [6]. Park and Mackenzie [7] have studied the thickness and microstructure effects on alcohol sensing of the tin oxide thin film prepared by the sol–gel process. Rosental et al. [8] and [9] have prepared ultra thin amorphous SnO2 layers on α-Al2O3(0 1 2) substrate using SnCl4 and SnI4 precursors by AL-CVD and ALD processes, which were sensitive to the reducing gases in air. A great challenges, however, offered by PE-ALD have practically, not yet been studied. PE-ALD is a promising technique to produce high purity and high density metal oxide thin films at low growth temperatures. The inherent growth kinetics considerably enhances the control over thickness and morphology. In this paper, we tried to optimize the reaction process conditions of PE-ALD, and studied physical, chemical and electrical properties as a function of thickness of the SnO2 thin film for gas sensing.