Grain size reduction in metal oxide films is one of the key factors to enhance the gas sensing properties of semiconductor layers. One of the basic ideas to create metal-oxide films with small grain size is to use a special regime of thin film deposition by the dc magnetron, ion-beam or r.f. sputtering of pure metal or metal oxide targets. The regime includes the thin film deposition with one or several interruptions of target sputtering. In this case "extra" interfaces are introduced into a thin film body. During the interruption of the sputtering process, an equilibrium surface is formed due to the free surface bond saturation by the atoms from residual atmosphere and/or the structural relaxation of the interface.
Among metal oxide semiconductors, tungsten oxide is a promising material for gas sensing. Therefore, we have checked the idea reported above by preparing WO₃ thin films using r.f. sputtering. The WO₃ films were reactively sputtered onto silicon wafers at an argon/oxygen flow ratio of 50/50, r.f. power of 200 W and a total pressure of 5 mtorr. This gave an average deposition rate up to 2.12 nm per min. The thickness was controlled by ellipsometry and stylus profilomentry and was calculated from AES profiles as well. Three types of films were prepared, i.e. using continual deposition, one and two interrupted depositions with a total deposition time of 40, 40.5 and 41 min, respectively. The interruption time was set to 30 s. It was found that the thickness of the WO₃ films decreased with the number of interruptions. The features of surface structures of these films have been investigated by XRD, AFM and TEM. The gas sensing properties of WO₃ thin films as a function of the number of interruptions have been studied as well. All these results will be presented and discussed in detail.

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