Influence of the composition of gas mixture on the stoichiometry of sputter-deposited compound films: The case of zircon
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Influence of the composition of gas mixture on the stoichiometry of sputter-deposited compound films: The case of zirconium nitrides M. Wautelet, J. P. Dauchot, F. Debal, S. Edart, and M. Hecq Laboratoire de Chimie Inorganique et Analytique, Universit´e de Mons-Hainaut, 23, Avenue Maistriau, B-7000 Mons, Belgium (Received 18 July 1995; accepted 12 December 1995)
By means of dc-reactive sputtering, it is possible to vary the stoichiometry of deposited zirconium nitrides, by varying the molar fraction of N2 in an Ar–N2 gas mixture. In order to understand the origin of this effect, a theoretical model of reactive sputtering is devised. It is based on the study of reaction kinetics taking place at the surfaces of the cathode and the chamber walls. In fitting the model with experimental data, it turns out that one has to introduce the roles of Ar, N2 , and N species. For reactive sputtering of ZrNz films, a good fit is obtained when it is assumed that the molar fraction of N is constant when the molar fraction of N2 increases up to about 75% (under our experimental conditions). Above this concentration of N2 , the concentration of N has to increase. By the analysis of the theoretical model, general scaling laws between experimental parameters (current, pressure, and areas of the cathode and the chamber walls) are easily obtained.
I. INTRODUCTION
Reactive sputtering is a widely used technique for synthesizing compound films, in various technological fields1 (microelectronics, glass coating, hard coating, . . .). It is also seen as an environmental benign technique, since one avoids the use of various chemical reactants. Despite the large number of applications, the fundamental processes are far from being understood. This is due to the fact that many interdependent mechanisms take place simultaneously. The starting point is the application of a large electrical potential difference between a cathode and an anode, separated by a gaseous medium. Because of this, some gas particles are ionized. As a result, a nonuniform plasma appears, made of various atoms, ions, molecules, and electrons. Complex chemistry also takes place in the plasma zone. Positive ions are accelerated to the cathode, where they collide and eject (sputter) cathode species. In turn, these interact with the plasma species. Some go to the anode, where they eventually condense and chemically react with adsorbates or substrate atoms. Also, some species may escape from the plasma zone and interact with the chamber walls. All these processes are obviously pressure-dependent. Moreover, the interactions are generally nonlinear.2 In the description of the overall process, the characteristics of the walls and the pumping system have also to be considered, since they influence the total gas consumption. Experimentally, the interdependence of the parameters is such that one has no direct control over all parameters, and it is hard to predict the behavior of a given J. Mater. Res., Vol. 11, No. 4, Apr 1996
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