Modeling the stability of reactive sputtering processes

Reactive sputtering of metal targets in a working gas / reactive gas mixture is a favorable method for the deposition of compound layers like oxides. One basic problem of the reactive sputtering process is the strong dependence of essential process parameters such as the sputtering yield upon the degree of target coverage with compound layers. Via positive feedback effects these dependences lead to the instability of the process in a range of reactive working points of the so-called transition mode. Often stoichiometric films can only be deposited in this unstable transition mode at high deposition rates. Therefore process stabilization including a control loop for the reactive gas flow is required. In this paper a model of the reactive sputtering process is presented that quantitatively predicts the dependencies of process variables like reactive gas partial pressure, deposition rate and target coverage on the reactive gas flow. Comparison with experimental data for deposition of alumina films shows, that the model is in good agreement with the experiments and describes for example the well known hysteresis loop. The novel approach of the model includes the dynamics of the process. It therefore enables us to investigate the stability of the reactive working point and to simulate the operation of a reactive gas control loop in the unstable transition mode. Practical requirements on typical control loops and gas systems like the maximum reaction time of the control devices or the maximum volume of the gas man fold are derived from the simulation.