Improved utility of microwave energy for semiconductor plasma processing through RF system stability analysis

Summary form only given. One of the major drawbacks of using microwaves for plasma processing is the inherent instability of typical microwave powered plasma systems. Any RF/microwave powered plasma system comprised of a nonlinear plasma, launched by a resonant antenna system, impedance matched by a resonant circuit or cavity, and powered by an RF/microwave generator with a complex source impedance is invariably unstable over some operating conditions. For microwave systems, this instability typically manifests itself as a propensity for the plasma to extinguish or rapidly change to an undesired intensity as the impedance matching device is brought to best tune or a process transits chemistries. This paper shows through modeling and control analysis why this instability exists, leading to knowledge of how a system can be designed to achieve a higher degree of stabilization in a preferred operating regime. This paper: (1) defines stability as it applies to an RF/microwave driven plasma-processing system; (2) defines a method of quantifying stability; (3) gives a brief background via a summary of past stability studies; (4) establishes a new set of variables and criterion for stability; (5) introduces a Matlab Simulink model to simulate a typical microwave driven plasma system and (6) shows how modifying an RF/microwave generator´s source impedance can modify plasma system stability.