Plasma-Assisted Chemical Vapor Deposition Processes

Abstract

Over the past two decades a vast number of publications have emerged from laboratories all over the world, describing the application of plasmas for preparing and processing materials. MRS symposia, scientific journals and books, and complete conference series are solely devoted to this specific topic.Modern VLSI integrated circuits, for instance, would simply not exist without sophisticated plasma etching techniques. But highly reactive, partly ionized and dissociated, quasi-neutral gases—plasmas—are not only useful for etching purposes, i.e., the removal of materials. They are also very valuable tools for the deposition of materials with unique structures and compositions at lower temperatures than for conventional thermally induced chemical vapor deposition processes. Backed by intensive research activities and more than a decade of practical experiences, plasma deposition technologies are now penetrating a number of industrial manufacturing processes.Plasmas can be classified into two basic categories — non-isothermal, and isothermal or thermal plasmas.Within the high electric fields applied for non-isothermal plasma generation at reduced pressure, free electrons are accelerated to energies that correspond to several thousand degrees in the case of thermal activation. The neutral species in the gas phase and the heavy ions are either not influenced by the fields or cannot follow changing fields fast enough. Their temperature stays low, resulting in a difference between electron and gas temperature. In these nonequilibrium plasmas, the collisions of high energy electrons and gas molecules result in dissociation processes that would only occur at very high temperatures of more than 5,000 K in the case of thermal equilibrium. Therefore, non-isothermal plasmas allow the preparation of materials and compositions that are difficult to obtain using thermally activated, conventional CVD. Due to the initiation of chemical reaction by collisions with “hot” electrons rather than hot gas molecules, the processing temperature can, in many cases, be kept lower than in conventional deposition processes.