Use of RF Bias in LAPPS (Large Area Plasma Processing System)

Abstract

Abstract : In the LAPPS processing scheme, RF bias can be used for either of two purposes: to provide energetic ion bombardment of a substrate, or to controllably raise the electron temperature (which is intrinsically very cool) to the desired value. The physics of RF bias in LAPPS differs from the situation in conventional processing reactors, for several reasons: (1) The plasma density adjacent to the substrate can be so high that the ion plasma frequency exceeds the microwave frequency. (2) Plasma transport to the substrate is across a magnetic field. (3) Ionization occurs only in a thin, well-defined planar sheet, and thus the volume occupied by plasma is very broad in two dimensions but thin in the third dimension. (4) The surface area of the substrate is comparable to that of the containment vessel. We discuss the modifications to the theory of RF bias that are needed to account for these factors. We examine the partition of RF power into various plasma channels and show that, for a given RF current, the presence of the magnetic field does not substantially change the ion bombardment energy or power, but does significantly increase the electron heating power. We address the issue of return current path for the RF current, and show that the presence of the magnetic field does not influence the return current flow. In conventional reactors, the vessel area is large compared to the substrate, and return current can be transported through the plasma and dissipated over the vessel walls. But in LAPPS, it may be preferable to provide a return current collector immediately adjacent to the plasma sheet.