MODULATED DISCHARGES - EFFECT ON PLASMA PARAMETERS AND DEPOSITION

The initial motivation for using amplitude modulated (kHz) rf discharges was as an aid for ascertaining the temporal evolution of various transient species which are formed from the feedstock commonly used for the deposition or etching of semiconductors. Such gases are easily dissociated and hence the density of some of the transient molecules may easily exceed the remaining fraction of the donor gas. From a diagnostic point of view, the use of “time” will be shown to be a cheap and a most valuable aid in ascertaining some of the kinetics of such glows. Most surprising was the fact that the use of a modulated discharge can have a profound effect on the processing goal itself. For instance, it has been shown that the bandgap of amorphous silicon deposited from SiH4is lower with a modulated discharge than that deposited from a continuous wave (cw) one; simultaneously, the amount of “dust,” as measured by Mie scattering, is also much lower. These experiments and others using etching gases point to a role for negative ions, which are usually confined by the sheaths in a cw discharge, but can be extracted during the off period of a modulated one. © 1990, American Vacuum Society. All rights reserved.