Plasma and Reagent Pulse Induced Transients in the Etching of Si by

Previously, etching experiments were performed in this laboratory by injecting short pulses of an etchant gas, such as NF3, into a slow stream of Ar passing through an active plasma cell, and by analyzing the resulting transients of inter-mediates and products mass spectrometrically. In the present study, these procedures were modified in several ways, so as to add to our understanding of the etching mechanism: (i) In some experiments, the plasma voltage was applied only for short time intervals to a previously etched sample, and discharge cleaning was observed, as one would expect. (ii) More surprisingly, when Si wafers were first etched, then partially discharge cleaned, and finally subjected to pulses of NF ~ after the plasma had been extinguished, small additional amounts of SiF4 were released. By varying the lengths of gas and voltage pulses and by adjusting the time delays between pulses, it was possible to estimate the rates at which ion bombardment damage built up and healed in near-surface r gions of the Si sample. (iii) Finally, in some ex-periments, the plasma was extinguished uring an NF3 gas pulse, and etching virtually stopped at once while the frag-ment concentrations decayed more gradually. The subtle effects described under (ii) were not discernible in these latter experiments. In recent years, it has become clear that plasma etching of a solid can lead to the formation of layers of intermedi