Bias voltage diagnostics during oxide etch in Drytek 384T

Abstract

This article will report the results of voltage measurements across a matching network capacitor in a commercial rf etch machine during oxide etching. It will be demonstrated that this ‘‘bias voltage’’ is an accurate way to measure endpoint, especially for patterned wafers with small open oxide areas which are difficult to measure with optical emission endpoint. It will also be shown that the impedance between the wafer and the wafer platen can have a strong effect on this bias voltage. The presence of an intervening layer of insulating oxide, either from back side wafer oxide or from platen anodization, can reduce the magnitude of the bias voltage. When the bias voltage is changed by changing the rf power, there is a linear change in the oxide etch rate. When the bias voltage is changed by changing the layer of intervening oxide, there is no corresponding change in the oxide etch rate. One interpretation is that the change in the bias voltage due to the voltage drop across the intervening layer of oxide does not affect the voltage difference between the wafer and the plasma, which is the critical voltage difference in terms of etching.