Suppression of electron shading effect by a counter radio frequency bias in plasma etching

Abstract

We investigated the suppression of the charge buildup caused by the electron shading effect using a counter rf bias, which produces high energy electrons reflected from the counter electrode. We expected that these high energy electrons could overcome the potential barrier formed at the entrance of high aspect-ratio hole patterns and to decrease the charge buildup at the bottom of these patterns. We directly investigated the charge buildup by measuring dc self-bias potential differences between high aspect-ratio hole patterns and open-space pattern. The dc self-bias potential difference increased, independent of the hole pattern aspect ratio, at the lower substrate rf bias voltage and they tended to saturate at the following higher substrate rf bias voltage. The dc self-bias potential difference reached about 100 V with a substrate rf bias voltage of 400 V for an aspect ratio of 2. The dc self-bias potential difference dramatically decreased from 100 to 20 V by increasing the counter rf bias voltage. In addition, we investigated this suppression effect by changing the phase difference between counter and substrate bias voltages when both bias frequencies were set to 13.56 MHz. We found that there is a variation of the suppression effect with respect to the phase difference. This indicates that high energy electrons, which were accelerated in an oscillation sheath, could reach the bottom of the holes.