Control of Ion Energy in a Capacitively Coupled Reactive Ion Etcher

Abstract

The energy of ions bombarding the wafer is proportional to the potential difference between the plasma and the powered electrode in reactive ion etching systems. This work seeks to control the ion energy without altering the applied radio‐frequency power or the chamber pressure since these variables are closely tied to other important quantities, such as reactive chemical species concentrations in the plasma and wafer etch uniformity. A variable resistor placed in parallel with the blocking capacitor allows the plasma self‐bias voltage (Vbias) to be arbitrarily varied between its nominal value and zero. Optical emission spectroscopy for a plasma reveals that the nominal plasma chemical concentrations do not change under this control method. The use of a Langmuir probe to measure the plasma potential shows that the ion energy changes by approximately one‐half of the change in Vbias. The potential uses of this ion energy control technique to plasma self‐bias voltage regulation, etch selectivity, and plasma cleaning of chamber walls are demonstrated. A potential drawback, namely, decreased plasma stability, is also indicated.