The effects of radio-frequency bias on electron density in an inductively coupled plasma reactor

The effect of radio-frequency bias on electron density in an inductively coupled plasma reactor was measured using a wave cutoff probe, over a wide range of conditions in pure Ar, pure CF4, and 50%-50% mixtures of Ar/CF4, at pressures of 0.7-4.0 Pa (5-30 mTorr), bias frequencies of 10-30 MHz, bias voltages up to 750 V, and inductive source powers of 50-300 W. Also, at selected experimental conditions, comparisons with Langmuir probe measurements were made. Two types of bias-induced changes in electron density were detected. First, at high source powers, we observed a bias-induced decrease in electron density, which had a slow time response (several minutes), a linear dependence on bias voltage, and little or no dependence on bias frequency or pressure. This decrease is a gas composition effect caused by etch or sputter products liberated from the wafer surface. Second, at low source powers, we observed a faster, bias-induced increase in electron density, which was proportional to the bias frequency and the square root of the bias voltage. This second effect was caused by absorption of bias power by electrons via stochastic heating. Simple models of each effect were derived and were shown to yield quantitative predictions in agreement with the observations. To obtain correct predictions, the effect of bias-induced electron heating cannot be considered by itself; rather, its effect on the efficiency of the inductive source must also be considered. (C) 2007 American Institute of Physics.