Comparison of advanced plasma sources for etching applications. III. Ion energy distribution functions for a helicon and a multipole electron cyclotron resonance source

Abstract

As part of our research effort in evaluating the etching performance of high density plasma sources, we measured ion energy distribution functions near the wafer surface for a helicon and a multipole electron cyclotron resonance source (ECR). The following two salient results stand out: first is the remarkable similarity in behavior of the two sources which was also observed in previous studies comparing etching rates, profile control, and Langmuir probe diagnostics; and second is the surprising level of coupling between the applied rf bias and the bulk plasma. For both sources, the ion flux increases strongly with source power, decreases by 20%–40% as the reactor pressure increases from 2.0 to 5.0 mTorr, and is weakly modified by the applied rf bias. The mean ion energy is strongly influenced by the applied rf-bias and is relatively insensitive to source power and pressure. The ion flux exhibits high uniformity for both sources, with the helicon exhibiting slightly better uniformity. However, we note that instabilities in the ECR discharge from mode jumps caused by different operating conditions and changing reactor wall conditions, such as temperature, result in poorer uniformity. The behavior of ions with respect to applied source and rf-bias powers follows roughly the trends expected of quiescent, high density plasmas in contact with a rf-biased electrode (i.e., independent control of ion flux and mean ion energy). However, there exist subtle effects upon the ion flux, such as bimodal energy distributions, brought about by the coupling of the rf-bias power into the bulk plasma. This coupling may be an essential parameter in wafer platen design that must be addressed in order to obtain high etching rate uniformity.