Measurements and simulations of ion energy distributions at rf-biased substrate electrodes in high density plasma reactors

In plasma etching and deposition processes, the energy distribution of ions incident onto the substrate strongly affects the surface reactions and film deposition and etching rates. A compact floating retarding-field ion energy analyzer and the accompanying electronics have been designed and built to measure the energy distribution of ions bombarding radio frequency (rf) biased electrodes in high-density, inductively coupled plasma (ICP) reactors. The analyzer was designed to be able to operate in the presence of several hundred volts of rf-bias and in the harsh conditions encountered in commercial high density plasma reactors. The operation and capabilities of the energy analyzer are demonstrated through ion energy distribution measurements conducted on a rf-biased electrostatic chuck in a high-density transformer coupled plasma (TCP) reactor. A Langmuir probe is used in conjunction with the ion energy analyzer to verify the accuracy of the analyzer measurements. The effects of plasma power, rf-bias power, gas composition, and ion mass on the ion energy distributions are demonstrated through Ar, Ne, Ar/Ne, O ₂ and CF₄/O₂ discharges. In the operating range studied, the average ion energy increases linearly with increasing rf-bias while the ion flux remains constant indicating that independent control of ion flux and energy is achieved in the TCP reactor. Bimodal ion energy distributions resulting from ion energy modulation in the sheath were observed and multiple peaks in the IEDs measured in gas mixtures were identified as ions with different masses falling through the sheath.; The magnitude and frequency of the rf-bias power applied to the substrate electrode determines the spatiotemporal variations of the sheath potentials and hence the energy distribution of the ions impinging upon the substrate. A self-consistent dynamic model of the sheath, capable of predicting ion energy distributions (IEDs) impinging on a rf-biased electrode, was developed. The model consists of equations describing the charge transport in the sheath coupled to an equivalent circuit model of the sheath to predict the spatiotemporal charge and potential distributions near the surface. Model predictions are compared to the experimental measurements over a wide range of plasma operating conditions in both Ar and Ne discharges. The predicted ion energy distributions are in very good agreement with the experimental measurements.