Theoretical and Experimental Investigation of Chlorine RF Glow Discharges and Polysilicon Etching.

A combined modeling/experimental study of a RF (13.56 MHz) chlorine plasma and etching of polysilicon was undertaken in an effort to better understand the intricate nature of the plasma and to use this understanding for improved and novel reactor design methods. A comprehensive mathematical model of the chlorine discharge and etching of polysilicon in a parallel plate reactor was developed. This is the first model that integrates detailed plasma kinetics and wafer heating with etchant transport and reaction. The electron energy distribution function and electron impact reaction rate coefficients were determined by solving the Boltzmann equation. The rate coefficients were subsequently used in a bulk plasma model and a transport and reaction model to calculate the time-dependent etchant concentration distribution, etch rate and uniformity. Emphasis was placed on the methodology so that similar approaches can be used to model other plasma systems. An experimental system was designed and built to facilitate a well-defined experimental investigation of plasma etching. Key plasma properties were measured using several diagnostics including spatially resolved optical emission spectroscopy (OES) and laser interferometry (LI), impedance analysis, ion flux and energy analysis and Langmuir probes. Specifically, spatially resolved LI and OES were developed as novel techniques to monitor etch rate and uniformity in situ. Spatially resolved LI proved especially useful for monitoring the spatiotemporal variation of the etch rate. Good agreement between the measured and predicted values of electron density and energy, self-sustained electric field, RF current, ion bombardment energy, and atomic chlorine concentration was obtained over a range of power, pressure and electrode spacing in an empty reactor. In the case of polysilicon etching, both experiments and model predictions were very sensitive to surface conditions and surface reaction kinetics. Consistent with model predictions, the polysilicon etch rate increased with time due to wafer heating and for most experiments did not reach a steady state before the film cleared. Multiple states of the chlorine plasma were discovered experimentally. At the same operating conditions, the plasma could exist in one of two states. The plasma properties, etch rate, and uniformity of etching depended on which state the plasma was operated.