Multidimensional Modeling and Simulation of Plasma Processing Reactors.

Etching and deposition of thin films using low -pressure nonequilibrium gas discharges are crucial for the fabrication of advanced microelectronic devices. Plasma reactor models are urgently needed for the rational design, optimization and control of plasma based processes. A new methodology was developed to model and simulate plasma etching or deposition of thin films. The overall problem was broken down into several strongly interacting pieces: electron velocity distribution function (EVDF), electromagnetic (EM) fields, transport and reaction of charged species, transport and reaction of neutral species, and plasma-surface interactions. This modular approach effectively coupled the disparate time scales between electrons (ns) and neutrals (100 ms), and made feasible the self -consistent spatiotemporal simulation of reactive plasma flow in complex multidimensional geometries, a hitherto unsolved problem. The methodology was applied to argon and chlorine glow discharges in both capacitively- and inductively-coupled radio frequency plasma reactors. The EVDF was calculated by a novel method referred to as the dynamic Monte Carlo simulation. Maxwell's equations for the EM fields and "fluid" equations for plasma species were integrated by a finite element method. Special "acceleration" techniques were used to speed-up convergence to the periodic steady state, thereby reducing the computation time by many orders of magnitude. Argon discharge simulations revealed that, for pressure of ~1 Torr, argon metastables played a major role in the discharge despite the fact that their mole fraction was less than 10^{ -5}. In particular, metastable (two-step) ionization was the main mechanism for electron production to sustain the discharge. The development of complex spatial structures and "hot" spots due to the interaction of axial and radial electric fields was observed, in agreement with experimental data. The plasma structure changed drastically when the pressure was lowered to 100 mTorr. Polysilicon etching with chlorine in an inductively -coupled reactor was also simulated. As power deposition increased, the electron density increased linearly, the plasma became less electronegative, the degree of gas dissociation increased, and the plasma potential remained constant in agreement with laboratory measurements. The radial uniformity of the Cl atom flux was better than that of the ion flux.