Electron and Chemical Kinetics in the Low-Pressure RF Discharge Etching of Silicon in SF6

Abstract

Monte Carlo simulation and Boltzmann equation solutions have been used to study the electron kinetics. All electronic excitation of SF6 is assumed to be dissociative in analogy with the known product channels in ionization and multiphoton dissociation. The electric-field-to-gas-density ratios are high (E/n ? 1000 Td, where 1 Td (Townsend) = 1 × 10-17 V . cm2) in low-pressure (p < 0.3 torr) radiofrequency (RF) discharges. At these high E/n values, the electron energy relaxation time is much shorter than the 74-ns period at 13.56 MHz. Furthermore, the time scale of the chemical kinetics is much longer than the period of the applied RF voltage. Therefore the electron energy distribution can track the time-varying electric field, and time- and space-averaged rate coefficients can be used in chemical kinetics models. A rate equation model has been used to study the chemical kinetic processes. Electron-impact dissociation and ionization are the dominant sources of chemically active species. An electron density of 1 × 108 cm-3 is estimated from the known average values of E/n and the discharge input power. Two limiting cases are studied for the positive and negative ion diffusion losses: a) trapped negative ions and positive ion loss at the ambipolar diffusion rate; and b) positive and negative ion losses at the free diffusion rates. Neutral particle diffusion losses are estimated by using an effective diffusion length which takes surface reflection into account and increases as the surface reflection probability increases.