Percent dissociation of Cl2 in inductively coupled, chlorine-containing plasmas

Abstract

The percent dissociation of Cl2 was determined for two configurations of a commercial transformer-coupled plasma (TCP) reactor (LAM Research Alliance metal etcher), using Cl2 and BCl3/Cl2 feed gases, during slow etching of SiO2 covered Si wafers. Emission from Cl2 at 305 nm was recorded as a function of TCP source power, along with emission from 1% Ar and Xe, added as part of an equal mixture of the five rare gases. Absolute Cl2 number densities were determined from the Cl2-to-rare gas emission intensity ratios. The Cl2 percent dissociation increases with power, reaching 70% between 1 and 2 mTorr at the highest power (900 W, 0.080 W/cm3). The percent dissociation decreases with increasing pressure between 1 and 10 mTorr. Decreasing the gap between the TCP window and the wafer chuck from 11 to 6.5 cm decreases dissociation at pressures between 0.5 and 2 mTorr, and increases dissociation slightly at 10 mTorr. The percent dissociation as a function of power, and for the most part as a function of pressure and gap, is reproduced by a zero-dimensional model that includes electron-impact dissociation and dissociative attachment of Cl2, and diffusion-controlled recombination of Cl at the walls. Addition of BCl3 to Cl2 increases the percent dissociation of Cl2, most likely due to a passivation of the chamber walls by adsorbed BClx, lowering the Cl-atom recombination coefficient.