Effect of chamber wall conditions on Cl and Cl2 concentrations in an inductively coupled plasma reactor

Abstract

The effect of chamber wall conditions on the Cl and Cl2 concentrations in a Cl2 discharge was studied in an inductively coupled plasma reactor. Cl and Cl2 mole fractions were determined using optical emission spectroscopy in conjunction with actinometry, while the state of the reactor walls was monitored using a surface probe that enables detection of films and adsorbates that deposit on these walls. Prolonged exposure of the chamber walls to a Cl2 plasma increases the Cl concentration in the discharge. This increase is due to the decreasing recombination probability of Cl atoms on the walls which with time are covered with a thin SiO2 film. The source of the SiO2 is the quartz dielectric window which is sputtered by ion bombardment. A SF6/O2 plasma etches the SiO2 film from the chamber walls and restores the chamber walls to a “clean” state. The Cl concentration in the reactor with these two different states of the wall conditions, under otherwise identical plasma operating conditions, was dramatically different and implied that the wall recombination probability of Cl atoms on the SiO2 covered walls is considerably lower than that on the clean anodized Al. Changing the state of the walls also changes the rate controlling step for Cl recombination from diffusion limited for the reactor with the clean walls to surface reaction limited for the SiO2 covered walls. This change in the rate controlling step changes the dependence of the plasma composition on the power, pressure, and gas flow rate.