Threshold Levels and Effects of Feed Gas Impurities on Plasma Etching Processes

The etching of polysilicon has been characterized as a function of feed gas impurity levels, to determine specifications for gas purity in plasma etching processes. CF 4 , CF 2 Cl 2 , CF 3 Br , and Cl 2 feed gases were blended with 10 to 100,000 ppm impurity gases. H 2 , O 2 , and N 2 impurities were added to simulate air and water contamination; C 2 H 4 was added to simulate hydrocarbon impurities; and CHF 3 and CF 3 Cl were added to test the effect of feed gas purification. Typically, changes in discharge properties (ion bombardment flux, discharge electrical impedance, and plasma‐induced emission) correlated well with changes in the etching rate. In all but one case, the impurity levels below 1000 ppm had no apparent effect on discharge properties. In Cl 2 discharges, H 2 + O 2 or O 2 addition was found to alter the etching characteristics such that after impurity addition ceased, approximately 1h was needed for the system to return to the initial etching rate of pure Cl 2 . This hysteresis is associated with the presence of photoresist and has been tentatively attributed to polymeric deposition on the chamber surfaces.