Effects of O 2 Feed Gas Impurity on Cl2 Based Plasma Etching of Polysilicon

The effect of low level, between 0.001 and 10%, O 2 addition on Cl 2 based plasma etching of polysilicon in a parallel‐plate etcher was investigated. Three strong effects on the etching rate of unmasked polysilicon were observed. Low level O 2 addition, 0.1–2%, enhanced the etching rate up to 4 times that of the pristine system. High level O 2 addition, ≥6%, stopped the etching altogether. Pure Cl 2 plasma etching rates were enhanced by previous runs with O 2 addition; a hysteresis effect. The hysteresis was reduced by subsequent system plasma exposure. Running a CF 4 plasma after O 2 addition runs cleaned the system and returned it to its pristine state. The stopping of etching at high O 2 levels was attributed to plasma oxidation of the polysilicon surface; since Cl 2 plasmas are very selective with respect to oxide, the etching is stopped by the surface oxide. Both the low level O 2 etching rate enhancement and the hysteresis effect can be attributed to the deposition of an oxychloride film on the etcher surfaces. The oxychloride film, formed by reaction between the etching products and the added O 2 , passivates the interior etcher surfaces against Cl surface recombination, and, therefore, reduces the major Cl loss mechanism. The gas‐phase Cl concentration increases, which in turn increases the polysilicon etching rate. These results demonstrate the importance of etcher surface condition and of the etching product deposition. Photoresist masked polysilicon films etched in the same configuration showed similar O 2 addition dependencies as unmasked polysilicon films. The main difference is that the O 2 addition effect thresholds are higher with the photoresist mask due to the consumption of the added O 2 by photoresist etching. Unmasked polysilicon film etched in a commercial etcher, Applied Materials Precision 5000, under magnetically enhanced reactive ion etching configuration, exhibited the same trends as in the parallel‐plate research etcher. This indicates that results obtained from the parallel‐plate research etcher can be generalized to other configurations.