Optical diagnostics of plasma chemistries and chamber conditions in gate oxide stack etch

Various kinds of plasma chemistries were used in the study of polysilicon gate stack etch. Different degrees of gate oxide surface roughness were observed. Stable gate oxide thickness and smooth surface were found when using fluorine-based plasma chemistries. In contrast, non-fluorine-based chemistry tends to give uneven gate oxide thickness and rough surface. The stability of the gate oxide thickness can be controlled by chamber seasoning when using non-fluorine-based chemistry. It is also noticed that fluorine-based chemistries always result in thicker remaining gate oxide than the one without fluorine. The type of wafer used for seasoning can also have influence on chamber condition and subsequently the etch rates and gate oxide thickness. From the trends of emission intensity of Si, it is believed that etch byproducts as well as chamber wall polymer have potential impacts on the observed variation of gate oxide surface roughness, thickness, and etch rates.