During polybuffered local oxidation of silicon processing the formation of pits in the polysilicon buffer layer is often observedafter wet nitride etch. While nitridation of whole polysilicon grains due to a chemical reaction of ammonia, water, and silicon is discussed in the literature, it is shown in this work that the pits originate from voids in the polysilicon film which are already created during oxidation. For the first time, a stress-induced migration effect in polysilicon, analogous to void formation in narrow aluminum interconnect lines, is proposed as the primary cause of the pit problem. Based on the experimental findings a process strategy is suggested to avoid substrate damage in the active transistor area.
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