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It is shown that anomalous off-current fluctuation may happen due to phosphorous channeling through gate polysilicon during source/drain implantation particularly for the first phosphorus double-implantation scheme. It is proven that increased grain size by phosphorus predoping is the main cause of channeling. A stacked polystructure with an amorphous top layer and a columnar bottom layer is proposed to suppress the phosphorus channeling and hence to prevent the off-current fluctuation. It is shown that the proposed stacked polystructure is highly efficient in suppressing the phosphorus channeling without degradation of device performance.

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