The motivation for forming rounded corners at the top of the shalllow trench is based on the fact that any sharp corners result in poor gate oxide quality. The gate poly-Si can wrap around the top corner, which can create a parasitic conduction path (slide 1).
Slide 2 shows two different approaches for the formation of rounded trench tops, the carbon polymer based approach and the silicon etch by-product based approach.
Resist striation is a challenge for the carbon polymer based approach. Top of resist is roughened during mask open etch. High bias power required during the mask overetch and the trench etch steps can lead to faceting of the resist. The roughness on the top of the resist can become transferred to the sidewall. More faceting leads to striations in the sidewall can extend to the nitride sidewall. Nitride mask striation leads to silicon striation (slide 3).
Both approoaches are compared in slide 4.
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