US Patent 7700434 Trench widening without merging

A semiconductor fabrication method. First, a semiconductor structure is provided. The semiconductor structure includes a semiconductor substrate, a trench in the semiconductor substrate. The trench includes a side wall which includes {100} side wall surfaces and {110} side wall surfaces. The semiconductor structure further includes a blocking layer on the {100} side wall surfaces and the {110} side wall surfaces. Next, portions of the blocking layer on the {110} side wall surfaces are removed without removing portions of the blocking layer on the {100} side wall surfaces such that the {110} side wall surfaces are exposed to a surrounding ambient.