A semiconductor fabrication method comprises steps of providing a semiconductor structure. The semiconductor structure includes a semiconductor substrate, a trench in the semiconductor substrate. The trench comprises 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. The method further comprises the steps of removing portions of the blocking layer on the {110} side wall surfaces 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.