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Changes in surface morphology have been studied for Si surfaces treated with CF4/O2 down-flow etching. It has been found that rough Si surfaces can be smoothed and Si trench corners can be rounded off using this CF4/O2 down-flow etching. A SiFxOy layer is formed on the Si surface etched by a down-flow discharged CF4/O2 gas mixture in high O2 concentration. A thick SiFxOy layer is formed at the concave part of the surface, which prevents fluorine atoms from reacting with Si. On the other hand, Si etching proceeds fast at the convex part covered with a thin SiFxOy layer. As a result, a rough Si surface is smoothed and trench corners are rounded off. By applying this treatment to a polycrystalline silicon surface, the leakage current of a SiO2 film grown on it is much reduced.

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