Patent ID: 8372722

Claim:
A method of fabricating a semiconductor device, comprising the following steps: forming in a crystalline silicon (Si) substrate having a horizontal upper surface with a <100> orientation, a recess having sidewalls extending downward from the upper surface and having a horizontal bottom; performing an anisotropic oxidation on the substrate having the recess to thereby form a first oxide layer having a first thickness on the sidewalls and a second oxide layer having a second thickness on the bottom of the recess, wherein the second thickness is greater than the first thickness; performing a first isotropic wet etching on the oxidized substrate which completely removes the first oxide layer on the sidewalls of the recess and only partially removes the second oxide layer on the bottom of the recess thereby leaving the bottom of the recess covered by a remaining part of the second oxide layer; performing an orientation selective wet etching on the recess while using the remaining part of the second oxide layer on the bottom of the recess as an etch stop layer to thereby obtain a Σ-shaped cross section for the recess wherein the Σ-shaped cross section includes a horizontally extending bottom due to the etch stop function of the remaining part of the second oxide layer; and after obtaining the Σ-shaped cross section, performing a second isotropic wet etching process to remove the remaining part of the second oxide layer and thus expose a horizontally extending Si bottom surface at the bottom of the recess.