Grave depth insulation and methods for their preparation

A method comprises performing the anisotropic etching on a semiconductor substrate for fabricating a trench. The trench has vertical side walls and a rounded bottom, which is connected to the vertical side walls. A damage eliminating step is performed to remove a surface layer of the semiconductor substrate, wherein the surface layer to the trench is exposed out. The rounded bottom of the trench is etched so that an inclined straight bottom surface is formed. The trench is filled to form a grave isolation region in the trench.