US Patent 10147728 Semiconductor device and method for fabricating the same

A method for fabricating semiconductor device includes the steps of: forming a first trench in a substrate; forming a first shallow trench isolation (STI) in the first trench; forming a first patterned mask on the substrate; and using the first patterned mask to remove part of the first STI for forming a second trench and remove part of the substrate for forming a third trench. Preferably, a bottom surface of the third trench is lower than a bottom surface of the second trench.