Patent ID: 8524093

Claim:
A method for forming a deep trench, comprising the steps of: providing a substrate having a pad layer formed on a top face of the substrate, wherein the pad layer comprises a first layer of a combination of oxide and nitride and a second layer of silicate glass, and the second layer is on a top surface of the first layer; performing a first etching process to form a first trench through the pad layer and into the substrate, wherein the first trench includes a sidewall and a bottom surface; after the first etching process, forming a liner to cover a top surface of the pad layer and the sidewall of the first trench, wherein the liner does not cover the bottom surface of the first trench, and wherein the sidewall of the first trench is aligned with an edge of the pad layer and an edge of the second layer; after forming the liner, performing a second etching process to etch the bottom surface of the first trench to form a second trench by using the liner on the pad layer as an etching hard mask; and after forming the second trench, enlarging a bottom portion of the second trench.