Patent ID: 8334189

Claim:
A method for forming a trench, comprising: providing a substrate having a hard mask and a pad nitride layer disposed under the hard mask, wherein the substrate has thereon a first shallow trench and a second shallow trench disposed thereon, and the hard mask comprises a silicon oxide layer; forming a patterned filling layer, wherein the patterned filling layer fills the first shallow trench while exposing the second shallow trench; and using the patterned filling layer as an etching hard mask to etch a bottom of the second shallow trench thereby forming a deep trench; removing the patterned filling layer; depositing an oxide layer to fill the first shallow trench and the deep trench; and performing a chemical mechanical polishing (CMP) process to simultaneously remove the oxide layer and the hard mask outside the first shallow trench and the deep trench until the exposure of the pad nitride layer.