Patent ID: 7307025

Claim:
A method for etching features in a silicon oxide based dielectric layer over a substrate, comprising performing an etch cycle, comprising: performing a lag etch partially etching features in the silicon oxide based dielectric layer, comprising: providing a lag etchant gas; forming a plasma from the lag etchant gas; and etching the silicon oxide based dielectric layer with the lag etchant gas, so that smaller features are etched slower than wider features; and performing a reverse lag etch further etching the features in the silicon oxide based dielectric layer, comprising: providing a reverse lag etchant gas, which is different from the lag etchant gas and is more polymerizing than the lag etchant gas; forming a plasma from the reverse lag etchant gas; and etching the silicon oxide based dielectric layer with the plasma formed from the reverse lag etchant gas, so that smaller features are etched faster than wider features.