A method for forming an opening in a semiconductor device is provided, including: providing a semiconductor substrate with a silicon oxide layer, a polysilicon layer and a silicon nitride layer sequentially formed thereover; patterning the silicon nitride layer, forming a first opening in the silicon nitride layer, wherein the first opening exposes a top surface of the polysilicon layer; performing a first etching process, using gasous etchants including hydrogen bromide (HBr), oxygen (O2), and fluorocarbons (CxFy), forming a second opening in the polysilicon layer, wherein a sidewall of the polysilicon layer adjacent to the second opening is substantially perpendicular to a top surface of the silicon oxide layer, wherein x is between 1-5 and y is between 2-8; removing the silicon nitride layer; and performing a second etching process, forming a third opening in the silicon oxide layer exposed by the second opening.