The method of improving critical dimension differences in different regions of a semiconductor device

The present invention relates to a method for improving critical dimension differences in different regions of a semiconductor device, applied to thick polysilicon hard mask etch process, comprising: providing a substrate having a polysilicon layer, the polysilicon layer and the upper surface is covered with a thick hard mask; wherein the thickness of the hard mask layer comprises silicon nitride and silicon oxide layer, the silicon nitride layer covering the upper surface of the polysilicon layer, the silicon oxide layer overlying the silicon nitride layer an upper surface; anisotropic plasma of the silicon oxide layer is etched by dry etching, removing the portion of the silicon oxide layer to the upper surface of the silicon nitride layer; continue the isotropic plasma said silicon nitride layer is dry etched to remove the silicon nitride layer to the upper surface of the polysilicon layer is exposed to the upper surface of the polysilicon layer for forming a mask pattern. The present invention can improve the overall appearance thick polysilicon hard mask, improve the uniformity of the critical dimensions, and improve the reliability of the final device.