The present invention provides a method for etching a poly-silicon layer of a semiconductor wafer. The semiconductor wafer comprises a dielectric layer, a poly-silicon layer situated on the dielectric layer and containing dopants to a predetermined depth, and a photo-resist layer having a rectangular cross-section above a predetermined area of the poly-silicon layer. The semiconductor wafer is processed in a plasma chamber. A first dry-etching process is performed to vertically etch away the dopant-containing portion of the poly-silicon layer not covered by the photo-resist layer. Then, a second dry-etching process is performed to vertically etch away the residual portion of the poly-silicon layer not covered by the photo-resist layer down to the surface of the dielectric layer. The etching gases used in the first dry-etching process differ from those used in the second dry-etching process, and the main etching gas of the first dry-etching process is C2F6.