Polysilicon resistor formation in a gate-last process

A method comprising:

• forming a polysilicon layer over a substrate;
• forming a hard mask layer over the polysilicon layer;
• forming a photo resist over the hard mask layer and patterning the photo resist;
• implanting the polysilicon layer with a first dopant to form a doped polysilicon region in the polysilicon layer;
• implanting the hard mask layer with a second dopant to form a doped hard mask region in the hard mask layer, wherein the steps of implanting the polysilicon layer and the hard mask layer are performed using the photo resist as an implantation mask;
• etching the hard mask layer, wherein portions of the hard mask layer not doped with the second dopant are substantially fully removed after the step of etching, and wherein the doped hard mask region has at least a bottom portion not removed after the step of etching;
• and
• replacing a portion of the polysilicon layer not covered by the doped hard mask region with a gate stack of a transistor.

A method comprising:

• forming a polysilicon layer over a substrate;
• forming a first photo resist over the polysilicon layer and patterning the first photo resist;
• implanting the polysilicon layer with a first dopant to form a doped polysilicon region in the polysilicon layer, wherein the polysilicon layer is implanted using the first photo resist as a mask;
• after the step of implanting the polysilicon layer, forming a hard mask layer over the polysilicon layer;
• etching the hard mask layer and the polysilicon layer to form a first opening over a well region in the substrate;
• forming a second photo resist into the first opening and over the hard mask layer;
• patterning the second photo resist to form a second opening in the second photo resist, wherein the second opening is over and overlapping the doped polysilicon region;
• implanting the hard mask layer with a second dopant to form a doped hard mask region in the hard mask layer, wherein the step of implanting the hard mask layer is performed using the second photo resist as an implantation mask;
• etching the hard mask layer, wherein portions of the hard mask layer not doped with the second dopant are etched through, and wherein the doped hard mask region has at least a bottom portion not removed;
• and
• replacing a portion of the polysilicon layer with a gate stack of a transistor.