Method of forming sharp beak of poly by nitrogen implant to improve erase speed for split-gate flash

A method of forming a bird's beak in a split-gate flash memory cell comprising the steps of:

• providing a semiconductor substrate having a gate oxide layer;
• forming a first polysilicon layer over said gate oxide layer;
• performing ion implantation on said first polysilicon layer to form a layer of impurity in said polysilicon layer;
• forming a mask layer over said first polysilicon layer;
• patterning said mask layer to define a floating gate region in said first polysilicon layer;
• removing said layer of impurity using said mask layer only in said floating gate region;
• forming a bird's beak by thermal oxidation in said floating gate region whereby the floating gate region oxidizes faster than other areas of said first polysilicon layer containing said impurity layer;
• etching said first polysilicon layer to form said floating gate;
• forming an oxide layer over said floating gate;
• forming a second polysilicon layer over said oxide layer;
• and
• patterning said second polysilicon layer to form a control gate to complete said memory cell.