Method of forming a three-gate non-volatile flash memory cell using two polysilicon deposition steps
A simplified method for forming a non-volatile memory cell using two polysilicon depositions. A first polysilicon layer is formed on and insulated from the semiconductor substrate in a first polysilicon deposition process. An insulation block is formed on the first polysilicon layer. Spacers are formed adjacent first and second sides of the insulation block, and with the spacer adjacent the first side is reduced. Exposed portions of the first poly silicon layer are removed while maintaining a polysilicon block of the first polysilicon layer disposed under the insulation block. A second polysilicon layer is formed over the substrate and the insulation block in a second polysilicon deposition process. Portions of the second polysilicon layer are removed while maintaining a first polysilicon block (disposed adjacent the first side of the insulation block), and a second polysilicon block (disposed adjacent the second side of the insulation block).
1. A method of forming a non-volatile memory cell comprising:
forming a first insulation layer on a semiconductor substrate;
forming a first polysilicon layer on the first insulation layer in a first polysilicon deposition process;
forming an insulation block on the first polysilicon layer, the insulation block having a first side and a second side;
forming insulation spacers adjacent the first and second sides;
reducing a width of the insulation spacer adjacent the first side;
removing portions of the first polysilicon layer while maintaining a polysilicon block of the first polysilicon layer disposed under the insulation block and the insulation spacers adjacent the first and second sides of the insulation block;
forming a source region in the substrate adjacent the first side of the insulation block;
removing the insulation spacers to expose end portions of the polysilicon block of the first polysilicon layer;
forming a layer of insulation material that at least extends along the exposed end portions of the polysilicon block of the first polysilicon layer;
forming a second polysilicon layer over the substrate and the insulation block in a second polysilicon deposition process;
removing portions of the second polysilicon layer while maintaining a first polysilicon block and a second polysilicon block of the second polysilicon layer, wherein:
the first polysilicon block of the second polysilicon layer is disposed adjacent the first side of the insulation block and over the source region,
the second polysilicon block of the second polysilicon layer is disposed adjacent the second side of the insulation block; and
forming a drain region in the substrate adjacent the second polysilicon block of the second polysilicon layer
wherein:
the first polysilicon block of the second polysilicon layer includes a first portion laterally adjacent to the polysilicon block of the first polysilicon layer, and a second portion that extends up and over the polysilicon block of the first polysilicon layer;
the second polysilicon block of the second polysilicon layer includes a first portion laterally adjacent to the polysilicon block of the first polysilicon layer, and a second portion that extends up and over the polysilicon block of the first polysilicon layer.
2. The method of claim 1 , wherein an amount of vertical overlap between the first polysilicon block of the second polysilicon layer and the polysilicon block of the first polysilicon layer is less than an amount of vertical overlap between the second polysilicon block of the second polysilicon layer and the polysilicon block of the first polysilicon layer.
3. The method of claim 1 , wherein the insulation block is formed of nitride, oxide, or a composite of layers including both oxide and nitride.
4. The method of claim 1 , wherein the first insulation layer is formed of oxide or nitrogen treated oxide.
5. The method of claim 1 , further comprising:
removing the second polysilicon block of the second polysilicon layer; and
forming a metal block adjacent the second side of the insulation block.
6. The method of claim 5 , further comprising:
forming a layer of high-K insulation material between the metal block and the second side of the insulation block.