Method of making select gate self-aligned to floating for split gate flash memory structure

Abstract

A process for making self-aligned split-gate non-volatile memory cell is disclosed. It includes the step of using a nitride photomask in conjunction with a photoresist to etch the nitride layer and cause it to become a stepped nitride layer having a high thickness region and a low thickness. Then a poly-1 photomask is used in conjunction with a photoresist to etch through a first portion of the low thickness region to expose an underlying poly-1 layer intended to be floating gate, wherein at the same time, a portion of the high thickness region adjacent to the first portion of the low thickness region is also etched to a reduced thickness. After poly-1 oxidation, a cell drain photomask is used in conjunction with a photoresist to etch through a second portion of the low thickness region using a nitride etch and an underlying poly-1 layer using a poly etch. The portion of the high thickness region adjacent to the second portion of the low thickness region is also etched to a reduced thickness during the nitride etch. With such a stepped nitride layer approach, the floating gate and cell drain are formed using the same nitride photomask, thus ensuring the self-alignment of the select gate relative to the floating gate and the cell drain.