Using ONO as hard mask to reduce STI oxide loss on low voltage device in flash or EPROM process

摘要

A new method of forming differential gate oxide thicknesses for both high and low voltage transistors is described. A semiconductor substrate is provided wherein active areas of the substrate are isolated from other active areas by shallow trench isolation regions. A polysilicon layer is deposited overlying a tunneling oxide layer on the surface of the substrate. The polysilicon and tunneling oxide layers are removed except in the memory cell area. An ONO layer is deposited overlying the polysilicon layer in the memory cell area and on the surface of the substrate in the low voltage and high voltage areas. The ONO layer is removed in the high voltage area. The substrate is oxidized in the high voltage area to form a thick gate oxide layer. Thereafter, the ONO layer is removed in the low voltage area and the substrate is oxidized to form a thin gate oxide layer. A second polysilicon layer is deposited over the ONO layer in the memory area, over the thin gate oxide layer in the low voltage area, and over the thick gate oxide layer in the high voltage area. The second polysilicon layer, ONO layer and first polysilicon layer in the memory cell area are patterned to form a control gate overlying a floating gate separated by the ONO layer. The second polysilicon layer is patterned to form a low voltage transistor in the low voltage area and a high voltage transistor in the high voltage area.