PBLOCOS with sandwiched thin silicon nitride layer

A method for fabricating semiconductor devices having silicon oxide isolation structures, comprising:

• providing a silicon substrate;
• forming a layer of silicon oxide on the surface of said silicon substrate;
• forming a first layer of silicon nitride having a thickness of between about 300 and 400 Angstroms on the surface of said silicon oxide layer, to prevent diffusion of oxygen into said silicon oxide layer and said silicon substrate during subsequent oxidation steps, thereby reducing said bird's beak length;
• forming a layer of polysilicon on the surface of said silicon nitride layer;
• forming a second layer of silicon nitride on the surface of said polysilicon layer;
• patterning said second silicon nitride layer, said polysilicon layer, and said first silicon nitride layer to form oxidation masks and to form openings over said silicon oxide layer;
• ion implanting said silicon substrate through said openings to form channel stop implants;
• oxidizing said silicon substrate in the area not masked by said oxidation masks, to form said silicon oxide isolation structures, during which a thin surface oxide layer forms on the surface of said second silicon nitride layer;
• removing said thin surface oxide layer;
• removing said second silicon nitride layer;
• oxidizing said polysilicon layer;
• removing said oxidized polysilicon layer;
• removing said first silicon nitride layer;
• removing said silicon oxide layer, to complete formation of said silicon oxide isolation structures;
• and
• forming said semiconductor devices on and in said silicon substrate, and between said silicon oxide isolation structures.

A method of forming a silicon oxide isolation region with reduced bird's beak length, on the surface of a silicon substrate, comprising:

• forming a layer of silicon oxide on the surface of said silicon substrate;
• forming a first layer of silicon nitride having a thickness of between about 300 and 400 Angstroms on the surface of said silicon oxide layer, to prevent diffusion of oxygen into said silicon oxide layer and said silicon substrate during subsequent oxidation steps, thereby reducing said bird's beak length;
• forming a layer of undoped polysilicon on the surface of said first silicon nitride layer;
• forming a second layer of silicon nitride on the surface of said polysilicon layer;
• patterning said second silicon nitride layer, said polysilicon layer, and said first silicon nitride layer to form an oxidation mask and to form an opening over said silicon oxide layer;
• ion implanting said silicon substrate through said opening to form a channel stop implant;
• oxidizing said silicon substrate in the area not masked by said oxidation mask, to form said silicon oxide isolation region, wherein during the oxidizing step a thin surface oxide layer forms on the surface of said second silicon nitride layer;
• removing said thin surface oxide layer;
• removing said second silicon nitride layer;
• oxidizing said polysilicon layer;
• removing said oxidized polysilicon layer, whereby said first silicon nitride layer prevents oxide thinning at the field oxide--gate oxide interface during subsequent gate oxide formation;
• removing said first silicon nitride layer;
• and
• removing said silicon oxide layer, to complete formation of said silicon oxide isolation region.