Method for self-aligned punchthrough implant using an etch-back gate

A method for forming a self-aligned MOS device consisting essentially of the steps of:

• forming a semiconductor substrate having a background conductivity, said background conductivity being of a first conductivity type,
• forming a first gate oxide layer over said semiconductor substrate,
• removing said first gate oxide layer from said semiconductor substrate,
• forming a second gate oxide layer over said semiconductor substrate,
• depositing a shielding layer over said semiconductor substrate such that said second gate oxide layer is disposed between said semiconductor substrate and said shielding layer,
• depositing a photoresist layer over said shielding layer such that said shielding layer is disposed between said photoresist layer and said second gate oxide layer,
• removing a portion of said photoresist layer and said shielding layer such that a first area of said semiconductor substrate is covered only by said second gate oxide layer, said first area extending in a lengthwise and widthwise direction across said semiconductor substrate,
• forming a first doped region having a higher doping concentration of said first conductivity type than said semiconductor substrate by;
• making a first punchthrough implant of a dopant of said first conductivity into said first area of said semiconductor substrate,
• making a second threshold voltage adjust implant of a dopant of said first conductivity type into said first area of said semiconductor substrate,
• removing said photoresist layer from said shielding layer,
• removing said second gate oxide layer from said first area of said semiconductor substrate,
• depositing or growing a third gate oxide layer over said first area of said semiconductor substrate,
• forming a gate electrode by depositing a single layer of polysilicon over said shielding layer and said third gate oxide layer covering said doped region of said semiconductor substrate, said layer of polysilicon having a depth which is at least half as great as the distance through which said first area extends in said lengthwise direction across said semiconductor substrate,
• implanting a dopant of a second conductivity type into said layer of polysilicon,
• selectively removing said layer of polysilicon having said dopant of said second conductivity type implanted therein such that said layer of polysilicon remains over only said first area of said semiconductor substrate,
• removing said shielding layer from said semiconductor substrate,
• forming a source region for said self-aligned transistor in said semiconductor substrate, said source region disposed on one side of said first doped region,
• forming a drain region for said self-aligned transistor in said semiconductor substrate, said drain region disposed on the other side of said first doped region such that said first doped region is disposed between said source and drain regions,
• forming a protective coating over the surface of said semiconductor substrate,
• opening contact holes through said protective coating,
• forming electrical contacts through said holes in said protective coating to said source region, said drain region, and said doped polysilicon remaining over said first area of said semiconductor substrate.