Buried contact process

The method of forming a buried contact to source/drain regions in a silicon substrate comprising:

• forming a silicon oxide layer over said silicon substrate;
• forming a first polysilicon layer over said silicon oxide layer;
• providing a resist mask having an opening over the planned said buried contact and isotropically etching said first polysilicon layer to form a tapered opening to said underlying silicon oxide which opening undercuts said resist mask;
• anisotropically etching said silicon oxide layer using the said resist mask which results in an opening to said silicon substrate corresponding to the planned said buried contact while leaving a silicon oxide border uncovered with said first polysilicon layer;
• depositing a second polysilicon layer over said first polysilicon layer, the opening to said silicon substrate and said border of uncovered silicon oxide;
• masking the planned said buried contact with a resist mask;
• forming openings to the planned said source and drain regions wherein at least one of said source and drain regions is adjacent to said opening of the planned buried contact;
• removing said resist mask;
• implanting ions through said second polysilicon layer in said opening to said silicon substrate to form said source and drain regions;
• and
• heating the structure to cause the formation of a buried region under said buried contact by outdiffusion from the first and second polysilicon layers and to make electrical contact to at least one of said adjacent source and drain regions.