Self-aligned low resistance buried contact process

Abstract

A buried contact is formed in a substrate implantation of phosphorous or arsenic through a window cut into the insulating silicon oxide layer and a superimposed thin silicon layer. The photoresist used to etch the window is cut back a limited amount prior to implantation. The peripheral margin of the buried contact implanted through the exposed part of the thin layer of silicon lowers the threshold voltage of any parasitic MOS device which may be created between the buried contact and the remote N+source or drain structure.

What is claimed is: 
     
       1. A process for forming a buried contact between a transistor element at the surface of a substrate and a remote diffusion region which comprises the steps of: growing an insulation layer of silicon oxide over said surface;   depositing a first layer of polysilicon over said insulation layer;   depositing a photoresist over said first polysilicon layer;   patterning a window in said photoresist;   etching said first polysilicon layer and oxide layer through said window;   descuming the edges of said photoresist window to expose a peripheral width of said first polysilicon layer;   implanting a doping element into said substrate through said descumed window and peripheral width to form a central zone of buried contact aligned with said window and a peripheral zone of buried contact substantially aligned with said peripheral width, wherein said peripheral width receives a lesser amount of doping element than said central zone;   removing said photoresist after said implanting;   depositing a second layer of polysilicon over said first layer; and   patterning said second layer to form the source, drain and other parts of said transistor, and access to said buried contact.   
     
     
       2. The process of claim 1, wherein the step of implanting comprises implanting into said substrate a doping element selected from a group consisting of arsenic and phosphorous. .Iadd. 
     
     
       3.  A process for forming a buried contact between a transistor element at the surface of a substrate and a remote diffusion region which comprises the steps of: growing an insulation layer of silicon oxide over said surface;   depositing a first layer of polysilicon over said insulation layer;   selectively forming an etch resist over said first polysilicon layer to define a window;   etching said first polysilicon layer and oxide layer exposed by said window;   exposing a peripheral width of said first polysilicon layer adjacent said window;   implanting a doping element into said substrate through said window and peripheral width to form a central zone of buried contact aligned with said window and a peripheral zone of buried contact substantially aligned with said peripheral width, wherein said peripheral zone receives a lesser amount of doping element than said central zone;   removing said etch resist after said implanting;   depositing a second layer of polysilicon over said first layer; and   patterning said second layer to form the source, drain and other parts of said transistor, and access to said buried contact..Iaddend..Iadd.4. The process of claim 3, wherein the step of implanting comprises implanting into said substrate a doping element selected from a group consisting of   
     
     
        arsenic and phosphorous..Iaddend..Iadd.5.  A process for forming a buried contact between a transistor element at the surface of a substrate and a remote diffusion region which comprises the steps of: growing an insulation layer of silicon oxide over said surface;   depositing a first layer of polysilicon over said insulation layer;   selectively forming a resist over said first polysilicon layer to define a window;   removing said first polysilicon layer and oxide layer exposed by said window;   exposing a peripheral width of said first polysilicon layer adjacent said window;   implanting a doping element into said substrate through said window and peripheral width to form a central zone of buried contact aligned with said window and a peripheral zone of buried contact substantially aligned with said peripheral width, wherein said peripheral zone receives a lesser amount of doping element than said central zone;   removing said resist after said implanting;   forming a selectively patterned second layer of polysilicon over said first layer to form source, drain and other parts of said transistor, and access to said buried contact..Iaddend..Iadd.6. The process of claim 5, wherein the step of implanting comprises implanting into said substrate a doping element selected from a group consisting of arsenic and   
     
     
        phosphorous..Iaddend..Iadd.7.  A process for forming a buried contact between a transistor element at the surface of a substrate and a remote diffusion region which comprises the steps of: forming an insulation layer over said surface;   depositing a polysilicon layer over said insulation layer;   selectively defining a window on said polysilicon layer;   removing said polysilicon layer and underlying insulation layer exposed by said window;   defining a peripheral width of said polysilicon layer adjacent said window;   implanting a doping element into said substrate through said window and peripheral width to form a central zone of buried contact aligned with said window and a peripheral zone of buried contact substantially aligned with said peripheral width, wherein said peripheral zone receives a lesser amount of doping element than said central zone; and   forming a selectively patterned conductive layer over said polysilicon layer to form source, drain and other parts of said transistor, and access to said buried contact..Iaddend..Iadd.8. The process of claim 7, wherein the step of implanting comprises implanting into said substrate a doping element selected from a group consisting of arsenic and phosphorous..Iaddend..Iadd.9. A process for forming a buried contact between a transistor element at the surface of a substrate and a remote diffusion region which comprises the steps of:   forming an insulation layer over said surface;   depositing a polysilicon layer over said insulation layer;   selectively removing said polysilicon layer and underlying insulation layer to expose said substrate through a window;   defining a peripheral width of said polysilicon layer adjacent said window for implantation of a doping element therethrough;   implanting said doping element into said substrate through said window and peripheral width to form a central zone of buried contact aligned with said window and a peripheral zone of buried contact substantially aligned with said peripheral width, wherein said peripheral zone receives a lesser amount of doping element than said central zone; and   forming a selectively patterned conductive layer over said polysilicon layer to form parts of said transistor, and access to said buried   
     
     
        contact..Iaddend..Iadd.10.  The process of claim 9, wherein the step of implanting comprises implanting into said substrate a doping element selected from a group consisting of arsenic and phosphorous..Iaddend.