Ion implant silicon nitride mask for a silicide free contact region in a self aligned silicide process

A method of forming a silicide free contact, comprising the steps of:

• providing a silicon substrate with a number of devices formed therein wherein each device comprises a source region, a drain region, a silicide free contact region within said source region, and a channel region;
• providing field oxide regions formed in said silicon substrate;
• providing gate oxide patterns formed on said silicon substrate over said channel regions;
• providing polysilicon gate electrodes having sidewalls formed on said gate oxide patterns;
• providing oxide spacers formed on said sidewalls of said polysilicon gate electrodes;
• forming a photoresist layer on said silicon substrate covering said polysilicon gate electrodes, said oxide spacers, said drain regions, said source regions, and said silicide free contact regions;
• forming contact hole openings in said photoresist layer directly over said silicide free contact regions whereby said contact hole openings expose said silicide free contact regions;
• implanting nitrogen ions into said silicide free contact regions by means of a beam of nitrogen ions using said photoresist layer with contact hole openings as a mask;
• removing said photoresist layer after implanting said nitrogen ions into said silicide free contact regions;
• annealing said silicon substrate using a first anneal after implanting said nitrogen ions and removing said photoresist layer thereby forming a layer of silicon nitride over said silicide free contact regions;
• forming a titanium layer on said silicon substrate covering said polysilicon gate electrodes, said oxide spacers, said source regions, said drain regions, and said silicon nitride layer after annealing said silicon substrate using said first anneal;
• annealing said silicon substrate and said titanium layer using a second anneal thereby forming titanium silicide over said polysilicon gate electrodes, said drain regions, and said source regions except said layer of silicon nitride;
• etching away those parts of said titanium layer not changed to titanium silicide, after annealing said silicon substrate and said titanium layer using said second anneal;
• forming a passivation layer over said silicon substrate after etching away those parts of said titanium layer not changed to titanium silicide;
• forming contact holes in said passivation layer thereby exposing part of said layer of silicon nitride and parts of said titanium silicide formed over said polysilicon gate electrodes, said source regions, and said drain regions;
• and
• filling said contact holes in said passivation layer with conducting material.