Method of making single polysilicon self-aligned bipolar transistor having reduced emitter-base junction

A method of fabricating a bipolar transistor comprising:

• providing an integrated circuit substrate comprising a semiconductor well region of a first conductivity type;
• forming thereon a thin dielectric layer;
• depositing thereon a bilayer comprising a first layer of silicon, and a second layer of an oxidation resistant material;
• patterning the bilayer to form an emitter mask and expose adjacent surface regions of substrate;
• selectively doping the adjacent surface regions to form heavily doped extrinsic base regions of a second conductivity type;
• exposing the structure to an oxidizing atmosphere to grow an oxide layer overall and partially oxidize sidewalls of the silicon layer under the oxidation resistant layer to form inwardly extending dielectric sidewall spacers;
• removing the oxidation resistant layer;
• removing any remaining unoxidized part of the silicon layer leaving the thin dielectric layer thereunder;
• selectively implanting the surface region between the sidewall spacers to form an intrinsic base region of the second conductivity type;
• selectively removing the thin dielectric layer from surface of the intrinsic base region, leaving the dielectric sidewall spacers and the oxide layer on extrinsic base regions;
• providing a layer of conductive material of the first conductivity type overall, and patterning and selectively etching said conductive layer to define an emitter structure extending laterally over the dielectric sidewall spacers;
• selectively removing oxide layer from the extrinsic base region adjacent the emitter structure to form base contact openings to extrinsic base region;
• forming contacts to the emitter and base regions.