Utilizing polysilicon diffusion sources and special masking techniques

A method of fabricating a bipolar transistor having base contacts formed of polysilicon material comprising the steps of:

• A. forming, on a semiconductor substrate of a first conductivity type having a collector region of opposite conductivity type disposed therein, at least two oxide isolation regions therein spaced apart a given distance over said collector region,
• B. applying a first layer of insulating material over said epitaxial layer and, using photo-lithographic techniques, delineating and etching to provide an opening in said insulating layer in the area between said oxide isolation regions,
• C. forming a layer of polysilicon material having first conductivity type impurities therein over said insulating layer and the opening therein, said polysilicon material thereby being in contact with said layer of epitaxial material in the open area of said first layer of insulating material,
• D. forming a second layer of insulating material over said layer of polysilicon material,
• E. forming a layer of metal oxide over said second layer of insulating material,
• F. forming a third layer of insulating material over said layer of metal oxide.
• G. using photo-lithographic techniques, delineating and etching to provide an opening in said third insulating material to provide an emitter window in the region above said opening in said first layer of insulating material,
• H. using said emitter window in said third layer of insulating material, etching to remove said layer of metal oxide, said second layer of insulating material and said layer of polysilicon material to provide an opening therethrough to said layer of epitaxial material,
• I. diffusing impurities of said first conductivity type into said layer of epitaxial material in between said at least two oxide isolation regions to form extrinsic base regions beneath and in contact with said polysilicon material and to form an intrinsic base region intermediate said extrinsic base regions and beneath the opening in said polysilicon material,
• J. forming a layer of insulating material over said intrinsic base region in said epitaxial material and, using said opening in said metal oxide layer as a mask, etching to remove said insulating material over said intrinsic base region to form an opening for an emitter,
• K. implanting impurities of said opposite conductivity type through said opening provided in step J into said intrinsic base region to form an emitter and removing said layer of metal oxide, thereby providing a bipolar structure having an emitter and having extrinsic base regions in contact with polysilicon material containing impurities of said first conductivity type.