Reducing base resistance of a BJT by forming a self aligned silicide in the single crystal region of the extrinsic base

A method of fabricating a bipolar transistor having emitter, base, and collector regions comprising the steps of:

• a) on a silicon substrate having a first surface, implanting dopants of a first conductivity type in a first region, said first region forming a portion of said collector;
• b) forming an epitaxial silicon layer on said first surface, said epitaxial layer doped to said first conductivity type;
• c) forming a first oxide layer on said epitaxial layer;
• d) forming a first nitride layer on said first oxide layer;
• e) masking said first nitride layer to define said base and collector sink regions;
• f) etching said epitaxial layer in regions not protected by said mask on said first nitride layer to form recesses in said epitaxial layer having substantially horizontal surfaces and sidewalls;
• g) removing said mask on said first nitride layer;
• h) forming a second oxide layer on said substrate;
• i) forming a second nitride layer on said second oxide layer and a third nitride layer on said second nitride layer;
• j) without a mask, etching said oxide and nitride layers to remove oxide and second nitride from said horizontal surfaces, but not said sidewalls and said base and collector region;
• k) oxidizing said horizontal surfaces to form field oxide regions;
• l) heavily doping said sink with dopants of said first conductivity type;
• m) forming a polysilicon layer on said substrate;
• n) implanting dopants of a second conductivity type into said polysilicon layer;
• o) diffusing said dopants of said second conductivity type into said base region;
• p) implanting dopants of said first conductivity type into at least a portion of said polysilicon region over said base region to form an emitter contact region of said first conductivity type;
• q) etching said polysilicon region to form said emitter contact, base contacts, and a sink contact;
• r) implanting dopants of said second conductivity type so as to increase a dopant concentration in said base region between said emitter and said base contacts;
• s) forming oxide on sidewalls of said emitter contact;
• t) forming metal silicide on exposed regions of silicon along an upper surface of said device, said metal silicide extending along an upper surface of said base region up to said oxide sidewalls;
• u) forming an oxide layer having contact apertures on said device;
• v) filling said apertures with tungsten;
• and
• w) forming an aluminum silicon contact layer on said oxide layer.

A method of forming a base region for a bipolar transistor comprising the steps of:

• a) providing a single-crystal region bounded by field oxide regions on a substrate, said single-crystal region having a first conductivity type, wherein said single crystal region and said field oxide regions define an active region;
• b) forming a polycrystalline region on said active region, said polycrystalline region covering substantially all of said active region, at least portions of said polycrystalline region being doped with a dopant of a second conductivity type;
• and
• c) diffusing said dopant of a second conductivity type into said active region to form said base region in said single-crystal region, wherein said diffusion occurs across substantially all of said active region.