Method to bridge extrinsic and intrinsic base by selective epitaxy in BiCMOS technology
View Patent ↗A method of forming a heterojunction bipolar transistor. The method includes providing a structure comprising at least an intrinsic base region and an emitter pedestal region. A stack is formed on the intrinsic base region. The stack comprises a polysilicon layer and a top sacrificial oxide layer. A trench is formed in the structure. The trench circumscribes the intrinsic base region and the stack. An extrinsic base is formed at two regions around the stack. The extrinsic base is formed by a selective epitaxial growth process to create a bridge over the trench. The bridge connects the two regions. An opening is provided in the stack. The opening exposes a portion of the intrinsic base region. An emitter is formed in the opening.
1. A method of forming a semiconductor device comprising:
providing a self-aligned sacrificial emitter process pedestal stack on a wafer;
growing a SiGe layer on said wafer;
forming a trench in said wafer, said trench circumscribing said pedestal stack and defining an intrinsic base of a bipolar transistor;
performing a selective epitaxial growth process to form an extrinsic base at two regions around said stack and a bridge over said trench, said bridge connecting said two regions, said selective epitaxial growth process ensuring substantially no growth originates from a bottom of said trench;
forming an emitter window in said pedestal stack;
performing in-situ doped emitter deposition in said emitter window; and
performing photolithographic deposition and etching to define an emitter of said bipolar transistor.
2. The method according to claim 1 , said wafer further comprising a silicon layer comprising a collector of said transistor, said collector being at a location aligned vertically with said emitter, and said collector being electrically isolated from a remainder of said wafer by shallow trench isolation.
3. The method according to claim 1 , further comprising:
before forming said trench, depositing a first nitride blanket on said wafer;
applying a first mask to said first nitride blanket;
etching said first nitride blanket according to said first mask, said etching forming a nitride spacer on said emitter pedestal;
removing said first mask;
depositing an oxide blanket on said nitride blanket;
applying a second mask to said oxide blanket; and
etching said oxide blanket according to said second mask, said etching forming an oxide spacer adjacent to said nitride spacer.
4. The method according to claim 3 , further comprising:
forming a second oxide layer in said trench;
depositing a nitride layer in said trench;
forming a spacer inside said trench by etching said nitride layer, said etching having sufficient overetch to leave no nitride over said oxide spacer on said stack; and
performing a wet chemical etch to remove said oxide spacer.
5. The method according to claim 4 , further comprising:
adding thickness to said nitride layer inside said trench and around said pedestal stack by depositing a second nitride blanket; and
end-point etching said second nitride blanket.
6. The method according to claim 1 , further comprising:
after performing said selective epitaxial growth process, depositing a third oxide layer on said wafer;
chemical mechanical polishing said third oxide layer; and
performing oxide recess etching using reactive ion etch processing to reduce thickness of said third oxide layer.
7. The method according to claim 1 , said forming an emitter window in said pedestal stack comprising:
performing a polysilicon etch to remove layers of said stack in order to provide an opening that exposes at least a portion of said intrinsic base, said polysilicon etch being highly selective to oxide and stops on said intrinsic base.