Silicon germanium heterojunction bipolar transistor structure and method
Disclosed is an improved semiconductor structure (e.g., a silicon germanium (SiGe) hetero-junction bipolar transistor) having a narrow essentially interstitial-free SIC pedestal with minimal overlap of the extrinsic base. Also, disclosed is a method of forming the transistor which uses laser annealing, as opposed to rapid thermal annealing, of the SIC pedestal to produce both a narrow SIC pedestal and an essentially interstitial-free collector. Thus, the resulting SiGe HBT transistor can be produced with narrower base and collector space-charge regions than can be achieved with conventional technology.
1. A method of forming a hetero-junction bipolar transistor comprising:
providing a substrate;
selecting a first dopant and a second dopant different from said first dopant;
forming a silicon collector layer on said substrate, said silicon collector layer having a top surface;
implanting said second dopant into said silicon collector layer to form a selective implant collector (SIC) pedestal at a predetermined depth below said top surface;
removing defects from said top surface of said silicon collector layer caused by said implanting and simultaneously activating said second dopant in said selective implant collector (SIC) pedestal with minimal diffusion of said second dopant into other regions of said silicon collector layer by performing a laser anneal; and
forming a silicon germanium base layer on said top surface, said silicon germanium base layer being any one of in-situ doped and subsequently doped with said first dopant,
said removing of said defects minimizing diffusion of said first dopant from said silicon germanium base layer into said silicon collector layer.
2. The method of claim 1 , said diffusion of said first dopant into said silicon collector layer being minimized by said removing of said defects such that a peak concentration of said first dopant in said silicon germanium base layer adjacent to a bottom surface of said silicon germanium base layer is at least 100 times greater than a concentration of said first dopant in said silicon collector layer.
3. The method of claim 1 , said forming of said silicon germanium base layer comprising epitaxially growing said silicon germanium base layer and in-situ doping said silicon germanium base layer such that a peak concentration of said first dopant in said silicon germanium base layer is greater than 1×10 19 cm −3 at 0.03 μm above said top surface.
4. The method of claim 1 , said diffusion of said first dopant into said silicon collector layer being minimized by said removing of said defects such that a concentration of said first dopant in said silicon collector layer near said top surface is less than 1×10 17 cm −3 .
5. The method of claim 1 , said second dopant being implanted into said silicon collector layer at 0.03 μm below said top surface to form said selective implant collector (SIC) pedestal and further being implanted such that said selective implant collector (SIC) pedestal has an uniform second dopant concentration of greater than 1×10 18 cm −3 .
6. The method of claim 1 , said laser anneal being performed at temperatures greater than 1100. degree. C. and further being performed using a technique that can achieve a thermal equilibrium in said silicon collector layer in less than 10 ps to minimize diffusion of said second dopant from said selective implant collector (SIC) pedestal.
7. The method of claim 6 , said diffusion of said second dopant being minimized such that a concentration profile of said second dopant outside of said selective implant collector (SIC) pedestal decreases from 1×10 18 cm −3 at 0.02 μm below said top surface to less than 1×10 17 cm −3 near said top surface.
8. The method of claim 1 , said first dopant comprising boron and said second dopant comprising one of phosphorous, antimony and arsenic.
9. The method of claim 6 , said hetero-junction bipolar transistor being formed with a current-gain cut-off frequency (F t ) of greater than 365.00 GHz, a maximum oscillation frequency (F max ) of greater than 255.00 GHz, a collector-base capacitance (Ccb) less than 3.40 fF and a sheet base resistance (Rbb) less than 110.00 Ohms.
10. A method of forming a hetero-junction bipolar transistor comprising:
providing a substrate;
selecting a first dopant and a second dopant different from said first dopant;
forming a silicon collector layer on said substrate, said silicon collector layer having a top surface;
implanting said second dopant into said silicon collector layer to form a selective implant collector (SIC) pedestal at a predetermined depth below said top surface;
removing defects from said top surface of said silicon collector layer caused by said implanting and simultaneously activating said second dopant in said selective implant collector (SIC) pedestal with minimal diffusion of said second dopant into other regions of said silicon collector layer by performing a laser anneal; and
forming a silicon germanium base layer on said top surface, said silicon germanium base layer being any one of in-situ doped and subsequently doped with said first dopant,
said removing of said defects minimizing diffusion of said first dopant from said silicon germanium base layer into said silicon collector layer such that a peak concentration of said first dopant in said silicon germanium base layer at a bottom surface of said silicon germanium base layer remains at least 100 times greater than a concentration of said first dopant in said silicon collector layer, said concentration of said first dopant in said silicon collector layer being less than 1×10 17 cm −3 and
said laser anneal further being performed at temperatures greater than 1100. degree. C. and using a technique that can achieve a thermal equilibrium in said silicon collector layer in less than 10 ps to minimize diffusion of said second dopant from said selective implant collector (SIC) pedestal.
11. The method of claim 10 , said diffusion of said second dopant being minimized such that a concentration profile of said second dopant outside of said selective implant collector (SIC) pedestal decreases from 1×10 18 cm −3 at 0.02 μm below said top surface to less than 1×10 17 cm −3 near said top surface.