Low thermal budget issues for Si/Si1-xGex heterojunction bipolar transistors and selective epitaxial Si bipolar transistors

Gregory, Haydn James (1995) Low thermal budget issues for Si/Si1-xGex heterojunction bipolar transistors and selective epitaxial Si bipolar transistors. University of Southampton, Doctoral Thesis.

Abstract

The use of low thermal budget processing is an important constraint in the fabrication of Si_1-xGe_x devices. In this thesis three issues associated with the low thermal budget processing of Si_1-xGe_x are investigated. The first is the effect of an emitter contact implant on the diffusion of an underlying in-situ doped Si_1-xGe_x or Si epitaxial base. It is shown that a direct As emitter implant leads to an enhancement of the boron diffusion coefficient by a factor of 2600 during a rapid thermal anneal of 30 seconds at 800^oC. Process simulations are carried out using a point defect distribution obtained from Monte Carlo simulation in an attempt to model the measured boron profiles. An enhancement of the boron diffusivity is predicted, but the enhancement is much smaller than measured. An empirical transient enhanced diffusion model is therefore used to interpret and quantify the observed boron diffusion, and it is shown to be consistent with the model. The maximum difference between the extracted and predicted enhanced diffusion coefficient is -38%, for the lowest anneal temperature of 800^oC, decreasing to -28% for 900^oC. Further results showed that the enhanced diffusion is eliminated by incorporating a polysilicon emitter instead of a directly-implanted emitter.

Electrical results are presented on low thermal budget polysilicon emitters for Si/Si_1-xGe_x heterojunction bipolar transistors. Rapid thermal anneals of 30s in the temperature range 775-900^oC are investigated, and arsenic and phosphorus emitter dopants are compared. The base current is shown to be very sensitive to the temperature of the emitter drive-in anneal. Unusually high base currents are observed for the lowest anneal temperatures, and a sudden drop in base current is observed at higher anneal temperatures, with the drop occurring at ≃875^oC for phosphorus-doped emitters, and ≃900^oC for arsenic. Doping profiles are presented which show that the drop in base current correlates with an increase in the dopant concentration at the polysilicon/silicon interface. This behaviour is explained by the segregation of dopant at the polysilicon/silicon interface, which passivates interface states and creates a low-high-low potential barrier. The collector current is shown to be, not only sensitive to the amount of boron out-diffusion due directly to anneal temperature, but also to the emitter dopant type. This is explained by the phosphorus emitter dopant causing an emitter-push type effect. It is shown that the use of phosphorus instead of arsenic as the emitter dopant allows the anneal temperature to be decreased by approximately 50^oC.

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