Impact of ex-situ and in-situ cleans on the performance of bipolar transistors with low thermal budget in-situ phosphorus-doped polysilicon emitter contacts

This paper investigates the effects of an in-situ hydrogen bake and an ex-s itu hydroflouric acid (HF) etch prior to polysilicon deposition on the elec trical characteristics of bipolar transistors fabricated with low thermal b udget in-situ phosphorus-doped polysilicon emitter contacts. Emitter contac t deposition in a UHV-compatible low pressure chemical vapor deposition (LP CVD) cluster tool is also compared with deposition in a LPCVD furnace. Tran smission electron microscopy (TEM) and secondary ion mass spectroscopy, (SI MS) are used to characterize the emitter contact material and the interface structure and a comparison is made with Gummel plots and emitter resistanc es on bipolar transistors. The SIMS results show that an in-situ hydrogen b ake in a cluster tool gives an extremely low oxygen dose at the interface o f 6.3 x 10(13) cm(-2), compared with 7.7 x 10(14) and 2.9 x 10(15) cm(-2) f or an ex-situ HF etch and deposition in a cluster tool or a LPCVD furnace, respectively. TENT shows that the in-situ hydrogen bake results in single-c rystal silicon with a high density of defects, including dislocations and t wins. The ex-situ BF etch gives polycrystalline silicon for deposition in b oth a cluster tool and a LPCVD furnace. The single-crystal silicon emitter contact has an extremely low emitter resistance of 21 Omega.mum(2) in spite of the high defect density and the light emitter anneal of 30 s at 900 deg reesC. This compares with emitter resistances of 151 and 260 Omega.mu m(2) for the polycrystalline silicon contacts produced using an ex-situ HF etch and deposition in a cluster tool or a LPCVD furnace, respectively. These va lues of emitter resistance correlate well with the interface oxygen doses a nd the structure of the interfacial oxide layer. The high defect density in the single-crystal silicon is considered to be due to the high concentrati on of phosphorus (>5 x 10(19) cm(-3)) in the as-deposited layers.