The behavior of Ti silicidation on Si/SiGe/Si base and its effect on base resistance and f_max in SiGe hetero-junction bipolar transistors

The relation between Ti silicidation and base resistance in SiGe hetero-junction bipolar transistors (HBT) was investigated. The Ti layer deposited on the Si/SiGe/Si base converted to Ti silicide during two-step annealing. The thickness of the Ti silicide, which was identified as the Ti(Si_1-xGe_x) phase of uniform composition, abruptly increased over the annealing temperature of 650/850°C, and as a result it accomplished a very low extrinsic base resistance. The Ti silicidation affected the base resistance of real devices (R_B), which was extracted from simulating the electrical data of SiGe HBTs such as I-V curves, forward Gummel plots, forward current gain curves, and s-parameter plots. It was shown that the R_B was compatible with the theoretical relation which included the small-signal unity-gain frequency (f_τ), the maximum oscillation frequency (f_max) and R_B. f_max varied more sensitively with R_B than f_τ, which was due to the inherent property of f_max being inversely proportional to √R_B. The f_max of the SiGe HBT reached 47.4 GHz when Ti silicidation was performed at the annealing temperature of 650/850°C. This silicidation condition is thought to be an appropriate temperature for Ti silicidation applicable to SiGe HBT fabrication.