Process design for SiGe-HBTs prepared using cold-wall UHV/CVD

摘要

We have studied the influence characteristics of a self-aligned SiGe-base heterojunction bipolar transistor (HBT) with a poly-crystalline silicon (polysilicon) emitter. Particular attention has been given to a collector-based undoped SiGe intermediate layer. This undoped layer acts as a spacer for boron outdiffusion of the P~＋-type SiGe to the n~- -type Si collector, during heat treatment after the SiGe base formation. The SiGe-base was selectively grown by a cold-wall ultra-high vacuum/chemical-vapor-deposition (UHV/CVD) technique. Using both low temperature oxide (LTO) as a passivation film, and in-situ phosphorus-doped emitter polysilicon for suppressing of boron out-diffusion, a 15-nm-thick undoped layer was an insufficient boron outdiffusion spacer, This was confirmed by measuring the cut-off frequency, f_T, characteristics. With a 30-nm-thick spacer layer, the f_T of the SiGe base bipolar transistor increased from 38 to 63 GHz.