Self aligned BM for VLSI using a new diffusion technique for shallow base formation

Considerable effort has been invested by researchers to find a suitable base formation technique that is compatible wirh self aligned bipolar transistor m c m s . Ion implantation is plagued with channeling, particlllarly for low dose boron implants. This channeling, coupled With defect enhanced boron diffusion during post implant anneals, ultimately limits the vertical scaling of ion imphted BJTs. In narrow emitter devices, shadowing during implantation can make extrinsic base contact resistance more sensitive to emitter window over etch, exainsic base diffusion, and spacer width, resulting in a difficult control problem. Formation of base profiles by diffusion has been shown to be a desirable alternarve to ion implantation l. However, dif&xlties arise in the integration of a base diffusion source into a self aligned bipolar process. Previously reported base diffusion work has involved either the use of the sidewall spacer marerial, or the use of the emitter polysilicon as a diffusion source2. Difficulties with these diffusion methods including reduced control of base profiles, base link-up. and emitter junction depth result from unpredictable impurity diffusion and an inability to selectively remove the diffusion source following base diffusion3. The present paper describes a method of using selectively deposited and selectively removed "polycrystalline" SixGel-x as a diffusion source for the fabrication of narrow base self aligned BJTs. We present, for the first time, bipolar transistors built using this technique.