The influence of BF/sub 2/ and F implants on the 1/f noise in SiGe HBTs with a self-aligned link base

A study is made of 1/f noise in SiGe heterojunction bipolar transistors (HBTs) fabricated using selective growth (SEG) of the Si collector and nonselective growth (NSEG) of the SiGe base and Si emitter cap. The transistors incorporate a self-aligned link base formed by BF/sub 2/ implantation into the field oxide below the p/sup +/ polysilicon extrinsic base. The influence of this BF/sub 2/ implant on the 1/f noise is compared with that of a F implant into the polysilicon emitter. Increased base current noise S/sub IB/ and base current are seen in transistors annealed at 975/spl deg/C, compared with transistors annealed at 950 or 900/spl deg/C. At a constant collector current, both the BF/sub 2/ and F implants reduce S/sub IB/, whereas at a constant base current, only the BF/sub 2/ implant reduces S/sub IB/. This result indicates that the BF/sub 2/ implant decreases the intensity of the base current noise source whereas the F implant decreases the base current. The proposed explanation for the increased 1/f noise is degradation of the surface oxide by viscous flow at 975/spl deg/C under the influence of stress introduced during selective Si epitaxy. The influence of the BF/sub 2/ implant on the noise is explained by the relief of the stress and hence the prevention of viscous oxide flow.