The influence of BF2 and F implants on the 1/f noise in SiGe HBTs with a self-aligned link base

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Description/Abstract

A study is made of 1/f noise in SiGe HBTs fabricated using selective growth of the Si collector and non-selective growth of the SiGe base and Si emitter cap. The transistors incorporate a self-aligned link base formed by BF2 implantation into the field oxide below the p+ polysilicon extrinsic base. The influence of this BF2 implant on the 1/f noise is compared with that of a F implant into the polysilicon emitter. Increased base current noise SIB and base current are seen in transistors annealed at 975C, compared with transistors annealed at 950 or 900C. At a constant collector current, both the BF2 and F implants reduce SIB, whereas at a constant base current, only the BF2 implant reduces SIB. This result indicates that the BF2 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C under the influence of stress introduced during selective Si epitaxy. The influence of the BF2 implant on the noise is explained by the relief of the stress and hence the prevention of viscous oxide flow.