Variation in emitter diffusion depth by TiSi ₂ formation on polysilicon emitters of Si bipolar transistors

The emitter diffusion depth of wider emitters was found to be shallower than that of narrower emitters for transistors with a TiSi ₂-formed in-situ phosphorus doped polysilicon (IDP) emitter. This dependence of emitter diffusion depth on emitter width W _E resulted in a large dependence of h _FE on W _E. We found that the emitter diffusion in emitters with TiSi ₂ formation was shallower than that in emitters without TiSi ₂ formation. We conclude that the dependence of emitter diffusion depth on W _E is due to variation in phosphorus diffusivity caused by the TiSi ₂ formation. We found by X-ray diffraction measurements that TiSi ₂ formation reduced the crystal lattice volume of the IDP emitter layers by 0.2-0.3%. We attribute this reduction to a reduction in packing density due to a supersaturation of vacancies caused by the TiSi ₂ formation. We believe that the vacancy supersaturation decreases the density of interstitials in the emitter regions, which decreases the emitter diffusion depth, and that the dependence of emitter diffusion depth on W _E is due to variations in the density of vacancies depending on W _E.