Fabrication of thin base bipolar transistors using pulsed UV laser processing

In this thesis, a new pulsed uv laser doping technology is investigated for fabrication of thin-base bipolar transistors. The new process, known as gas immersion laser doping (GILD), incorporates a pulsed XeCl excimer laser to melt the silicon surface. Dopants are incorporated into the molten silicon from the gas phase during the melt/regrowth cycle. The GILD process allows very precise control of junction depth, does not damage the crystalline structure of the silicon and yields electrically active dopant species without high temperature anneals, enables completely independent control of emitter width, base width, and base doping concentration, and allows simple fabrication of uniformly doped, abrupt impurity profiles. These aspects of the process make it ideal for fabrication of ultra-narrow base regions ({dollar}Wsb{lcub}B{rcub}{dollar} {dollar}<{dollar} 500A), where base resistance and punchthrough become significant problems in bipolar devices. In order to fabricate thin-base transistors we first characterize the melt/regrowth step. Using unique equipment designed as a part of this thesis, the metallurgical junction depth is studied as a function of laser fluence and silicon melt duration. Experimental results are compared with computer simulations to demonstrate that in-situ junction depth prediction is possible using the GILD process. Redistribution during the rapid melt/solidification cycle is also studied. The high solidification velocity is shown to have significant effects on dopant redistribution. To accurately predict these effects, a new diffusion simulator is developed. Dopant incorporation is also studied in an effort to adequately control the impurity dose. Using the results of these initial studies thin-base bipolar transistors are fabricated. The npn transistors exhibit good electrical behavior with current gains that range from 50 to 200. Finally, we demonstrate a laser induced epitaxial process that allows fabrication of high-quality, selective Ge{dollar}sb{lcub}x{rcub}{dollar}Si{dollar}sb{lcub}1-x{rcub}{dollar}/Si layers. These structures have potential applications to very high speed heterostructure bipolar transistors.