Shaping of Dopant Concentration Profiles in Silicon by Multiple–Pulse Laser Processing

Abstract

Over the past ten years, the performance of bipolar transistors in high speed integrated circuits has been steadily improved by fully exploiting the complex solid state diffusion behaviour of dopants in silicon to make successively closer approaches to the ideal dopant profiles in the emitter and base regions of the transistor. Further optimisation requires a new approach, and in this paper an entirely new method of shaping the dopant profiles by exploiting the large ratio (about 10⁸) of diffusion coefficients in liquid and solid silicon near the melting point is described Melting is effected by irradiation with a homogenised pulse from a Q-switched ruby laser. Completely new features of the technique are the controllable fabrication of both quasi–Gaussian and quasi-rectangular dopant profiles independent of dopant (arsenic or boron), dopant concentration (10¹³−10¹⁶atom cm⁻²) or presence of other dopants. This allows for the first time independent optimisation of emitter profile shape, emitter doping level, and base profile in bipolar transistor structures.