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ArF EXCIMER LASER DOPING INTO AMORPHOUS SILICON THIN FILMS M. ELLIQ, A. SLAOUI, E. FOGARASSY, H. PATTYN*R. STUCK and P. SIFFERT Centre de Recherches Nucl6aires (IN2P3), Laboratoire PHASE (UPR 292 CNRS), F67037 Strasbourg Cedex, France IMEC, Kapeldreef 75, B-3030 Leuven, Belgium

ABSTRACT Doping of amorphous silicon (a-Si or a-Si:H) coated by a spin-on oxide film containing the dopant (phosphorus or boron) using an ArF excimer laser has been investigated as a function of laser fluence, number of pulses and dopant film thickness. From these results, it is found that the surface concentration and the junction depth vary with the number of pulses, and that the doping process is rate limiting. Sheet resistance lower than 10 kfl/c have been obtained. It is also shown that for a-Si:H films, laser irradiation produces exodiffusion of hydrogen from the molten layer resulting in rough surface. This one-step process seems suitable for polycrystalline silicon thin film transistors (TFT's) fabrication.

INTRODUCTION The polycrystalline silicon thin film transistors (poly-TFT's) require shallow, high concentration doping for source-drain fabrication. They also require low-temperature processing in order to permit use of low-cost glass substrates. However, with the conventional ion-implantation process followed by heat treatment in a furnace above 9000C for time of the order of minutes to hours, it is difficult to achieve a high dose with low-energy doping. In order to form such a shallow junction, new doping technique using high power lasers have been performed for doping from solid or gas sources [1 - 3]. Among these techniques the combination of excimer laser and a solid doping source deposited at the silicon surface seem to be a promising tool for such shallow junctions. Indeed, since silicon has a large absorption coefficient (-. 10 cm- 1 ) in the ultraviolet region, the excimer laser light is strongly absorbed in the near surface region. Thus, laser irradiation of silicon substrate coated by a film containing dopant can be used to melt locally a near surface region and produce significant incorporation of dopant from the surface into the molten surface layer. The dopant distribution is driven by a simple liquid state diffusion. This feature gives an advantage in forming shallow junctions. In this work, we report the results of ArF excimer laser doping of phosphorus and boron into hydrogenated and non hydrogenated amorphous silicon deposited onto quartz or silicon substrates. The thin silicon films are coated by a spin-on-glass (SOG) film containing high concentration of dopant (P or B). Laser irradiation simultaneously induces crystallization and doping of the amorphous silicon layer. EXPERIMENTAL DETAILS The samples used in these experiments were 200nm hydrogenated (a-Si:H) and non-hydrogenated (a-Si) amorphous silicon films. The a-Si:H layers were deposited by plasma-enhanced CVD (PECVD) at 250°C while the a-Si films were deposited by low-pressure CVD (LPCVD) at 550'C. The substrates are quartz or monocrystalline Mat. Res