Pulsed-Laser Annealing of Silicon Films
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PULSED-LASER ANNEALING OF SILICON FILMS T.SAMESHIMA Sony Research Center, 174 Fujitsuka-cho, Hodogaya-ku, Yokohama 240, Japan. Abstract Phase transition between crystalline and amorphous states was studied through 30ns-pulsed XeC! laser induced melting of silicon films. Crystallization occurs through interface controlled growth for laser energy above surface melting threshold. Grain size is smaller than 100nm because of short melt duration (1OOcm 2 /Vs) at a low temperature of 250 0 C. Introduction Pulsed laser-induced melt followed by regrowth is a powerful technique for surface modification of semiconductor such as crystallization activation of ion implanted species and amorphization. Many researchers have been working to clarify these phenomena[l-8). We have reported that pulsed laser-induced melt followed by regrowth is also useful for crystallization of thin silicon film formed on a glass substrate. This method made fabrication of polycrystalline thin film transistors (poly-Si TFTs) at temperature below 300 0 C possible[9,10]. Moreover, we recently reported thin poly-Si films fabricated on quartz substrates can be amorphized by irradiation using a XeC! excimer laser with a long pulse of 30 ns [11-15]. This paper discusses the characteristics and mechanism of pulsed laser-induced crystallization and amorphization of silicon films, and also electrical and optical characteristics of these films. It also discussses fabrication of poly-Si TFTs and amorphous silicon(a-Si) TFTs using laser crystallization and laser amorphization. Pulsed laser-induced melt-regrowth of silicon films Irradiation with a XeCI excimer laser with a 30ns pulse (full width at half maximum) provides rapid heating of hydrogenated amorphous silicon (a-Si:H) formed on glass substrates. Undoped a-Si:H films with thickness from 6 to 36nm deposited on quartz substrates at 250 oC using radio frequency glow discharge (ff-GD) were irradiated with XeCI excimer laser pulses at room temperature in a vacuum chamber. The laser energy density was increased from 100 mJ/cm 2 in steps of 10 mJ/cm 2 in order to release hydrogen atoms without explosive eruption. The pulsed laser-induced melting of silicon films can result in the formation of crystalline and amorphous states following solidification. Figure 1 shows threshold energies for crystallization and amorphization as a function of film thickness. The crystallization threshold energy was almost independent of film thickness between 10 and 36nm. Silicon films are heated to the melting point of a-Si:H during irradiation and crystallization initiates[10]. Figure 2 shows electrical conductance change in a 30nm-thick film during and after laser-induced melting with increasing laser energy. The conductance change is associated with change of melt depth. The similarity in shape of the conductance curves indicates that the melt-regrowth occurred in the same way at energy density between 160 and 240mJ/cm 2 . The solidification velocity is obtained by the down-slope of the conductance. It was about 0.6m/s. On the othe
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