The Effects of Selectively Absorbing Dielectric Layers and Beam Shaping on Recrystallization and FET Characteristics in
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THE EFFECTS OF SELECTIVELY ABSORBING DIELECTRIC LAYERS AND BEAM SHAPING ON RECRYSTALLIZATION AND FET CHARACTERISTICS IN LASER RECRYSTALLIZED SILICON ON AMORPHOUS SUBSTRATES
G. E. Possin, H. G. Parks, S. W. Chiang* and Y. S. Liu General Electric Research and Development Center Schenectady, New York 12301 ABSTRACT Selective absorption, using patterned dielectric films, and beam shaping were used as means for improving the recrystallization of LPCVD polysilicon islands on fused quartz. IR imaging of the laser heated region was used to optimize and control the recrystallization. MOSFETs were fabricated in laser-recrystallized silicon islands on amorphous substrates using Devices with various channel lengths and widths a standard n-channel poly-gate process. were fabricated, and the dependence of threshold voltage, channel mobility, and leakage on recrystallization conditions and device dimensions was studied. INTRODUCTION This paper will describe an investigation of the use of a CW Argon ion laser to recrystallize polysilicon islands on quartz substrates. Both shaped beams and selective absorption using dielectric layers were used to control the shape of the melt front during recrystallization. Infrared imaging of thermal radiation from the heated region was used to optimize the beam shape and dielectric structures and to improve the control of the recrystallization conditions. The quality of the achieved recrystallization was investigated using grain boundary etching. MOSFETs were fabricated in the recrystallized islands using a standard self-registered polysilicon gate technology. The influences of channel length and width on the device properties were investigated. IR IMAGING AND TEMPERATURE MONITORING The laser annealing system consisted of an Ar ion laser and 40 mm microscope objective for beam focusing. The wafers were scanned under the beam using a numerically controlled table with an accuracy of one micron and a maximum scanning speed of 10 cm/sec. Beam power, beam blanking, and table scanning were controlled using a HP85 computer. The beam power was set using a voltage applied to the feedback reference of the plasma supply. In the 1 to 2 watt range used for these experiments, the power was stable to better than 11/0. The power was checked between scans and reset automatically as required. A silicon vidicon camera with 1000 mm lens viewed the sample through a dichroic mirror and the same microscope objective. IR transmitting glass filters (Corning 7-69 and 7-57) were used to provide an infrared image of the laser heated region. Because of the 1 micron wavelength cutoff of the silicon vidicon, only a narrow IR band around 1.0 micron was imaged. Quantitative monitoring of the IR vidicon signal during set-up and scanning provides a useful reference for determining the laser power needed for good recrystallization. A beam expander consisting of a converging-diverging lens pair whose separation could be controlled was used to control the beam size. By introducing a small additional beam divergence, large spot sizes
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