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EXCIMER LASER CRYSTALLIZED AMORPHOUS SILICON FILMS: EFFECTS OF SHOT DENSITY AND SUBSTRATE TEMPERATURE R. I. JOHNSON, G. B. ANDERSON, S. E. READY, and J. B. BOYCE Xerox Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA 94304 ABSTRACT Laser crystallization of a-Si thin films has been shown to produce materials with enhanced electrical properties and devices that are faster and capable of carrying higher currents. The quality of these polycrystalline films depends on a number oT parameters such as laser energy density, shot density, substrate temperature, and the quality of the starting material. We find that the average grain size and transport properties of laser crystallized amorphous silicon films increase substantially with laser energy density, increase only slightly with laser shot density, and are unaffected by substrate temperatures of up to 4000C. The best films are those processed in vacuum but films of fair quality can also be obtained in air and nitrogen atmospheres. INTRODUCTION Considerable progress in laser crystallization has been achieved during the last five years [1-91. The major thrust of these efforts has been to improve the electrical properties of thin amorphous silicon films using pulsed laser systems to rapidly crystallize the films without appreciably effecting the substrate. Here we discuss the grain growth and the electrical properties as a function of laser energy density, laser shot density (the number of laser pulses to which any point on the sample surface is exposed), the substrate temperature, and process atmosphere. We have characterized the processed films using x-ray diffraction, TEM, and Hall mobility. EXPERIMENTAL CONSIDERATIONS Both low pressure chemical vapor deposited (LPCVD) and plasmaoenhanced chemical vapor deposited amorphous (PECVD) silicon films of 1000-1250 A thickness were laser crystallized using a system described elsewhere (1,9]. The PECVD films contain about 10 atomic % hydrogen which evolves upon laser crystallization, fracturing the films. As a result, the PECVD films were annealed at 450°C for one or more hours, in order to remove most of the hydrogen before crystallization. With this preprocessing, the results on LPCVD and PECVD are essentially identical, so only data on the LPCVD are presented here. All samples were crystallized in vacuum, except for the experiments testing the effects of air and nitrogen atmospheres. A XeCl excimer laser with an energy output of 150 mJ at a wavelength of 308 nm and a pulse length of 17 ns was used. Thermally biased samples were processed at temperatures up to 400'C, using a small heating element attached to the substrate holder inside the vacuum chamber. A chromel-alumel thermocouple embedded in the substrate holder provided the substrate temperature. Substrates were held in thermal equilibrium for several minutes before exposure to the laser beam. A constant temperature for the substrate was maintained during crystallization by lowering the heater controller voltage in order to compensate for the heating effec