Comparison of the Processes Induced by Mercury Lamp and ArF Excimer Laser Photoassisted CVD of a-Si:H Films
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COMPARISON OF THE PROCESSES INDUCED BY MERCURY LAMP AND ArF EXCIMER LASER PHOTOASSISTED CVD OF a-Si:H FILMS C. FUCHS and E. FOGARASSY Centre de Recherches Nucl~aires - Laboratoire PHASE 23, Rue du Loess F-67037 STRASBOURG, France ABSTRACT We compare, in this study, the photoassisted processes for silicon deposition using both a low pressure mercury lamp and an ArF excimer laser for the specific case where the SiH 4 gas is sealed in the reaction chamber. INTRODUCTION The growing interest in amorphous materials in many fields of technology has stimulated research for new low temperature deposition techniques. Among the new methods, photoassisted processes are particularly attractive. Different groups have recently produced good quality hydrogenated amorphous silicon films by photodissociating silane (SiH 4 ) or disilane (Si 2 H6 ) using ultraviolet light from either a low pressure mercury lamp Ii-5d or a pulsed excimer laser 16-101. With the UVlamp (Hg photosensitization process) the decomposition mechanism of silane is thought to involve the collisional energy transfer from the UVexcited HgA (3PI) state to the reactive species. By comparison, high energy photons provided by the ArF excimer laser are able to dissociate silane directly by multiphoton excitations of electronic molecular states. In the present work, the deposition processes using both coherent and incoherent UVlight are studied for the specific case where the active gas is sealed in the reaction chamber. The basic energy transfer mechanisms and free radical chemistry involved when these two different types of light sources are employed to dissociate SiH4 are also examined. EXPERIMENTAL Pure SiH, gas sealed in the reaction chamber, at pressures ranging between 0.5 t6 500 torr, was irradiated under normal incidence through a suprasil quartz window with two different types of UV excitation sources. The incoherent light was provided by the 254 nm resonance line of a low pressure mercury lamp, giving a light intensity of a few mW/cm 2 at the substrate surface. In this case the active gas was mixed with mercury vapor from a Hg reservoir usually held at a temperature of 500 C or less. A 20 nsec pulsed ArF (193 nm) excimer laser, operating at a repetition rate of 50 Hz, was used as a coherent light source. The energy density per pulse available close to the surface of the substrate was varied from 5 to 100 m J/cm2 by focusing the laser beam with a lens system. In order to evaluate the deposition at ambient temperature on the inner side of the incident window, the film thickness was monitored by optical transmittance at 360 nm provided by a high pressure mercury lamp. At this wavelength, where no photochemical reactions are induced in SiH4 ,follow the strong absorption the initial coefficient of amorphous silicon is well adapted to stages of the deposition (typically < 200 A). The different optical data, already reported in the literature I11,12,131 show that in the UVrange Mat. Res,. Soc. Symp. Proc. Vol. 75. 1987 Materials Research Society
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(< 400 nm) th
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