The effects of processing conditions on the density and microstructure of hot-pressed silicon powder

Silicon powder was hot pressed into polycrystalline wafers 1.5 in (similar to 3.8 cm) diameter using various processing conditions. The submicrometre powder used was a by-product of the fluidized bed decomposition process of silane [SIH,) in the production of silicon pellets. The effect of temperature (1250-1300 degrees C), pressure (2000-3000 p.s.i.; 13.18-20.67 N mm(-2)) and ambient (argon, hydrogen, vacuum) on the density of the hot-pressed powder was studied. All wafers processed had densities > 92% of the theoretical density of silicon as determined by Archimedes' density measurements. Hydrogen was found to increase the densification rate of powdered silicon. The mechanism by which this occurs is believed to be the reduction of the native oxide layer of the powders resulting in increased surface transport. The microstructure of the polycrystalline wafers was examined by scanning electron microscopy, and transmission electron microscopy. The general microstructure of the polycrystalline wafers consisted of micrometre-sized grains with twins, stacking faults, and dislocations within the grains. However, under hot-pressing conditions of 1300 degrees C, 2000 p.s.i., and a hydrogen ambient, the grains of the wafer were on the order of 1 mm. The silicon wafers contained iron, aluminium, carbon and oxygen impurities as determined by secondary ion mass spectroscopy.