Kinetics and mechanism of laser-induced photochemical deposition from the group 6 hexacarbonyls

Abstract

A quartz crystal microbalance has been used to measure the rates of photochemical deposition from Cr(CO)6, Mo(CO)6, and W(CO)6 induced by a focused frequency-doubled argon ion laser (257 nm). The kinetic data indicate that deposition occurs via single-photon dissociation of the hexacarbonyl in the gas phase, yielding products that subsequently condense on the substrate surface. This interpretation is supported by the observation of material deposited well outside the area of the substrate that is directly illuminated by the laser beam. A calculation of the deposit thickness versus radial distance from the center of the laser beam, using a simple gas-phase model of the deposition process, provides an adequate fit to the observed thickness profile of a typical deposit. The presence of ripples in material deposited within the focal spot of the laser beam suggests that secondary photoinitiated dissociation of the gas-phase photoproducts occurs after they condense on the substrate surface. Adsorption measurements performed with the microbalance show that the group 6 hexacarbonyls do not adsorb on SiO2 and nickel surfaces under the conditions of the deposition experiments, up to their respective room-temperature vapor pressures.