Visible laser-induced nucleation and growth of Cr, Mo, and W films from the hexacarbonyls. Reactivity of CO on film surfaces

Abstract

Continuous visible laser irradiation is used to deposit Cr, Mo, and W films from the hexacarbonyls by thermal excitation. Detailed analyses of the films by scanning Auger microscopy show that the Mo and Cr films are spatially inhomogeneous, having clean metal centers surrounded by contaminated metal rings. W films are completely pure. Depth profiles reveal that, prior to formation of clean Mo and Cr, an interfacial layer of contaminated material is deposited which becomes progressively metal rich as thickness increases. If such a layer is formed for W, it is very thin. The measured composition profiles are compared to calculated surface temperature distributions in order to extract information on the kinetics of CO dissociation and desorption during growth. The temperature and mechanism simulations show that CO desorption dominates above about 450 K, ensuring that pure metal is deposited. Below that temperature contamination occurs. Previous investigations of film growth from metal hexacarbonyls have led to the conclusion that film purity will only be high at temperatures exceeding 800 K, where recombinative desorption of CO is fast. The origins of the large discrepancy in temperatures are discussed. Chemical processes leading to growth of metal oxycarbide material at the film edges and film-substrate interface are explored using data from the catalysis literature. The strong parallels found between the laser-deposited material and supported catalysts formed from the hexacarbonyls suggest that the oxycarbide films may have significant Lewis acid-base character, leading to faster CO and metal-CO bond dissociation rates. Thus, formation of contaminated material appears to depend on local surface composition as well as temperature. © 1992 American Chemical Society.