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SYNTHESIS AND PROCESSING: MORPHOLOGICALLY SPECIFIC METHODS. 43 Cluster Beam Deposition Although multilayer materials are usually prepared using chemical deposition methods or by effusion from a thermal evaporative source, another alternative involves preparation from clusters. Clusters have been used to prepare thin films of metals, insulators, and semiconductors. The clusters may be prepared by a number of methods, with the inert gas condensation and free jet expansion method being the most viable. The general subject of clusters and cluster-assembled materials, beyond the application to multilayer structures, has been recently considered in some detail elsewhere in a Department of Energy panel report (Andres et al., 1989). A great variety of source designs have been developed to produce cluster beams. Figure 11 shows schematically four generic types of sources: free jet source, pulsed laser vaporization source, continuous thermal vaporization source, and supercritical solvent extraction source. The free jet source is the most widely studied cluster source because the flow field is fairly well understood and some reasonable cluster kinetic models have been developed. Pure or mixed species can be expanded; as they cool in the rapid expansion, they form clusters. Figure 11 Cluster sources: (a) standard free jet source (G = gas, C = coolant, D = divergent section); (b) pulsed laser vaporization source (V = pulsed valve, L = laser, MR = rod of material); (c) thermal vaporization into helium carrier source (M = material inside graphite source, F = electron bombardment filament); (d) supercritical solvent extraction source (S = solvent, HPP = high-pressure pump, O = oven, M = material, C = capillary) (J. M. Weare, D. R. Miller, and J. E. Crowell, 1988, private communications).
