Ionized cluster beam deposition

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Abstract
An experimental cluster beam facility of the type used by Takagi was developed and tested. Since the cluster generation rate (of silver clusters in graphite crucibles) was small and not consistently observable, a basic study of the generation process was undertaken. Theoretical evaluation has revealed that clusters are formed on crucible internal surfaces and not in the vapor stream which passes out of the crucible through a nozzle, as had been assumed earlier. Experiments have confirmed this heterogeneous cluster growth model, showing that clusters occur consistently only when a temperature distribution is maintained within the crucible which favors cluster formation on surfaces near the crucible nozzle. A remaining problem is the low rate of cluster generation. This rate needs to be raised if metal clusters are to be used in practical film deposition apparatus. An alternative process which involves gaseous rather than metallic clusters is also under study. It is well known that isentropic expansion of gases, pressurized to a few atmospheres, results in homogeneous nucleation and can produce appreciable quantities of clusters with thousands of atoms. In order to utilize such gaseous clusters for film deposition applications, an assist mode is used in which films are grown by simultaneous vapor deposition and bombardment with energetic noble gas clusters. So far, a suitable cluster beam source has been developed, and deposition of goldfilms on GaAs substrates has been initiated, whereby ionized argon cluster beams provide the energy.

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