Analysis of ionized cluster beam thin film deposition

Ionized cluster beam (ICB) deposition has received considerable attention since its introduction in 1972 by Takagi et al. at Kyoto University because of its potential for low temperature film growth. The Kyoto University group has conducted three major experiments that indicated the presence of large clusters in varying concentrations. These three experiments form the basis for all subsequent ICB research and are supplemented by a body of indirect evidence of film formation effects that have been attributed to the presence of large clusters. Many other groups have tried to corroborate these cluster size experiments with only a few suggesting the presence of large clusters. While further investigation of many aspects of ICB deposition is warranted, it is first necessary to determine with some certainty whether large clusters are being produced. A complete analysis of the Eaton ICB source involving computer calculation of the potential fields and computer simulation of the electrons and ions as they react to and influence these fields provides an in depth understanding of the dynamics that influence the final beam characteristics. A high resolution time-of-flight mass spectrometer was developed to investigate the most important parameter, the cluster size distribution. Great effort was put toward developing a system that could detect even the smallest concentrations of large clusters and computer simulation of all aspects was done to assure the accuracy of the spectrometer. No evidence of large clusters was found down to a level more than two orders of magnitude below what the Kyoto University group has claimed. This motivated a review of the primary evidence for large clusters. A computer analysis of the three Kyoto University cluster size experiments has identified severe problems with each of the three experiments where the potential fields in the ionization areas are critically distorted by either space-charge effects or design flaws, both of which are serious enough to invalidate the experiments. The two highest resolution experiments, the one presented here and a laser ionized time-of-flight experiment, have turned up absolutely no evidence for the existence of large clusters. The theory behind large cluster production and the body of indirect evidence attributed to the presence of large clusters are not convincing by themselves. It is therefore concluded that a Takagi-type source does not produce large clusters in quantities capable of affecting film growth