Cluster Size Measurements in an Ionized Cluster Beam System
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CLUSTER SIZE MEASUREMENTS IN AN IONIZED CLUSTER BEAM SYSTEM D.E. TURNER, K.M. LAKIN, AND H.R. SHANKS Microelectronics Research Center, Iowa State University, IA 50011.
Ames,
ABSTRACT Ionized Cluster Beam (ICB) deposition has received considerable attention in recent years because of its potential application to low temperature semiconductor and metal film growth. To better understand this deposition process, however, it is necessary to determine the properties of the cluster source and the cluster size distribution obtained. This paper discusses the design of a time-of-flight mass spectrometer with single atom resolution for the measurement of cluster size in a conventional ICB deposition system. Results have been obtained for silver and germanium deposition. In both cases most of the clusters have been found to consist of less than 35 atoms. With the high resolution spectrometer it has been possible to show that certain cluster sizes are more stable than others. TIME-OF-FLIGHT MASS SPECTROMETER The time-of-flight mass spectrometer uses a one kilovolt pulse generator to pulse electrons from a tungsten filament across the path of the cluster beam, singly ionizing a narrow band of clusters as shown in figure 1. Under a constant acceleration potential, the large clusters accelerate more slowly than the smaller clusters creating a spatial distribution of the clusters based on size. A faraday cup receives the ionized cluster distribution and creates a signal that is then amplified and filtered. A digital oscilloscope records the arrival of the clusters and averages the signals from 256 individual pulses to produce a time-of-flight distribution. This distribution can then be sent over to a computer for further manipulation, cluster size analysis, mass flux analysis, and storage. Single atom resolution has been achieved for small cluster sizes.
Mat. Res. Soc.Symp. Proc.Vol.128.-1989Materials Research Society
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Figure 1 - Illustration of the Time-of-Flight Mass Spectrometer
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S:LVER CLUSTERS
SILVER MASS FLUX
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Figure 2 - Silver cluster size distribution
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Figure 3 - Silver mass flux distribution
The mass spectrometer was designed so that it doesn't interfere with the normal operation of the source or any of its film growth capabilities. The source and chamber have not been altered except for the addition of a faraday cup mounted on the shutter. A measurement of the cluster distribution can then be taken at the beginning of each film growth after which the faraday cup can simply be swung out of the way. This will in the future allow for direct studies of the film growth characteristics of different cluster distributions. SILVER Silver was chosen as a test case for several reasons. First of all, Takagi's group has shown that large clusters of 500-2000 silver atoms can be produced by the ICB techn
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