Development of New Materials by Ionized-Cluster Beam Technique
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DEVELOPMENT OF NEW MATERIALS BY IONIZED-CLUSTER BEAM TECHNIQUE
T. TAKAGI Ion Beam Engineering Kyoto 606, Japan
Experimental
Laboratory,
Kyoto
University,
Sakyo
ABSTRACT In the Ionized-Cluster Beam (ICB) technique, deposition by macroaggregates consisting of 500 to 2000 atoms loosely coupled together involves low ratios of charge to mass. Consequently, high mass density beams at low equivalent energy per atom in an optimum range for film formation can be transported without problems due to space charge repulsion forces and deposition onto insulating substrates is easily possible due to low accumulation of ion charge. The presence of ionic charge has great influence upon film formation mechanisms in spite of low content of ions in the total flux. Also, because of the kinetics of cluster breakup upon impact, enhancement of migration of adatoms upon a substrate surface can be achieved by increasing the acceleration voltage. It is possible to control the mechanical, crystallographic, optical and magnetic properties of films over three dimensions by variation of acceleration voltage or ion content in the total flux. Films of many materials have been formed at low temperatures with well-controlled characteristics. Among examples of interest are included metal and semiconductor material films for functional devices and VLSI applications, intermetallic compound films for magnetic or thermoelectric uses and organic material films. Results suggest that ICB offers exceptional potential for applications involving formation of new materials.
INTRODUCTION Clusters are created by condensation of supersaturated vapor atoms produced by adiabatic expansion through a small nozzle into a high vacuum region. Once formed, the cluster beam can be partially ionized to positive single charge state by electron impact. Energy can then be added to ionized clusters by use of acceleration potentials [1,2]. Unique capabilities of ICB deposition are attributed to the properties of the cluster state. Enhancement of the adatom migration effect is one of the most significant properties of cluster beam deposition. A number of other properties of ICB, such as extremely low charge to mass ratios which prevent space charge problems and high equivalent current, low energy ion beam transport in a range from above thermal energy up to a few hundred eV, are also important characteristics of this technology for film formation purposes. The importance of low energy ion beams for film formation can be easily understood when we recognize that binding energies of the atoms in solids are of the order of lOeV. Atomic kinetic energies corresponding to thermal energies given by kT/e are only 0.01-0.1eV in the temperature 0 range below 1000 C and are consequently much less than the binding
Mat. Rs. soc.
Syrp. Proc. Vol.
27 (1984) @Elsevier Science Publishing Co.,
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energies. A strong effect can be expected however as the result of bombardment using accelerated ion beams, even at energies of only a few eV approximately corresponding to binding ene
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