Ion beams are used extensively in materials research for processing and synthesis as well as for characterization. In the last few years, enormous advances have been made regarding the use of ion beams for processing or synthesis, and this issue of the MRS BULLETIN will review some of those advances. (The use of ion beams for materials characterization will be the subject of a future issue of the BULLETIN.) The areas covered in this issue are ion implantation, ion beam mixing, ion-assisted deposition, and direct ion beam deposition. For each area, recognized experts in the field prepared overview articles that should be very interesting to those who are not active in the field, and that should be useful to other experts in the field.

The first large-scale use of ion beams for materials modification took place in the semiconductor industry more than 20 years ago when ion implantation began to be used to dope the near-surface region of silicon with Group III or Group V dopants. The use of ion implantation in the semiconductor industry has undergone explosive growth, and today almost all electronic devices are fabricated utilizing at lest one ion implantation step.

In addition to the semiconductor area, research is being carried out using ion implantation in a multitude of other areas which include ceramics, metals and alloys, insulators, etc. The article on “Ion Implantation” by S.T. Picraux and P.S. Peercy provides an excellent overview of current research activities involving ion implantation of a wide spectrum of materials.

In the last quarter of a century, modification of the near-surface region of materials has become of major technological importance. The principal surface modification technique utilized in integrated circuit technology is ion implantation, a technique which has more recently been applied in the metal-processing industry as well. The very high doses required for applications such as increasing the hardness of steel or forming buried oxide layers in silicon have pushed ion implantation to its limits. Ion beam mixing, the intermixing of surface layers by the penetration of energetic ions through them, was developed to overcome these limits. Additionally, ion beam mixing has been able to produce new phases, amorphous and crystalline, which have technologically and scientifically interesting properties.

Ion beam mixing was studied extensively in silicide forming systems, due partly to applications to electrical contacts for silicon devices. In intermetallic alloy systems, research has concentrated on determining the interplay between the formation of amorphous and crystalline structures and that between equilibrium and metastable phases. Although over 50 alloy systems have been studied, this article will concentrate on the Al-based alloys. These alloys, particularly the near-noble-metal alloys, demonstrate nearly all the features associated with ion-induced phase formation. Further, Al-rich refractory metal alloys form quasicrystalline icosahe-dral alloys. Ion-beam mixing results parallel those of splat-quenching, the technique first used to produce the fivefold symmetric structure.

Concurrent energetic particle bombardment during film deposition can strongly modify the structural and chemical properties of the resulting thin film. The interest in this technique, ion-assisted deposition, comes about because it can be used to produce thin films with properties not achievable by conventional deposition. Bombardment by low energy ions occurs during almost all plasma-based thin film deposition techniques. Bombardment of a growing film, particularly by accelerated ions, can also be combined with non-plasma-based deposition techniques, such as evaporation, to simulate some of the effects observed with sputtering. The bombarding particle flux is usually controllable so that the arrival rate, energy, and species can be independently varied from the depositing flux. Thus, a basic aspect of ion-beam-based deposition techniques is the “control” often absent in plasma-based techniques. In plasmas, the voltage, current, and pressure are all interdependent. The energetic bombardment at the substrate-film interface depends on the various properties of the plasma, as does the deposition rate. It is often difficult, or even impossible, to decouple these processes. With ion-beam-based deposition techniques, the ion bombardment is essentially independent of the deposition process, and both can be more easily controlled.

The incident energetic particle contributes some of its energy or momentum to irreversibly change the dynamics of the film surface. The incident particle may also be incorporated into the growing film, changing the film's chemical nature. The changes induced by particle bombardment during deposition are often not characteristic of equilibrium thermodynamics because the incident particle's energy is often many times the local adsorption or binding energy.

Ion beam processing of materials has a tradition at Oak Ridge National Laboratory that is as old as the laboratory itself. Consequently, when we began looking for a competitive way to participate in the excitement and new physics beginning to emerge from the fabrication and study of artificially structured materials, it was natural to look for a growth technique that incorporated ion beam processing. Our division, the Solid State Division, has a variety of ion implantation and ion beam analysis accelerators which are integrated with pulsed-laser sources into ultrahigh vacuum (UHV) surface analysis and processing chambers. These facilities allow us to do ion beam and laser processing of materials in UHV at temperatures from liquid helium to several hundred degrees centigrade and to study these alterations in situ by a variety of ion beam (ion scattering, ion channeling, nuclear reactions, etc.) and surface analysis (low energy electron diffraction, Auger, etc.) techniques. Since isotope separation has been done continually at ORNL for almost 45 years, the idea and advantages for altering this technique to do materials fabrication in UHV were immediately obvious. In the following article we will briefly review the history of the ion beam deposition (IBD) concept, describe our preliminary apparatus, and point out the inherent advantages of IBD for fabricating and studying artificially structured materials. Recent results obtained by IBD will be presented.