Introduction To Medium‐Energy Ion Beam Analysis

Abstract

Abstract The earliest applications of ion beam techniques for materials analysis, typified by the work of Tollestrup et al. (1994), were carried out with mega‐electron‐volt beams in the course of basic research in nuclear physics, and the mega‐electron‐volt beam techniques still form the indispensable core of the subject. In the broadest sense, the importance of these techniques lies in the fact that the information they yield depends upon nuclear properties and interactions and is, therefore, relatively insensitive to perturbation by local properties such as the chemical environment and electronic structure. The relevant nuclear physics and scattering theory are well known experimentally and often theoretically, and in consequence, important parameters such as interaction cross‐sections are known from first principles. This is in sharp contrast to the situation for some other important techniques (e.g., secondary ion mass spectrometry) where the fundamental measurement interactions may be very vulnerable to the unique idiosyncrasies of a specimen. It is essential to stress the unity of ion beam analysis even as one seeks to differentiate the relatively newer medium‐energy techniques from their more mature high‐energy predecessors. First it is important to understand what is meant by “medium energy.” There are two answers, one historical and one based upon the physics of scattering. For practical purposes, medium energy encompasses ion beam energies from a few tens of to a few hundreds of kilo‐electron‐volts, and even a bit higher for heavy ions being used in forward‐recoil work. Below this range, ion scattering becomes increasingly surface specific. Above it lies the full range of conventional elastic scattering and nuclear reaction analysis. Historically, medium energy has been defined by technology. The field of ion beam analysis owes its ubiquity, if not its existence, to a single device, the silicon surface‐barrier particle detector. The articles in this category follow describe some medium‐energy ion beam analytical techniques that are optimized for different problems. Time‐of‐flight medium‐energy backscattering and forward‐recoil spectrometry, are quite similar to the high‐energy techniques of high‐energy ion beam analysis but offer increased depth resolution, sensitivity, and surface specificity at the expense of total analyzable depth and ease of use. Heavy‐ion backscattering spectrometry is a variant of time‐of‐flight backscattering that is optimized for the highest possible sensitivity, specifically for the measurement of ultra‐low‐level metallic contaminants on device‐grade Si wafer surfaces. Along with total‐reflection x‐ray fluorescence spectrometry, it is the definitive method currently available for this measurement. Medium‐energy techniques are appropriate for specialized problems and circumstances as described in the articles.