Focused MeV Ion Beams for Materials Analysis and Microfabrication
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ence a small focused-beam spot size.2-3 While it is relatively difficult to focus MeV ions to small spots, the beneficial side effect is that the beam tends to stay focused through tens of microns into the sample, owing to the large ion mass. This issue of MRS Bulletin describes applica tions in materials analysis and microfabri cation using focused MeV ion beams in a nuclear microprobe. Many of these ex amples make good use of this property of MeV ions to stay well focused. Also, many of these new methods of materials charac terization require only an extremely small beam current of ~ 1 fA for analysis. These methods are used to image ■ variations in charge diffusion and drift in solar cells and radiation detectors, ■ variations in radiation-induced softupset sensitivities in integrated circuits, ■ the structure of working devices, ■ different types of crystal defects, ■ and to measure much smaller lattice strains than previously possible. The tendency for MeV ions to stay focused as they travel through matter has also enabled their use for fabricating highaspect-ratio microstructures, which is a growing area of microprobe technology. The first article in this issue, by H. Schöne and D.N. Jamieson, reviews applications of ion-beam induced charge (IBIC) microscopy. This has become a commonly used microprobe technique in recent years because it provides a method of analyzing working integrated circuits, radiation detectors, solar cells, and so on, through any thick surface layers. The resulting IBIC images can be interpreted to give information on physical and electronic properties such as layer thickness, the distribution of the underlying p-n junc-
tions, mask-misalignment errors, radia tion hardness, and upset sensitivity. Imaging crystallographic defects using a focused MeV ion beam relies on the channeling effect, which occurs when a lattice direction is aligned with the beam. The ions are steered by the rows and planes of atoms and undergo a lower energy-loss rate and a smaller chance of collision with the lattice atoms than nonchanneled ions. Defects within the crystal lattice disrupt the regulär atomic arrangement and so affect the channeling process. This allows the production of images of defects using a scanned, focused MeV ion beam transmitted through a crystal 50 /um thick. P.J.C. King gives a comprehensive introduction to all relevant aspects of ion channeling and the production of spatial ly resolved channeling images, and he demonstrates the types of defects that can be studied in this manner. A new approach, called "beam rock ing," which also uses the ion-channeling process in conjunction with a microprobe, has recently been developed for the analysis of small strains from micronsized areas of a crystal. A focused MeV ion beam is rocked in angle about a stationary point on the crystal surface, enabling very small angular shifts to be measured from different areas. The article by D.G. de Kerckhove describes how this process is implemented on a microprobe and gives examples of beam rocking for m
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