Ion Microscopy and Secondary Ion Mass Spectrometry
Since the early 1970s, Riccardo Levi-Setti and several generations of graduate students have been involved in the theory, development and application of finely focused ion beam systems, with lateral dimensions of the ion probe well into the submicrometer range. Key to this development is the use of point-like field ionization sources. Cryogenic, gas-supplied tips were used at first, to be supplanted by brighter liquid metal ion sources. These new sources yield ion probes suitable for scanning ion microscopy, focused ion beam microfabrication, and imaging microanalysis by secondary ion mass spectrometry (SIMS). Even though the attainable focused ion beam spot size, and consequent image resolution, cannot reach the subnanometer range attained with electron beams, the phenomena that accompany the interaction of fast ions with solids can be exploited to provide information that is difficult or impossible to retrieve with electron probes. Of distinguishing value is the emission of sputtered atomic and molecular ions from the surface of the bombarded solid that enables the identification by SIMS of all isotopic components of the material, inclusive of the light elements. Thus fundamental studies of the ion-solid interaction, in particular of secondary ion emission phenomena, have played an important role in this development, while providing challenging thesis topics to our physics students.
A second generation, high resolution scanning ion microprobe (UC SIM) was built and brought into operation in 1984. This instrument uses a 40-60 keV Ga+ or In+ focused ion beam with useful spot size ranging from 70 to 20 nm. The resulting unprecedented performance in terms of spatial resolution and analytical sensitivity have paved the way toward the exploration of entirely novel and useful SIMS applications; so far, only a small range of exploration has been traveled. Even at the highest analytical image resolution (close to the limits of the SIMS method imposed by the width of the induced collisional cascade) high quality isotopic images can still be obtained for samples rich in elements of low ionization potential (collecting positive secondary ions) or high electron affinity (negative secondary ions). Segregated impurities in domains as small as 20 nm can be visualized, corresponding to bulk concentrations in the ppm region.
A substantial upgrade of the microprobe is presently underway. The RF quadrupole mass filter that performs the SIMS function is being replaced with a high-performance, magnetic sector mass spectrometer. This addition will improve by large factors the transmission (and consequently the sensitivity) as well as the mass resolution of the secondary ion analysis system. It will become feasible to distinguish molecular ions from isotopes having the same mass number, a task that was precluded until now by the limited mass resolution of the RF quadrupole. Essentially, we will investigate completely new science.
A number of advanced technologies are employed in our microprobe facility and are used in the operation and the continued improvement of the related instrumentation. Aside from the scientific content of the many interdisciplinary investigations, mentioned below, these technologies provide a valuable training to our students in a broad range of modern experimental methods. In addition to ion optics design, essential for the understanding of the instrument operation, experimental skills include close knowledge of ultra high vacuum techniques, high voltage engineering, liquid metal ion sources, nuclear detectors and fast counting electronics, computerized instrument control, mass spectrometry, digital image processing and quantitative image analysis.
An expansive network of collaborative, cross-disciplinary research initiatives has been established to utilize the UC SIM. Many universities and industrial concerns, in this country and Europe, participate in this effort. Exciting breakthroughs in many areas of basic research in the natural sciences, and many questions of scholarly interest relevant to applied science have been addressed.
Engineered materials present a host of problems that require the acquisition of information at a basic level. Problems have been chosen that could best or uniquely be attacked by matching the specific capabilities of our facility to the nature of the problem itself. Thus, for example, the exceptional SIMS sensitivity to the alkali metals, including lithium, has spurred a long standing collaboration with Lehigh University on the study of Al-Li alloys, important materials for the aerospace industry. Coupled with the high resolution of our instrument, the high sensitivity for Li, some 105 times that of the electron microscope, has made it possible to correlate microstructure with the microchemistry of grain boundary segregation and intermetallic precipitates. The oxidation properties of these materials has also been explored in detail.
In a collaborative program sponsored by BP America, the study of advanced ceramic materials and both metal matrix and ceramic matrix composites has proven rich in problems that could be elucidated by use of the UC SIM. Particularly successful is the identification of mineral phases and compositional segregations that lie at the submicrometer interface between matrix and reinforcing ceramic fibers. These discoveries play a key role a key in understanding the mechanical properties of these materials.
Modern photoemulsion microcrystals are engineered as sophisticated miniature solid state devices. In collaboration with ILFORD Ltd. of England and ILFORD AG of Switzerland, research continues on the imaging chemical microanalysis of layered AgBr-halide photoemulsion crystals in a variety of habits. The distribution of sensitizer and dye additions has been determined over the surface of submicrometer-sized grains. Also of interest are the interference fringe patterns, made out of developed Ag, throughout the depth of exposed holographic films, as well as the fibrillar structure of emulsion gelatin.
Biological applications continue to be the focus of much of the research activities performed at the microprobe facility. Detailed physiological studies of mineralized tissue such as bone are part of a long standing collaboration with the University of Rochester. Dental tissue and titanium implants are studied in collaboration with Chalmers University, and the University of Göteborg, Sweden. It is now possible to understand the changing composition and morphology of normal, pathological and stressed hard tissues, at the microscopic level.
Among soft biological materials, SIMS imaging of chromosomes provides exciting opportunities for advances in cytogenetics. Labelled molecules such as bromodeoxiuridine (BrDU, a thymine analog) and 14C-thymidine are used to map the distribution of specific DNA bases in metaphase human chromosomes, in collaboration with scientists at the University of Chicago. BrDU is also used to label polytene chromosomes of the fruit fly, in collaboration with the Vollum Institute of Portland, Oregon. The chromosomes are imaged with the ion microprobe by selecting the tracer isotope signal, either 81Br- or the abundantly emitted 14C14N- ion. The resulting maps reveal banding patterns arising from varying concentrations of the labelled nucleoside, valuable for the understanding of DNA packing and a new tool for gene mapping. When the upgraded instrument is complete, further experiments with labelled nucleosides will vastly enrich our understanding of chromosome structure and function.
