SECONDARY-ION MASS-SPECTROMETRY FOR QUANTITATIVE SURFACE AND IN-DEPTHANALYSIS OF MATERIALS

Secondary ion mass spectrometry (SIMS) is a technique based on the spu ttering of material surfaces under primary ion bombardment. A fraction of the sputtered ions which largely originate from the top one or two atomic layers of the solid is extracted and passed into a mass spectr ometer where they are separated according to their mass-to-charge rati os and subsequently detected. Because the sputter-yields of the indivi dual species, coupled with their ionization probabilities, can be quit e high and the mass spectrometers can be built with high efficiencies, the SIMS technique can provide an extremely high degree of surface se nsitivity. Using a particular mode like static SIMS where a primary io n current is as low as 10(-11) amp, the erosion rate of the surface ca n be kept as low as 1 Angstrom per hour and one can obtain the chemica l information of the uppermost atomic layer of the target. The other m ode Like dynamic SIMS where the primary ion current is much higher can be employed for depth profiling of any chemical species within the ta rget matrix, providing a very sensitive tool (similar to 1 ppm down to ppb) for quantitative characterization of surfaces, thin films, super lattices, etc. The presence of molecular ions amongst the sputtered sp ecies makes this method particularly valuable in the study of molecula r surfaces and molecular adsorbates. The range of peak-intensities in a typical SIMS spectrum spans about seven to eight orders of magnitude , showing its enormously high dynamic range; an advantage in addition to high sensitivity and high depth-resolution. Furthermore, the high s ensitivity of SIMS to a very small amount of material implies that thi s technique is adaptable to microscopy, offering its imaging possibili ties. By using this possibility in static SIMS or dynamic SIMS mode of analysis, one can obtain a two-dimensional (2D) surface mapping or;a three-dimensional. (3D) reconstruction of the elemental distribution, respectively within the target matrix. Secondary ion yields for elemen ts can differ from matrix to matrix. These sensitivity variations pose serious limitations in quantifying SIMS data. Various methods like ca libration curve approach, implantation standard method, use of relativ e sensitivity factor, etc. are presently employed for making quantitat ive SIMS analysis. The formation of Secondary ions by ion bombardment of solids, is relatively a complex process and theoretical research in this direction continues in understanding this process in general. Th e present paper briefly reviews the perspective of this subject in the field of materials analysis.