Microscopy 101
Table 2: Sputter target metals for various applications (indicated by x). Recommendations assume that a 1–2 nm thick film was properly deposited using appropriate equipment.
Sputter Material Tabletop SEM Au
Au/Pd Pt Ir
Cr W Ca
aCarbon deposited to 10 nm thickness by vacuum evaporation or ion-beam sputtering
coater manufacturer may have guidelines for specific targets (that is, sputter time, current setting, vacuum), which may be used a for a “ball park” thickness determination. Also visual evaluation of the film color and opacity can be useful in esti- mating the thickness of the sputtered film. If X-ray microanalysis of the sample is required, select a
coating (target) material that is not present in the sample. Tis should avoid interfering peaks in the X-ray spectrum of the sample. Also consider all the X-ray lines possible from your sample (that is, K-series, L-series, M-series) and from the sputtered film. It must be kept in mind not only what X-ray lines might be present, but also what lines could be enhanced at the electron beam accelerating voltage (kV) to be used in your study. If all possible interferences must be avoided, then classic carbon deposition (evaporation) is the recommended approach to render a non-conductive sample amenable for X-ray microanalysis. The rule of thumb for selecting a sputter target for coat-
ing an SEM specimen is to choose the metal that produces the smallest grain size consistent with capabilities of the SEM available. Thus, Au may be acceptable for a table-top SEM for magnifications below 5000×; Au/Pd and Pt would be useful for general purpose SEM imaging; and Cr or W would be appropriate for high-resolution, high-magnifica- tion imaging with an FE-SEM (Table 2). Once the target metal has been selected, the effort should be to produce the thinnest metal film that mitigates charging effects, ideally in the range of 1–2 nm.
Conclusion Te ease with which sputter coater target metals can be
changed allows for flexibility in preparing SEM specimens for imaging and microanalysis. Targets are available for low- magnification and high-magnification work, and they may be changed to facilitate elemental analysis. Use the metal coating that produces the smallest grain structure consistent with the capabilities of the SEM available. Different sputter targets vary in cost, and some require additional infrastructure such as the increased pumping/vacuum capabilities of a high-resolution sputter coater.
Acknowledgments Te authors thank Jack Vermeulen of Micro to Nano, Haarlem, Netherlands, for helpful discussions.
36
References [1] JI Goldstein et al., Scanning Electron Microscopy and X-ray Microanalysis, Kluwer/Plenum/Springer, New York, 2003.
[2] DE Newbury et al., Advanced Scanning Electron Micros- copy and X-Ray Microanalysis, Plenum/Springer, New York, 1986.
[3] P Echlin, Handbook of Sample Preparation for Scanning Electron Microscopy and X-ray Microanalysis, Springer, New York, 2009.
[4] Y Lin and DC Joy, Surf Interface Anal 37 (2005) 895–900.
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