NanoFab SIMS


Accelerating voltage effects. In the above


discussion, an acceleration voltage of 25 kV was implied, but it should be pointed out that this parameter


influences performance as follows:


the primary beam-size increases with decreasing acceleration voltage, while the width of the second- ary-ion escape region decreases with acceleration voltage, so optimization requires a compromise. Operators have a range of 10 to 30 kV acceleration voltage available to optimize their experimental results for the material under analysis, in terms of effective spot size. Te sputter rate typically does not vary dramatically for Ne+


irradiation over this


voltage range, so effects on analysis time are not severe. Analysis time. Te analysis time depends


mainly on the beam current used, typically 1–10 pA, and on data-acquisition settings such as pixel dwell- time, image size (total number of pixels in a map), and SIMS acquisition parameters. Te latter include mass-range analyzed, dwell-time per analysis, and the number of scan-steps to acquire the mass spec- trum. Typical acquisition times for a mass spectrum are 10 to 20 minutes. Typical acquisition times for chemical maps, using four ion detectors in paral- lel-detection mode, range between 10 minutes and 1 hour. For depth-profiling or 3-dimensional map- ping, the analysis time depends strongly on the total volume of sample material removed and may range from several minutes to several hours, depending on the application.


Results SIMS image resolution. Figure 4 shows


results obtained from the well-established BAM- L200 certified reference sample [13]. Te NanoFab SIMS was used to map aluminum and gallium in the alternating GaAs and Al0.7


Ga0.3 As layers of


the test sample. Te finest details are in the form of line pairs of detected Al signal with a specified pitch (for example, P12 with 31 nm pitch) and soli- tary lines (for example, W11 with a narrow width). Figures 4b and 4c show an intensity line-profile taken through the aluminum map of the sample for certain line pairs (P9 through P14), with pitches from 76.5 nm to 17.5 nm, and these line-pairs were all clearly resolved. Te profiles are suggestive of a single edge-resolution performance of about half the resolved pitch, say 10 nm, which is close to the beam-sample interaction limit. Note also that the system was sufficiently sensitive to image the soli- tary line W11 with a width of 3.5 nm. Historical Metallurgy. Te specimen examined was an


Figure 4: Spatial resolution of analysis. (a) SIMS chemical map generated using the 27 69Ga+


peaks from a BAM L-200 standard sample. (b) 27 Al+


Al+


and image showing analysis region across


lines P9 to W11. (c) Image intensity profile taken from the sample region highlighted in (b). Line pairs P9 to P14 with pitches ranging from 76.5 nm to 17.5 nm, respectively, are resolved. Also detected was the narrow single line W11 (3.5 nm wide). A 10 keV Ne+ generate these data.


primary beam was used to


aluminum-copper alloy (with small amounts of iron and other elements) that was modeled aſter the alloy used in the 1903 Wright Flyer crankcase. Te latter alloy, produced and applied before the formal discovery of precipitation hardening, can be considered the first aerospace aluminum alloy [14]. SE images, secondary ion mass spectra, and SIMS elemental maps were


26


acquired with the NanoFab SIMS. Figure 5 shows a global mass spectrum from a flat-polished section of the alloy, with adja- cent elements clearly separated (note the separation of 39 40Ca+


K+ of 63Cu+ and


). Figure 6 combines a SE image (in grayscale) with maps (blue) and 56Fe+


(red). Tis composite image highlights


several features of the microstructure. Most of the sample con- sists of α-aluminum dendrites (grains), shown as the gray back- ground provided by the SE signal. Note that there appears to


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