Microscopy 101
Target Metal Selection Gold is perhaps the most widely for non-con-
used coating material
Figure 1: Secondary electron SEM images of various sputter target materials. All coatings were 2 nm thick deposited on glass and imaged at 10 keV. Image widths=140 nm.
Instrumentation. Te sputtered films for this article were
produced using a CCU-010 HV (turbo-pumped) Safematic Coating System on glass slides. Pure argon was used as a backfill “process gas.” In general, there are two types of sput- ter coaters. Te system above could be described as a “high- resolution” sputter coater because a turbo pump is employed to obtain a higher (and cleaner) vacuum environment, and pure argon gas is backfilled in the chamber to remove air and increase sputter efficiency. Te second type of sputter coater may be described as a more basic unit, developing only a mod- est vacuum with a mechanical pump and sometimes replac- ing argon backfill gas with room air. Tis basic sputter coater may be acceptable for coating Au and Au/Pd films, but not for coatings with finer grain sizes. Use of a system with a poorer vacuum and air backfill results in lower sputter efficiency and deposited films that are not as clean. Film thickness mea- surements were obtained using the quartz thickness monitor (operating at 6 MHz) inherent with the system. Coatings in Figure 1 were imaged with a Zeiss Merlin FE-SEM. Te images in Figure 2 were obtained using a Zeiss EVO 15 LS employing tungsten source.
ductive SEM samples, but it is not recommended as a sputter coating for research purposes where high-magni- fication images are required (see Au/ Pd below). Gold has a high secondary electron yield and sputters relatively rapidly, but the coating structure is composed of large islands (grains) that can be observed at high magnifi- cations in most modern research-level SEMs (Figure 1 and Table 1). Tus, it should only be used for imaging at low magnifications, say less than 5000×, where the coating structure will not interfere with the structural details of
the sample. An advantage shared by most other precious-metal coatings, Au coatings do not oxidize in laboratory air. X-ray emission lines of the Au M-series (2.12 keV) may interfere with X-rays from S and Nb, while the Au L-alpha line (9.71 keV) may interfere with X-rays from Ge. If the Au coating is appro- priately thin, however, there should not be significant problems with qualitative X-ray microanalysis. Gold/palladium sputtered alloys (60/40 and 80/20) have
smaller grain size and are the recommended metal coatings for general research purposes. Secondary electron yields are high, and sputter rates for Au/Pd are only slightly lower than for pure Au. Te Pd L-series X-ray lines at 2.84 keV do not overlap important lines from other elements; thus, no additional inter- ference with X-ray microanalysis would be expected beyond that mentioned above for Au. Platinum has a finer grain size than either Au or Au/Pd,
which makes it more suitable for higher-magnification applica- tions. A sputtered Pt coating exhibits a high SE yield, but Pt has a lower sputtering rate than Au (Table 1). Pt has been observed to crack. Tis effect could be “stress cracking” and could be attrib- uted to oxygen deposition in the sputtered coating, indicating
Figure 2: Secondary electron SEM images at 5 keV of ePTFE tape. (a) Uncoated tape shows significant charging with bright saturated area and image distortions. (b) Sputter coated with 5 nm of Au/Pd shows charging mitigated. Image widths=50 μm.
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www.microscopy-today.com • 2019 July
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