Microscopy101
Keeping It Clean! Tom Levesque Technology Business Consulting, Lewisville, Texas 75077
tomlevesque424@gmail.com
Introduction Microscopists are looking at smaller features every day,
and the potential to see artifacts that degrade or obscure these small features has never been greater. Because many current microscopy techniques are conducted in vacuum environments, residual gaseous components in the vacuum chamber may have substantial effects on the data collected. Tis brief review will describe some approaches to keep the quality of data collected in electron and ion-beam microscopes at the highest possible levels. In short, we will attempt to “keep it clean.”
Carbon Contamination Te problem of hydrocarbon contamination inside the
electron microscope is well documented and has been an issue from the earliest days of electron microcscopy. Tis artifact is the result of the electron (or ion) beam striking unwanted contaminant molecules and promoting the growth of carbonaceous materials on the surface of the sample. Because fewer low-energy, secondary electrons reach the detector from the sample surface, the contamination region oſten appears as a darkened area in the secondary electron image. Typically, in a beam-scanning instrument, this contamination layer is in the shape of the rastered pattern on the sample—a rectangle. Te effect is more pronounced at high magnifications and lower accelerating voltages—just the conditions under which smaller surface features are oſten analyzed. Further, if the contaminated area is measured in an atomic force microscope (AFM), the build-up of this material has been shown to be quite pronounced [1]. Of course, the first method of reducing contamination is to use lint-free or powder-free gloves when handling specimens and any microscope part inside the vacuum. Cleaning the specimen before placing it in the scanning electron microscope (SEM) helps, but there is always a small amount of hydrocarbon in the system coming from the scope itself (machining processes in chamber manufacture, lubricants in the stage, O-rings, etc.) that are difficult to eliminate. Hence, there is the requirement for chamber cleaning.
Methods for Removing Contamination Conventional plasma cleaners.
Te original work by Zaluzec [2] resulted in a patent and commercial products that use plasma to clean
40 (a)
samples and parts destined for use in electron microscopes. Tis has allowed scientists to remove substantial contamination artifacts from their images. Follow-on work, such as that of Isabell and Fischione [3], continued to demonstrate the value of plasma cleaning to remove problematic hydrocarbons and even to clean contaminated samples if they have already suffered beam-induced carbonaceous polymerization on their surfaces. Most of these units use argon or argon/oxygen mixtures. In these systems, the energetic ions interact with the surfaces to eliminate the unwanted artifacts. Plasma cleaners are now common in the electron
microscope (EM) suite with users in many disciplines cleaning both samples and sample holders prior to microscopy. Tese ex-situ plasma cleaners are available from several manufacturers at various levels of sophistication and price. However, these instruments do not address the internal surfaces of the microscope, which may harbor miniscule amounts of mobile hydrocarbon contaminants that cannot be completely removed. Disassembly and manual cleaning is not very practical, and, needless to say, highly labor intensive (read: expensive). Other methods. In reducing contaminants inside the
microscope, early work centered on the use of cryo surfaces or cold traps to capture the hydrocarbons and sequester them in areas where they would do no immediate harm. Still used in many tools, this method does provide some relief from the visible build up of contamination (see Figure 1), but the cold surfaces eventually become saturated, and the adsorbed contaminants must be removed, usually by warming them up and removing the materials with other cleaning methods. Clearly, it would be best to remove the unwanted materials completely. Some success was shown using prolonged purging with dry nitrogen. However, this was a slow and inefficient process
(b)
(c)
Figure 1: Carbon contamination on a sample in the SEM. (a) The before image shows the build-up after scanning for 10 minutes at 5 kV and 10 pA with no cryo trap. (b) Contamination is less but still noticeable when the cryo trap was used during the same 10-minute exposure. (c) After downstream plasma cleaning technique alone. This result was obtained after cleaning both the chamber and the sample after the same exposure and contamination as (a).
doi:10.1017/S1551929511001349
www.microscopy-today.com • 2011 November
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