Microscopy101
that required overnight, or even several-day, periods of flowing gases.
Downstream Plasma Cleaning In 1999, XEI Scientific patented and introduced a radio
frequency (RF) plasma product that used the technique of secondary or downstream plasma cleaning to address the problem of cleaning internal surfaces of vacuum chambers in electron microscopes. Te current XEI Scientific product is called the Evactron® De-Contaminator, and a schematic of its downstream plasma process is shown in Figure 2. How it works. Tis type of system produces the active plasma
in a remote chamber (called a Plasma Radical Source or PRS) and transfers the active species to the cleaning chamber via gas flow, relying primarily on the chemical activity of the reactive radicals produced by the plasma for the cleaning action (Fig- ure 2). Experiments with different gases to create the plasma have shown room air to be an excellent source of oxygen to create reactive radicals and efficiently crack hydrocarbon molecules. It has the benefits of being available, free, and safe. Also, via the choice of other non-corrosive gases for producing radicals, different chemical etch processes may be selected and benign regimes for sensitive components may be obtained as well as optimized chemistries for the fast removal of unwanted contaminants. While the energetic ions are contained in the external PRS,
reactive gas radicals are allowed to driſt through the vacuum chamber and come into contact with the sample and internal surfaces. Photons in the plasma are in the Vacuum UV (VUV) wavelengths, and VUV energy is very effective in breaking most organic bonds, that is, C-H, C-C, C=C, C-O, and C-N. Tus, high molecular weight contaminants are broken into smaller components. A second cleaning action is carried out
by the various oxygen species created in the plasma (O2+, O2−, O3, O, O+, O−, ionized ozone, meta-stably-excited oxygen, and free electrons), which combine with organic contaminants to form H2O, CO, CO2, and low molecular weight hydrocarbons. Exhibiting relatively high vapor pressure, these compounds are easily pumped out of the microscope by the vacuum system. Cleaning cycle. Cleaning is done at higher pressures
than those that typically exist when the microscope is in operation. However, the process is quite fast and oſten can be accomplished immediately aſter a vent or sample exchange cycle. Also, once a system is initially cleaned, maintenance can usually be accomplished with a weekly cleaning of 10 minutes or less. Tis obviously depends on the type and cleanliness of samples being inserted into the scope. Figure 3 shows an example of improvement in the image of a gold on carbon SEM resolution sample. Te image on the leſt was taken before cleaning with the Evactron process, and the image on the right is the result aſter plasma cleaning. Use in metrology. Workers at NIST are strong believers in
removing all contamination from both samples and chambers. Te work of the NIST nanoscale metrology group needs highly accurate scanning electron and helium ion microscopy down to sub-1 nm resolution [4], and this demands contamination-free operation. Repeatable results require a high degree of cleanliness so that over the few minutes of measurement, the sample does not change noticeably. Tis also includes the need for consistent secondary electron emission (yield). Te group is one of the leading proponents of plasma-based SEM cleaning. Prior to using XEI’s Evactron, NIST had tried nearly every known cleaning method, including a liquid nitrogen trap, clean nitrogen gas bleeding, cryo, special pump oil, oil free methods, and so on. No method worked to the standards required at NIST with respect to its SEM research. Te Evactron downstream plasma cleaning method allows the NIST contamination specifications to be met. By implementation and regular use of these methods, it is possible to get rid of electron beam induced contamination. (Author’s note: NIST does not endorse any specific products or brands that it may use.) CD measurements. Comparison imaging to examine
the effects of contamination on critical dimension (CD) measurements has shown that these image artifacts can affect dimensional measurements [5]. In CD work, modification of dimensions by the SEM imaging process causes a loss of
Figure 2: Schematic representation of process.
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the downstream plasma cleaning
Figure 3: Removal of contamination from a “gold on carbon” resolution test sample. The image on the left was taken on a “well used” standard that had be exposed for a prolonged period in a modern and supposedly clean FE SEM. It was removed and cleaned for several minutes in a chamber equipped with a downstream plasma source and reinserted into the microscope for examination (seen in the photo on the right). No damage occurs to the carbon substrate, and the resolution on the sample is clearly improved.
www.microscopy-today.com • 2011 November
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