Microscopy101
Figure 4: A CD test pattern began to show filling-in of the holes after a 20-minute scan (left). After Evactron in situ cleaning of the chamber and the specimen, a repeat of the measurement (right) showed no filling of the holes and a much-reduced scan mark.
precision in the measurement. Using a very clean Hitachi 6280 (Figure 4 leſt), the test pattern began to show filling-in of the holes aſter a 20-minute scan. Aſter Evactron in situ cleaning of the chamber and the specimen, a repeat of the measurement showed no filling of the holes and a much-reduced scan mark (Figure 4 right). Other applications. It is also well established that cleaner
vacuum systems can assist in removing spurious analytical artifacts. Oſten, carbon analysis can contain contributions that do not originate from the sample but can be due to contamination. Work by Strein and Allred [6] showed that the antechamber of an X-ray photoelectron spectroscopy system was introducing a thin layer of carbon onto samples, making carbon analysis unreliable. Downstream plasma treatment in the antechamber removed the contamination. Horiuchi et al. [7] have also shown that analytical
transmission electron microscopy (TEM) results on polymer brush samples could be accomplished with a system cleaned using downstream plasma. Electron energy loss spectrometry (EELS) in imaging mode could be used for high-resolution carbon mapping. Having pristine surfaces is an absolute requirement for
nanomanipulation and nanofabrication. Recent work by Mancevski has shown that downstream plasma cleaning was essential for successful vapor phase cutting of carbon nanotubes using a nano-manipulator system [8]. Also, electrical measurements, made by positioning minute probes on circuits, using nanopositioning systems located inside SEMs and FIBs, require that the probes be free of con- tamination in order to make good contacts. In situ cleaning of these devices is a requirement for accurate measurements. Plasma radical sources are rapidly evolving to solve new problems and are now available in a number of configurations
for most makes and models of electron and ion microscopes. Standard PRS units commonly allow for air, pure oxygen, and oxygen/argon mixtures. Tese units have KF 40 flanges adapted for most SEMs and Dual Beam FIB/SEMs. Further, because Evactron units are portable, these systems may be moved around the laboratory, cleaning a number of different electron microscopes. An ultra-high vacuum (UHV) Conflat flange version is available for surface analysis tools or other HV chambers and provides for the use of hydrogen gas. Versions of the PRS specific for TEMs, where a patented hollow cathode is inserted through the sample insertion port, are becoming available on most major TEM models. Tis unique design for the TEM delivers the cleaning capability directly to the hard-to-reach area where beam and sample interact in the TEM. Two of the most recent configurations are summarized in Figure 5. Tese show a TEM Wand and a PRS mounted in a benchtop unit. Te downstream plasma technique has proven extremely
useful and is now well accepted. Tere are over 1,100 installations of the XEI tool on nearly all makes and models of SEM and Dual Beam FIB/SEMs. In fact, today most new high-resolution tools come equipped with some form of downstream plasma cleaning upon delivery from the factory. Further, service personnel oſten carry a portable version of the Evactron system when they make service and preventative visits in the field in order to maximize SEM performance.
Conclusions Using reactive gas plasma systems has proven to be one of
the most effective methods to remove contamination artifacts that hamper imaging and analysis in electron microscopes. Te latest systems combine downstream cleaning in microscopes with benchtop sample cleaning, bringing together the attributes of clean samples and clean microscope environments in a single product. Now it is easier than ever for electron microscopists to “keep it clean.”
References [1] M Amman et al., J Vac Sci Technol B 14(1) (1996) 54–62. [2] Simultaneous Specimen and Stage Cleaning Device for Analytical Electron Microscopy, US Patent # 5,510,624, Argonne National Laboratory and the University of Chicago, 1996.
[3] TC Isabell and PE Fischione, Micros Microanal 5 (1999) 126–35.
[4] A Vladar, C Archie, and B Ming, Meas Sci Technol 22 (2011) 024004, doi: 10.1088/0957-0233/22/2/024004.
[5] A Vladar, NIST, personal communication. [6] L Strein and D Allred, “Use of commercial RF Plasma Cleaner in eliminating adventitious carbon contamination in an XPE system,” Microscopy and Microanalysis poster, 2008.
[7] S Horiuchi et al., ACS Nano 3(5) (2009) 1297–1304.
Figure 5: Examples of two different types of plasma radical source (PRS) configurations: the TEM Wand (left) and the SoftClean benchtop system (right). Images courtesy of XEI Scientific, Inc.
2011 November •
www.microscopy-today.com
[8] V Mancevski and PD Rack, Materials Science &Technology 2010 Conference and Exhibition, Oct. 17–21, 2010, Houston, TX.
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