XEI Scientific, Inc.
The EVACTRON® Anti-Contaminator and De-Contaminator Updated August 2007
XPS EVALUATION OF SAMPLES SURFACE CLEANED BY
Abstract of paper presented at M&M 2001
With the advent of field emission gun equipped SEMs
and TEMs with clean vacuum systems, it has been shown repeatedly that
the surface cleanliness of samples exposed to the small, high current
density electron beam available in these instruments is critical with
respect to controlling hydrocarbon contamination under the electron
beam. In previous studies of dedicated plasma cleaners for TEM
samples by one of the authors (SDW), it was shown that surface
analytical techniques are the best methods for determining the
effectiveness of such cleaners in removing hydrocarbons from the
samples' surfaces.1, 2 In the current study, some of the experiments
that were used in these previous studies were repeated using the XEI
Evactron® 3 system in order to compare the relative effectiveness
of this system for surface cleaning of EM samples.
Figure 1 shows a picture of the sample introduction
chamber of a VG Scientific ESCALAB Mk II system (XPS) with the
Evactron® in place. The Evactron® system consists of an RF
head unit that attaches to the vacuum chamber with an adaptable
flange, an RF impedance matching unit, and an RF generator/vacuum
controller. Although a 100% O2 system can be used and is recommended,
for safety reasons, a 20% O2, 80% Ar by volume gas mixture was selected
because of the hydrocarbon oil used in the mechanical pump. Cleaning
was also performed using laboratory air. The major difference between
the dedicated plasma cleaners and the Evactron® configuration is
that the sample is not immersed in the plasma but is exposed to the
activated oxygen species in the downstream flow.
Samples of Si, Ti, and Ni were first solvent cleaned
(SC) and then intentionally contaminated by either a low temperature
melting wax (LT-Wax) in acetone solution or exposure to the exhaust
of a mechanical pump with a hydrocarbon oil (Oil), and then plasma
cleaned (PC) with the Evactron® system with increments of 20 min
of time. XPS analysis was done on the SC samples, intentionally
contaminated samples, and Evactron® cleaned samples. One
important difference in the present study compared to the earlier
studies was that the sample was processed in the sample introduction
chamber of the VG XPS system and thus was not exposed to ambient air
prior to analysis.
Table 1 gives the XPS concentration results of the
samples intentionally contaminated cleaned with the Evactron®
with air and Ar O2 for 20 min. The Ar O2 mixture typically reduced
the carbon concentration better than when air was used. Although the
Evactron® has significantly decreased the hydrocarbon level on
the surfaces, it has not completely eliminated all of it. In
comparison, the dedicated plasma cleaners were able to remove all of
the contaminates with a 20 min treatment.1 Increasing the time to 40
min and 60 min further decreases the oil, but again did not eliminate
it (not shown). Figure 2 shows the C 1s peak for the oil contaminated
Si sample after a 20 min PC treatment. The spectrum from the oil
contaminated sample shows a typical aliphatic carbon peak
characteristic of C-C and C-H bonds. After the PC treatment, an
additional carbonyl peak and carboxyl shoulder are present indicating
the oxidation of the hydrocarbon species. Figure 3 shows the Si 2p
spectra of a Si SC sample before and after a 20 min PC treatment. It
can be seen that there are little relative changes between the oxide
and the metal peaks indicating that the Si surface is not oxidizing.
In contrast, in the previous study involving dedicated plasma
cleaners, oxidation of the solvent cleaned Si surface was quite
apparent and could also be detected in the TEM.1
In conclusion, the Evactron® unit does decrease
the hydrocarbons present on sample surfaces. With properly prepared
and handled samples, it would most likely remove modest amounts of
contaminates even with the reduced O2 partial pressure used in this
study. It was unfortunate that 100% O2 could not be used because it
probably would have produced better results. It is also evident from
the variable geometry possible with an Evactron® that care should
be taken with the design parameters if it used as a sample cleaner,
such as distance and position of the sample relative to the RF head
and pump ports. The lack of oxidation of the Si surface suggests that
it is a more gentle cleaning solution for contamination problems than
the dedicated plasma cleaners.
References
1J T Grant, S D Walck, et al., MRS Proceedings, Vol.
480, pp. 49-71, 1997.
2 S P Roberts, N J Zaluzec, S D Walck, J T Grant, MRS
Proceedings, Vol. 480, pp. 127-136, 1997.
3 R A Vane, US Patent Number 6,105,589, Aug 22, 2000.
RF Plasma Cleaning Systems for Electron Microscopes
and High Vacuum Systems
Stops Artifacts and Removes Hydrocarbons and Organics.
THE XEI EVACTRON®
Scott D. Walck***, Brian R. Strohmeier***,
Edward G. Goralski***, and Ronald A. Vane
***PPG Industries, Inc., Pittsburgh, PA 15238
Long Beach, CA August 2001