Argonne National Laboratory Plasma Cleaning Technology
US Patent No 5,510,624 issued April 23, 1996
Technique for Simultaneous Specimen & Stage Cleaning
for Analytical Electron Microscopy
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To download a copy of the Patent in Adobe PDF format click here.
Introduction
A new technique has been developed which allows simultaneous
cleaning of a sample and a specimen stage or the internal components
of a microscope (polepieces, apertures etc..) which minimizes and, in
some cases eliminates, contamination of inorganic specimens analyzed
via SEM, TEM, STEM and/or AEM. The technology involves subjecting
the specimen and the specimen stage to a reactive plasma gas (either DC or RF excited) which
efficiently removes a wide range of contaminants from critical
surfaces. The procedure may be carried out prior to inserting the
specimen and specimen stage into the EM by either mounting the
specimen holder in the supplied put or by inserting the entire sample
or sample assembly into a plasma reaction chamber. The system can be used with any side
entry TEM holders, top entry TEM holders or with any sample that will
fit inside the large, easily accessible chamber.
Background
Microcharacterization of materials by electron microscopy is
ultimately limited by the ability to observe, detect and analyze the
constituent materials present in micro volumes of specimen in the environment
of the electron microscope, the
interaction of surface borne contaminants with the high energy
incident electron probe can create deposits on the surfaces of a sample
which can prohibit these analyses These contaminants may be
introduced directly by the specimen, deposited on the specimen via
the specimen stage or deposited on the specimen via the microscope
system. Electron microscope manufacturers attempt to minimize the
last of these by judicious design. However, the first two sources of
contaminant introduction are out of their direct control.
Surface borne contamination is introduced in the preparation of
inorganic specimens (metals, ceramics, semiconductors etc.) for EM
work. The materials to be studied are frequently subjected to chemical
or electrochemical polishing followed by solvent rinsing and air drying.
This typically leaves residual organic material on the specimen
surface Attempts to clean the specimen stage include rinsing the
stage with various solvents. Improper or poor techniques used to store
both specimens and/or stages prior to insertion in the microscope can
introduce additional organic residue. Finally, contamination
derived within in the instrument can mitigate analysis. While the
majority of the organics involved m these processes dissipate, a small
amount generally remains on the surfaces and is sufficient to cause
problems when the specimen is subsequently examined in modern
analytical microscopes. Although these organic residues are widely
distributed and generally at low concentrations on the various
surfaces, they can become mobile in the microscope environment
and are attracted to the periphery of any focused electron probe,
forming deposits (figure 1). Since these contaminants can travel large distances
over the surface of the specimen, it is important to remove or
immobilize them as much as prior an analysis without disturbing the
microstructure of the specimen. It is important to note that once the
deposits are formed, they are not easily removed. Analysis of a
"corrupted" area is precluded
![](example1.gif)
Figure 1. Contamination formed on a specimen due to focussed electron probe
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Procedure
It has been well documented that low temperature plasmas
can be used to reactively etch ash organic materials found on the
surface of inorganic materials. This has been used by the Industrial
community to clean semi-conductor wafers and other bulk materials
for many years. In this procedure, a related technique is
employed for the cleaning of EM specimens. Instead of subjecting only
the material of interest (i e. specimen) to the plasma, the entire stage
and sample is plasma cleaned (figure 2) The nature of the gas selected is based
upon the desired effect Although an argon - oxygen mix is generally
used, specific gases ( BCl3, CF4, .. .) may be used to tailor a reaction.
Cleaning time is typically 10-30 minutes. Specimens that previously
contaminated m minutes can after treatment, be studied for several
hours (Fig. 3 & 4).
![](cleaner.gif)
Figure 2. Experimental arrangement to clean specimens
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![](example2.gif)
Figure 3. Illustration of contamination and it's elimination
using the ANL Technology. Large dark marks caused by focussed electron
probe on a virgin sample, followed by 5 Minutes of treatment using
an Argon plasma, followed by 5 Mintues of treatment with an Oxygen plasma
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![](example3.gif)
Figure 4. Quantitative Measurements in the reduction of contamination
as a function of plasma processing time and gas composition.
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To obtain additional detailed information about the
process you can download a copy of the ANL Patent, an article from Microscopy & Microanalysis-97 Meeting or
a poster from Materials Research Society Spring-97 Meeting, or the following Book Chapter PlasmaCleaningPaperFinal.pdf. All these documents are stored in
Adobe Acrobat Format on the ANL WWW site. You may download a FREE Adobe Reader/Viewer to
be able to read these documents on Mac, Windows, or Unix Platforms.
The following Commerical Organizations have Licensed this Technology from ANL and produce
a product which implements this procedure in TEM's or SEM's
Additional information about licensing of the ANL Technology can be
obtained by contacting the ANL Technology Transfer Office
or calling the Technology Transfer Office at 630-252-9771.
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