Cleaning of AFM Samples
Several customers have developed and shown that carbon dioxide
snow cleaning is effective for AFM samples. The work includes
studies by William Morris at GE R&D, Digital
Instruments, and Ray Eby of ThermoMicroscopes. Since then, many other
scientists have used CO2 snow cleaning as a sample preparation
for AFM samples. Applications include cleaning standards and sample preparation.
We believe we can clean particles off of AFM tips with proper process attention.
In fact, we did it as a demo.
Bill Morris of GE R&D was the first to use carbon dioxide snow cleaning as
an aid in cleaning AFM samples. He found that it removed particles,
and also, reduced the "nanoscum" on the surface. Bill said that
he got better images and less tip-substrate artifact events.
Ray Eby found better imaging and cleaner samples on polymers and Si wafers. Data
interpretation is limited on these images since we were not
familiar with the sample histories.
Applied Surface Technologies also thanks Dr. Karin Jacobs for the elegant quartz work shown below
and the Park Scientific employee at a trade show
for the gold images.
There are 7 image pairs before and after carbon dioxide cleaning.
Of special interest are the image pairs from the quartz surface showing
removal of 40 nm sized particulates.
Sample List
AFM Height Standard
Gold Pattern - Image Improvement
Quartz Surface Evidence of 40 nm particle removal
Epitaxial and Bare Si Wafers AFM Images
Polymer AFM Images Ureathane and a PMMA copolymer
AFM Height Standard
This sample is a step of known height. Before cleaning, the sample looked as if it had
nodules and debris; after cleaning, it looked better defined. Others have cleaned AFM
standards and noted excellent cleaning. In one case, a user with a heavily contaminated
(useless) Pt/Si step height standard cleaned it with CO2 and noted somewhat increased RMS
value compared to the new standard values. We do not know if the RMS changes were related
to the cleaning or the extensive contamination and damage from handling.
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Gold Pattern Image
This sample was an evaporated gold on silica sample. The gold patterns
was defined as a series of dots regularly spaced.
Sample history is unknown and was a demo sample for Park
Instruments at a trade show. The initial image showed the gold dots but had about 4 or
more major artifacts from tip-surface phenomena along several smaller streaks
and some debris. After CO2 snow cleaning, only one small streak
was seen and the image has a clearer overall look. This image improvement is typical of
CO2 Snow Cleaning and is expected in most examples. The image was taken on a trade
show floor and are raw data, no image improvements were done.
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Quartz Surfaces
These images are courtesy of the research group of Prof. Dr. Stephan
Herminghaus of the Max Planck Institute
for Colloid and Surface Research. The researchers were studying polymer
film dewetting from silica surfaces and the first step in their process
was to cut and clean the quartz substrates. They ultrasonically cleaned
the quartz samples in acetone, ethanol, and toluene. Initial AFM
examination of the quartz films revealed extensive contamination all
over the samples. In this image, many particles ranging in size as small
as 40 nanometers are seen in the 2µ by 2µ image.
Cleaning with spectroscopic solvent grades led to these dirty
surfaces. Dr. Jacobs, of the above group, used the CO2 snow jet and was able to clean the
surfaces to acceptable levels as seen in the above image. We
observe that particles smaller than 0.04µ (40 nanometers) are removed.
This is direct evidence that snow cleaning can remove particles smaller
than 0.1µ, the previous limit quoted in the literature.
The importance of clean surfaces in research can not be neglected as
illustrated in the image set below from Dr. Jacob's thesis. The top
image was typical of some of the ultrasonically cleaned surfaces.
Wiping removed some particles, snow cleaning led to almost total cleaning.
After spin coating with polystyrene and annealing the films, extensive
"dewetting" or polymer film decohesion is seen for the "wiped"
surface while the CO2 snow cleaned surface shows no signs of film failure.
The results were used to question the standard spinodal dewetting
mechanisms for polymer film failure, and instead, a mechanism related to
surface defect densities was suggested. Further details are in Langmuir, Vol14,
pages 965-969, 1998.
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Silicon Wafers
This AFM sample was a small section from a bare silicon wafer that has a thin native
oxide, all other details are unknown.
The small features at the bottom of the image after cleaning are believed
to be growth features that were not observed in the initial scan.
They were probably there before cleaning.
As for all images, the exact same areas could not be used for before and
after cleaning - too hard to reproduce a location.
The next AFM image pair was a small section from an epitaxial silicon wafer.
Details on the wafer are unknown.
The RMS heights before and after cleaning are within experimental uncertainties.
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Polyurethane sample
This was a small section sliced from a polyurethane polymer sample.
This was a small section sliced from polycarbonate - PMMA co-polymer.
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