Re: Dark rectangular area in SEM
From: Gary G (see.signature_at_bottom)
Date: 10/13/04
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Date: Tue, 12 Oct 2004 19:14:07 -0700
On 12 Oct 2004 08:28:58 -0700, yjlin@ttu.edu.tw (Yung-Jen Lin) wrote:
>Hello, everyone,
>when I wanted to take SEM pictures of a Au-coated ceramic surface, I
>often encounter one problem. I used a higher magnification image to
>get a better focus. When I turned down to the desired (lower)
>magnification, I usually found a rectangular dark area, which was the
>scanning area of higher magnification. People suggested me to take
>pictures of the nearby area.
>My question is: what causes such a darken area after scanning of
>e-beam? Some said it was caused by the contamination of the sample
>surface. Others said it was caused by the defects of ceramics due to
>high energy electron bombardment. Which one is correct? Or, what is
>the real reason for it?
>Thanks in advance.
This is organic contamination of the specimen. Very common. Quite
often frustrating.
Your sputter coater probably uses an oil mechanical pump. This pump
will backstream into the chamber and put a thin coating of pump oil on
the specimen. This hydrocarbon material polymerizes under a strong
electron beam. The other problem is that even without coating,
specimens will collect atmospheric organics over time and exhibit the
same dark area problem. The upper left of the dark area will usually
be bright since this is where the beam rests and starts from.
Depending on your SEM, it too will have an oil roughing pump. This
can put more oil into the chamber and onto the specimen. An oil
diffusion pump is a really bad actor in this regard. The better
systems use an LN2 freeze trap to keep the oil out of the chamber.
But LN2 is really a hassle to deal with in a small scale lab.
If you are going to take high mag images, the rule of tumb is to take
low mag images first and then do high mag. Do not go from high mag to
low mag. The other option is to clean the specimen in the coater if
it has an etch mode. A short etch clean will make a big difference.
Coat slightly thicker than normal and then etch. This will pull out
the organics.
The other option is to adjust KV and probe current (condenser and/or
final aperture size) to reduce specimen current. You will lose S/N
but may be able to make up for it by doing a longer integrated or
averaged line scan image capture...depending on what capture options
your scope has.
Finally, keep all specimens in a vacuum container until ready for use.
I use a Ladd stainless steel tub and pump it out to about 30mT-40mT
with all specimens inside. When needed, purge the vacuum and put the
specimen in the SEM. Also, keeping the new specimen under SEM vacuum
for a period of time will pull out the hydrocarbons and reduce the
dark area.
If you do an EDS line spectra of the dark area it will show C big
time. The H-C gives way to C which is all that can be seen with the
best low Z EDS. H is not to be seen. No problem. C is the problem.
Gary Gaugler, Ph.D.
Microtechnics, Inc.
Granite Bay, CA 95746
916.791.8191
gary@microtechnics dot com
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