Re: Dark rectangular area in SEM
From: Yung-Jen Lin (yjlin_at_ttu.edu.tw)
Date: 10/14/04
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Date: 13 Oct 2004 20:22:53 -0700
Gary G <see.signature@bottom> wrote in message news:<p13pm0ltdmr32hh0onoeuin70r41c2rf23@4ax.com>...
> 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
Thank you very much for all the responses. Special thanks to Dr. Gary
Gaugler. Your explanation is so clear and detailed. It will help a
lot.
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