I didn't respond to this because I just don't know what a PDMS structure
is or the material.  It sound like it must be a polymeric material from
the response that I saw below. Even if it is not, I would like to make a
few comments on high resolution SEM imaging.

Imaging should be done at relatively low accelerating voltages for best
results.  This gives you the surface detail because the signal is
originating close to the surface.  If a field emission inlens or
semi-inlens detector is available, then very good work can be done at
1kV accelerating voltage or less without coating the sample.  Even with
these types of SEM's, though, it is common to put a high resolution
sputtered coating on the sample.  It is generally accepted by the
electron microscopy community that an ion sputtered Cr coating gives the
best resolution because the grain size is very small.  You do have to
have a special coater to coat a sample with any topography with a
continuous, uniform film that is less than 1 nm thick.  My company makes
such a high resolution coater.  Because Cr oxidizes over a relatively
short period, people have gone to Ir and W coatings.  These coatings
also have fine grain structure and can be coated with less than 1 nm of
a continuous and uniform coating but do not oxidize as readily.
Sublimed Os coatings are an alternative fine grain size coating that has
been used successfully.  If the grain size of the coating is not fine
enough, then at magnifications in the microscope, you can see the grain
structure of the coating and it can interfere with the structure that
you are trying to determine.

The response that was given below is incorrect for a high resolution SEM
coating application.  First, 2 nm of gold is too thick.  Evaporated gold
also has very large crystalline grains and forms islands that are not
continuous on polymeric materials.

In the original posting, the painting is typically silver or carbon
paint that is applied around the edges of the area of interest and helps
make a good contact from the coated region to the microscope stage.  The
drying time was done to avoid the ougassing of solvents.  This painting
is not done over the area of interest.

If there are any questions, please feel free to respond to me.

-Scott

-----Original Message-----
From: Cristi Lepadatu
Subject: Re: [mems-talk] any special trick for taking SEM of PDMS

You could try to evaporate maximum 2 nm of gold over your structures in
rotational mode. Fixed mode (for lift off) may work as well if you don't
want to tilt your specimen. Less than2 nm of gold should be transparent
to electron beam and shouldn't form crstals.

Make sure you have a good contact between your sample and the SEM stage
in order to dissipate the charges.