Hi,

I second your comments about Au coating for SEM. I do a lot of
cross-sectional SEM for features in submicron scales. I have done
systematic test on Au coating thickiness for good image, as well as
ordering sample from sputter coater company with Pt and Cr coatings. I
found that the Au coating thickness should be of the order of your
recommendation. Also, both Au and Pt coatings give good image under
identical conditions but little blurred with Cr coating.

About the other comment about accelerating voltage, my experience tells me
that it really depends on what sample you are looking at. Low accelerating
voltage has a chance of being deflected by neighboring charges and causes
"diffused"  image. For hard surfaces like metals, it's better to use
higher accelerating voltage like 10-15 kV. For soft surfaces like organic
materials, lower voltage like 5 kV is better because high voltage causes
electrons to penetrate deeper into the material. This creates a large
interaction volume, leading to blurred image as well. You also want small
aperture, smaller beam current (but not too small depending on background
noise), and short working distance. The chamber pressure is also an
important factor for crystal clear image. With 10^5-10^6 Torr, it's
difficult to get anything far below 100nm. If you want 5-10nm resolution,
you should avoid the conductive coating technique and go for SEM systems
that can inject low-energy positive ions to compensate for the negative
charging effect. This way you don't have to worry about the conductive
coating thickness to affect your resolution. Getting an impressie
high-resolution SEM image is a very delicate art work...


Yours sincerely,

Isaac Chan, Ph.D.
University of Waterloo


On Wed, 17 Aug 2005, Cristi Lepadatu wrote:

> Hello,
>
> I have seen sputtered Cr on an ordinary sputtering machine, and it
> looks rough, very rough. What roughness are you getting Scott? How did
> you measure it? Maybe there are ways to obtain wery smooth thin Cr
> films. They may require special sputtering or evaporating machines,
> but I think there are alternatives.
>
> Evaporation of thin gold indeed forms crystalline grains. There should
> be no problem with it if your layer is thin enough since it will be
> totally transparent to electron beam. I have already found this in a
> paper where SiO2 was coated with gold and believe me, it looks nice. I
> can send you the reference.
>
> Jason you may also check if your PDMS sample is well connected to the
> ground. This helped me alot while doing images on SiO2 without any
> coating.
>
> Jason, if you want you can send me a sample. I will try to find some
> PDMS around, and see how this takes place in a SEM FEG.
>
> Scott I work with people who are expericenced with SEM and have
> already used gold for coating insulating substrates. They are part of
> what you call "electron microscopy community".
>
> Best wishes,
> Cristian
>
> On 8/17/05, Scott Walck  wrote:
> > 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.
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