Re: Environmental SEMs: competitive resolution?

From: Fred Chen (flipsu5_at_comcast.net)
Date: 10/22/04 

Date: 22 Oct 2004 00:13:17 -0700

Gary G <see.signature@bottom> wrote in message news:<qandn0d6q88kf6pbegq8pmurma8hrr7pug@4ax.com>...
> On 20 Oct 2004 11:58:50 -0700, flipsu5@comcast.net (Fred Chen) wrote:
>
> >Gary G <see.signature@bottom> wrote in message news:<cmtcn01rncfd7lf5gt4v020ioo51rntk6s@4ax.com>...
> >> On 3 Oct 2004 17:55:32 -0700, flipsu5@comcast.net (Fred Chen) wrote:
> >>
> >> >Do environmental (low vacuum) SEMs offer resolution that is
> >> >competitive with conventional SEMs? They seem ideal for imaging
> >> >insulating samples which cannot be coated with conducting material.
> >> >
> >> >Fred
> >>
> >> True ESEMs use W filaments and mechanical pumps. These SEMs also
> >> include a cold stage. If mag is reasonably low, images are good.
> >> Resolution is probably between 300-500A. My Zeiss Supra 55VP is a
> >> Schottky FE variable pressure SEM and does 17A rez at high vacuum, 1KV
> >> and 20A VP at 30KV. Getting good rez at VP is not easy. I doubt I am
> >> getting 20A. More like 100A. But it is very dependent on WD and KV.
> >> Since the detector collects photons rather than electrons, the trick
> >> is to generate as many photons as possible for good S/N but also keep
> >> rez good.
> >
> >Photon generation? Is this for spectroscopy? How would secondary
> >electron imaging perform?
> >
> >Is the measurement repeatable? That is most key for me..
> >
> >Thanks,
> >Fred
>
> The SEM has standard E-T SE detector, in-lens detector and VPSE
> detector. The VP detector is sort of like half of a E-T detector.
> The E-T detector has a Faraday shield and covers a scintillator disc.
> Next to the disc is a light pipe to carry the photons to a photo
> multiplier tube, then electrical signal to an amplifier and on to the
> SEM for display. The VP detector just has a light pipe sticking into
> the chamber and then the rest of the items like in an E-T detector.
>
> The way this VP system works is that it collects photons (light) from
> SE electrons hitting gas molecules. Earlier systems (perhaps some
> still today) used electron induced current to form an image, but it
> really was not an SE image. This is an SE image. however, in a high
> vacuum system, there is no gas. Hence, the electron beam hitting the
> specimen creates bounced SEs. These are captured by the E-T detector.
> In high pressure systems, there are no (very few) normal SEs generated
> since the chamber is full (so to speak) of gas. The traditional E-T
> SE detector does not work. Hence, the VPSE method. This allows
> imaging and analysis (EBSD in my case, mostly) of uncoated specimens.
> At 20KV-30KV there is no charging. I typically run at 50Pa.
>
> It is very repeatable. The VPSE does not affect SEM measurement
> ability--only as long as there is enough resolution in the image to
> make a good measurement. I'm running between 50KX-120KX and do at
> times have difficulty getting really stunning images. In these cases,
> the image is not the product, so it does not matter. By adjusting WD
> to around 8mm, it seems optimum for E-T specimen positioning, in-lens
> high rez images at 1KV and then VP EBSD at 30KV.
>
> So much depends on your specimen and how much mag you need and then,
> corresponding resolution.
>
> Gary Gaugler, Ph.D.
> Microtechnics, Inc.
> Granite Bay, CA 95746
> 916.791.8191
> gary@microtechnics dot com

Thanks for the tips, Gary. The next step for me would be to suggest
this approach to the responsible group. Hopefully we can arrange a
demo (with the glass samples we would use) on a system nearby.

Thanks again,
Fred

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