Re: Environmental SEMs: competitive resolution?

From: Gary G (see.signature_at_bottom)
Date: 10/20/04 

Date: Wed, 20 Oct 2004 15:01:33 -0700

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

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