How an SEM works

[T3D <JValaVIDI@xxxxxxx>]

   Someone who works with these has to back me up here.  But.... back when I
was working with SEM's the principle of operation was very simple, and not at
all related to the way an optical camera captures an image.

  Imagine an electron beam several electrons in diameter (very very tiny)
shooting down a steady stream of electrons toward the sample the same way an
electron gun in a CRT scans the screen of the monitor your looking at right
now.  Im an SEM the beam travels about 2 or 3 feet instead of 6 to 12
inches,the beam sweeps an area of "your sample size" which is very very tiny,
unlike your CRT which scans an area "your screen size"!  So, you see the
angles involved in an SEM are so close to zero (source real far away scanning
a very small area) we can call them collimated.

  Next, we CANNOT think of this electron beam as a light source where the
"light" is reflected off of the sample, collected by a lens and focused on a
detector.  The beam is swept very slowly over the sample, the electrons
traveling toward the sample are either absorbed by the sample or are
"reflected" off in some direction.  The electrons which are absorbed travel
through the sample, to the stage, on to an amplifier which strokes the
electron gun of a CRT which is sweeping the face of a CRT at exactly the same
(very) slow rate that the electron beam is sweeping the sample.  And after a
while the two beams complete thier X/Y sweep on the sample and CRT and......
 Bob's you Uncle!  You have a complete image of the sample.  Magnification is
accomplished very simply by restricting the X/Y sweep of the sample beam, the
less area you sweep the higher the magnififation.  Electrons which are
deflected/reflected are collected by a detector which has similar circuitry
as the stage, the visual effect is a bit different though since this signal
is less dependent on the conductivity of the sample and more dependent on the
topography of the sample.  The last step of the process is to point a
conventional camera at the CRT and take the light from the phosphor and put
it on film.

   What I am trying to stress here is that we can't take what we know about
conventional optics and stereography and try to apply it to SEM stereography.
 Does anyone else know how SEM's work?  Do I know how SEM's work?!

John Vala


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