In the line drawing on the
left, we see that the IBM VG Microscopes STEM has the electron gun at the
bottom. Above that are a set of lenses
which focus the beam to 0.08 nm at the specimen. Beyond the specimen, there are two major
detectors – 1) an annulus for sensing electrons which have scattered to large
angles by interaction with an atom or atom column, and 2) an electron energy
loss spectrometer, which separates electrons according to their energy to
obtain information about the identity, bonding and electronic structure of
atoms within the specimen. A CCD TV camera records electron shadow maps, or
Ronchigrams [1], of amorphous areas for measurement of aberration parameters. A
photo of the column is shown at the right. It is enclosed in a magnetically and
acoustically shielded room, and supported by vibration isolation mounts. The
original oil diffusion pumping system has been replaced with a large ion pump. This instrument is unusual in having high
voltage connections [2, 3] at both the
top and bottom of the machine to allow EELS spectroscopy with very high
accuracy and resolution. Also in this
machine the gun chamber has been modified to allow insertion of a monochromator
[4] which will improve the EELS resolution to 60 meV or better. [1] V. Ronchi, Forty years of history
of a grating interferometer, Applied Optics 3, 437-450 (1964). [2] P.E. Batson, High Resolution Electron
Energy Loss Spectrometer for the Scanning Transmission Electron Microscope, Rev. Sci. Inst. 57, 43-48 (1986). [3] P.E. Batson, Parallel Detection for
High Resolution Electron Energy Loss Studies in the Scanning Transmission
Electron Microscope, Rev. Sci. Inst. 59,
1132-1138 (1988). [4] H.W. Mook, P.E.
Batson, and P. Kruit, Monochromator for high brightness electron guns, in 12th European
congress on electron microscopy, Vol. III, (2000) pp. 315 -316.
