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The IBM Instrument



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. 

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