Gatan, Inc.

Michael Karl Kundmann

John Andrew Hunt

Stuart L Friedman

Alexander Jozef Gubbens

Ondrej L Krivanek

Henri Adriaan Brink

Electron optical aberrations of an energy filtering system of an energy filtering transmission electron microscope (EFTEM) are automatically corrected under computer control to set up the EFTEM for use. Optics of the electron microscope preceding an energy filter are used to scan the beam at the entrance to the filter in a pattern corresponding to a defined geometry. The beam can either be finely focused to yield a spot at each position visited during the pattern scan, or the beam can be spread out and imprinted with a well-defined intensity distribution, such as normally occurs due to passage of the beam through a specimen, so that its relative scanned displacements can be assessed using cross-correlation techniques. In the case of the finely focused beam, electron images of the scanned pattern directly yield a spot pattern image. Deviations of the recorded spot pattern image from the defined scan geometry reflect the imaging aberrations introduced by the energy filter. In the case of the spread out beam, post-filter electron images of the scanned beam are cross-correlated with an image of the beam taken without scanning yielding cross-correlation peak images that give the effective displacement of each scanned beam position due to the aberrations/distortions of the filter. Summing the cross-correlation peak images again yields a spot pattern image that is equivalent to that obtained in the focused beam case. Deviations of the recorded spot pattern image from the defined scan geometry are analyzed to assess and subsequently correct aberrations introduced by the energy filter.

Automated set up of an energy filtering transmission electron microscope

An energy filtering system of an EFTEM is automatically adjusted using a computer. The computer inserts an energy-selecting slit into the beam path and begins monitoring the position of the electron beam through a combination of the current sensors integral to the slit and the readout of an electron camera. The beam is centered within the slit by adjusting an energy dispersing element while monitoring beam sensors. After initial alignment, the slit is retracted and a reference aperture is inserted at the entrance to the energy filter. The electron camera captures an image of the reference aperture and the computer analyzes the deviations of the aperture image from its known physical dimensions in order to evaluate the electron optical distortions and aberrations of the filter. The computer uses the determined optical parameters to adjust the distortion and aberration correcting optical elements of the filter, whose effects are known due to previous calibration. After correcting the imaging aberrations, the reference aperture is withdrawn, the slit reinserted, and an isochromatic surface of the filter at the plane of the slit is measured by scanning the beam across a slit edge while integrating the transmitted beam intensity on the electron camera. The isochromatic surface thus collected by the electron camera is analyzed by the computer to extract additional aberration coefficients of the filter system. These measured aberration coefficients are used to make calibrated corrections to the filter optics.

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Downers Grove, IL, United States of America

Michael Karl Kundmann