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Technical Details
Electron Optical System
The FOCUS IS-PEEM is a three-lens electrostatic microscope column imaging the lateral
electron distribution being emitted from the sample. It consists of an electrostatic tetrode objective lens, a contrast aperture located in the backfocal plane (piezomotor-driven aperture
exchange and adjustment optional), an octopole stigmator/deflector for maximum resolution (optional), a continuously variable iris aperture for enhancement of contrast and resolution,
background reduction in high-resolution mode as well as area selection for micro-analysis and two projective lenses. The final image is intensified by a multichannelplate and made visible by
a fluorescent screen (YAG crystal).
Iris Aperture for Microspot Selection (optional)
The IS-PEEM can be equipped with a continuously variable iris aperture which can be
adjusted in UHV by means of the rotary motion feedthrough on the base flange. This ‘microspot selector’ allows to select well defined sample areas of down to about 1m m
diameter for microspectroscopy such as Micro-XAS, Micro-AES or Micro-ESCA. Furthermore, the iris aperture serves for a general enhancement of contrast and resolution as
well as background reduction especially in the high-resolution mode.
Since the iris is located in the plane of the first real image (Zwischenbild) produced by the
objective lens, it’s hexagonal shape will appear on the fluorescent screen after being magnified by the projective lenses. When being closed, it acts as an efficient sample
microspot selector, because both the image of the sample surface and the image of the hexagonal shape of the iris in the ‘Zwischenbild’-plane are simultaneously focused onto the
screen. The iris also serves as an easy reference for the ‘Zwischenbild’-plane during adjustment of the lenses.
For all microspectroscopy techniques (see below) the iris aperture is a necessary precondition.
Piezomotor-Driven Exchange and Adjustment of the Contrast Aperture
(optional)
The contrast aperture of the IS-PEEM is located in the backfocal plane of the objective lens. In
the standard version it is fixed in the centre of the beam and it’s diameter is factory preselected, taking into account the customer’s needs. The diameter of the contrast aperture
determines both the resolution of the instrument (through the spherical aberration coefficient) and also the intensity of the image (through it’s cross section). Hence it is very advantageous,
to be able to select different sizes of the contrast aperture in situ during the experiment, thus optimising the interplay between spatial resolution and image intensity (analogously to the
selection of the slit-widths of a spectrometer).
In the IS-PEEM this in-situ selection of various contrast apertures is made possible by a
piezomotor-driven multiaperture mount. Five different sizes can thus be easily and rapidly selected. Especially in Synchrotron radiation applications and for microspectroscopy this is a
valuable option.
Besides the size selection there is another advantage of the piezomotor movement facility of
the contrast aperture: The position of the aperture relative to the beam axis can be varied, which allows to make use of ‘orientational contrast’. The orientational contrast arises in off-axis
position of the contrast aperture and resembles the influence of sample illumination in optical microscopy and of sample rotation in scanning electron microscopy (SEM).
Octopole Stigmator and Deflector (optional)
In order to reach the ultimate resolution every electron microscope must be operated with a
stigmator thus correcting non-spherical aberrations of the optics. The IS-PEEM uses an electrostatic octopole stigmator located in the backfocal plane of the objective lens. The
octopole arrangement ensures that an astigmatism can be corrected in any rotational orientation, independently in the x/y orientation as well as in the xy/-xy orientation (i.e. along a
direction rotated by 45 degrees with respect to x/y).
In addition, the octopole can be used as an x/y deflector in order to shift the field of view without
moving the sample. This can be useful for fine adjustment of the sample position as well as for optimization of the focussing of the field of view (compensation of small misalignment of the sample).
Imaging Assembly
The final image is intensified by a multichannelplate having a special thickness of 800 m m
and is made visible by means of a YAG single-crystal fluorescent screen. As compared with double-channelplate arrangements, the special thick single plate causes significantly less
increase of spot-resolution, so that the ultimate lateral resolution can be reached by a plate of 20 mm dia. which is less fragile and much less sensitive to the humidity of ambient air than
larger plates.
In addition, the use of a single-crystal fluorescent screen further reduces increase of
spot-resolution in the imaging device, because there are absolutely no effects of grains of fluorescent material (in contrast to conventional screens).
In typical applications (e.g. work function contrast using the Hg source) at moderate resolutions
the image is easily visible with the bare eye and can be photographed similar to a LEED-screen. A simple CCD video camera can make such images visible on a monitor or
take data in real time using a video recorder. For high-resolution imaging or applications of Synchrotron radiation etc., a time integrating slow-scan peltier-cooled CCD camera is
recommended. The advantages of real-time imaging and the sensitivity of slow-scan capability are combined in ‘dual-scan’ cameras.
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