Macroscopic fields in EMs are produced by macroscopic elements, including the solenoids, magnetic multipoles or by the voltages applied to conducting devices, for example cylinders or apertures.
In conventional EM systems, the existence of spherical aberration requires the utility of very small apertures to maximize the spatial resolution, while the resolution will also be limited by diffraction (see Airy disc) if the apertures are too small.
Fresnel fringes are produced along the edges of TEM objects when the objects are out of focus under a coherent illumination. For instance, the edge between the carbon film and vacuum, or the aperture edges in the TEM present such fringes.
In STEM operation, to observe the Ronchigram, the apertures after the specimen are removed and a large probe convergence angle (>100 mrad) is selected, for instance by inserting the largest STEM objective aperture (condenser aperture in CTEM).
The columns of high voltage TEMs are normally higher than 3 m so that the aperture drives and specimen airlock are difficult to reach during microscope operation. Fortunately, computer or remote control devices are provided for modern microscopes; otherwise, a ladder is needed.
Table 3623. Apertures selected by operator in modern TEM systems.
|Condenser (or C2) Apertures
||Right below condenser
|Define the electron beam in both TEM and STEM modes
||At the back focal plane, is placed immediately below the specimen
||i) Cut off peripheral electrons in BF-, DF- and HR-TEM modes.
ii) t/λi, from EELS spectra, should be recorded with the objective aperture withdrawn. λi is the inelastic mean
iii) t/λe can be obtained simply by recording the low-loss
spectrum of the specimen with a small objective aperture. λe is the elastic mean
|Selected Area Diffraction (SAD) Aperture
||At the image plane of the objective lens
||Select an area so that we can limit the region which generates the diffraction pattern in TEM mode
|High Contrast Aperture
||At a image plane of the back focal plane, go through the lower polepiece
||Permits high contrast imaging (similar function to that of objective aperture), and simultaneous EDS and high angle tilt of TEM specimen in TEM mode
||In GIF camera
||Controls collection angle in EFTEM mode: lead to an improvement in the edge signal-to-background (and thus signal-to-noise) ratio, due to relatively broad angular distribution of the background signal
|Slits for EELS
||In GIF camera or spectrometer
||Select part of the spectrum in EELS mode