Introduction to STEM

Some TEMs have scan coils which allow them to be used as a scanning transmission electron microscope (STEM). In STEM, a conical electron beam is focussed through the specimen by a lens in front of the specimen (C2 and the objective lens pre-field), as shown below.

The optical configuration is roughly the same as simply forming an image of the filament by focussing C2 – the very first experiment we did in the electron microscope. In fact, if your microscope has a twin objective lens, the optics are a little bit more complicated, but lets start thinking about things as simply as possible.

The tight beam cross-over at the specimen plane is usually called an electron ‘probe’. Images can be formed by scanning the probe across the specimen (by the double- deflection shift coils) and detecting the transmitted electrons, which are either on the optic axis (to form a bright-field image) or have been scattered to high angles, to form a dark-field image.

In order to make an image, we have to display the signal coming out of one or more detectors in some way. This is usually done on a slow-scan television screen (although it is nowadays also done by computer). The signal detected is used to modulate the intensity of the image on the display screen, while the scan of the display is synchronised accurately with the position of the probe on the specimen. Usually, the same ‘scan generator’ is used to control both the x-y position of the beam (or probe) shift deflection coils and the coils that control the display screen. The principle is the same as a conventional scanning electron microscope.

A STEM doesn’t actually need any lenses below the specimen at all, because everything important happens before the electrons reach the sample. In practice, a TEM/STEM has all the usual objective lens and projector lenses below the sample (shown as a dotted box above), but in STEM mode these are just used to change the effective camera length between the specimen and the detector plane. They can also be used to form an image of the electron probe.

Although there are certain benefits of STEM imaging (which have not been generally been recognised until quite recently), the main advantage of this geometry is the fact that the probe can be used to irradiate a very small volume of specimen in order to obtain all sorts of other signals such as characteristic X-rays, Auger or secondary electrons, or electron energy-loss spectra. All these signals can be spatially resolved at a resolution corresponding to the width of the probe, thus allowing for high resolution micro- analysis. For the material scientist, analytical signals like these can be much more useful than simply images, say to identify the elements present in an inhomogeneous sample.

Copyright J M Rodenburg