1. Principle

Observing the micro-relief of a surface is impossible at high magnification with a conventional ("photonic") optical microscope. This is because the separating power (or lateral spatial resolution) is limited to around 0.2 μm and, at maximum magnification of 1,500, the depth of field is limited to around 1 μm. Both depend on the wavelength of the visible radiation and the numerical aperture of the beam, and cannot be improved. This is why the idea of imaging a sample from :

• or a beam of electrons sufficiently accelerated for the associated wavelength to be less than a nanometer (transmission electron microscopy, generally referred to as TEM);

• or a very fine, almost parallel brush of electrons that sweeps across the sample, using the secondary electrons emitted to form a point-by-point image (scanning electron...