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. The values of these two parameters depend on the wavelength of the visible radiation, and the beam aperture, and cannot be improved. This is why the idea of forming an image of a sample from :

• or a beam of electrons sufficiently accelerated for the associated wavelength to be less than a nanometer: this is transmission electron microscopy (TEM);

• or a very fine, almost parallel electron beam, which scans the sample and, from the secondary electrons emitted, forms a point-by-point image: this is scanning...