DEep in… nanowires

The Scanning Electron Microscope (SEM) offers a power of magnification that optical microscopes cannot reach as the SEM uses electrons, in place of visible light.

Resolution or resolving power is the ability to clearly distinguish two neighboring points, and it depends on the wavelength of the used radiation.

In the case of optical microscopes, wavelength of visible radiation ranges from 700 to 350 nm, therefore the diffraction limit makes it difficult if not impossible resolve two points spaced less than 350 nm (1 nanometer = 10 exp(–9) m).

Electron microscopes – using electrons instead of photons – allow to get up to millions of magnifications and to resolve points few nanometers apart, because the electrons wavelength is of the order of the picometer (1 picometer = 10 exp(–12) m).

Most of the primary electron beam incidents on the sample spread into a volume of the material which depends on the polarization working and induces secondary electrons which are collected by a detector, converted into electrical impulses and sent on time real to a screen.

A minimum part of the incident electron beam comes back on the same beam axis and it is collected from the backscatter detector. This mode does not possess very high resolution but it is highly dependent on the atomic weight of the elements that are hit, allowing you to obtain information on the composition of the samples.

The electron beam source is a filament typically made of tungsten. The electrons, accelerated up to 30 kV from a polarized electrode, pass through magnetic lenses. Finally an electromagnet deflects the electron beam on the sample which surface is irradiated with a rectangular scan.

The result is a black and white image characterized by high resolution and large depth of field, which has features similar to those of a normal photographic image. For this reason SEM images are immediately intelligible, and intuitive to understand.

The scanning electron microscope can get images that appear almost three-dimensional.