X-ray Production

 As the scanning electron microscope uses a high energy electron beam to illuminate a specimen, one of the by-products is the generation of x-rays as primary beam electrons interact with specimen electrons. The production of x-rays occurs in two basic ways (see figure1).

Inelastic scattering has the effect of generating x-rays in two ways:
(i)
By causing primary electrons to slow down as they travel through the 'volume' of an atom. In order for this to occur the                 electrons must give up energy and this can be accomplished by the emission of x-ray radiation. This type of radiation is known as breaking radiation and is observed as a continuous spectrum. This continuous spectrum is regarded as background radiation for EDXA spectrometers.
(ii)
By collisions between primary beam electrons and electrons within specimen atoms. The consequent rearrangement of electrons within electron shells, as atoms strive to reach their lowest energy states, results in the release of energy in the form of x-ray photons; a process known as fluorescence. As the energy of these photons is related to the energy difference between electron shells, the x-ray photons are characteristic of the elements present in the specimen. By collecting and analysing these x-rays, qualitative and quantitative information about the component elements of a specimen may be obtained.

The additional hardware required to detect and measure the energy of the characteristic x-rays is shown in figure 2. Energy dispersive x-ray analysis (EDXA) is a very widespread technique which is regularly applied to biological or chemical as well as physical problems in material science. Normal electron microscope imaging is usually performed during the course of the analytical procedure.

Energy Dispersive spectrometers employ a solid-state detector usually involving a single crystal of silicon. The idea behind the detection system requires the use of an intrinsic semi-conducting material i.e. a material with a full valence band and empty conduction band. As there are no electrons available for charge transport in the conduction band and no holes in the valence band, no current can be conducted in an applied electric field unless the material absorbs energy causing electrons to be promoted to the conduction band. Such materials make useful radiation detectors.

The function of the detecting crystal is to convert the energy of an x-ray photon into an electrical signal of proportional magnitude. When an x-ray photon collides with the intrinsic layer, electrons are excited up into the conduction band leaving behind holes in the valence band. An applied voltage bias has the effect of sweeping the charges to the electrodes on the opposing faces of the crystal. The more energetic the incident x-ray, the greater the charge difference generated across the crystal.

A thin, opaque window (usually of beryllium approximately 8mm thick) protects the detector crystal from the SEM vacuum system (10-5 Torr).  The window places a limit on the lightest detectable element. The characteristic x-rays must have enough energy to pass through the window in order to be detected by the crystal. Normally the lightest detectable element is sodium with atomic number 11.
When an incident x-ray generates charge formation in the crystal the signal has then to be processed to form a useful electrical signal which can ultimately be fed to a multi-channel analyser (computer) for sorting.

The  multichannel analyser sorts the detected signals (pulses) using an  array of computer ‘pigeon holes’ or channels. Each channel is assigned 20eV of energy in a range of 200eV to 20 000eV. Each incoming pulse is assigned a channel according to its magnetude (i.e. x-ray energy). The multichannel analyser display shows a histogram of Number of x-ray counts against x-ray energy (see figure 3).