The signals used by a scanning electron microscope to produce an image result from interactions of the electron beam with atoms at various depths within the sample. Various types of signals are produced including secondary electrons (SE), reflected or back-scattered electrons (BSE), characteristic X-rays (EDS) and light (cathodoluminescence) (CL), absorbed current (specimen current) and transmitted electrons.
Secondary electron (SE) detectors are standard equipment in all SEMs, but it is rare for a single machine to have detectors for all other possible signals.
Secondary Electrons (SE)
Secondary electrons have very low energies on the order of 50 eV, which limits their mean free path in solid matter. Consequently, SEs can only escape from the top few nanometers of the surface of a sample. The signal from secondary electrons tends to be highly localized at the point of impact of the primary electron beam, making it possible to collect images of the sample surface with a resolution of below 1 nm.
Backscattered Electrons (BSE)
Back-scattered electrons (BSE) are beam electrons that are reflected from the sample by elastic scattering. They emerge from deeper locations within the specimen and, consequently, the resolution of BSE images is less than SE images. However, BSE are often used in analytical SEM, along with the spectra made from the characteristic X-rays, because the intensity of the BSE signal is strongly related to the atomic number (Z) of the specimen. BSE images can provide information about the distribution, but not the identity, of different elements in the sample.
Energy Dispersive Spectrometry (EDS or EDX)
Characteristic X-rays are emitted when the electron beam removes an inner shell electron from the sample, causing a higher-energy electron to fill the shell and release energy. The energy or wavelength of these characteristic X-rays can be measured by energy-dispersive X-ray spectroscopy and used to identify and measure the abundance of elements in the sample and map their distribution.
X-ray Fluorescence (SEM-XRF)
The technological progress in the fields of small-spot low-power X-ray tubes and associated polycapillary X-ray optics has enabled the development of compact micro-focus X-ray sources that can be attached onto a SEM-EDS (scanning electron microscope with energy dispersive X-ray spectrometer) system. As such, micro-spot X-ray fluorescence (μXRF, microEDXRF) can be performed with an SEM so that the analytical capabilities of SEM are considerably extended. Implementation of SEM-XRF is especially attractive due to the possibility of using many existing features offered on existing SEM-EDS systems (e.g. acquisition and identification of the X-ray fluorescence spectra). For example, sample stage control can be used for carrying out X-ray fluorescence (elemental analysis) spectral maps in the manner that is well known in SEM-EDS. By combining the analytical information obtained from the X-ray spectra excited with electrons and with X-ray photons respectively, trace elements of low and high atomic numbers may be quantified, albeit with different spatial resolutions.*
Magnification & Depth of Field
Due to the very narrow electron beam, SEM micrographs have a large depth of field yielding a characteristic three-dimensional appearance useful for understanding the surface structure of a sample. A wide range of magnifications is possible, from about 10 times (about equivalent to that of a powerful hand-lens) to more than 500,000 times, about 250 times the magnification limit of the best light microscopes.