Editing Scanning electron microscope (section)

==Detection of secondary electrons==
The most common imaging mode collects low-energy (<50 eV) secondary electrons that are ejected from conduction or valence bands of the specimen atoms by inelastic scattering interactions with beam electrons. Due to their low energy, these electrons originate from within a few [[nanometer]]s below the sample surface.<ref name="Goldstein-1981" /> The electrons are detected by an [[Everhart–Thornley detector]],<ref name="Everhart-1960">{{cite journal |last=Everhart |first=T. E. |author2=Thornley, R. F. M.  |year=1960 |title=Wide-band detector for micro-microampere low-energy electron currents |journal=Journal of Scientific Instruments |volume=37 |pages=246–248 |doi=10.1088/0950-7671/37/7/307 |issue=7|bibcode = 1960JScI...37..246E |url=http://authors.library.caltech.edu/12086/1/EVEjsi60.pdf }}</ref> which is a type of collector-[[scintillator]]-[[photomultiplier]] system. The secondary electrons are first collected by attracting them towards an electrically biased grid at about +400 V, and then further accelerated towards a phosphor or scintillator positively biased to about +2,000 V. The accelerated secondary electrons are now sufficiently energetic to cause the scintillator to emit flashes of light (cathodoluminescence), which are conducted to a photomultiplier outside the SEM column via a light pipe and a window in the wall of the specimen chamber. The amplified electrical [[signal (electrical engineering)|signal]] output by the photomultiplier is displayed as a two-dimensional intensity distribution that can be viewed and photographed on an analogue [[video]] display, or subjected to [[Analog-to-digital converter|analog-to-digital conversion]] and displayed and saved as a [[digital image]]. This process relies on a raster-scanned primary beam. The brightness of the signal depends on the number of secondary electrons reaching the [[sensor|detector]]. If the beam enters the sample perpendicular to the surface, then the activated region is uniform about the axis of the beam and a certain number of electrons "escape" from within the sample. As the angle of incidence increases, the interaction volume increases and the "escape" distance of one side of the beam decreases, resulting in more secondary electrons being emitted from the sample. Thus steep surfaces and edges tend to be brighter than flat surfaces, which results in images with a well-defined, three-dimensional appearance. Using the signal of secondary electrons [[image resolution]] less than 0.5&nbsp;nm is possible.