Editing Electron microscope (section)

=== Secondary electrons ===
[[File:Electron-matter_interaction_volume_and_various_types_of_signal_generated_-_v2.svg|thumb|Electron–matter interaction volume and types of signal generated in a SEM]]
In a SEM the signals result from interactions of the electron beam with atoms within the sample. The most common mode is to use the [[secondary electrons]] (SE). Secondary electrons have very low energies on the order of 50 [[Electronvolt|eV]], which limits their [[Inelastic mean free path|mean free path]] in solid matter, a few [[nanometer]]s below the sample surface.<ref name=":1" />  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 |url=http://authors.library.caltech.edu/12086/1/EVEjsi60.pdf |journal=Journal of Scientific Instruments |volume=37 |issue=7 |pages=246–248 |bibcode=1960JScI...37..246E |doi=10.1088/0950-7671/37/7/307}}</ref> which is a type of collector-[[scintillator]]-[[photomultiplier]] system. 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 [[Nanometre|nm]], and with specialized instruments at the atomic scale.<ref>{{Cite journal |last=Ciston |first=J. |last2=Brown |first2=H. G. |last3=D'Alfonso |first3=A. J. |last4=Koirala |first4=P. |last5=Ophus |first5=C. |last6=Lin |first6=Y. |last7=Suzuki |first7=Y. |last8=Inada |first8=H. |last9=Zhu |first9=Y. |last10=Allen |first10=L. J. |last11=Marks |first11=L. D. |date=2015-06-17 |title=Surface determination through atomically resolved secondary-electron imaging |url=https://www.nature.com/articles/ncomms8358 |journal=Nature Communications |language=en |volume=6 |issue=1 |doi=10.1038/ncomms8358 |issn=2041-1723 |pmc=4557350 |pmid=26082275}}</ref>

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.<ref name=":1" />