|Fig. 1: (Top) Backscattered Electron Generation. (Bottom) Secondary Electron Generation.|
The Scanning Electron Microscope (SEM) images the topography and composition of a sample surface using a high-energy beam of electrons. The microscope operates by shining the electron beam onto a sample in a sequence of horizontal strips. The electrons interact with the atoms on the sample surface, and signals from these interactions are interpreted by a computer as information about the properties of the sample. This webpage describes the history, theory, and operation of the SEM.
The first crude version of the Scanning Electron Microscope (SEM) was constructed in 1935 by German scientist Max Knoll . Years later, Siemens Berlin hired Manfrad von Ardenne to make significant advances to the technology, though the company quickly discontinued support due to the invention of the supposedly superior Transmission Electron Microscope (TEM) . The availability of field emission guns and desktop computing in recent decades has revived the use of SEM, and in fact, today SEM is prevalent in many areas of scientific and medical research .
The four types of signals that are recorded from SEM scans include backscattered electrons, secondary electrons, x-rays, and cathodoluminescence (visible light). The following sections describe each type of signal, the solid state physics phenomena that produce them, and how they are detected and processed by the SEM.
As the SEM's electron beam scans the sample, the high energy electrons in the beam produce inelastic interactions with the nuclei of atoms on the surface. During this process, many electrons undergo are pried from their atoms and scatter from the surface. Of these scattered electrons, those that have experienced very low energy loss are called backscattered electrons . Backscattered electrons can have scattering angles that range up to 180 degrees, though they tend to have a very narrow focus. The fraction of backscattered electrons is dependent on the atomic number of the atoms on the sample surface. The radius of a hemispherical region from which backscattered electrons are produced is given below .
Information about the fractions and locations of backscattered electrons are sent to the SEM's computer and interpreted as images. The resolution of the images is limited by the radius in which the backscattered electrons are produced; the resolution is limited to the order of 2 x Radius, irrelevant of the diameter of the incident electron beam . The intensity of the backscattered electron signal is also affected by the composition, in particular any inhomogeneity, in the sample .
|Fig. 2: Scintillator-Light-Guide-PMT Detector.|
The backscattered electrons are detected by Scintillator-Light-Guide-PMT Detectors. The backscattered electrons emerge from the sample surface and strike a phosphor coating on the detector . This collision produces photons, which travel through a visible light guide to the photomultiplier tube (PMT). The PMT detector multiplies the signal from the incident light and resolves single photons . Information about the distribution of photons is relayed to the SEM computer for imaging on a screen.
A more complicated backscattered electron detector is the Solid State Diode Detector. This detector uses an applied voltage to collect and amplify electron-hole pairs that are produced by backscattered electrons with energies above 4 kV in a junction diode . This method has a larger acceptance angle and area than that of the Scintillator-Light-Guide-PMT Detector because the sample can be brought very close to the lens pole . However, the detector response is slower than the Scintillator-Light-Guide-PMT Detector response, so it is not useful for fast scan displays on television screens.
The most popular SEM imaging is done by interpreting secondary electrons. When the electron beam scans the sample surface, high-energy electrons from the incident beam interact with valence electrons of the sample atoms. The valence electrons are released from the atom and emerge from the surface, often after traveling through the sample. The emergent electrons with energies less than 50 eV are called secondary electrons . These are labeled SE (I) in the figure. Of the secondary electrons, 90% have energies less than 10 eV, and most have energies from 2 to 5 eV . A second method of producing secondary electrons is when the high-energy electrons from the beam interact with backscattered electrons (labeled SE (II) in figure). In some cases, secondary electrons are produced when backscattered electrons collide with the SEM equipment in range of the specimen (labeled SE (III) in figure). The three ways that secondary electrons are produced are shown in the image below.
The secondary electrons are detected by an instrument on the SEM that is made of a photomultiplier tube followed by a light guide. The light guide is coated by a phosphor and a layer of aluminum . When a potential of 10-15 keV is applied to the aluminum, it attracts secondary electrons . The secondary electrons are then elevated in energy level and activate the phosphor. The levels and directions of secondary electrons attracted to the detector relay information about the sample surface. Unlike the images from backscattered electrons, the secondary electron images do not show deep or dark areas and tend to resemble the black and white photographs of the surface .
Some SEMS also have an energy-dispersive X-ray spectroscope that is used to detect X-rays . The X-rays are formed by interactions between the electrons and sample surface. The X-ray detector converts X-rays into voltage signals, and these signals are sent to a processor that analyzes and displays the data.
SEMs can also collect information about visible light reflected from the surface of the sample . Cathode ray tubes are used to illuminate the sample, and cathodolumniscence detectors sample and display images of the sample in color.
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