Scanning electron microscope

A scanning electron microscope scans a focused electron probe across a specimen and assigns a detector signal to each probe position. Secondary electrons leaving the irradiated region often reveal surface relief; backscattered electrons can provide composition-dependent contrast. Image brightness records the selected signal, rather than a transmitted projection enlarged by two lenses. Display enlargement is therefore set by the ratio of display width to scanned specimen width. Surface geometry and composition both affect brightness.^[ref]

Composition-sensitive contrast associated with the generally larger electron backscattering yield of higher-atomic-number regions. Detector geometry, crystal orientation, surface topography and beam energy can also affect intensity, so a bright region is not a unique elemental identification.

Image contrast from changes in secondary-electron escape and collection as surface slope, edges and detector-facing geometry vary. Secondary electrons originate near the surface, but brightness also depends on material, charging and the beam conditions.

A scanning electron microscope permitting a controlled gaseous specimen environment while maintaining the electron column at lower pressure by differential pumping. Ionization of the gas supplies positive ions that can neutralize negative specimen charging, and a suitable gaseous detector collects electron signals. This enables examination of some nonconducting uncoated specimens, with gas scattering and detector response included in interpretation.

A secondary-electron detector using amplification through collisions in the specimen-chamber gas. Its field drives signal electrons toward collection while positive ions can move toward a negatively charging specimen. Operating pressure and electrode geometry control amplification and background.

An Everhart–Thornley detector converts emitted electrons into an amplified optical signal in a scanning electron microscope. A positively biased collector attracts low-energy secondary electrons toward a scintillator; their impact produces light carried through a light guide to a photomultiplier.

The measured brightness depends on how many emitted electrons reach the collector, not just on how many leave the surface. Changing specimen tilt or collector bias can therefore change an image without changing composition. Comparing such settings helps identify collection-geometry effects in secondary-electron topographic contrast. ^[ref]

The fraction of incident electrons emerging from a specimen as backscattered electrons.

Scanning transmission electron microscopy forms an image by scanning a focused electron probe across a thin specimen and recording transmitted or scattered electrons at each probe position. The detector's angular range determines which interaction contributes to the recorded brightness.

A detector including the direct beam gives a different image contrast mechanism from an annular detector collecting scattered electrons. High-angle annular dark-field imaging can emphasize composition and mass-thickness differences, but specimen thickness and channeling still affect interpretation. The probe size limits spatial detail, whereas the displayed-to-scanned length ratio sets magnification; increasing the latter cannot sharpen the probe.

An electron microscope that scans a focused probe across an electron-transparent specimen and records transmitted-electron signals at each scan position. Its image magnification is set by the displayed-to-scanned length ratio, separately from angular detector collection.

Detection of transmitted electrons within an angular annulus excluding the direct beam. Contrast can include diffraction, atomic-number and thickness contributions depending on the collection angles.

A scanning transmission electron microscopy mode collecting electrons scattered to sufficiently large angles. It often gives approximately monotonic atomic-number and mass-thickness contrast, with detector angles and multiple scattering still important.