Useful Terms

From Electrons to Images

Electron Beam: A column of electrons ending in a cone-shaped “probe” used to image a sample.

Spot Size: The size of the electron probe at the sample surface. It correlates with the probe current.

Interaction Volume: An approximately teardrop-shaped volume of beneath the sample surface throughout which the electron beam scatters and generates signal.

Charging: The buildup of electrons (charge) just beneath the sample surface resulting in unwanted interactions that may appear very bright or otherwise distort images.

Beam Damage: May result from too high energy or current on sensitive or nonconductive samples. Typically appears as “burning” or dark boxes over a rastered region.

Contamination: The deposition of carbon or other elements on the sample as the electron beam scans an area.

Takeoff Angle: The emission angle of various signals from the sample surface.

Secondary Electron: An electron inelastically scattered multiple times in the sample then emitted through the surface. Provides topographic detail.

Backscattered Electron: An electron elastically scattered by atoms in the sample. Provides material or phase contrast.

Characteristic X-Ray: An x-ray produced by the energy change of a higher energy electron filling the vacancy left when the incident beam displaces an atomic electron. Provides elemental information.

Noise: Random signal variation.

Signal: Any particle or photon collected by a detector that relates to some property of the material.

Raster: The way the beam scans point-by-point across a region.

Focus: The process of attaining minimum probe diameter at the sample surface for the sharpest images.

Signal-to-Noise Ratio: The relative amount of noise in an image, also called SNR. Longer scan times, either slower scan speed or more frames, yield high SNR and clearer images.

Inside the SEM

Accelerating Voltage: The beam energy from the source.

Emission Current: The beam current at the source.

Probe Current: The beam current at the surface of the sample, related to the spot size (probe size.) This is usually displayed as an arbitrary number.

Scanning Electron Microscope: A characterization tool that uses a high energy beam of electrons to raster (scan) an area and create an image, or micrograph, based on the relative intensity of signals produced at each point.

Source: The start of the SEM, where the electron beam originates.

Column: The portion of the electron microscope that focuses the electron beam before reaching the sample.

Aperture: A small pinhole in the beam path which filters out a portion of electrons to a given aperture size (typically 10s of um.)

Stigmator: An aberration correction mechanism that allows a user to control the shape of the probe (ideally, round.)

Lens: An electromagnetic coil that focuses the electron beam.

Pole Piece: The end of the column where the beam is emitted.

Focal Plane: The imaginary plane where the electron beam yields a focused image. This is at the crossover point of the beam after the pole piece, but realistically includes a small region around the narrowest part of the probe.

Working Distance: The distance between the pole piece and the focal plane, measured from the pole piece down to the sample, so a smaller working distance is closer.

Depth of Focus: The region (in z) around the focal plane where the specimen is still in focus. High depth of focus is better for highly textured samples.

Stage: Where the sample sits and how it is manipulated in the chamber.

Detector: A device that collects signals to be converted into images.

Chamberscope: A typically IR camera mounted inside the sample chamber to observe the stage and ensure the sample does not collide with the pole piece or detectors.

Operating the SEM

Sample: The specimen being imaged.

Feature: Details on the sample. This can refer to anything, even dust or scratches.

Features of Interest: Specific features that you want to image.

Stub: The disk, sometimes with a pin, that interfaces the sample to the sample holder.

Sample Holder: The interface between the stub and the microscope stage.

Set Screw: A small screw, typically on the side of a sample holder, used to secure the stub and reduce motion while imaging.

Drift: Small typically continuous motion of the sample during imaging. May result from an unsecured sample or charging.

Conductivity: The likelihood or ability of the sample to transport electrons. SEM works best on conductive samples. Otherwise, additional sample preparation or operating conditions may be needed to achieve good images.

Alignment: The process or state of ensuring the beam is round, focused, and passing straight through the column.

In Focus: The state when the focal plane is at the sample surface. This is also where the image appears sharpest.

Wobbler: An automatic function that moves the focal plane to help align the aperture.

Astigmatism: The misshapenness of the electron probe typically due to lens aberrations. This results in images that appear stretched or smeared when out of focus, or very blurry when in focus.

Reduced Area: A smaller scan area useful for alignment.

Scan Speed: The rate at which the beam rasters across the sample. Fast speeds are useful for moving, magnifying, and rough focus. Slow speeds are best for alignment.

Dwell Time: The amount of time the beam spends on a single point while scanning.

Scan Rotation: A way of reorienting the image without moving the stage.

Stage Rotation: A way of reorientating the image by rotating the stage.

Compucentric Rotate: An option to rotate the stage around the center of the current image rather than the stage center.

Photo: A common preset that saves an image at a user-defined speed and resolution.

Scale Bar: A reference line in the data bar of an image used to show feature sizes regardless of the displayed size.

Magnification: The degree of image zoom. Magnification is only accurate (calibrated) for images as displayed on the control monitor.

Resolution: May refer to: 

• the number of pixels in an image; 

• the smallest resolvable distance between features; 

• or, colloquially, the sharpness of an image.

Contrast: The range of greyscale distribution in an image, helps to enhance adjacent features. Increasing contrast widens the difference in greyscale values.

Brightness: The relative center of the greyscale histogram of an image. Increasing brightness makes everything brighter.

Annotation: Any additional measurements, text, or indicators placed and then saved on an image.