Scanning Electron Microscope (SEM)
An electron microscope that rasters a finely focused electron beam across a specimen's surface point by point, building an image from the electrons scattered or emitted back from each point — delivering highly detailed, deeply three-dimensional-looking images of surface topography at magnifications and resolutions far beyond any light microscope.
Resolution
~1 – 20 nm typical
Imaging signal
Secondary / backscattered electrons
Specimen requirement
Vacuum-compatible, often conductive coating
Image character
Surface topography, high depth of field
Overview
- An electron gun generates a beam of electrons, which is focused by a series of electromagnetic lenses into a very fine probe spot, then deflected by scan coils to sweep systematically across the specimen surface in a raster pattern, similar in concept to how an old CRT television scans an image
- At each point the beam strikes, it knocks loose low-energy secondary electrons from the specimen's surface (and/or reflects higher-energy backscattered electrons); a detector counts these emitted electrons at each scan position, and that intensity is used to build up a point-by-point image on a display, synchronized with the beam's scan position
- Secondary electron imaging is exceptionally sensitive to surface topography — since the yield of secondary electrons depends strongly on the local surface angle relative to the beam — producing images with an intuitive, almost photographic sense of 3D surface relief and remarkable depth of field compared to optical microscopy at comparable magnification
- Because electrons scatter heavily in air, the entire electron path (gun, lenses, specimen chamber) must be held under high vacuum, and many specimens — especially non-conductive biological or polymer samples — need a thin conductive coating (commonly sputtered gold or carbon) applied first to prevent charge buildup that would otherwise distort the image
- Since imaging relies on the specimen's outer surface interacting with the beam, SEM inherently images surface topography and composition rather than internal structure — for looking inside a specimen, a TEM (which transmits electrons through a thin sample) is the appropriate instrument instead
Key Features
Exceptional surface detail & depth of field
Secondary electron imaging renders surface topography with a strong, intuitive 3D quality and depth of field far exceeding light microscopy at comparable magnification.
Nanometer-scale resolution
The short effective wavelength of accelerated electrons enables resolution roughly two to three orders of magnitude finer than visible-light optical microscopy.
Vacuum & sample prep requirements
Specimens must tolerate high vacuum and are often conductively coated first, meaning live or wet biological samples generally can't be imaged directly.
Design and Construction
Electron column
- Electron gun (thermionic tungsten/LaB6 filament or field-emission source) generates and accelerates the beam
- Electromagnetic condenser and objective lenses focus the beam to a fine probe spot; scan coils deflect it in a raster pattern across the specimen
Specimen preparation
- Non-conductive specimens typically receive a thin sputter-coated conductive layer (gold, gold-palladium, or carbon) to prevent charge buildup that would otherwise deflect the beam and distort the image
- Specimens must be dried or otherwise stabilized to survive the high-vacuum chamber environment, a significant constraint for biological samples
Applications
Materials Science
Surface & fracture analysis
Widely used to examine material surfaces, fracture patterns, coatings, and nanostructures at extreme resolution.
Semiconductor Inspection
Chip & component quality control
Standard tool for inspecting semiconductor structures and detecting nanometer-scale manufacturing defects.
Forensics & Biology
Fiber, particle & surface examination
Used to examine trace evidence, insect and pollen surface structures, and other specimens where fine surface detail is critical.
Why choose an SEM
Unmatched surface resolution
Reveals nanometer-scale surface detail far beyond any optical microscope's capability.
Intuitive 3D-like imaging
Secondary electron images provide a naturally interpretable sense of surface topography and depth.
Frequently asked questions
Here are some common questions about achromatic lens.
Electrons scatter very readily off air molecules — far more than photons of visible light do — so a beam traveling through even ordinary atmospheric pressure air over the distances inside an electron microscope column would lose focus and coherence before ever reaching the specimen, making a usable focused probe spot impossible. Evacuating the column and specimen chamber removes essentially all the air molecules the beam would otherwise collide with, allowing the electrons to travel from the gun to the specimen and into the detector in a straight, tightly focused path — which is why standard SEM operation requires a functioning vacuum system, though some specialized "environmental SEM" variants use a differentially pumped chamber to image at higher pressures for specific applications.
