SEM Detectors and Signals

Quick answer

Secondary electron detectors and backscatter electron detectors answer different SEM questions.

Use secondary electron, or SE, imaging when you need surface morphology, texture, edges, and a visually intuitive topographic image. Use backscatter electron, or BSE, imaging when you need contrast related to composition, average atomic number, phases, inclusions, grains, or orientation effects.

Neither signal is universally superior. A strong SEM workflow often uses SE to find and understand the surface, then BSE to test whether brightness differences are related to material differences.

Signal origin

Secondary electrons are low-energy electrons emitted from near the specimen surface. They usually provide high surface sensitivity and strong topographic contrast.

Backscattered electrons are primary beam electrons that enter the specimen, undergo elastic scattering, and leave the specimen with relatively high energy. Because the probability of backscattering increases with atomic number, BSE imaging can reveal material contrast that is nearly invisible in SE mode.

What SE detectors show best

SE detectors are often the first choice for:

• surface morphology
• particles and fibers
• fracture surfaces
• insect and biological surfaces
• coatings and contamination
• microfabricated structures
• navigation at low and medium magnification

SE images can be beautiful and immediately readable. They often show relief, pores, ridges, cracks, and edges with strong apparent depth.

The caution is that brightness in an SE image is not simply height, composition, or conductivity. It is affected by detector position, local electric fields, sample tilt, charging, edge emission, and the mixture of SE1 and SE2 signal.

What BSE detectors show best

BSE detectors are often the first choice for:

• multiphase alloys
• polished geological sections
• ceramics and composites
• inclusions and precipitates
• heavy element contamination
• phase distribution before EDX
• orientation contrast in crystalline materials

In many BSE images, heavier regions appear brighter and lighter regions appear darker. For example, a heavy metal inclusion in a lighter matrix may stand out clearly.

BSE imaging is especially useful before EDX analysis because it helps locate compositionally distinct regions worth measuring.

Comparison table

Feature	Secondary electron detector	Backscatter electron detector
Main contrast	Surface topography and edges	Composition, average atomic number, phase, orientation
Signal energy	Low	High relative to SE
Surface sensitivity	Very high	Lower than SE, depends on beam energy and sample
Spatial resolution	Often higher for surface detail	Often lower because signal comes from a larger interaction volume
Best specimen geometry	Rough or textured surfaces	Flat, polished, or moderately smooth samples
Common detector position	Chamber side, in-lens, through-the-lens	Annular or segmented detector near pole piece
Typical first use	Find morphology	Find phases or material contrast

Detector placement

SE detectors are frequently mounted in the chamber or integrated into the electron column. A classic Everhart-Thornley detector collects secondary electrons using a biased grid and scintillator. In-lens and through-the-lens systems collect secondary electrons near or inside the column and are often favored for high-resolution imaging.

BSE detectors are commonly placed below the pole piece, often as annular or segmented solid-state detectors. Their position favors collection of high-energy electrons leaving the specimen at relatively high angles.

Segmented BSE detectors can also create directional topographic contrast. By comparing segments, the operator can distinguish some topographic effects from average atomic number contrast.

Practical selection guidance

If the sample is rough, unknown, or biological, start with SE. It will usually give the fastest visual understanding of the surface.

If the sample is polished, multiphase, geological, metallurgical, ceramic, or compositionally complex, start with BSE or capture BSE immediately after SE.

If brightness differences in SE imaging may be composition-related, verify with BSE and EDX. SE alone is usually not enough to make a compositional claim.

If brightness differences in BSE imaging may be caused by topography, reduce tilt, improve polishing, lower the working distance if appropriate, and compare detector segments or SE images.

BSE and atomic number contrast

The backscatter coefficient generally rises with atomic number. This is why BSE is widely used for Z-contrast imaging.

The useful phrase is "average atomic number contrast." A phase containing heavier elements tends to appear brighter than a phase dominated by lighter elements, assuming comparable geometry and imaging conditions.

There are limits. BSE brightness is not a complete chemical analysis. Density, crystallography, surface angle, beam energy, detector geometry, and charging can all influence the image. Use EDX or another analytical method when elemental identity matters.

SE and topographic contrast

SE images are often interpreted as morphology because low-energy secondary electrons escape from near the surface. Edges and features facing the detector commonly appear bright.

This visual style is useful, but it is also why SE images can exaggerate roughness. Two features with the same material and height can appear different if one faces the detector and the other is shadowed.

For quantitative height, use profilometry, AFM, stereo SEM methods, or calibrated 3D workflows rather than raw SE brightness.

When to use both

Many serious SEM sessions should include both SE and BSE images of the same field of view.

Use the paired images this way:

Observation	Likely interpretation
Bright in SE, not distinct in BSE	Topography, edge effect, charging, or local geometry
Bright in BSE, not especially raised in SE	Higher average atomic number or phase contrast
Bright in both	Could be a raised feature, heavier material, or both
Dark in SE and BSE	Depression, light phase, poor collection geometry, or charging

Notes on beam energy

Higher accelerating voltage increases interaction volume and often strengthens BSE production. This can improve compositional contrast but reduce surface specificity.

Lower accelerating voltage improves surface sensitivity and can reduce charging or beam damage in some samples. It may also reduce X-ray generation for EDX and change the BSE contrast mechanism.

There is no universal best voltage. Choose beam energy according to the signal you need, the sample thickness and conductivity, and whether imaging or analysis is the priority.

Bottom line

SE is the morphology workhorse. BSE is the composition and phase contrast workhorse.

For research-grade interpretation, use them as complementary signals. SE tells you what the surface looks like. BSE tells you whether the material itself may be changing.