Your SEM has two imaging modes—are you using the right one? Secondary electron (SE) and backscattered electron (BSE) detectors reveal different information about your sample. Understanding when to use each is fundamental to effective electron microscopy.

Most modern SEMs, including the SNE-Alpha, come with both SE and BSE detectors as standard. Switching between them takes seconds, but choosing correctly can mean the difference between seeing exactly what you need and missing critical information.

The Fundamental Difference

Secondary Electron (SE) Imaging

Secondary electrons are low-energy electrons knocked out of sample atoms by the primary beam. Because they have low energy, only SE generated very close to the surface can escape and be detected.

SE Characteristics

• Surface sensitive: Signal comes from top 5-50nm
• Edge brightness: Edges and protrusions appear bright (more SE escape)
• 3D appearance: Images look like you’re viewing the surface with illumination from the detector
• High resolution: Small interaction volume gives sharp detail

When to Use SE

• Surface morphology and texture
• Particle shape and size
• Fracture surfaces
• Surface roughness
• Biological structures
• Any application where surface topography matters

Backscattered Electron (BSE) Imaging

Backscattered electrons are primary beam electrons that have bounced back out of the sample after interacting with atomic nuclei. The key insight: heavier atoms (higher atomic number Z) backscatter more electrons than lighter atoms.

BSE Characteristics

• Composition sensitive: Brightness correlates with atomic number
• Deeper information: Signal comes from 50-300nm depth
• Flat appearance: Less topographic contrast, more uniform lighting
• Lower resolution: Larger interaction volume than SE

When to Use BSE

• Identifying different phases or compounds
• Finding heavy metal inclusions
• Mapping compositional variations
• Distinguishing materials of different density
• Geology and mineralogy
• Metallurgy (phase identification)
• Forensics (gunshot residue particles appear bright)

Practical Examples

Example 1: Metal Alloy

SE image: Shows surface scratches, polishing marks, and topography from sample preparation. Grain boundaries might be visible if etched.

BSE image: Different phases appear as different gray levels. Precipitates with heavier elements appear bright against the matrix. Compositional variations are immediately obvious.

Example 2: Polymer Composite

SE image: Shows fiber-matrix interface, surface texture, fracture morphology.

BSE image: If fillers contain heavy elements (like glass with barium or antimony), they appear bright against the organic matrix.

Example 3: Geological Sample

SE image: Crystal faces, surface texture, morphology.

BSE image: Different minerals appear as different gray levels based on their average atomic number. Zoning within crystals becomes visible.

Combining SE and BSE

The most complete analysis often uses both detectors:

1. Start with SE: Get oriented, find areas of interest, assess surface condition
2. Switch to BSE: Look for compositional variations, identify different phases
3. Add EDS: Confirm elemental composition of features identified in BSE
4. Return to SE: High-resolution imaging of specific features

Technical Considerations

Accelerating Voltage

BSE imaging often benefits from higher accelerating voltage (15-30kV) because:

• More BSE are generated
• Better Z-contrast
• Stronger signal

SE imaging can work well at lower voltages (1-10kV) for surface-sensitive applications.

Sample Preparation

For BSE imaging, sample flatness matters more than for SE. Topographic effects can mask compositional contrast. Polished samples work best for BSE phase identification.

Detector Position

BSE detectors are typically positioned directly above the sample (solid-state detector) or in a ring around the beam (annular detector). SE detectors are usually off to the side, which creates the shadowing effect that gives topographic contrast.

Summary: Quick Reference

Additional Resources

• How SEM Works
• Bruker XFlash EDS – Add elemental analysis
• Backscattered Electrons – ScienceDirect