Understanding SEM Controls

The Scanning Electron Microscope (SEM) uses a focused beam of electrons to create high-resolution images of sample surfaces. Understanding how each control parameter affects imaging is crucial for obtaining optimal results. This simulator demonstrates the internal electron optics and how your adjustments change the beam-sample interaction.

Accelerating Voltage (1-30 kV)

The accelerating voltage determines the energy of electrons in the beam. Higher voltages accelerate electrons to greater speeds, affecting penetration depth and signal generation:

  • High voltage (20-30 kV): Electrons penetrate deeper into the sample, generating more backscattered electrons (BSE). Useful for compositional contrast and imaging through contamination layers.
  • Low voltage (1-5 kV): Electrons interact primarily at the surface, producing more secondary electrons (SE). Ideal for surface-sensitive imaging and minimizing beam damage to delicate samples.
  • Penetration depth: Approximately 0.1 μm per kV in typical materials. A 15 kV beam penetrates about 1.5 μm deep.

Aperture Size (30-200 μm)

The aperture is a physical opening that controls the beam's convergence angle. It's one of the most important settings for balancing resolution and signal strength:

  • Small aperture (30-50 μm): Reduces beam convergence angle, producing a narrower beam with better resolution. However, less current means weaker signal and longer acquisition times.
  • Large aperture (100-200 μm): Allows more electrons through, increasing beam current and signal strength. The larger convergence angle reduces resolution but improves signal-to-noise ratio.
  • Trade-off: Resolution vs. signal strength. Choose smaller apertures for fine detail, larger for better signal or when working with low-contrast samples.

Working Distance (5-20 mm)

Working distance (WD) is the vertical gap between the objective lens and the sample surface. It significantly affects resolution and depth of field:

  • Short WD (5-7 mm): Provides the best resolution because the beam is most focused at short distances. Results in shallow depth of field—only features at similar heights appear sharp.
  • Long WD (15-20 mm): Sacrifices some resolution but provides much greater depth of field, allowing rough or tilted samples to appear uniformly focused. Necessary for large or tall specimens.
  • Practical use: Short WD for flat, polished samples requiring maximum resolution. Long WD for rough surfaces, fractured samples, or when using specialized detectors.

Spot Size (1-7)

Spot size controls the final beam diameter at the sample. It's adjusted by changing the condenser lens strength:

  • Small spot (1-3): Produces the finest beam diameter for maximum resolution. Best for imaging nanoscale features, but provides weak signal requiring longer scan times or multiple frame averaging.
  • Large spot (5-7): Increases beam current for stronger signal and faster imaging. Resolution is reduced, but excellent for quick surveys, charging samples, or low-magnification work.
  • Beam current relationship: Beam current increases with both larger spot size and larger aperture. In this simulator: Current (nA) = (aperture² × spot size) / 2000.

Secondary Electron (SE) Detector

The SE detector collects low-energy electrons (< 50 eV) ejected from the sample's surface:

  • Signal origin: Secondary electrons are knocked out of atoms near the surface by incoming primary electrons. They can only escape from the top few nanometers.
  • Topographic contrast: SE imaging excels at showing surface morphology. Edges and raised features appear bright because more SEs can escape.
  • Best for: Surface texture, particle morphology, fracture surfaces, and general-purpose imaging.

Backscattered Electron (BSE) Detector

The BSE detector collects high-energy electrons that bounce back from deeper in the sample:

  • Signal origin: Primary beam electrons that undergo elastic scattering events with atomic nuclei and reflect back out. Their energy remains close to the original beam energy.
  • Compositional contrast: Heavier elements (higher atomic number Z) backscatter more electrons, appearing brighter. This allows phase identification and compositional mapping.
  • Best for: Distinguishing materials, identifying inclusions, mapping grain structure in metals, and quality control applications requiring compositional information.

Interaction Volume

The interaction volume (shown as the teardrop shape beneath the sample) represents the region where beam electrons scatter and generate signals. Its size depends primarily on accelerating voltage and sample composition. Understanding this helps explain which signals come from where in your sample.