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ID: past-exam-of-the-mphil-in-micro-and-nanotechnology-enterprise-course-of-the-university-of-cambridge/2023/paper-1/section-b/8/i/solution

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Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2023 / Paper 1 / Section B / 8 / i / Solution by

Codex  0 2026-10-20

An incident SEM probe undergoes elastic deflections by atomic potentials and inelastic interactions that excite or ionize the specimen. Repeated scattering generates an interaction volume wider and deeper than the incident probe. The measured resolution depends both on where a signal is generated and on whether it can escape.

Signal	Origin and representative spatial scale
Secondary electrons	Low-energy electrons escape mainly from the upper few nanometers. The directly excited SE1 signal can resolve a few nanometers with a suitable probe; secondary electrons generated by backscattered electrons add a broader component.
Backscattered electrons	Incident electrons escaping after mainly elastic scattering sample a larger volume. Bulk-specimen compositional resolution is commonly tens to hundreds of nanometers, and can be worse at high voltage or better in favorable low-voltage heavy materials.
Characteristic X-rays	Inner-shell ionization and radiative relaxation occur throughout a substantial interaction volume. Conventional bulk SEM elemental resolution is often several tenths of a micrometer to a few micrometers.
Auger electrons	Surface-sensitive electron relaxation products have a short escape depth. Lateral resolution additionally depends on probe size and the analyzer geometry.
Cathodoluminescence	Electronic excitation followed by photon emission can sample the interaction volume plus carrier diffusion, so it is not automatically resolved at the probe diameter.

Transmitted electrons from a sufficiently thin specimen can instead support scanning transmission electron microscopy. The values above are orders of magnitude, not intrinsic constants of the signal: beam energy, atomic number, specimen thickness, probe size and detector geometry all matter. The spatial origin explains why a fine topographic SEM image does not guarantee equally fine X-ray mapping.

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