Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2022 / Paper 1 / Section A(i) / 1 / ii / b

Use a low-dose, dose-efficient acquisition rather than assuming that a lower accelerating voltage is always gentler. Focus and align on a sacrificial neighbouring region, blank the beam between exposures, illuminate only when recording and use an efficient detector. A broad, low-current-density transmission electron microscopy exposure can avoid a high instantaneous local dose; scanning can also work if probe current, dwell time and repeated visits are controlled.

Radiolysis starts with inelastic scattering and ionization. For an appropriate thin specimen, increasing the electron energy can reduce the inelastic interaction probability and energy deposition per incident electron, so a relatively high voltage can help. However, increasing voltage can also activate knock-on displacement. The optimum therefore depends on the material and the amount of useful information obtained per damaging event; $150\mathrm{nm}$ alone does not determine it. Cooling may reduce diffusion-driven chemical changes but does not eliminate primary ionization. An experimental study of nonbiological layered polymer films explicitly found that the voltage choice must balance damage with image signal.^[ref]