Scanning Electron Microscope

Overview

A scanning electron microscope makes images with electrons instead of light. Because electrons have a far shorter wavelength than visible light, an SEM resolves detail a thousand times finer than an optical microscope, individual pollen grains, fracture surfaces, microchip features, all rendered in crisp three-dimensional relief. It does this not by projecting a whole image at once but by sweeping a finely focused beam across the specimen and measuring, point by point, the electrons that come back.

The instrument is really an electron-optical bench in a vacuum. The vacuum matters: electrons scatter off air molecules, so the entire beam path and sample chamber must be pumped down before imaging can begin.

How it works

Electrons come from the Electron Gun, where a sharp emitter releases them and an accelerating anode pulls them into a beam. The Electron Column then shapes that beam: a pair of electromagnetic Condenser Lens stages demagnify it and the Objective Lens focuses it to a spot a nanometre wide on the sample. Apertures trim the beam's edges and Scan Deflection Coils coils sweep it in a raster.

Where the beam lands it knocks electrons out of the specimen. The Detector Set collect them: a Secondary-Electron Detector gathers low-energy secondary electrons for surface topography, while a Backscatter Detector senses high-energy backscattered electrons whose yield rises with atomic number, giving compositional contrast. The brightness at each scan position becomes one pixel.

The sample rides the Specimen Stage, a motorized five-axis stage that tilts and translates it inside the chamber. All of it runs under high vacuum drawn by the Vacuum System, a turbo pump backed by a roughing pump. The HV & Control Electronics supply the accelerating voltage and lens currents and digitize the detector signal, and the Operator Workstation assembles the pixels into the live image and stores it.