Scanning electron microscope arrives at TSL
Torr Scientific’s new scanning electron microscope
Towards the end of last year, TSL took delivery of a Hitachi S-3200N scanning electron microscope (SEM) with an energy-dispersive X-ray (EDX) spectrometer. The SEM allows high-resolution imaging of samples while the EDX provides elemental analysis and mapping. The system had spent many years at University of Surrey where it served in the Microstructural Studies Unit (MSSU), being considered the ‘workhorse’ system among the increasingly sophisticated (and expensive!) new arrivals. It is a familiar system to TSL, being the very instrument on which TSL’s Dr Dave Stupple took his first tentative steps in electron microscopy. Three of TSL’s finest attended Surrey over two days for the not‑trivial task of disconnecting and transporting the instrument down to sunny Bexhill on the East Sussex coast in the south-east of the UK.
In the optical microscopes that most people are familiar with, a sample is flooded with light, some of which bounces back and is focussed onto the eye of the operator or a digital sensor. In an SEM, a beam of electrons is accelerated through a field of many thousands of volts then focussed onto the sample, dislodging a cloud of secondary electrons that are hoovered up and counted by a detector. This happens sequentially in a series of parallel rows covering the area being viewed (hence the ‘scanning’), with a high electron count at each position giving a bright image pixel and a low count a dark pixel. This happens under vacuum in the sample chamber of the SEM as electrons would otherwise be deflected by air molecules with a considerable weight advantage (a factor of 50 000 or so for a stationary electron).
Our company scientist, Dr. David Stupple is examining the damaged surface of a customer’s X-ray anode that has been sent in for recoating. The EDX spectrometer protrudes at an angle from the sample chamber beside him, while examples of false‑colour element maps are visible on the monitor in the background.
The geometry of the electron source (directly above the sample) and the detector (typically off to the side of the sample) results in an image with the sample viewed from above and apparently lit from the detector direction. More electrons are collected from a surface facing the detector than from a surface facing away, so the facing surface will appear brighter in the image. SEM images typically have the appearance of a black‑and‑white (greyscale) photograph complete with highlights and shadows and a pleasing depth of focus. Electrons, of course, do not discriminate colour though some image contrast might result from different materials in the sample. It is easy to fixate on the considerable level of magnification available from an SEM, but magnification is pointless without resolution, the ability to discriminate fine detail. This is the main point of electron microscopy: the need for a vacuum system and a high-voltage power source and the lack of colour are more than compensated by the resolving power.
Slamming high-energy electrons into a sample happens to excite X‑ray photons from the atoms of the sample. As well as a broadband background spectrum of X-ray photon energies, there are distinct peaks whose energy corresponds to characteristic emission from different elements. With a suitable X-ray spectrometer and data processing (a process commonly known as EDX*) the elements present (except for the very lightest) and their location in the sample can be determined.
We hope the SEM will prove invaluable for inspecting damaged anode films sent in for recoating and repair, materials identification and for quality control of coatings applied by us for our customers. Inspection of the microstructure of applied thin-film coatings will help with process optimization.
Finally, we send our thanks to Dave Jones of Surrey’s MSSU for his continuing support and guidance with this instrument, and best wishes to James Whiting, also of MSSU, who is recovering from surgery.
*I say EDX, you say EDS… let’s call the whole thing off. X for X-ray, S for spectrometry: both terms are in common use, with EDS perhaps being the choice of the majority. Let’s not fall out over it.
