What do ancient artifacts, fragments from celestial bodies, microchips and gunshot residue have in common? Their innermost secrets can all be unveiled with the same method. Rutherford backscattering spectrometry (RBS) uses a beam of charged particles to analyze their composition by counting the atoms in thin-film materials.
But now RBS is even better, having been proven accurate to the satisfaction of the metrologists and the University of Surrey now has the first laboratory to achieve accreditation for accurate RBS. This may sound like a small, technical step, but it is a leap that could have huge returns.
Reliable Atom Counting
Lord Rutherford was the first to explain, in 1911, that atoms are made up of heavy nuclei surrounded by orbiting electrons. He was amazed to see alpha particles (the nucleus of a helium atom) bounce backwards (“backscattering”) after reacting with the electrons in an ultra-thin gold foils. But he also solved the physics, finding an elegant formula for how the alphas scatter. Essentially, RBS counts how many atoms of the various elements are present in a thin-film.
RBS is now a standard tool for chemical analysis. For example, it was famously used by Anthony Turkevich in 1967 to analyse the composition of rocks from the moon during the Surveyor V mission. RBS is always accompanied by particle-induced X-ray emission, that is, X-rays characteristic of the elements in the target. RBS plus this X-ray emission is known as ion beam analysis.
Turkevich claimed 1 percent accuracy for his RBS measurements. So if he measured a thin-film thickness of 100nm (nanometers), his uncertainty would be less than 1nm. However, in the 50 years since Turkevich’s claim only very careful analysts did better than 5 percent. But now we know how to reliably measure accurately.
Ion beam analysis was also used recently to discover organic “fossils” in remnants of meteorite impacts and to determine the painting techniques of famous artists. At the Louvre Museum in Paris they have an accelerator laboratory dedicated to ion beam analysis of art treasures.
‘Doping’ Microchips
The electronic magic going on in the ultra-thin surface layers of microchips is at the heart of nearly all the technology we see around us today. An important part of it is the process of doping, a way to directly manipulate the electronic behavior of a material by implanting ions (atoms or molecules that have lost one or more of their electrons) into it.
An ion implanter typically consists of an ion source, an accelerator that boosts the ions’ energy and a target chamber where the ions are implanted into the material. Implanting a few arsenic atoms here and a few boron atoms there into a silicon wafer forms the array of transistors that become the brains of “silicon chips.”
