Made2Measure Heaven: Plasma FIB

Who are you? Dr Mark J. Whiting

What is your role? I’m an Academic in the Department of Mechanical Engineering Sciences at the University of Surrey.

What is your work about? My research centres on the role of microstructure and especially interfaces in the manufacture and performance of advanced metallic materials. One way to study interfaces is to use a plasma focused ion beam or PFIB.

I beg your pardon? The engineering performance of many materials depends on the nature of interfaces as well as the physical processes that occur at these boundaries. Many current and next-generation structural materials bring together diverse materials in composites, combing a metal and an inorganic material. These include glass-to-metal seals, previously featured on this site. Another example, also featured here, are SiC monofilament reinforced titanium matrix composites. The interfaces not only determine some aspect of performance, but their exact nature depends on processing and manufacture. Even monolithic materials owe some of their performance to interfaces created during manufacture—additive manufacturing processes for metallic materials have a molten metal/gas interface throughout production. Gas absorption and oxidation etc. can all alter this interface prior to its becoming part of the bulk.

A new method for studying interfaces as well as other microstructural features is plasma focused ion beam, PFIB, technology. My two colleagues, Professor John Watts and Dr David Cox, and I were recently awarded an EPSRC grant to purchase such an instrument for the University of Surrey. This venture is in partnership with the NPL who are interested in the materials metrology capability the instrument.

Mark

Figure 1 – The author discussing plasma technology with a Cyberman (Cyber-leader 768HY98)

What is a PFIB? FIB, or focused ion beam, techniques have become firmly established in the last decade as ways to make both devices and specimens for characterisation. In these traditional FIB methods liquid gallium is ionised and used to remove material. In this way, thin slices of many functional and structural materials can be made for study by techniques such as transmission electron microscopy. The selective removal of material down to the nanometre scale means that very small devices can be fabricated from a multiplicity of materials. The study of interfaces by electron microscopy and allied advanced characterisation techniques requires greater effort to gain nanometre-scale information reproducible over reasonable length scales. The use of gallium to remove material poses some problems. Gallium can react with many metals, in some cases forming low melting point eutectic alloys. More fundamentally, there is a limit to the ion current and therefore speed with which a gallium ion beam can remove or ‘cut’ material. The recently commercialised plasma FIB uses xenon ions. Xenon is not only inert but can also offer significantly increased ion currents and sputter yield. The two current manufacturers both quote at least a sixty-fold increase in throughput. In most cases this enhanced milling speed will offer the opportunity to fabricate bigger devices with nanometre-scale detail or the production of larger more representative samples.

And? Much materials characterisation studies specimens which are so small that there are often significant doubts as to whether the specimen is representative of the bulk material. Rather than study a handful of grains and three grain boundaries, PFIB means that hundreds of grains and thousands of boundaries can be studied. The speed of PFIB also makes possible the 3D characterisation of materials on a sensible timescale.

So what? The University of Surrey’s PFIB will be used for a multiplicity of engineering projects. The work that I will do with it offers science which will enable (i) the manufacture of lighter, stronger and stiffer structural materials, and (ii) the additive manufacture of advanced structural materials which produce less waste than materials made by subtractive manufacture.

Final Thought: Plasma FIB offers the ultimate in Made2Measure Materials. It offers new possibilities for the manufacture of devices, new capabilities for materials metrology as well as being a technique that offers characterisation capability informing the science underpinning modern materials manufacture.