Since the invention of the MOSFET transistor sixty year ago, the chemical element silicon on which it is based has become an integral part of modern life. It ushered in the computer age, and by now the MOSFET has become the most produced device in history. Silicon is readily available, cheap, and has ideal electrical properties, but also one important drawback: it is very brittle and, therefore, breaks easily. This can become a problem when trying to make micro-electro-mechanical systems (MEMS) from silicon, such as the acceleration sensors in modern smartphones.
At ETH in Zurich, a team led by Jeff Wheeler, Senior Scientist at the Laboratory for Nanometallurgy, together with colleagues at the Laboratory for Mechanics of Materials and Nanostructures at Empa, has shown that, under certain conditions, silicon can be much stronger and more deformable than was previously thought. Their results have recently been published in the scientific journal Nature Communications.
“This is the result of a ten-year effort”, says Wheeler, who worked as a researcher at Empa prior to his career at ETH. To understand how tiny silicon structures can deform, within the framework of an SNF project, he took a closer look at a widely used production method: the focused ion beam. Such a beam of charged particles can mill desired shapes into a silicon wafer very effectively, but in doing so leaves behind distinct traces in the form of surface damage and defects, which cause the material to break more easily.
Lithography with final cleaning
Wheeler and his collaborators had the idea to try a particular type of lithography as an alternative to the ion beam method. “First, we produce the desired structures – tiny pillars in our case – by etching away un-masked material from the areas of the silicon surface using a gas plasma”, explains Ming Chen, a former PhD student in Wheeler’s group. In a further step, the surface of the pillars, some of which are narrower than a hundred nanometres, are first oxidized and then cleaned by completely removing the oxide layer with a strong acid.