Making Silicon Stronger and Deformable for More Capable MEMS ComponentsJune 15, 2020 by Luke James
Researchers have demonstrated that silicon processed by modern lithography procedures exhibits an ultrahigh elastic strain limit, potentially enabling enhanced functional properties that will lead to better MEMS components.
Due to its relatively low cost and abundance, silicon is used extensively in microelectronics and has been described as the “backbone” of the information age. At ambient temperatures, however, silicon’s brittleness limits its use in micro-electro-mechanical system (MEMS) components.
Now, new research by a team at ETH Zürich, in collaboration with colleagues at the Laboratory for Mechanics of Materials and Nanostructures at Empa, reportedly shows that under specific controlled conditions, silicon can be manipulated to be much stronger and more deformable, which could enable enhanced functional properties that will lead to better MEMS components.
Micrometer silicon columns under an electron microscope. Photograph credited to Laszlo Pethö / Empa
A 10-Year Effort
The joint team’s research was a 10-year effort that focused on the production methods of silicon components by using lithography as an alternative to the focused ion beam production method. Although the ion beam method can be used to mill desired shapes into a silicon wafer, it leaves behind surface damage and defects which can cause the material to break more easily.
As an alternative, the research team tried a particular type of lithography. "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", explained Ming Cheng, a former Ph.D. student of Jeff Wheeler, the research team’s lead.
The research team then tested the strength and plastic deformability of the silicon ‘pillars’ that were produced. According to them, they say that they achieved ultrahigh elastic strain limit and near-ideal strength—the pillars produced using lithography can reportedly deform at sizes 10 times greater than what has been recorded in previous studies.
The production process the ETH Zurich researchers used for silicon pillars. The pillars are etched through a resist before being oxidized and cleaned. The right-hand image shows the end result under an electron microscope. Image and illustration credited to ETH Zurich and Laszlo Pethö / Empa
Achieving ‘Absolute Purity’
Not only did the results show that the pillars deform at sizes ten times greater than what was previously seen but that the strength of the pillars also reached levels that were previously thought of as merely theoretical for ideal crystals. What makes the pillars so strong here, according to Wheeler, is the “absolute purity” of the surfaces of the pillars, achieved through a final cleaning step. This culminates in much fewer surface defects from which a fracture could originate and decimate the material.
Wheeler also notes that his team’s results could have an immediate and profound impact on the fabrication of silicon MEMS: "In this way, the gyroscopes used in smartphones, which detect rotations of the device, could be made even smaller and more robust."
The industry is already using the combination of etching and cleaning that Wheeler and his team investigated, so their work can be used to improve current processes and be applied to other materials with crystal structures similar to that of silicon.
Another advantage of more elastic silicon is that electrical properties could be improved. Applying a large strain to the silicon can improve electron mobility which can, among other things, lead to shorter switching times by using structures integrated into a semiconductor chip.