Thin diamond films provide new material for micro-machines

By Jared SagoffJuly 31, 2014

ARGONNE, Ill. – Airbags, inkjet printers and video projectors may not seem to have much in common, but all three rely on the action of tiny, microscale devices in order to work properly.

These devices, known as microelectromechanical systems (MEMS), are of growing interest to researchers because of their wide range of applications, from microphones to biosensors.

Most of the current generation of MEMS are made primarily of silicon, which is due in large part to the relative ease of making silicon-based devices with current technology. However, silicon-based MEMS have a few significant drawbacks: they wear out very quickly due to friction and they are not biocompatible – preventing possible future medical applications within the human body.

Researchers at the Center for Nanoscale Materials at the U.S. Department of Energy’s Argonne National Laboratory and a handful of other institutions around the world have directed their focus to exploring MEMS made of a relatively new material known as ultrananocrystalline diamond (UNCD), which are smooth and wear-resistant diamond thin films.

“Robust and reliable MEMS are needed for sliding and rotating actions on the small scale,” Argonne nanoscientist Anirudha Sumant said. “Silicon does not have nearly as good mechanical or wear properties as diamond.”

But the relative difficulty of trying to fashion a MEMS device from UNCD complicates matters. MEMS have to be made extremely precisely, and their components cannot shift out of place.

The goal of the experiment was to make the part of a MEMS device known as the actuator, which in this case converts heat energy into mechanical displacement or motion. The actuator looks like a web of diamond wires that expands and contracts like a bellows as it is heated and cooled. This web of diamond filaments is attached to a long shaft, which can then in turn be attached to a microgear system to provide rotational motion.

Unfortunately, the diamond material is beset by stresses that scientists need to find ways to work around. The material is especially notorious for what Sumant calls “compressive stress,” a phenomenon that occurs because the diamond’s atomic network does not expand that much during the hot phase when the film is deposited on another material. “The main question we’re trying to solve is how to reduce the intrinsic stress in this film,” Sumant said.

Fortunately, several of the properties of the UNCD film help to relieve the stress. UNCD consists of tiny diamond grains connected by grain boundaries. “You can think of these grain boundaries as a badminton net; they are flexible instead of rigid, which is good for accommodating stress,” Sumant said. “There’s also uniform grain size throughout from bottom to top, which is important for keeping the stress low.”

The researchers were able to tune the intrinsic stress by optimizing the grain boundary materials and the thickness of the films.

“This really opens the door for using diamond for fabricating advanced MEMS devices,” Sumant said.

A study based on the research, “Electrically conducting ultrananocrystalline diamond for the development of a next generation of micro-actuators,” appeared in the May 2 issue of Sensors and Actuators A: Physical. The research was performed in collaboration with Merlijn van Spengen at Tu Delft University from the Netherlands and was supported by the U.S. Department of Energy’s Office of Science.

Argonne National Laboratory seeks solutions to pressing national problems in science and technology. The nation's first national laboratory, Argonne conducts leading-edge basic and applied scientific research in virtually every scientific discipline. Argonne researchers work closely with researchers from hundreds of companies, universities, and federal, state and municipal agencies to help them solve their specific problems, advance America's scientific leadership and prepare the nation for a better future. With employees from more than 60 nations, Argonne is managed by UChicago Argonne, LLC for the U.S. Department of Energy's Office of Science. For more visit www.anl.gov.

The Center for Nanoscale Materials at Argonne National Laboratory is one of the five DOE Nanoscale Science Research Centers (NSRCs), premier national user facilities for interdisciplinary research at the nanoscale, supported by the DOE Office of Science. Together the NSRCs comprise a suite of complementary facilities that provide researchers with state-of-the-art capabilities to fabricate, process, characterize and model nanoscale materials, and constitute the largest infrastructure investment of the National Nanotechnology Initiative. The NSRCs are located at DOE’s Argonne, Brookhaven, Lawrence Berkeley, Oak Ridge and Sandia and Los Alamos National Laboratories. For more information about the DOE NSRCs, please visit the Office of Science website.