Amorphous metallic glass for high-sensitivity MEMS microphones

November 1, 2017, Science and Technology of Materials, Interfaces, and Processing
This transmission electron microscope (TEM) picture shows an amorphous cobalt tantalum boron (CoTaB) film on a silicon dioxide (SiO2) substrate. Credit: MaiPhuong Nguyen

Advanced microphones using microelectromechanical systems (MEMS) are capable of supporting new user interactions with "smart" devices, like chatting with Apple's Siri, or Amazon's Alexa. The key to achieving the high sensitivity desired for these microphones, you might be surprised to learn, is tied to the "admittance" or "compliance" of its membrane components.

Polysilicon is the material most commonly used as a membrane for devices today. But, in general, single-crystal and polycrystalline-silicon-based devices are brittle and prone to fractures that can cause interior defects during the fabrication processes. This has lead researchers to search for a replacement material.

During the AVS 64th International Symposium & Exhibition, being held Oct. 29-Nov. 3, 2017, in Tampa, Florida, researchers from WPI-Advanced Institute for Materials Research/Micro System Integration Center at Tohoku University, in Japan, will present their work with a potential replacement material that shows promise for MEMS microphones: amorphous metallic glass.

"One of the most important specifications to evaluate these devices is signal-to-noise ratio (SNR)," said Mai Phuong Nguyen, assistant professor at Tohuku University. "SNR is a good indicator of the minimum sound level that a microphone can detect within a quiet environment."

Until now, most studies focused on improving the SNR, according to Nguyen, which is proportional to the "compliance" of the membrane. "But no significant results have been achieved, due to the limitations of polysilicon," Nguyen said.

Metallic glasses, however, share the properties of both metals and glasses. Like glasses, they have a "supercooled liquid region," in which the viscosity can be controlled.

"[T]heir amorphous structure means that are free of crystalline defects—dislocation, point defects, stacking faults, etc.," Nguyen said. "So metallic glasses possess high strength and low stiffness, which gives them the ability to store elastic strain energy and release it."

The MEMS fabrication processes are typically done at a temperature of 200 degrees C or higher, and the materials must undergo chemical corrosion during the stripping processes in lithography steps, which also etch for patterning or structuring of the . "Thermal budgets and chemical resistance may potentially have a strong impact on device performance and this needs to be explored further," Nguyen said.

The group prepared a cobalt tantalum boron (CoTaB) amorphous metallic glass by a sputter technique, a simple method commonly used by the semiconductor industry. The final thickness ranges from 100 nanometers to several micrometers, controlled by simply changing sputter conditions. They were able to confirm the most important properties of metallic glass: an amorphous structure with metallic behavior.

"We also explored the mean and gradient stress, which appear to have a strong impact on the sensitivity of devices," Nguyen said. "And we further studied compressive and tensile stress under different deposition conditions, as well as their compatibility with MEMS manufacturing processes."

Microphones are already well established in a full range of consumer products. "But there is clearly room to increase their reach within the audio supply and value chain, which would enhance audio capabilities," Nguyen said.

Explore further: Studying entropy in metallic glasses

Provided by: Science and Technology of Materials, Interfaces, and Processing

6 shares

Related Stories

Studying entropy in metallic glasses

October 10, 2017

A team led by Caltech recently solved a decades-old materials science mystery by tracking down the origin of entropy in metallic glasses.

Stretchable, transparent heater made from metallic glass

January 7, 2016

(Phys.org)—Researchers have fabricated a stretchable and transparent electrode that can be used for applications such as heating parts of the body and defrosting the side view mirrors on cars. It is the first stretchable ...

Making sense of metallic glass

February 8, 2016

If you freeze any liquid fast enough, even liquid metal, it becomes a glass. Vitrified metals, or metallic glasses, are at the frontier of materials science research. They have been made by rapidly cooling alloys of various ...

Overcoming brittleness: New insights into bulk metallic glass

November 15, 2013

From the production of tougher, more durable smart phones and other electronic devices, to a wider variety of longer lasting biomedical implants, bulk metallic glasses are poised to be mainstay materials for the 21st Century. ...

Recommended for you

Tangled magnetic fields power cosmic particle accelerators

December 13, 2018

Magnetic field lines tangled like spaghetti in a bowl might be behind the most powerful particle accelerators in the universe. That's the result of a new computational study by researchers from the Department of Energy's ...

Stretched quantum magnetism uncovered by quantum simulation

December 13, 2018

By studying ultracold atoms trapped in artificial crystals of light, Guillaume Salomon, a postdoc at the Max-Planck-Institute of Quantum Optics and a team of scientists have been able to directly observe a fundamental effect ...

The secret life of cloud droplets

December 13, 2018

Do water droplets cluster inside clouds? Researchers confirm two decades of theory with an airborne imaging instrument.

Networking goes quantum

December 13, 2018

A scientist involved in expanding quantum communication to a network of users, is continuing his work at the University of Bristol.

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.