Researchers at the University of Alabama in Huntsville (UAH) invented a new way to deposit thin layers of atoms as a coating onto a substrate material at near room temperatures.
Dr. Moonhyung Jang, postdoctoral research associate at UAH working in the laboratory of Dr. Yu Lei, an associate professor in the Department of Chemical Engineering, got the idea to use an ultrasonic atomization technology to evaporate chemicals used in atomic layer deposition (ALD) while shopping for a home humidifier.
ALD is a three-dimensional thin film deposition technique with exceptional control of nanometer features while depositing materials uniformly on large silicon wafers for high volume manufacturing. Each cycle deposits a layer a few atoms deep. The ALD process repeats the deposition cycle hundreds or thousands of times. The uniformity of the thin films relies on a surface self-limiting reaction between the chemical precursor vapor and the substrates.
While browsing online for a safe and easy-to-use home humidifier, Dr. Jang observed that humidifiers on the market use either direct heating at high temperature or ultrasonic atomizer vibration at room temperature to generate the water mist. He suddenly realized that the latter could be a safe and simple way to generate vapors for reactive chemicals that are thermally unstable.
Typically, ALD processes rely on heated gas-phase molecules that are evaporated from their solid or liquid form. Some chemical precursors in these processes are not stable and can decompose before reaching a sufficient vapor pressure for ALD.
“In the past, many reactive chemicals were considered not suitable for ALD because of their low vapor pressure and because they are thermally unstable,” says Dr. Lei. “Our research found that the ultrasonic atomizer technique enabled evaporating the reactive chemicals at as low as room temperature.”
The UAH scientists’ ultrasound invention makes it possible to use a wide range of reactive chemicals that are thermally unstable and not suitable for direct heating. The group’s ultrasonic atomization process supplies low vapor pressure precursors by evaporating them by ultrasonically vibrating the module.
“Like a household humidifier, ultrasonic atomization generates a mist consisting of saturated vapor and micro-sized droplets,” Dr. Lei says. “The micro-sized droplets continuously evaporate when the mist is delivered to the substrates by a carrier gas.”
Small liquid droplets in the mist are evaporated in the gas manifold under vacuum and mild heat treatment, leaving behind an even coat of the deposition material.
The research demonstrated proof of concept by comparing titanium oxide ALD using thermally evaporated and room-temperature ultrasonic atomized chemical precursors and found the thin film quality is comparable.
“Using the room-temperature ultrasonic atomization reported by our manuscript, new ALD processes could be developed using low volatility and unstable precursors,” Dr. Lei says. “It will open a new window to many ALD processes.”
The paper published on the invention was selected as an editor’s pick in the Journal of Vacuum Science & Technology A.
For more information:
University of Alabama in Huntsville
https://www.uah.edu/
