In 1995, University of Illinois Professor J. Gary Eden was approached by James Frame and David Wheeler, two graduate students in the Department of Electrical and Computer Engineering.
They showed him a block of silicon and asked, “Do you mind if we drill a small hole in this and see if we can fill it with a gas and generate a plasma?”
“Of course not, go ahead,” Eden told them.
Coming on 30 years later, the descendants of that initial experiment constitute a completely new technology: microcavity plasma devices.
As discussed in the October cover story of Plasma Processes and Polymers, they gave rise to a host of compact and affordable devices manufactured by three companies—EP Pure, Eden Park Illumination, and Cygnus Photonics—that stand to revolutionize water purification, air and surface disinfection, and electronics fabrication. And all because the University of Illinois researchers took on a project that just seemed interesting.
Microcavity plasmas: smaller is better
Like other plasma technologies—including neon signs and fluorescent lightbulbs—microcavity devices operate by applying a high voltage to pull electrons away from the atoms or molecules of a contained gas. The result, a plasma, can be used to either generate light or drive chemical reactions.
However, instead of housing it in a large tube, the new technology confines the plasma in an array of small cavities, each less than a millimeter in size. This gives the devices a couple of key differences that make them especially appealing for applications.
Microcavity devices operate at atmospheric pressure, whereas the gases in standard plasma devices need to be vacuum-pumped. This greatly simplifies the manufacturing process, since the housing unit does not need to account for the pressure difference.
In addition, the small cavity size also means that they use much less electrical power than standard devices, significantly prolonging their operating lifetimes.
Ozone production and water purification
The most prominent application of microcavity plasma devices to date has been producing ozone for drinking water disinfection. Chlorination is not viable in many parts of the world and poses hazards to human health and the environment, making ozonation an appealing alternative.
The traditional barriers of cost and power consumption are overcome by miniature chemical reactors using microcavity plasmas developed in Eden’s lab. These units produce ozone from room air at a rate of 0.3 grams—enough to disinfect 10 gallons of water—per hour. Because they are small and consume less than 15 watts of electrical power, they have proven ideal for disinfecting drinking water in off-grid communities in more than 20 countries.
The largest facilities to use these devices are operating in the Kisumu region of western Kenya. Constructed and installed by a partnership between the University of Illinois at Chicago, the Safe Water and AIDS Project of Kenya, and the Eden Park Foundation, each of two self-sufficient “kiosks” produces 2000 liters of clean drinking water every day from contaminated river or surface water, and they are operated, maintained, and managed by local Kenyans.
The application of microcavity plasma light sources to disinfect air and surfaces in public spaces has attracted intense attention in the last three years.
Lamps that emit ultraviolet light to kill microbes—so-called “germicidal lamps”—are a well-established technology but the wavelength they use, 254 nanometers, is known to be carcinogenic to humans. Based on University of Illinois technology, Eden Park Illumination designed a microcavity lamp that emits 222-nanometer light which kills viral and bacterial pathogens. However, the wavelength cannot penetrate the outer layer of human skin, so it is safe for human exposure.
Sales of these lamps were modest prior to the Covid-19 pandemic, but demand boomed thereafter. They are now used in locations around the world, including restaurants, the Seattle Space Needle, and a US military base.
“Magic lamps” for electronics fabrication
An unexpected application of microcavity plasma lamps is coming to commercial maturity now: electronics fabrication.
Early research by Dane Sievers, an engineering teaching laboratory coordinator, and Andrey Mironov, a research assistant professor in ECE, resulted in a lamp that operates at 172 nanometers—in the far ultraviolet spectrum. They realized that the photons it generates can break most chemical bonds. So, if a patterned template is placed over virtually any polymer surface, the lamp will destroy the exposed polymers and etch the pattern.
This makes the devices ideal for printing integrated circuits. Standard methods for etching circuit patterns are costly, take place in a vacuum, and require toxic substances. In contrast, a process facilitated by microcavity-based lamps is inexpensive, works in a nitrogen atmosphere at room temperature, and the final development stage only uses rubbing alcohol.
The improvements are so vast that Sievers calls the microcavity plasma devices “magic lamps.” He and Mironov are working to bring these techniques to industry with their startup company Cygnus Photonics.
Because they could do better
Microcavity plasma devices have given rise to a trove of applications, but they began as a purely fundamental investigation. Eden encourages his students to devote 20% of their time to study topics of their choice. So, when Frame and Wheeler read a paper in which plasmas were shown to exist in small cavities and thought, “We can do a lot better,” they were allowed to pursue it for its own sake.