Microdischarge devices shed light on variety of applications
By Laura Schmitt
January 1, 2002
- Using standard micromachining and silicon processing technology, ECE researchers have developed light-emitting gas-discharge devices small enough to be integrated on a millimeter-sized chip.
- "We've made an entire family of devices that are about five orders of magnitude smaller than fluorescent tubes used in overhead lighting," said ECE Professor Gary Eden.
Using standard micromachining and silicon processing technology, ECE researchers have developed light-emitting gas-discharge devices small enough to be integrated on a millimeter-sized chip. These devices, which are able to produce light from the far infrared to the soft x-ray region, may someday replace much larger and more expensive light sources for a variety of applications, including cancer detection and treatment and new gas and liquid sensors.
"We've made an entire family of devices that are about five orders of magnitude smaller than fluorescent tubes used in overhead lighting," said ECE Professor Gary Eden. "The emission we generate depends on the gas or gas mixture we use."
According to Eden, a major application area for the gas-discharge devices is flow cytometry, a medical technique for sorting healthy cells from bad cells by illuminating them with visible or ultraviolet light and then examining the spectrum they produce. Currently, a large laser costing as much as $50,000 is used as the light source.
"Our technique appears to be a very attractive way of mounting a light source directly into a cytometry system on a chip," Eden said. Not only that, but Eden's inexpensive optical array could produce light at various wavelengths, resulting in the capability to also destroy the undesirable cells.
Another medical application for Eden's devices is photodynamic therapy (PDT), a fairly new treatment that uses a fixed-frequency laser light and a photosensitizing agent to destroy certain types of cancerous cells.
"PDT is used all over the world, but its use has been limited because it requires a laser and not too many clinics can afford that," Eden said. "What we have in mind is a flexible array that would be inexpensive and could be wrapped around a patient's arm or chest with a Velcro strip. [The light source] lifetime is not the key issue. It doesn't have to last a thousand hours, it has to last two."
Eden is collaborating with ECE Assistant Professor Chang Liu, postdoctoral researcher Sung-Jin Park, and graduate research assistants Clark Wagner, Nels Ostrom, Scott McCain, Min Cai, and Jack Chen. They make one type of their discharge devices by chemically etching square pyramids upside down in a silicon wafer. A cavity having the same dimensions as the pyramid is also formed through an insulating film (dielectric) and a conducting film (anode). They then fill the cavity with a gas or mixture of gases and apply a voltage from the conducting film to the silicon, producing light.
"It lights up like a Roman candle," Eden said. "For 40 years, researchers have been trying to develop an intense optical source in silicon. The irony is that we've effectively done that, using the plasma or gas as an energy converter."
Eden's team has arranged multiple devices together in as many as four 15 x 15 arrays. These arrays have a radiating area of two square millimeters and include 900 individual devices, each measuring 50 microns on a side. In addition to silicon, the devices can be integrated on ceramic and other material substrates.
For example, Eden, ECE Adjunct Professor Bruce Vojak, and researchers at Motorola in Tempe, AZ, are developing ceramic devices based on Motorola's multi-layer technology originally developed for use in wireless circuit components.
"Because we use standard processing technology, these devices are straightforward to make and they're inexpensive," Eden said.
Packaging the arrays or devices is a challenge that Eden and his students are currently investigating. Laminating an array between two sheets of plastic is one inexpensive, yet not quite perfect, solution.
"Our devices currently last only 15 minutes not because they fail, but because the gas inside the devices becomes poisoned with organic vapors that bleed out from the plastic laminate," said Eden, who thinks this is not a major problem. "We need to deposit a transparent barrier on the laminating sheet to block the diffusion of the vapors into the microdischarge devices."
Eden's former student Cy Herring, who earned his PhD in physics in 1998, cofounded a company in Urbana, to explore commercial applications for the devices. According to Herring, Caviton is developing microdischarge devices for a variety of sensors that could be used in an environmental pollutant monitoring system, an oil and engine analysis instrument, and a portable gas and liquid analyzer.
"These sensors are small, lightweight, and use little power, so portable devices are a prime use of the technology," said Herring, who is working with Ju Gao (PhD '98) and biochemistry doctoral candidate David Kellner at Caviton.
Eden has three patents for his micro- discharge technology already, and he and his colleagues have filed for four more. Several display manufacturers have recently expressed an interest in Eden's technology, as have some lighting companies.
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