Compact UV source rivals excimer

17 Jun 2002

US scientists have developed compact UV-emitting lamps that will rival excimer sources used in lithography.

By injecting a low-energy electron beam into a mixture of gases, scientists in the US have come up with an efficient way to produce UV light. With efficiencies up to 50%, the researchers claim that their compact sources are candidates for future small-feature lithography.

The new sources are smaller than excimer lasers and emit light down to 80 nm.

Lead researcher Daniel Murnick of Rutgers University, US, told Optics.org: "Our technique involves the injection of low-energy electrons into a cold, high-pressure gas. We have produced a variety of lamps and demonstrated prototype infrared and visible lasers using this technique. UV lasers are in development."

Most existing UV lamps and lasers use a high-temperature electrical discharge to spark the emission in the gas. As an alternative, the team uses a compact electron gun, similar to those in cathode ray tubes (CRTs) or microwave tubes.

The CRT sources are 2 cm in diameter and 10 cm long while the microwave tube sources are smaller still.

Murnick and colleagues have also managed to reduce the size of the light-emitting gas volumes. Conventional excimer laser systems, such as argon-fluoride and fluorine, are synonymous with large dimensions.

However, Murnick said: "In our small lamps, the volumes are of the order of millimeter in size and can be scaled to large-area flat lamps. The lasers are of the order of 10 cm in length."

Both sources are based on a mixture of the rare gases helium, neon, argon, krypton and xenon. Light is emitted from 80 nm to 357 nm, with a unique narrow band at 121 nm. The output wavelength depends on the gas mixture used.

"Efficiences as high as 50% from electrical energy to UV light have been measured," said Murnick.

The lamps can operate in both continuous-wave (CW) and pulsed mode. Point CW lamps have powers of around 100 mW while large-area sources emit approximately 20 mW/cm². Higher powers are achieved in pulsed mode.

Murnick anticipates that the lithography sector will be the first to benefit from these sources. "Inspection and cleaning are near-term applications," he said. "As the lasers are developed, they may have a narrower linewidth than existing 193 nm and 157 nm excimer lasers for chip manufacture."

Rutgers University is currently licensing the technology. Prototype lamps have been tested by next-generation lithography consortium International Sematech.

Author
Jacqueline Hewett is news reporter on Optics.org and Opto & Laser Europe magazine.