November 2000 Meeting

High-average-power, High-pulse-rate Ultraviolet Lasers and their Applications

The Optical Society of San Diego and this evening's host, Rhonda Mason, are pleased to present a talk by Daniel J. W. Brown, Cymer, Inc., entitled "High-average-power, High-pulse-rate Ultraviolet Lasers and their Applications."

New demands for practical and efficient UV lasers are being driven by the escalating requirements of applications such as high-precision micromanufacturing and optical microlithography for semiconductor manufacturing. As a consequence, there is substantial effort worldwide being directed into the development of UV laser sources with output characteristics (wavelength, power, pulse rate) that match these applications. Of recent interest for both micromachining and photolithography applications are laser technologies that produce high average power (>10W) at multi-kilohertz pulse rates, as this combination of features makes industrial-scale implementation feasible.

This talk will describe recent developments in two areas of high-average-power UV laser technology within the author's direct experience. The first of these is the development of a high-power UV (255nm) laser system based on nonlinear frequency conversion (frequency doubling) of the visible output of copper lasers (research undertaken at the Centre for Lasers and Applications at Macquarie University in Sydney, Australia). Particular attention will be paid to the thermo-optical and beam shaping issues that need to be addressed for high efficiency, high average power frequency conversion to the ultraviolet. Several applications of frequency doubled copper laser sources will also be discussed, including high-speed micromachining of polymers and rapid fabrication of fibre Bragg gratings (FBGs). The second UV laser technology area to be discussed will be excimer lasers, which are now well established as light sources for photolithography in the semiconductor industry. An overview of excimer laser technology for photolithography and recent developments at Cymer will be presented.

Biography: Daniel Brown was born in Queanbeyan, New South Wales, Australia. He received the B.Sc.(Hons) and Ph.D. degrees from the University of New England, Armidale, Australia in 1984 and 1989, respectively. His doctoral research involved experimental studies of metal vapor laser kinetics. In 1988, he joined the Centre for Lasers and Applications, Macquarie University, Sydney, Australia, where he undertook research into new-generation metal vapor laser systems and trace impurity effects in metal vapour lasers. From 1993 to 1998, he was an Australian Research Fellow at the Centre for Lasers and Applications, where he was involved in the development of high-beam-quality copper laser systems. In 1998, he was awarded an Australian Senior Research Fellowship working on the nonlinear frequency conversion of high pulse rate laser sources such as copper vapour lasers and diode-pumped Nd:YAG lasers. In Jan 2000, he joined Cymer, Inc. where he is currently the manager of the Optics Technology Development Group.

November 2000 Meeting Review
by Steve Grove

Our November dinner meeting featured a talk by Dr. Daniel J.W. Brown of Cymer, Inc., entitled "High-average-power, High-pulse-rate Ultraviolet Lasers and their Applications." Daniel's research in UV lasers began at Macquarie University's Center for Lasers and Applications (CLA) in Sydney, Australia, where he helped develop high-power, frequency-doubled (255 nm) copper vapor lasers. Currently, Daniel manages optical technology development for Cymer's line-narrowed excimer lasers. His talk described the technology and applications for both types of lasers. The audience was entertained by Daniel's engaging speaking style, amusing anecdotes, and delightful Australian accent (and I'm not just saying that 'cause he's my boss!).

The challenges in making UV copper lasers include finding suitable non-linear crystals and delivering a high quality beam to the crystal. CLA's 1995 discovery of "kinetic enhancement" -- adding trace amounts of HCl and H2 to the standard Cu vapor / Ne buffer -- dramatically improved output power, efficiency, and beam quality. In 1998, CLA began testing newly developed CLBO crystals as an improvement over the standard BBO for frequency doubling. Daniel described how, the day after his team achieved record output power at 255 nm, they found their CLBO cube had disintegrated to a pile of dust! Simultaneously, they received notice from the supplier that CLBO was being taken off the market because its hydroscopicity made it mechanically unstable. It was over a year before this problem was resolved and work with CLBO could continue.

Applications for frequency doubled copper lasers include writing fiber Bragg gratings and micromachining. Daniel's face beamed with Aussie pride as he told how a copper laser was used to drill the nozzle holes in the Olympic torch that Cathy Freeman carried up the platform at the 2000 Summer Games in Sydney!

The talk concluded with a discussion of Cymer's krypton fluoride (248 nm) and argon fluoride (193 nm) excimer lasers. The current KrF systems achieve 20 W of line-narrowed output power at 2 kHz. Cymer builds these lasers exclusively for the microlithography industry, where excimer lasers are the only suitable light source for patterning advanced semiconductor chips. A typical projection lens for lithography contains over 20 elements and has a 0.7 NA. The system must achieve better than l/10 wavefront error across a 30 mm field, yet the lack of suitable UV materials means there is little or no chromatic correction. Consequently, Cymer must narrow the spectral linewidth to less than half a picometer.