Higher Repetition Rate Makes ArF Laser More Production-Ready
Aaron Hand, Managing Editor -- Semiconductor International, 1/1/2001
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Cymer Inc. (San Diego) has taken the argon fluoride (ArF) laser to the next level, increasing the laser's power and stability, and decreasing its bandwidth. The company expects to begin shipping the NanoLith 7000 (Figure) to exposure tool manufacturers next quarter.
The NanoLith 7000's repetition rate of 4 kHz is double what was previously available and four times what some chipmakers began using in 1999 for process development. The higher repetition rate, combined with a 5 mJ pulse energy, gives the laser an average output power of 20 W. The high output power is what enables it to keep up with the high-throughput scanners needed for volume chip production.
 The high output power and narrow bandwidth of Cymer's NanoLith 7000 brings 193 nm lasers for lithography applications into the realm of volume chip production. (Source: Cymer) |
To achieve the 20 W output, Cymer engineers had to make considerable improvements to the laser's thermal management system. The 193 nm ArF photon energy is twice what the 248 nm KrF photon energy is, and the gain efficiency is half. This means that a 20 W ArF excimer has to have four times the cooling capacity that a 20 W KrF laser does.
In addition to the 20 W average output power, volume 193 nm circuit printing is expected to require an energy stability of <±0.5% and a bandwidth of <1 pm at 95% energy integral. The NanoLith 7000 maintains the energy dose stability of £±0.3% that Cymer's most recent model has, and achieves a smaller bandwidth of £0.35 pm at full-width, half maximum (FWHM).
Typically, when a laser's power is increased, its bandwidth broadens. Despite this, Cymer's developers have achieved a bandwidth of 0.95 pm at 95% energy integral. This is the first time a 193 nm excimer laser has achieved sub-1 pm bandwidth at 95% energy integral, according to David C. Brandt, senior director of marketing. This parameter plays the biggest role in contrast loss through the lens train, he said. The symmetric, narrow line that the laser pulse produces — its spectral purity — decreases lens aberration to provide high-contrast imaging for systems with high-NA lenses.
One factor that Brandt emphasized was the NanoLith 7000's low cost of operation. For example, compared with the current capabilities of the ELX-5000A, the 1 kHz ArF laser that Cymer introduced in 1996, the NanoLith 7000 cuts the cost of consumables by about 80%.
This is achieved primarily by increasing the lifetime of three modules: the laser discharge chamber (whose lifetime has increased from 1.2 billion to 6 billion pulses), line narrowing module (1.2 billion to 6 billion pulses) and stabilization module (1.2 billion to 10 billion pulses). Although a negligible figure in terms of consumables, a gas life of >100 million pulses helps to decrease system downtime.
Cymer has sold about 50 ArF lasers to universities, research institutes and lithography system manufacturers since 1989. The company's first 193 nm model, CX-1A, was a broadband laser with a 100 Hz repetition rate and an output power of 4 W. The 10-11 years it has taken to reach a laser worthy of volume production is about the same cycle time it took for the company's KrF laser, Brandt said.
"In 1996, the world didn't believe the 1 kHz KrF laser was possible," he said. "Now we have a greater than 99% uptime on those systems." •
For additional information on lithography, go to www.semiconductor.net/lithography
