Japanese partners develop new laser for EUV lithography

01 Oct 2013

Gigaphoton and Mitsubishi Electric claim ‘critical step forward’ in development of production-worthy extreme ultraviolet light sources.

Lithography light source specialist Gigaphoton says that engineers at the firm have, with collaborators Mitsubishi Electric, developed a 20 kW carbon dioxide drive laser to support the expected switch by chip makers to extreme ultraviolet (EUV) lithography.

Beset by development problems, EUV technology is slated to start appearing in semiconductor fabrication facilities in a big way from next year onwards. Initial shipments of the systems by ASML will feature sources provided by its own Cymer division, based on CO₂ lasers from Trumpf, but Gigaphoton has long been working on a rival approach.

“Initial experiments conducted on the jointly developed CO₂ drive laser has produced output power of over 20 kW, signifying that a key piece of technology necessary for producing EUV output levels of 250 W, which is necessary for high volume manufacturing (HVM), has been realized,” announced Gigaphoton.

At the moment, however, the EUV source power is nowhere near that HVM level, with Gigaphoton only just realizing a prototype with an average 15 W output at the key 13.5 nm wavelength.

Earlier this year, ASML and Cymer reported stable operation of their source at 55 W, but Gigaphoton is playing the long game, believing that its own laser-produced plasma (LPP) design will eventually pay dividends when the EUV lithography market becomes much more significant.

Efficiency drive
Gigaphoton has been working on LPP light sources since 2002, using technologies such as a separate pre-pulse laser to provide more efficient EUV conversion in the plasma, and magnetic debris mitigation to ensure clean operation in the source chamber.

Its CEO Hitoshi Tomaru says that the latest work with Mitsubishi shows that sufficient output levels have been achieved for the production environment, and that the company’s LPP light source is “showing firm progress towards high volume production level output”.

“This also demonstrates progress towards realizing our vision for a green, environmentally friendly LPP light source,” he continued. “Our efforts will help to bring the industry closer to realizing EUV lithography scanners for high volume manufacturing.”

Those “green” credentials relate to efforts designed to improve the efficiency of EUV generation, which suffers from huge energy losses in generating the necessary CO₂ laser power and then converting that drive laser power into EUV emission.

In the past, the approach has been likened by those closely involved in the EUV field to “using a nuclear power station to provide electricity to a village” – hardly surprising, given that Gigaphoton set a conversion efficiency world-best of just over 5% last year.

“Improving the conversion efficiency of the EUV light generation and achieving efficient and effective mitigation of debris from the collector mirror are key issues that must be resolved for production level EUV light sources,” admits Gigaphoton.

It believes that highly efficient EUV output is possible through its technology, which uses a 1 micron wavelength pre-pulse laser to increase the size of the plasma created from fine droplets of tin before it is hit by the main CO₂ drive laser.

“Efficient and effective debris mitigation is also possible through Gigaphoton’s unique method of utilizing a superconducting magnet to generate a powerful magnetic field that guide the unwanted debris resulting from the thermal expansion of the tin droplets towards the tin catcher,” says the Japanese firm. “This results in further reduction of cost and downtime.”