The semiconductor industry is on the verge of a major technological breakthrough. From 2009 onwards, circuits will be created by exposure to extreme ultraviolet (EUV) rays. Researchers from Aachen were awarded the Stifterverband Science Prize – not only for their EUV source.
Since the earliest days of semiconductor fabrication, chips have been created by exposure to light. A stepper projects light through a mask on to the surface of a silicon wafer coated with photoresist, thus transferring the tiny structures. For more transistors to fit on a single chip, the structures have to be made smaller still. Manufacturers are therefore compelled to turn to light sources with ever-shorter wavelengths, and indeed, the use of sophisticated technology has enabled them to produce structures that are even smaller than the wavelength. Today, processors are fabricated using 90-nanometer technology in lithography plants where excimer lasers operate at the 193-nanometer wavelength in the ultraviolet range. However, conventional optical lithography reaches the limits of its physical capability at about 50 nm.
This situation calls for the development of entirely new lithographic methods. Since the tremendous cost of research and development is beyond the reach of individual manufacturers, the semiconductor industry has investigated various techniques in existence such as extreme ultraviolet EUV, x-ray, electron-beam and ion-beam lithography, and chosen the most promising technique: EUV. This technology now features on the roadmaps of all semiconductor manufacturers. Intel, for example, plans to use it in mass-producing chips from 2009 onwards.
EUV lithography uses radiation with a wavelength of only 13.5 nanometers. However, this leap in technology represents a greater challenge than any previous move to a new generation because it means developing completely new light sources, optical components and photoresist finishes. Since EUV is absorbed by all materials – including air – the entire lithographical process has to be performed in a vacuum. Conventional optical systems cannot focus light of such short wavelengths, so multi-layer mirrors have to be used. Semiconductor manufacturers thus face a multitude of unsolved technical problems and a gigantic investment risk.
“The crux of EUV lithography is to have an efficient and economic EUV source available,” stresses Klaus Bergmann of the Fraunhofer Institute for Laser Technology ILT. EUV radiation can be produced in extremely hot plasma in two different ways: by gas discharge or by laser induction. The researchers at the Aachen-based institute proposed a hollow-cathode gas discharge method, which earned them an outstanding rating in an international comparison. In recognition of the close and fruitful cooperation between research and industry, the alliance comprising Fraunhofer ILT, the department of laser technology at the RWTH Aachen and the two industrial companies AIXUV and Philips Extrem UV, is to be awarded the Science Prize of the Stifterverband (Donors’ Association for the Promotion of Humanities and Science) at the annual general assembly of the Fraunhofer-Gesellschaft.
The foundations for the patented concept were laid at the ILT and the department of laser technology in the period from 1997 to 2000. An international comparison revealed that the “Aachen lamp” was superior to the solutions proposed in the US. However, it needed strong partners in industry to enable it to compete effectively. In the year 2000, Rainer Lebert – who at that time was employed by Fraunhofer ILT – spun off a new company, AIXUV. The company manufactures and sells compact, reliable EUV sources for basic investigations in laboratories, and follow-up systems for EUV measurement engineering and quality assurance. “We supplied Infineon with an EUV photoresist exposure device already capable of producing structures at 50 nanometers. And then for Schott, we developed an reflectometer for mask qualification,” says Lebert, recalling two of his previous assignments.
The researchers took a further important step towards industrial-scale production in 2001 with the foundation of Philips Extreme UV GmbH, a joint venture of the Fraunhofer-Gesellschaft and Philips. The company’s mission is to develop EUV sources for series production of semiconductors. Its first step was to sign a contract with ASML, the leading manufacturer of lithographic equipment in the Netherlands, for the delivery of four prototypes. “We started off very well compared to our rivals in the USA and Japan,” says Joseph Pankert, CEO of Philips Extreme UV. “At the present time, though, Intel in particular is applying high pressure. The great advantage of our concept is that it is the cheapest, the simplest and the most compact. We plan to deliver the next prototype of a high performance EUV source to ASML next year.”
The semiconductor industry imposes very high demands. The EUV source must produce a light output of at least 100 W. Researchers working for Philips Extreme UV have meanwhile succeeded in improving the world record to about 30 W. Cost-effective chip production calls for the exposure of about 120 wafers per hour. The plasma must not exceed one millimeter in size, and must reach a temperature of 220,000 °C. Such extreme temperatures can only be controlled in short high-energy pulses so as not to destroy the materials the light source is made from.
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