Putting EUV Optics to the Test
Brian Dance, Contributing Editor -- Semiconductor International, 7/1/2001
One of the most promising candidates for next-generation lithography is the use of EUV radiation with a wavelength of ~13 nm to produce feature sizes of <70 nm on the chip. Each EUV station is expected to employ about 10 mirrors in its optical path, so a degradation in the mirror reflectivity of only 1% is the uppermost acceptable limit. The prevention of contamination is a key factor in preserving this reflectivity limit. The rate at which the special optical materials degrade under EUV illumination is therefore a critically important factor in the lifetime, performance and cost of ownership of future EUV systems.
A European collaborative group is using synchrotron radiation to systematically investigate the influence of different vacuum conditions on the performance of EUV optical systems during intense 13 nm illumination. This group has also developed a basic understanding of the relevant physical or chemical mechanisms responsible for the degradation of the mirror reflectivity. This work should aid the development of strategies for the prevention or reduction of contamination.
The research was performed by R. Klein, et al, of the Physikalisch-Technische Bundesanstalt (PTB, Berlin), M. Wedowski and F. Stietz of Carl Zeiss (Oberkochen, Germany) and B. Mertens et al of TNO TPD Netherlands Organization for Applied Scientific Research (Delft, Netherlands). They used radiation from the BESSY II synchrotron in Berlin.
The PTB used undispersed radiation from a long-period undulator, deflected by an angle of 13deg by a platinum-coated mirror, to simulate the irradiation conditions likely to be present in future lithography tools. The first undulator harmonic has the required wavelength of about 13 nm, with smaller contributions from higher harmonics than would be obtained with shorter undulators. Higher harmonics were further suppressed by the mirror reflection. If necessary, specially designed radiation filters based on silicon-zirconium can be used to reduce the influence of the higher harmonics and suppress any stray light in the lower range of photon energies. The irradiance at the sample is typically several milliwatts per square millimeter.
The optical performance of the samples was determined both before and after the illumination using a PTB reflectometer. The latter could measure the reflectance with a relative uncertainty of 0.25%, ensuring that even small trends present after some hours of illumination could be unambiguously qualified. Various gases were introduced via precision leak valves, and their partial pressures were monitored by a residual gas analyzer with pressures as high as 10-3 mbar.
Interest in New Superconducting
Material Is Worldwide
Scientists from around the world are intensely interested in the superconducting properties of magnesium diboride, MgB2. This simple metallic-like compound retains its superconductivity at higher temperatures than any other non-ceramic superconductor yet examined. Ceramic compounds of barium, yttrium, copper and oxygen can superconduct at temperatures of up to 160 K, but there are practical difficulties in making wires from these ceramic materials, which consist of microcrystalline grains.
MgB2 is relatively cheap compared with the widely used niobium-based materials that must be cooled to 4 K, requiring costly liquid helium. MgB2 can be cooled using electrical methods rather than liquid coolants, which further reduces costs. Unfortunately, it is available only as a pure powder, which is difficult to produce as a bulk material or in thin films.
MgB2 is readily available, but its superconductivity potential was surprisingly overlooked until it was shown to be a superconductor last January. Japanese workers at the Aoyama Gakuin University and the Japanese Science and Technology Corp. demonstrated that MgB2 shows superconducting properties at temperatures of up to 39 K (-2340°C).
Researchers at the Applied Superconductivity Center at the University of Wisconsin-Madison, working with chemists from the Materials Institute of Princeton University, have shown that MgB2 may be able to carry higher current densities than the ceramic superconductors currently in use. This is because the grain boundaries that separate the crystals in ceramic superconductors and thereby obstruct the current do not seem to do this in MgB2, so greater currents can flow in it.
Workers at the Blackett Laboratory of Imperial College, University of London, found that the critical current density in MgB2 decreases rapidly with an increasing magnetic field. They have suggested that adding defects by chemical doping or by injecting protons into it may help to raise the field at which it can remain superconducting.
