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Characterization of a laser produced plasma source for a laboratory EUV reflectometer

: Scholze, F.; Scholz, F.; Tümmler, J.; Ulm, G.; Legall, H.; Nickles, P.-V.; Sandner, W.; Stiel, H.; Loyen, L. van


Engelstad, R.L. ; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.:
Emerging Lithographic Technologies VII : 25 - 27 February 2003, Santa Clara, California, USA
Bellingham/Wash.: SPIE, 2003 (SPIE Proceedings Series 5037)
ISBN: 0-8194-4842-7
Conference "Emerging Lithographic Technologies" <7, 2003, Santa Clara/Calif.>
Conference Paper
Fraunhofer IWS ()

With the development of EUV lithography there is an increasing need for high-accuracy at-wavelength metrology. In particular, there is an urgent need for metrology at optical components like mirrors or masks close to the production line. Sources for metrology have to fit different demands on EUV power and spectral shape than sources for steppers systems. We present the results of the radiometric characterization of a laser produced plasma (LPP)-source, newly developed at Max-Born-Institute Berlin for use in an EUV reflectometer. It is operated with a hig hpower pointing-stabilized laser beam (energy per pulse up to 700 mJ, 10 ns puls e duration, < ± 25 µrad pointing stability) at 532 nm which is focussed on a rotating Au target cylinder. The incident angle of the laser beam is set to 63°, the detecting angle 55° to the target normal. The source has been characterized regarding spectral photon flux, source size and source point stability. Two independently calibrated instruments, an imaging spectrometer and a double multilayer tool for in-band power measurements were used to obtain highly reliable quantitative values for the EUV emission of the Au-LPP source. Both instruments were calibrated by Physikalisch-Technische Bundesanstalt in its radiometry laboratory at the electron storage ring BESSY II. We obtained a source size of 30 µm by 50 µm (2ó horizontal by vertical) and a stability of better than 2ó=5 µm horizontally and 2ó=9 µm vertically. A spectral photon flux of 1*1014 /(s sr 0.1 nm) at 13.4 nm at a laser pulse energy of 630 mJ is obtained. The shot-to-shot stability of the source is about 5% (1ó) for laser pulse energies above 200 mJ. For pulse energies between 200 mJ and 700 mJ, there is a linear relation between laser pulse energy and EUV output. The spectrum shows a flat continuos emission in the EUV spectral range, which is important for wavelength scanning reflectometry. High stability in total flux and spectral shape of the plasma emission as well as low debris was only obtained using a new target position for each shot. There is als o a trade off between source size and EUV power. For a slightly defocused laser, an increase in EUV power up to a factor of two is obtained, while the source size also increases by about a factor of two. It is shown that an Au-LPP source provides spectrally flat reproducible emission with sufficient power at low debris conditions for the operation of a laboratory based EUV reflectometer.