LTB delivers Echelle-based spectrometers for characterizing lithographic lasers
Berlin, Germany, July 22, 2004--LTB Lasertechnik Berlin GmbH has delivered the first units of its newly developed spectrometer, the ELIAS III, to several lithographic-laser manufacturers. The instrument can measure a minimum linewidth of 20 femtometers (fm) FWHM (full-width at half maximum) at a wavelength of 193 nm. This corresponds to a spectral resolution of 10 million.
To be able to measure such a narrow linewidth, it is necessary to verify the resolution capability or system function of the spectrometer with a laser that has a significantly narrower spectral bandwidth than 20 fm. Consequently, LTB acquired the first unit sold in Europe of a 193-nm Indigo-DUV laser produced by Coherent Inc. (Santa Clara, CA) with a FWHM linewidth of 6 fm. (Because this laser is expensive, LTB Lasertechnik will allow those interested to commercially use the laser in the company's applications laboratory.)
The ELIAS III spectrometer series' spectral resolution of 20 fm FWHM at a 193-nm wavelength can be restated, in other units, as 150 MHz or 0.005 cm-1; the instrument can resolve a spectral purity of 175 fm at a 95% energy content. (For the characterization of the spectral purity of lithography lasers, it has become internationally accepted to measure the spectral bandwidth into which 95% of the energy goes. Such a measurement can only be made at a very high signal-to-noise-ratio--typically 20,000, or greater than 40 dB. Contrary to Fabry-Perot spectrometers, the Echelle spectrometer in the ELIAS series makes it possible to quantitatively analyze the wings and the spectral shape of the laser lines.) The ELIAS III instrument was developed in collaboration with the Institute for Spectrochemistry and Applied Spectroscopy Berlin.
The main application area of these high-resolution spectrometers is the spectral characterization of excimer lasers for microlithography at 248, 193, and 157 nm. This type of characterization is important as it indirectly determines the quality of integrated circuits produced by means of optical lithography. The quality of the exposure process is determined by the spectral properties of the excimer lasers, as well as by the optical lens systems. Improving the spectral bandwidth of the lasers (by narrowing the line and raising the spectral purity) makes smaller pattern sizes possible. With an ELIAS instrument, manufacturers of lithography lasers can improve their lasers so that feature sizes of 65 nm can be achieved.
Optical lithography is, however, only one application area for the ELIAS series. Other applications include atom-absorption spectroscopy and the development of laser diodes.