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14 September 2001 Advanced VUV spectrometer for F2 laser metrology
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The advent of 157 nm F2 lasers in lithographic application implied new challenges in spectral metrology. The approaches for the lithographic imaging system, that have been suggested so far, differ in the requirements of the spectral bandwidth of the laser. However, even designs with less stringent demands will require high-resolution spectral metrology in order to enable comprehensible spectral bandwidth and purity measurements or specifications. Ideally, narrowband calibration sources in the VUV range should be used to precisely determine the instrument function of the spectrometer, enabling correct spectral de-convolution. However, most schemes for generation of appropriate light sources are rather complex and thus expensive. Stability and lifetime of solid state sources, i.e. nonlinear optical devices, are expected to be not satisfying too. The principal approach for a spectrometer design should be to increase the inherent spectral resolution of the instrument above the required specification limits of the laser systems under investigation, avoiding or at least significantly reducing the necessity of spectral de-convolution. Following this path, the optical layout of an existing Echelle grating based spectrometer has been investigated and re-designed. Collimation and imaging quality of the spectrometer could be considerably improved with the implementation of an aspheric focusing mirror.
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Eckehard D. Onkels, German Rylov, and Richard L. Sandstrom "Advanced VUV spectrometer for F2 laser metrology", Proc. SPIE 4346, Optical Microlithography XIV, (14 September 2001);

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