Eckehard D. Onkels

Feasibility of highly line-narrowed F2 laser for 157-nm microlithography

Highly line-narrowed F2 laser operation in the VUV has been achieved for the first time by means of a master oscillator/power amplifier laser design. Different concepts have ben investigated experimentally for the master oscillator (MO) in order to obtain narrowband spectra. The diffraction grating based design showed to be limited to a FWHM of approximately 0.4 pm. The spectral FWHM of the MO could be further reduced to below 0.3 pm with a double etalon-based resonator. Single pass amplification was employed to increase the beam energy density of the beam up to 50 mJ/cm2. The spectral FWHM of the amplified light is slightly larger than the FWHM of the correspondent MO radiation, indicating saturation and/or inhomogeneous broadening of the F2 amplifier medium. Experimental data obtained from broadband operation and ASE measurements suggests that the free running bandwidth of F2 lasers result form spectral gain-narrowing of the laser medium.

Advanced VUV spectrometer for F2 laser metrology

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.