Long-term testing of optical components for 157-nm lithography

Photolithography utilizing 157-nm excimer lasers is a leading candidate technology for the post-193-nm generation. A key element required for successful insertion of this technology is the near-term performance and long-term reliability of the components of the optical train, including transparent bulk materials for lenses, optical coatings, photomask substrates, and pellicles. For instance, after 100 billion pulses at an incident fluence of 0.5 mJ/cm(2)/pulse optical materials, of which the primary candidate is calcium fluoride, should have an absorption coefficient of less than 0.002 cm(-1), and antireflective layers should enable transmission of 98.5% for a two-sided coated substrate. Modified fused silica has emerged as a viable option as a transparent photomask substrate, and several approaches are being explored for transmissive membranes to be used as pellicles. This paper presents results on wide-ranging studies of optical components for 157 nm. These include absorption measurements and lifetime evaluation of crystalline fluoride materials, antireflectance and high reflector coatings, and pyroelectric detectors. Optical properties of several grades of materials hom worldwide suppliers have been evaluated in-situ under 157-nm irradiation at realistic fluences (0.1-3 mJ/cm(2)/pulse) for pulse counts exceeding I x 10(9). Materials characterization under these long-term irradiation conditions has been performed in three test chambers, one equipped with a 200-Hz 157-nm laser and two operating at 600-1000 Hz. Oxygen- and moisture-free environment inside the chamber is assured with a nitrogen purge and continuously monitored with chemical sensors with sub-ppm resolution.