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<SUP>2</SUP>/pulse optical materials, of which the primary candidate is calcium fluoride, should have an absorption coefficient of less than 0.002 cm<SUP>-1</SUP>, and antireflective layers should enable transmission of 98.5 percent 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.
We have completed a comprehensive evaluation of bulk materials designed for 193-nm lithographic applications. These studies are performed at realistic fluences and pulse counts in excess of 6 X 10<SUP>9</SUP>. The outcome of the study shows that most calcium fluoride materials should meet the industry lifetime targets for use in lens applications. Some fused silica material also appears to meet lifetime expectations of the industry; however, large grade-to-grade variability in both absorption and laser-induced densification has been observed. We also report on the impact of transient absorption in fused silica on lithographic dose control.
We have undertaken a systematic evaluation of both bulk material sand optical coatings designed for 193-nm lithographic applications. These studies are performed at realistic fluences and pulse counts in excess of 10<SUP>9</SUP>. Measurements of absorption is fused silica show a large variation in performance for different samples in both initial and laser-induced absorption. Calcium fluorides samples show less variation in laser-induced absorption and appear to be more stable under irradiation of 0.2-1 billion pulses. Laser-induced densification of fused silica appears to follow an empirical power law; however, an order of magnitude spread in densification is observed among grades. For optical antireflectance coatings, we have characterized the initial 'laser-cleaning' phenomenon for various coatings. We have observed that laser-cleaned coatings deposited on CaF<SUB>2</SUB> substrates exhibit higher initial optical losses at 193 nm than their counterparts on SiO<SUB>2</SUB> substrates. However, the losses for coatings on CaF<SUB>2</SUB> substrates are reduced over irradiation times of 0.2-1 billion pulses to final values comparable to their SiO<SUB>2</SUB> counterparts. Finally, we have characterized various catastrophic failures of coating material, such as induced losses, adhesion failure and laser-induced thinning.
We present an assessment of bulk fused silica and calcium fluoride, and of antireflective coatings for 193-nm lithographic applications. In the course of extensive marathon studies we have accumulated 1-5 billion laser pulses on over 100 bulk material samples at fluences from 0.2 to 4 mJ/cm<SUP>2</SUP>/pulse. The result show large variation in both initial and induced absorption of fused silica and in densification of fused silica. For antireflective coatings, there are samples that undergo no appreciable degradation when irradiated for > 1 billion pulses at 15 mJ/cm<SUP>2</SUP>/pulse. However, initial losses in some coatings may be unacceptably high for lithographic applications.
We investigated laser-induced damage of pellicles for 193-nm lithography. We surveyed 193-nm-optimized material from three pellicle suppliers. Pellicles were irradiated under realistic reticle plane conditions (0.04 mJ/cm<SUP>2</SUP>/pulse - 0.12 mJ/cm<SUP>2</SUP>/pulse for up to 100 million pulses). Pellicles from two suppliers were found to meet lifetime requirements of the industry. Pellicles from the third supplier do not appear to meet the lifetime requirements. We present fluence scaling of pellicle damage and discuss effects of the ambient on pellicle degradation rates. We present results of the outgassing studies of pellicle material under irradiation using a separate gas chromatograph-mass spectrometer-based detection apparatus. From the results of these studies, we suggest possible photochemical pathways for pellicle degradation as a function of ambient.
We present an assessment of antireflective coatings for 193-nm lithography. Coatings from nine suppliers were exposed in a nitrogen ambient for up to 1.5 billion pulses at 15 mJ/cm<SUP>2</SUP>/pulse at 400 Hz. Sensitive metrology, developed for this study, included reflectance/transmittance measurements, in-situ ratiometric transmission measurements, and interferometric calorimetry for absorption measurements. The coatings from at least two suppliers withstood greater than 1 billion pulses with no observable degradation. Catastrophic damage observed on some samples included blistering and a dramatic transmission drop. Such damage occurred rather early (less than 100 million pulses).