Ray S. Uttaro

Materials issues for optical components and photomasks in 157 nm lithography

Photolithography using 157 nm pulsed fluorine lasers has emerged as the leading candidate technology for the post-193-nm generation. Preliminary data have indicated that at 157 nm there are optical materials transparent enough to enable the fabrication of refractive elements, both in the projection and illumination part of the optical train. However, a number of critical issues still remain. Optical materials must show no appreciable degradation with laser irradiation. The availability of transparent photomask substrates must be ascertained. Optical coatings must be developed and qualified. At this short wavelength, interface effects, subsurface damage, and adsorbate effects become increasingly prominent. We present recent experimental results on the durability tests of calcium fluoride, modified fused silica, and optical coatings for 157 nm applications. Our initial assessment of several grades of modified fused silica demonstrates that at least one grade already meets transparency and durability require...

157 nm: Deepest deep-ultraviolet yet

Lithography at 157 nm is rapidly emerging as the industry-preferred technology for the post-193 nm era. Its target application is for the 100 to 70 nm generations, and it is therefore widely viewed as a “bridge” technology before the next-generation lithographies are ready for insertion into manufacturing. Its attractiveness stems from the overlap in many areas with current practice and shared infrastructure developed for longer wavelengths. This article will review the present status of 157 nm lithography, emphasizing the technological challenges in the various subsystems: lasers, optical materials and coatings, photomask materials, photoresists, and projection tool development. Viewed as a whole, recent developments in these diverse areas are cause for cautious optimism that indeed 157 nm lithography will be ready in time, without encountering unforeseen obstacles.

Excimer-laser-induced degradation of fused silica and calcium fluoride for 193-nm lithographic applications.

We report the initial results of a large-scale evaluation of production-grade fused silica and calcium fluoride to be used in 193-nm lithographic applications. The samples have been provided by many different suppliers of materials. A marathon irradiation chamber permits simultaneous exposure of as many as 36 samples at 800 Hz, at fluences from 0.2 to > or =4 (mJ/cm(2))/pulse and pulse counts in excess of 10(9) . The initial absorption and the laser-induced absorption are found to vary over a wide range. The compaction of each fused-silica sample follows a power law, but its parameters can differ widely from sample to sample.

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/cm2/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 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.

Performance of excimer lasers as light sources for 193-nm lithography

The performance of argon fluoride excimer lasers is an important issue in determining the practical feasibility of 193-nm exposure systems. This paper presents a summary of the experience gained at MIT Lincoln Laboratory regarding the long-term performance of 193-nm lasers, used under conditions similar to those expected in production-type lithographic systems.

Long-term 193-nm laser-induced degradation of fused silica and calcium fluoride

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 109. 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.

Assessment of optical coatings for 193-nm lithography

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/cm2/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).

Marathon damage testing of pellicles for 193-nm lithography

We investigate the effect of 193-nm radiation on commercially available pellicles for 248-nm lithography. Pellicles from two suppliers were irradiated at a realistic reticle plane fluence (0.1 mJ/cm2/pulse) for 50 million pulses. Analysis of transmission spectra revealed loss of pellicle material, decreased refractive index and increased absorption in various combinations depending on pellicle type and ambient. Although one of the two materials may be suitable for use at 193 nm, the other showed unacceptable degradation. We also quantified outgassing rates of organic species during irradiation, and observed greatly accelerated material loss in a pure nitrogen ambient compared with air. Yield rates of perfluorinated fragments and polymer product exhibited two-photon scaling behavior.

Testing of optical materials for 193-nm applications

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/cm2/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/cm2/pulse. However, initial losses in some coatings may be unacceptably high for lithographic applications.