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.

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 requirements for reticle substrates for 157 nm applications. For both bulk calcium fluoride and antireflectance coatings our tests to date show no degradation for 300 million pulses at fluences up to 3 mJ/cm 2 /pulse. We do observe degradation of beam steering mirrors in our experimental setup. Detailed damage analysis of these coatings is presented.

No abstract

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.

Fluid management issues in an immersion lithography system include the retention of the liquid ͑i.e., prevention of residual liquid on the wafer͒ and the possible entrainment of gas bubbles within the immersion fluid. Three key parameters strongly influence the control of fluid within the lens gap: the static liquid/resist contact angle, the contact angle hysteresis, and the dynamic contact angle characteristics. This article presents a comprehensive set of data for these parameters on silicon wafers coated with six different photoresists and describes the experimental apparatus and data reduction techniques used to collect the data. Measurements for six candidate photoresists, one with a top-coat, indicate that air entrainment due to contact line motion is highly unlikely for typical immersion lithography systems. However, significant contact angle hysteresis does exist that may lead to meniscus failure and to the deposition of droplets at low to moderate wafer velocities. In addition, the receding dynamic contact angle can approach zero on some surfaces, leading to liquid film deposition.