Jan H. C. Sedlacek

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...

Liquid immersion lithography: Why, how, and when?

Liquid immersion lithography, especially at 193nm, is a serious candidate for extending projection optical lithography to the 65nm node and beyond. This article reviews the status of this technology, the potential pitfalls that it may still encounter, and also the potential to extend it to 157nm and to higher-index liquids. At 193nm, no fundamental obstacles have been found yet, although defect control and materials compatibility must still be worked out. At 157nm, significant progress has been made in developing suitable liquids. The next hurdle is to increase their refractive index, in order to make the transition in wavelengths cost-effective.

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.

Laser patterning of metal oxide superconductor films by reactive solid-state transformation

A planar submicrometer-resolution patterning method has been developed for fabrication of thin-film Ba/sub 2/YCu/sub 3/O/sub x/ devices without photoresist, water, or solvent exposure. The method is based on a rapid transformation from the superconductive to a dielectric phase. The phase change is induced by controlled changes in the oxygen stoichiometry which are induced thermally by local-area laser irradiation of the thin film in a gaseous ambient. Both extended-area pattern projection and scanned-beam direct writing have been demonstrated with a spatial resolution in the submicrometer range and are presently limited by the grain size of available films. Negligible thickness loss is observed in patterning. The method circumvents lithographic techniques which tend to degrade the electronic quality of Ba/sub 2/YCu/sub 3/O/sub x/ superconductors.<<ETX>>

Photolithography at 193 nm

Photolithography at 193 nm is a natural continuation of the progression from 436 to 365 to 248 nm in lithography, dictated by the requirement for continually higher resolution. It is anticipated that 193‐nm lithography will enable 0.25‐μm patterning in volume production with conventional masks, and 0.18‐μm resolution with phase‐shifting masks. The main issues related to lithography at this new wavelength are being addressed. It has been shown that highly transparent optical materials are available at 193 nm. Also, they are damaged by the laser radiation at a slow enough rate that high‐quality projection optics are expected to perform within specifications for ten years of full‐time operation. Consequently, a 193‐nm step‐and‐scan system is being constructed, and it has been designed to attain 0.25‐μm resolution over a 22 by 35 mm field. A range of 193‐nm photoresist schemes has been demonstrated. They include semitransparent single‐layer resists, positive‐tone surface imaging (silylation), and negative‐ton...

Reversible laser chemically induced phase transformations in thin‐film Ba2YCu3Ox superconductors

Phase transformations of a thin film of Ba2YCu3Ox were induced with a focused laser beam in chemical ambients. The transformations, involving superconductive and nonsuperconductive phases, are achieved rapidly and with a high degree of spatial control. They are fully reversible, and the appropriate processing parameters have been studied. These effects are interpreted within present models, which relate the superconducting properties of Ba2YCu3Ox to its oxygen content and crystalline structure.

Laser‐direct‐writing processes: Metal deposition, etching, and applications to microcircuits

The laser‐directed discretionary deposition of metal films broadens the scope of electronics fabrication and provides new capabilities for building and restructuring microelectronic devices. When such depositions are combined with local laser‐guided etching processes, multilevel modifications are enabled. This mode of processing allows the modification of individual circuits on a micrometer scale, as well as permitting the interconnection of larger circuits composed of multiple devices, both on a single substrate (‘‘wafer‐scale’’ integration) and in multichip hybrid configurations. In this paper, recent advances in the interconnect deposition processes useful for these applications are described, as well as some principal demonstrations on functioning circuits.

Laser photochemical etching of molybdenum and tungsten thin films by surface halogenation

Laser direct‐write etching of the refractory metals Mo and W has been developed using reactions in Cl2 and NF3 vapors. Rates and high spatial resolution are simultaneously optimized using a two‐vapor halogenation/development sequence, based on surface modification. Local‐area laser chlorination of the metal surface is used to predispose areas to subsequent bulk etching.

Optical materials for use with excimer lasers

Synthetic UV-grade fused silica, crystalline fluorides, and dielectric coatings have been evaluated for transparency and durability at 193 nm. Most bulk materials eventually develop color centers, and fused silica also changes its density and index of refraction. However, the rate at which these changes occur and their magnitude vary strongly with material, grade, and other more subtle details. Careful selection and possibly pretesting are recommended, in order to ensure optimal matching between the intended application and the material properties.