Yasuhiro Ohmura

High-NA and low-residual-aberration projection lens for DUV scanner

This paper describes several kinds of new technologies, which are introduced into newly developed 0.78 NA ArF projection lens for Nikons latest DUV scanner, the NSR- S306C. A new lens configuration for an ArF projection system is obtained as a result of a minute survey of the space of the aspheric optical design. The new configuration uses fewer elements and less volume of calcium fluoride (CaF2) than a conventional type. Lens mounting performance and its stability is another key issue to realizing a high performance imaging system, because lens element deformation due to lens mounting degrades imaging performance severely. Reduction of the number of the elements of a new optical design can increase room for the opto-mechanical system. Even complicated mechanisms, such as kinematic lens mounting, can fit in the space. A pure kinematic lens mounting is developed for the new ArF projection lens system to minimize lens deformation due to lens mounting. The same mechanism is applied to the positioning scheme of a lens element for high precision lens adjustment. Simultaneous use of the new lens positioning system and a lens controller can perform high precision and rather complex lens fine-tuning. Intrinsic birefringence of calcium fluoride (CaF2) is a new item, which is a hot issue in F2 optics. Even for ArF projection lens system, the intrinsic birefringence is one of the most critical issues in terms of impact upon lens performance. Special treatment is required to avoid the degradation of imaging performance due to the intrinsic birefringence effects. Resist image comparison between an ArF lens with the treatment and that without is reviewed. Finally, actual lens performance is shown.

Immersion Lithography Ready for 45 nm Manufacturing and Beyond

Enhanced resolution capability, defined in Rayleighs criterion as: R = (k1*lambda)/NA (1); where R = minimum resolution, lambda = exposure wavelength, and k1 = process dependent factor is the key motivation for the transition to immersion lithography, and the continued push for higher numerical apertures (NA). Regardless of the imaging enhancements made possible by immersion lithography though, this technology would not have been implemented in volume manufacturing if two potential showstoppers identified early on, overlay and defectivity performance, were not successfully overcome. Fortunately, intense collaboration between scanner and track suppliers, resist vendors, and IC manufacturers has yielded significant progress in the critical areas of immersion defectivity and overlay. As a result, immersion lithography is experiencing rapid adoption into mainstream semiconductor manufacturing. Hyper-NA immersion scanners, such as the Nikon NSR-S609B (NA=1.07), began shipping in early 2006 for use in 55 nm production and 45 nm process development. These systems are already being used successfully for 56 nm NAND flash manufacturing. Aggressive industry integration continues, and scanners such as the NSR-S610C (NA=1.30) are fully capable of delivering the critical performance metrics required for 45 nm half-pitch production and beyond. Current areas of industry investigation now focus on the feasibility and practicality of extending immersion lithography to 32 nm applications using new lens and resist materials, as well as exploring alternative immersion fluids to push immersion lithography as far as possible.

Progress of Nikon's F2-exposure tool development

Progress of Nikons F2 tool development is described. Intrinsic birefringence of CaF2 reported in the middle of last year by NIST had large impact on F2 optics designing. However, we believe Nikon has already overcome it, and the imaging performance of our newest design is almost the same level to the performance without the intrinsic birefringence. Several methods to correct the intrinsic birefringence are discussed in this paper. Evaluation software for the intrinsic birefringence is also developed, and simulated performances of the newest optical designs, which correct for the intrinsic birefringence, are shown. Among them, simulated CD uniformity of 35nm width gate is a good measure to evaluate the optical design performance. We have also made a steady progress on gas purging. Purging of 02 and H20 concentration less than O.lppm and lppm respectively has been attained.

Current status and future prospect of immersion lithography

Immersion lithography is rapidly approaching the manufacturing phase. A production-quality exposure tool system with NA=1.07 (Nikon NSR-S609B) was constructed to target the start of immersion lithography for IC manufacturing in 2006. Its projection optics have very small wavefront aberration and lowest local flare levels. The overlay issue has been analyzed, and its cause was found to be evaporation cooling. With the tandem stage and local fill nozzle implemented in the S609B, we have successfully avoided the evaporation cooling so that the good wet-to-dry mix-and-match overlay data have been obtained. The major part of immersion specific defects is caused by dried water-droplets, i.e. water-marks. The local fill nozzle has eliminated this defectivity by avoiding air flow in the nozzle. In the future, water immersion with NA=1.30 optics will be used for half-pitch 45nm manufacturing. Finer pattern imaging down to 32nm seems to need high-index material immersion or nonlinear double patterning, but these have several issues and concerns to be solved.

An aberration control of projection optics for multi-patterning lithography

In order to realize further improvement of productivity of semiconductor manufacturing, higher throughput and better imaging performance are required for the exposure tool. Therefore, aberration control of the projection lens is becoming more and more important not only for cool status performance but also heating status. In this paper, we show the improvements of cool status lens aberration, including scalar wavefront performance and polarization aberration performance. We also discuss various techniques for controlling thermal aberrations including reduction of heat in the lens, simulation, compensating knob, and adjusting method with actual imaging performance data during heating and cooling.

Catadioptric lens development for DUV and VUV projection optics

According to the International Technology Roadmap for Semiconductors (ITRS), the 65nm technology node is forecast to appear in 2007. In this paper, we propose two specifications for the projection optics at 65nm nodes. The one is over 1.0 numerical aperture (NA) at 193nm lithography by liquid immersion. The other is 0.85 NA at 157nm lithography. Since it almost impossible for traditional dioptric optics to realize these specifications, catadioptric is supposedly the leading optics for an extreme optical lithography, like 65nm node. Described in the paper are feasibility study for catadioptric optics, and our assembly strategy. Emphasis is placed on our selection methodology among a variety of catadioptric configurations.

Thermal aberration control for low-k1 lithography

For many years, we have used a lens aberration controller that works via positioning elements of the projection lens assembly. While this has worked well, its disadvantage is that controllable aberrations are only relatively low order components and not enough for the degree of compensation of thermal aberrations required by leading-edge lithography. We have developed two methods to overcome thermal aberrations specific to dipole illumination exposure. One scheme is process-dedicated aberration control by the conventional aberration controller. The other is aberration control system using infra-red irradiation. This system can compensate uniform astigmatism which is generated by asymmetric setting of illumination light sources, such as dipole illumination schemes. Theses two techniques allow us to increase productivity by reducing pattern imaging performance degradation due to thermal aberrations. These schemes are applicable not only to current systems but also to next generation very low k1 lithography systems with very high throughput.

Nikon F2 exposure tool development

Present status of development of F2 (157nm) exposure tool in Nikon is described. Key points of F2 exposure tool are reported; low aberration projection optics, CaF2 quality, coating durability and gas purging of the pellicle space. We also report the measurement of refractive index inhomogeneity inside CaF2 crystals, which is suspected as the cause of local flare. Characteristics of high NA optics over 0.9 are investigated by imaging simulations for both 193nm and 157nm wavelengths, which are compared NA=0.85 imaging.

Imaging optics setup and optimization on scanner for SMO generationprocess

Source & Mask Optimization1 (SMO) is a promising candidate to realize further reduction of k1 factor to achieve 22nm feature lithography and beyond. To make the SMO solutions feasible all imaging-related parameters should be closer to the designed parameters used in SMO process. In this paper, we discuss how we realize this in the imaging system setup on the scanner. The setup process includes freeform pupilgram generation, pupilgram adjustment and thermal aberration control. For each step the important factors are speed and accuracy.

Current status of high-index immersion lithography development

High index immersion lithography (HIL) is one candidate for the next generation lithography technology following water immersion lithography. This technology may require only moderate changes of chip making processes and result in lower cost of ownership (CoO) compared with other technologies such as double processing, extreme ultra violet lithography (EUVL), and nano-imprinting, and other technologies. In this paper, the current status of high index lens material and immersion fluid development compared with our requirements is discussed considering microlithographic lens design feasibility and attainable NA.