Soichi Owa

Immersion lithography: its potential performance and issues

Imaging performance and issues of immersion lithography are discussed with the results of the recent feasibility studies. Immersion lithography has advantage in the numerical aperture of optics by a factor of refractive index n of the liquid filled into the space between the bottom lens and wafer. In case of 193nm exposure, water (n = 1.44) has been found as the best liquid. It is shown, by using imaging simulations, that ArF (193nm) immersion lithography (NA=1.05 to 1.23) has equivalent performance to F2 (157nm) dry (NA=0.85 to 0.93) lithography. Six fundamental issues in the ArF immersion lithography are investigated and studied. Results of the study indicate that there are no “show stoppers” that prevent going into the next phase of feasibility study.

Advantage and feasibility of immersion lithography

Immersion lithography has an advantage in the numerical aperture of optics by a factor of refractive index n of the liquid filled into the space between the bottom lens and wafer. In case of 193-nm exposure tools, water (n = 1.44) has been found as the best liquid. It is shown, by using imaging simulations, that ArF (193-nm) immersion lithography (NA = 1.05 to 1.23) has almost equivalent performance to F2 (157-nm) dry (NA = 0.85 to 0.93) lithography. Issues in the ArF immersion exposure tools are discussed with fluid-dynamic and thermal simulations results. In the fundamental issues, there seems to be no showstoppers so far, however, there exist several challenges to realize viable exposure tools.

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.

Immersion lithography extension to sub-10nm nodes with multiple patterning

This paper investigates the possibility of 193 nm immersion lithography extensions to sub-10 nm technology nodes using the patterning scheme of unidirectional (1D) grating lines and cuttings. Technological feasibility down to 5 nm nodes is examined with experimental data of self-aligned multiple patterning method (SAxP) and Litho-Etch (LE) cuttings. For the cutting by LE repetition, relationship between node definition and the repetition number n (LE^n) is discussed. Cost is evaluated for SADP, SAQP and SAOP to generate unidirectional grating formation, and the following LE^n cutting process. Finally, schemes of gridded cutting and trim are introduced, and found to be advantageous to keep the scaling merit of transistor cost at 7 and 5 nm technology nodes.

Immersion lithography: its history, current status and future prospects

Since the 1980s, immersion exposure has been proposed several times. At the end of 1990s, however, these concepts were almost forgotten because other technologies, such as electron beam projection, EUVL, and 157 nm were believed to be more promising than immersion exposures. The current work in immersion lithography started in 2001 with the report of Switkes and Rothschild. Although their first proposal was at 157 nm wavelength, their report in the following year on 193 nm immersion with purified water turned out to be the turning point for the introduction of water-based 193 nm immersion lithography. In February, 2003, positive feasibility study results of 193 nm immersion were presented at the SPIE microlithography conference. Since then, the development of 193 nm immersion exposure tools accelerated. Currently (year 2008), multiple hyper NA (NA>1.0) scanners are generating mass production 45 nm half pitch devices in semiconductor manufacturing factories. As a future extension, high index immersion was studied over the past few years, but material development lagged more than expected, which resulted in the cancellation of high index immersion plans at scanner makers. Instead, double patterning, double dipole exposure, and customized illuminations techniques are expected as techniques to extend immersion for the 32 nm node and beyond.

Development of polarized-light illuminator and its impact

Nikon has developed an illuminator with special options for RET (Resolution Enhancement Technique). For one of the solutions of RET, Nikon has pursued the development of a loss-less polarized illumination system. When the polarization direction is the same as the direction of the printed pattern, this technique improves image contrast and extends the process margin. We have simulated the impact of the RET with polarized illumination, in the case of dipole illumination and phase-shift masks, and we have estimated the dominant parameters for high performance polarized illumination. In addition, we have constructed a polarized-light illuminator and installed it in an ArF full-field scanner. We have measured and optimized the degree and distribution of polarization at the wafer plane with a special tool, and we have investigated image performance with polarized dipole illumination. Results show that the new polarized-light illuminator has extended the process margin, especially with respect to dose latitude. The results of the image simulations and experiments will be reported.

Feasibility of immersion lithography

Feasibility of ArF (193nm) immersion lithography is reported based on our recent experimental and theoretical studies. Local fill method of water, edge shot, high NA projection optics, focus sensing, water supply, polarization effect, polarized illumination and resist are investigated. Although we recognize there are some remaining engineering risks, we have judged that ArF immersion lithography is basically feasible and is a very promising method that can reach the half pitch required for the 45nm node. On this basis we have planned our development schedule of immersion exposure tools.

Tolerancing analysis of customized illumination for practical applications of source and mask optimization

Due to the extremely small process window in the 32nm feature generation and beyond, it is necessary to implement active techniques that can expand the process window and robustness of the imaging against various kinds of imaging parameters. Source & Mask Optimization (SMO) 1 is a promising candidate for such techniques. Although many applications of SMO are expected, tolerancing and specifications for aggressively customized illuminators have not been discussed yet. In this paper we are going to study tolerancing of a freeform pupilgram which is a solution of SMO. We propose Zernike intensity/distortion modulation method to express pupilgram errors. This method may be effective for tolerancing analysis and defining the specifications for freeform illumination. Furthermore, this method is can be applied to OPE matching of free form illumination source.