LITHOGRAPHY SERIES, PART 1: Low-k1 imaging for contacts and lines using immersion ArF
02/01/2005
Immersion lithography has the potential to extend current 193nm argon-fluoride technology to the 45nm process node by effectively reducing the wavelength of exposure light for improved depth-of-focus and enabling lens designs with numerical apertures >1.0. To prove the potential benefits of immersion technology, experimental 0.75 NA ArF dry imaging data have been compared to those of a 0.75NA ArF immersion scanner. The combination of resolution enhancement techniques and immersion lithography showed a substantial gain in DOF for dense patterns as well as the so-called forbidden pitches.
Interest is rapidly growing in the use of 193nm immersion lithography to print the critical layers of the 45nm process node. In general, the resolution of printed patterns can only be improved by increasing the numerical aperture (NA) or decreasing the Rayleigh k1 factor and/or the wavelength (λ). The NA and k1, however, have limits defined by physics for dry imaging: NA cannot go beyond 1.0, and for conventional optical lithography, the lower limit of k1 is 0.5. By using adequate resolution enhancement techniques (RET), this limit in k1 can be further extended toward its theoretical limit of 0.25. To further enhance the resolution, a wavelength change is required, involving completely new lens materials and design.
Immersion lithography offers an adequate alternative because it enables an effective wavelength reduction by replacing the air between the final imaging lens and the wafer with water or another liquid. The effect is twofold:
- It allows lens designs with higher NA - even >1.0 - therefore allowing improved resolution.
- It enhances the depth-of-focus (DOF) for a given NA.
Recent progress in 193nm immersion tool and process development has made this relatively simple extension of 193nm ArF lithography very attractive, especially in view of the difficulties encountered with 157nm scanner development (the economics of lens material and the development of adequate resist materials). Researchers worldwide are exploring immersion-specific processing effects and immersion-induced imaging effects in an effort to develop adequate solutions for the 45nm technology node. In addition, using high-NA lenses requires a profound investigation of the impact of polarization. Indeed, at these very high NAs, the light will couple under high angles into the photoresist.
Work covered by this article demonstrates the gain in DOF that is achieved with 193nm immersion lithography for printing both lines and contact holes at a wide range of pitches.
Low-k1 imaging of critical patterns
In generating the critical patterns of an IC with the desired resolution, mask type and illumination condition are among the most crucial parameters. To approach the ultimate resolving power of a given lithography toolset, various RETs are widely used in combination with dry lithography. Apart from the traditional binary masks, attenuated phase-shift masks (PSM) and alternating PSMs are now largely available. The combination of these reticles with off-axis illumination techniques, and the variation of NA/sigma (sigma being the partial coherence or fill factor) settings that the scanners permit, offer the optical scientist a wide range of possibilities when optimizing the printing conditions of a given design. Still, for each critical layer (i.e., gates and contact holes), the preferred enhancement technique needs to be selected and implemented.
For each optical enhancement technique or combination thereof, the corresponding resolution limit can be defined, and through-pitch solutions can be explored. In general, good resolution and DOF are obtained for the denser patterns. For more relaxed pitches, however, there is usually a pitch range, called the “forbidden pitch,” for which the DOF is greatly reduced. With this study, IMEC has demonstrated that 193nm immersion lithography can significantly improve the DOF for dense as well as sparser pitches, for both lines and contact holes. Illumination and mask settings were first optimized for a dry 0.75NA ArF scanner, and their corresponding resolution limits were determined. These dry experimental data were then compared with those of a 0.75NA ArF immersion scanner to prove the gain in DOF.
Line imaging
Using ASML Holding NV’s PAS 5500/1100 0.75NA ArF dry scanner, lines were printed at a range of pitches, starting at 160nm pitch (or a k1 factor = 0.31). Several mask types were used, including traditional binary masks and attenuated phase-shift masks for single-exposure patterning. For double-exposure patterning, alternating phase-shift masks and double-dipole binary masks were selected. Different types of off-axis illumination, including annular, quadrupole (ASML’s Quasar and cQuad, which is Quasar with the center of the poles on the x and y axes), and dipole illumination, were used. Subresolution assist features were applied to improve process capability of semidense through isolated lines, and sigma settings were selected to either enhance the printability of the densest pitch or to provide good through-pitch pattern printability.
All experiments were performed with the same resist process (at 150nm resist thickness), exposure tool, and metrology. At each illumination condition and mask type, researchers at IMEC identified the resolution limit, process capability, and forbidden pitches based on process windows and mask error factors (MEF) at different pitches. The outcome showed the optimal enhancement technique for printing a given pitch range [1].
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Through-pitch solutions were explored, starting at three different resolution limits that were defined based on the capabilities of the various enhancement techniques. The data show that up to a 190nm pitch (k1 = 0.37), Quasar illumination in combination with a binary reticle represents the best through-pitch solution. Attenuated phase-shift masks and annular illumination provide a through-pitch, single-exposure solution if the densest pitch on the design is 180nm (k1 = 0.35). The high MEF at the 180nm-pitch resolution limit may be a concern. A through-pitch imaging solution starting at the most aggressive 160nm pitch as resolution limit is only obtained by double exposures using double-dipole illumination or alternating PSMs (k1 = 0.31). Images of printed logic designs are shown in Fig. 1. A difficulty here is caused by the required design split.
For every enhancement technique, the pitch range studied revealed a forbidden pitch, which has an almost complete first-order capture but is just before or at the second order entering the lens pupil. The reduced DOF is critical for this forbidden pitch.
The 0.75NA ArF dry imaging data were then compared to experimental data of a 0.75NA ArF full-field immersion scanner (ASML’s AT1150i) at the same sigma settings. Based on the results for dry imaging, five different illumination conditions were selected. At every illumination condition, a dense pitch and a forbidden pitch were measured (Fig. 2). When switching to immersion lithography, the target critical dimension (CD) was maintained over a larger DOF. The results show a 40-100% gain in DOF compared to a conventional dry ArF scanner. Only for the extreme off-axis cQuad illumination was immersion lithography unable to generate a good process window for the forbidden pitch. For Quasar illumination, multiple pitches were measured for dry and immersion imaging (Fig. 3). Again, the DOF gain by immersion was 60-100%.
Since the next technology nodes probably will make use of high-NA immersion designs, the pitch range that can be covered with the current extension techniques has been extrapolated to a 0.85NA immersion system. From these simulations, pitch resolution limits that can be achieved using 0.85NA ArF immersion lithography are expected to be ~30nm lower than the corresponding dry resolution limits for the three through-pitch imaging solutions proposed (see table). Whereas for a dry 0.85NA tool the DOF is expected to be lower than for a corresponding dry 0.75NA tool, the loss in DOF by going from 0.75 to 0.85NA will be compensated by immersion lithography.
Contact-hole imaging
Contact-hole patterning is regarded as one of the biggest challenges in the extension of 193nm lithography, because the Rayleigh k1 factor shrinks to 0.4 and below. The state-of-the-art 6% transmission attenuated PSM with a conventional illumination setting is a typical solution for semidense through isolated contacts. At the targeted resolution limit (k1 = 0.39 or 200nm pitch), however, gaining both the resolution at the 200nm pitch and an acceptable DOF for the looser pitches is impossible. It raises the need for more aggressive RETs.
An exploration of the various single-exposure imaging solutions [2] showed that off-axis illumination (OAI) in combination with nonprinting assist features is a very attractive technique. Nonprinting assist features are enabled by both binary intensity mask (BIM) and chromeless phase lithography (CPL). This work summarizes the main results obtained with both BIM and CPL in combination with 193nm dry lithography at 0.75NA (ASML PAS5500/1150 scanner) and compares the experimental results with those obtained with immersion lithography (ASML AT1150i scanner). Here, 100nm contact holes were targeted from a 200nm pitch through an isolated process. Again, all experiments were performed using the same resist process at 300nm resist thickness, and the same metrology. The through-pitch behavior of the most critical metrics, such as MEF, process windows, forbidden pitch, and assist slots printing were investigated.
BIM with OAI shows good DOF performance in the dense pitch range, but demonstrates the need for assist slots from semidense to isolated pitches (pitch >325nm). For pitches >325nm, a DOF >0.31µm could be obtained at 8% exposure latitude (EL). The minimum DOF is measured at the 325nm pitch itself with 0.29µm at 8% EL. Acceptable MEF values were measured through pitch, except for the 325nm forbidden pitch (MEF slightly above 3). These results were compared with data measured using immersion lithography, showing a 70% gain in DOF at the 325nm pitch, and between 70% and 100% DOF benefit at the other pitches (Fig. 4).
A second technique that has been studied is CPL, which is based on interference mapping lithography (IML). Details on the technique and maskmaking process can be found in other works [3]. First, a field around the contact hole is mapped, allowing slot size and position and their optimum transmission and phase to be assigned. The lighter or darker spots in the map indicate respectively whether 180° phase or 0° phase light would have the strongest positive interference at the contact location. Gray regions indicate where a Cr shielding is better (i.e., where light would degrade the image quality at the contact location). Using this CPL technique, we measured values <3 for the MEF, which now has become a multidimensional metric depending on Cr, slots, and contact bias variations. When using 193nm immersion lithography, >0.40µm DOF (at 8% EL) could be measured through pitch. At the critical 300nm pitch, a DOF of 0.43µm could still be measured, showing again an improvement using immersion lithography instead of dry lithography (Fig. 5).
Through a k1 extrapolation at higher NA values, these experiments have proved useful for the 65nm technology node and beyond.
Conclusion
The combination of 193nm exposure systems and immersion lithography has recently shown promise for printing critical patterns in 45nm node processes. The results of this study demonstrate a significant gain in DOF obtained with 193nm immersion lithography at 0.75NA for both lines and contact holes at optimized illumination and mask conditions. Through-pitch solutions have been obtained, even at forbidden pitches. Extrapolation of these data toward higher (0.85) NA lens designs has led to prediction of the possible pitch range for the next technology node. It is expected that even higher NA designs (NA = 1 or larger) will be enabled, which allow denser feature printing. However, the higher-NA designs also will require better understanding of the imaging effects specific to high NA.
The results of this exploratory study are encouraging in view of increased interest in 193nm immersion lithography for the 45nm technology node. More research is needed and will be performed within IMEC’s industrial affiliation program on immersion lithography, which was launched in mid-2004. Within this program, immersion-specific processing and imaging effects will be further investigated, as well as high-NA polarization issues. The results of such R&D in 193nm technology may also be useful for 157nm immersion lithography at future technology nodes.
Acknowledgments
CPL, IML, and Quasar are trademarks of ASML Holding NV.
References
- E. Hendrickx, P. Monnoyer, L. Van Look, G. Vandenberghe, “Optical Extensions Towards the 45nm Node,” Proc. SPIE, Vol. 5377, Optical Microlithography, pp. 357-368, 2004.
- V. Wiaux, P. Montgomery, G. Vandenberghe, P. Monnoyer, K. Ronse, et al., “ArF Solutions for Low-k1 Back-end Imaging,” Proc. SPIE, Vol. 5040, Optical Microlithography XVI, pp. 270-281, 2003.
- V. Wiaux, J. Bekaert, F. Chen, S. Hsu, K. Ronse, et al., “Through Pitch Low- k1 Contact Hole Imaging with CPL Technology,” Photomask Japan 2004, Symposium on Photomask and Next Generation Lithography Mask Technology XI, 5446-109.
Mieke Van Bavel received his PhD in physics and is scientific editor at IMEC, Kapeldreef 75, B-3001 Leuven, Belgium; ph 32/16-288-010, fax 32/16-281-637, e-mail [email protected].
Eric Hendrickx received his PhD in physical chemistry and is a member of IMEC’s Optical Extensions and Imaging Group.
Vincent Wiaux received his PhD in physics and is a member of IMEC’s Optical Extensions and Imaging Group.
Geert Vandenberghe received his PhD in inorganic chemistry and is the program manager for IMEC’s Immersion Industrial Affiliation Program.