Kiwamu Takehisa, Lasertec Corp.
Although EUV lithography (EUVL) has widely been developed, technical challenges remain to be addressed, as well as the high cost of the production tool. The lithography industry has therefore been working to extend ArF immersion down to 22nm by using double-patterning and complex computational lithography. We are proposing a new, next-generation lithography (NGL) system that utilizes a Xe2 laser oscillating at 172nm or an ArCl laser oscillating at 175nm.
172nm/175nm laser radiation
There are many kinds of excimer radiations in the wavelength region of deep-ultra violet (DUV) and vacuum-ultra violet (VUV). DUV excimer radiation includes KrF at 248nm and ArF at 193nm, while VUV excimer radiations have wavelengths shorter than 193nm. Xe2 radiation from an excimer lamp has been used for surface cleaning and though the Xe2 laser uses a non-toxic gas, a rather high excitation to the laser gas is necessary to realize lasing [1]. An ArCl laser can be operated by conventional discharge pumping [2]. Unfortunately, both of these VUV lasers are not commercially available because there is no market demand. However, they have the potential to supply a large average power as they are a family of excimer lasers such as KrF and ArF.
Figure 1: Measured transmittance for BaLiF3 [3].
Fortunately, the high refractive index lens material, BaLiF3, has a high intrinsic transmission at 172-175nm (Figure 1) with a refractive index of ~1.67 [4]. Although a high refractive index liquid (HIL) has a higher absorbance than at 193nm, it has much higher transmission than at 157nm (Figure 2). Because these materials have a slightly higher refractive index than that at 193nm, we can expect a higher NA than at 193nm. Another advantage over an F2-based lithography system is that a soft pellicle can probably be used because there is much less absorption than at 157nm (Figure 3) [6].
Figure 2: Calculated absorption spectra of HIL [5].
Figure 3: VUV absorption of polymers [6].
Comparison among other DUV lithography systems
The expected performance of various lithography system options are listed in the Table below along with other DUV lithography systems including the defunct F2-based systems. The third-generation ArF immersion solution has been abandoned mainly because of the low transmission of LuAG at 193nm and the inability to develop a high-index resist.
Comparison among various optical lithography systems.
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Considerations
Although the proposed new NGL system can be used only for a single generation immediately after ArF immersion tools, EUVL exposure tools can also only be used for a single generation because the NA of the optics and the EUV power will need to be increased as the line width decreases. Considering the high R&D costs for achieving a practical EUVL system, we anticipate that our proposed NGL would realize major R&D cost savings for the lithography industry because it can be realized using already developed materials and technologies developed for the second generation ArF immersion and F2-based lithography system.
References
1. J. Kawanaka, S. Kubodera, et al., “New Xenon Excimer Lamps Excited by Quasi-cw Jet Discharges,” IEEE J. Selected Topics in Quantum Electronics vol. 1, no. 3, pp. 852-858, Sep. 1995.
2. R. W. Waynant, “A Discharge-pumped ArCl Superfluorescent Laser at 175.0nm,” Applied Physics Letters, Vol. 30, No. 5, pp. 234-235, 1977.
3. Y. Aimoto, Y. Inui, et al., “Development Status of BaLiF3 Single Crystal,” SEMATECH Litho Forum, Bolton Landing, May, 2008.
4. Y. Inui of Tokuyama Corp., private communication.
5. Y. Wang, T. Miyamatsu, et al., “Material Design for Highly Transparent Fluids of the Next Generation ArF Immersion Lithography,” 2nd Int. Symposium on Immersion Lithography, 12-15 Sep., 2005.
6. R. H. French, et al., DuPont Photomasks Inc., “Development of Polymeric Pellicles for 157nm Photolithography,” 157nm Technical Data Review, Dec. 2001.
Contact Kiwamu Takehisa, Lasertec Corp., 2-10-1 Shin-yokohama, Kohoku-ku, Yokohama, 222-8552 Japan, Fax: +81-45-478-7443, e-mail: [email protected].