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Abstract
Plasmonic focusing was investigated in concentric rings with a central pillar under linearly polarized illumination with a specific incident angle. When changing the incident angle of linearly polarized beam between 6 and 15 degree away from the normal direction, the focal spot size can keep a steady value of 37 nm, smaller than the focal spot with the radially polarized beam at the same excited condition, 45 nm. Combining this with the high-speed near-field photolithography technology, we demonstrated a plasmonic lithography with 16.85 nm linewidth on both organic and inorganic photo-resists in large scale at scanning speeds up to 11.3 m/s. This inclined linearly polarized illumination is easy to realize in a prototype of near-field photolithography system, and it opens a new cost effective approach towards the next generation lithography for nano-manufacturing.
© 2017 Optical Society of America
1. Introduction
Photolithography can create super-fine nanoscale patterns with high throughput and promise broad applications in integrate circuit manufacturing, nano-electronics, and data storage [1,2]. Optical diffraction limit, however, poses a critical challenge to the down-scaling of nano-scale manufacturing. Although focused ion beam (FIB), extreme ultra violet (EUV) and 193 nm immersion lithography are expected to deliver sub-10 nm and smaller nodes by optimizing photochemical reaction mechanism, adopting quadruple patterning lithography, and the most important shorter wavelengths, they still cannot effectively address the responsible working conditions and cost issues required for mass production [3–6]. Near-field photolithography, as illustrated in Fig. 1, is a newly emerging technique differing from the above-mentioned methods, which can process the substrate with high throughput and low-cost at a large scale [7]. It realizes the ultrahigh resolution beyond the Rayleigh diffraction limit with a specially designed plasmonic focusing lens in the near-field, which is fabricated on the bottom surface of a plasmonic flying head. The specific structure of plasmonic lens can excite surface plasmon polaritons (SPPs) and/or localized surface plasmons (LSPs), concentrating short-wavelength surface plasmons into sub-50 nm spots with high field enhancement effect [8–13]. The focused and enhanced beam at the vicinity of the lens center acts on the photoresist film at the focal point. The near-field photolithography could achieve nanopatterning on a large scale by high-speed scanning or sweeping across the substrate. To achieve high-speed scanning while maintaining the effective nanogap, the flight principle of the magnetic head in hard-disk drives (HDDs) has been used [14]. The unique air bearing surface (ABS) structure of the head with an array of plasmonic lens, generating an aerodynamic lift force between the rotating substrate and the ABS, results in a plasmonic flying height (FH). A consistent FH can be maintained by the high air bearing stiffness and small actuation mass of suspension. The self-adaptive flying control of the gap in tens of nanometers between the plasmonic flying head and the high speed revolving disk is insensitive to environmental vibration. With the disk coated by an inorganic TeOx-based thermal photoresist film, Xiang Zhang’s group patterned the film with 80 nm linewidth in 2008 using near-field photolithography [15], and the result has been updated to 22 nm in 2011 by progressive focusing scheme with radially polarized illumination [16]. However, this technology still faces key obstacles such as super-small focal spot, which can be controlled by changing the optical excitation condition.
Fig. 1 Near-field photolithography schematic. (a) Schematic shows the plasmonic lens focusing 355nm laser beam onto the rotating substrate to concentrate SPPs into sub-50 nm spots, which is only produced in the near field of plasmonic lens. Hence, a distance control system is needed to maintain the gap between the substrate and the lens. (b) Cross-section of the plasmonic head flying 30 nm above the rotating substrate which is coated with photoresist.
For a plasmonic lens, the progressive coupling of SPPs and LSPs of multi-stage scheme has shown that it is capable of efficiently compressing the optical energy at deep sub-wavelength scale [17–21]. The reason is that the SPPs is suitable for providing enough energy throughput, and LSPs can achieve deep sub-wavelength optical confinement due to its resonating nature. Radially polarized illumination is normally used in the multi-stage scheme in order to achieve the small and sole focal spot with high field enhancement intensity [22–24]. However, the complex optical system of radially polarized illumination and strict requirement for the centering between the plasmonic lens and the polarized beam severely affect the focal spot size and the energy throughput during the near-field photolithography. On the other hand, although the linearly polarized beam is easy to adjust in system, its two focal spots larger than that of radially polarized beam are not suitable for the near-field photolithography due to its destructive interference in the center of the symmetrical multi-stage scheme.
In this letter, we proposed and investigated a new method, exciting SPPs at special incident condition, to concenter the large two focal spots of linearly polarized beam into a sole point. The sole focal point could be smaller than that of the radially polarized illumination at the same excited condition. The finite difference time domain (FDTD) simulations and lithography experiment results over both organic and inorganic photoresists proved that the focal spot is steady when the incident angle of linearly polarized beam is in a suitable range, and near-field photolithography with higher resolution and stronger stability is achievable.
2. Simulation methods and results
A symmetric plasmonic lens, several concentric rings with a central pillar, was used to excite both SPPs and LSPs. Through this progressive focusing scheme, as illustrate in Fig. 2(a), that combines SPPs focusing and LSPs conversion, the incident light can be squeezed into the deep sub-wavelength scale with normal radially polarized illumination [25]. When the polarization state of incident light is changed from radially polarization to linearly polarization (transverse magnetic mode), the destructive interference of the concentric rings results in a dark spot in the center of the plasmonic lens, as shown in Fig. 3c, and the size of the two side focal spots, 62 nm [Fig. 3(d)], is much bigger than the radially polarized illumination, 45 nm [Fig. 3(b)]. However, it is worth to note that the lithography patterns can be controlled by changing the polarization and incident angle of the illuminating beam. The period of interference fringes by vertically incident light is , where is the wave vector of SPPs. While, if polarized light is incident at angle to the film normal [Fig. 2(b)], the oscillatory term in the intensity will be changed. In the + x direction, the fringe period is , which is larger than that when it is in the opposite (−x) direction, , whereis the wave vector of space [26]. The two SPPs with different propagating periods in ± x directions can be coupled to a sole focal point by the concentric rings, resulting in the beam-focusing of the linearly polarized beam with an incident angle.
Fig. 2 The structure of the concentric rings with a central pillar (a), and the plasmonic lens is illuminated by linearly polarized beam with incident angle (b).
Fig. 3 The focusing effect illustrated by radially polarized beam and linearly polarized beam without or with incident angle under TM mode. Figure (a) and (b) are the effect excited by radially polarized beam. Figure (c) and (d) are the effect excited by linearly polarized beam without incident angle. Figure (e) and (f) are the incident angle is equal to 1°. Figure (g) and (h) are the incident angle is equal to 8°.
The finite-difference time-domain (FDTD) software was used to analysis the focal effect with changing the incident angle. Three sets of calculations without and with an incident angle of linearly polarized beam are shown in Fig. 3, comparing with the radially polarized illumination. Figures 3(a) and 3(b) are the focusing results of radially polarized beam. Its focal spot size is 45 nm. Figure 3c is the simulation result of the linearly polarized beam illuminating without incident angle (i.e., ), Fig. 3(d) is the cross-section of this focal spots, the full width at half maximum (FWHM) are both 62 nm. When the angle of is equal to 1° (Figs. 3(e) and 3(f)), the focusing effect was redistributed, and the focal size is smaller than the perpendicular incident. If is increased to 8°[Figs. 3(g) and 3(h)], there is only one focal spot, the size of which is decreased to 37 nm, smaller than the radially polarized illumination, 45 nm. Figure 4 shows the focal size simulation result with continuous variation of incident angle. When the incident angle is between 6°and 15°, FWHM can be stable within the minimum size, 37nm. In addition, in the investigated cases, we found that the field enhancement effect gets weaker as the focal size becomes smaller when increasing the incident angle. The results imply that the linearly polarized beam can achieve better focusing effect than radially polarized beam and maintain the small size under a certain condition. Moreover, the reduced field enhancement effect is beneficial to weaken the stripping of the high energy beam to the metal layer and to maintain the long-term stability of the system.
Changing the transverse magnetic (TM) mode to the transverse electric (TE) mode, the focal size and field enhancement intensity show different trends. As illustrated in Figs. 5(a)-5(d), the linearly polarized incident angle are 1°, 3°, 5°, and 8°respectively under TE mode. Unlike the TM mode, two spots always exist under TE mode. In addition, the field intensity of the two spots decreases as the angle of incidence increases, but the intensity of side lobs increases. The results imply that TE mode cannot lead to effective SPPs phase change and desired field enhancement effect with a focal spot.
Fig. 5 The focusing effect illustrated by linearly polarized beam with incident angle under TE mode. (a)-(d) correspond to incident angle are 1°, 3°, 5°, and 8°respectively.
3. Experimental setup
In the near-field photolithography experiment, as illustrated in Fig. 6, a 355 nm continuous laser was uesd as the light source, which was modulated into pulsed waves by EOM. The incident light focused down to a spot of 3μm by the lens onto the plasmonic lens, which further focused the beam to less than 50 nm by the plasmonic lens on the spinning disk. The disk rotated at 5400 rpm, corrsponding to a linear speed of 11.3 m/s at the 20 mm radius. With the linearly polarizer installed on a high precision 3-axis stage, the illumination angle of linearly polarized beam was adjusted before the near-field photolithography. The plasmonic flying head was installed in the rotating stage driven by a voice coil motor (VCM) via a spring suspension. With the high bearing stiffness and small actuation mass of the suspension, the self-adaptive flying ability can provide a steady FH between the head and the rotational substrate. The use of the VCM eliminates the need for a feedback control and therefore overcomes the major technical barrier for high-speed scanning. The inorganic photoresist TeOx was sputtered on the surface of the glass disk and developed in 0.5% KOH solution after photolithography. The organic photoresist FPT-8Boc was spin-coated on the disk at 3000 rpm and baked at 100°C for 180 seconds. After photolithography, the FPT-8Boc was baked at 100°C for 60 seconds to refrain the over diffusion of photoacid generator firstly, and then developed in the solvent of propylene glycol 1-monomethyl ether 2-acetate (PGMEA). A transition zone [14] was formed to ensure the plasmonic flying head taking off steady over the soft photoresist.
4. Experimental results and discussion
We fabricated the plasmonic lens with a diameter of about 7 μm on a 50 nm chromium (Cr) metal film. The Cr film is selected because its good mechanical properties can prevent the lens from damages when the lens flies within a few nanometers above the photoresist at high speeds of several meters per second. The whole geometric parameters of the plasmonic lens are optimized through simulations [17]. The ring pitch of the lens is 350 nm, groove width is 120 nm, ring number is 9, and the central pillar diameter is 40 nm [see Fig. 7(d)]. High throughput writing requires the using of a large number of lens arrays at high speed. As illustrated in Figs. 7(a) and 7(b), the lens array was fabricated at the bottom surface of the head by focused ion beam (FIB) milling on Cr film coated on the ABS.
The sensitivity of photoresist can be controlled by adjusting the concentration of the acid generator in the photoresist formulation. Narrower linewidth than the focus size can be obtained by adjusting the sensitivity so that the photoresist exposes in a high energy region. This can be illustrated in Fig. 8.
Using advanced ABS design and micro-fabrication technology, the plasmonic flying head made of quartz can provide 30 nm FH above a resist surface with a speed of 10~12 m/s. Our near-field photolithography system could pattern the substrate by using a pulsed laser or a continuous laser with an electro optic modulator (EOM). Figures 9(a) and 9(b) show the scanning electron microscope (SEM) images of photolithography results on inorganic TeOx photoresist illuminated by linearly polarized beam with 10° incident angle. The 16.85 nm linewidth reveals that the linearly polarized illumination with a suitable incident angle can achieve better lithography result than the radially polarized illumination, which has been reported in ref [16]. The 50.71 nm half pitch is generated by a pulsed laser, which can be reduced further by adjusting the FH of ABS and the frequency of EOM synchronously. Figures 9(c)-9(e) show the results for an organic photoresist, which is the molecular glass compound of octa-tert-butyl (thiophene-2,3,4,5-tetrayltetrakis (benzene-4,2,1-triyl) octacarbonate, abbreviated with FPT-8Boc. The sensitivity of FPT-8Boc could be enhanced by increasing the proportion of the acid-sensitive groups Boc so that it could be used in a very short exposing time under high rotating speeds at the near-field photolithography [27–30]. The 28nm linewidth and 75 nm half pitch, characterized with atomic force microscope (AFM), were patterned by continuous-wave laser and pulsed laser respectively. In Fig. 9(e), large lens arrays write the organic photoresist synchronously with 37 nm linewidth. To obtain the 1μm pitch, a voice coil motor (VCM) with a positioning accuracy of about 1μm was used in the experiment. The pattern feature size would be greatly improved by the optimizations of the resist exposure threshold [31] and flying height.
Fig. 9 The patterning results of near-field photolithography. (a) SEM image of a pattern with 16.85nm linewidth on the TeOx inorganic photoresist by continuous-wave laser. (b) SEM image of a pattern with 50.71 nm half pitch resolution on the TeOx inorganic photoresist by150 MHz pulsed laser. (c) AFM image of a pattern with 28 nm linewidth on the FPT-8Boc organic photoresist by continuous-wave laser. (d) AFM image of a pattern with 75 nm half pitch resolution on the FPT-8Boc organic photoresist by150 MHz pulsed laser. (e) AFM image of patterning of the large arrays with 37nm linewidth and 1μm gap.
5. Conclusions
In summary, we have demonstrated that linearly polarized beam can achieve only one focal spot with high resolution by illumination with certain incident angle, because the concentric rings can couple the different frequencies of SPPs from ± x directions. This allows the highly efficient transmission and focusing of near-field spot to be realized easier and better than the radially polarized illumination. Using this method, the near-field photolithography can pattern a 2.5-inch wafer coated with an inorganic or organic photoresist in minutes by a single plasmonic flying head carrying up to several plasmonic lens with 16.85 nm high resolution. By using lower flying heights of ABS and higher frequencies of EOM, the half pitch would be potentially further reduced with the near-field photolithography. This makes the near-field potolithography an attractive low-cost and high-throughput method of nanomanufacturing.
Funding
National Natural Science Foundation of China (NSFC) (91623105, 91123033). State Key Laboratory of Tribology, China Tsinghua University (SKLTKF16B14).
References and links
1. S. Okazaki, “Resolution Limits of Optical Lithography,” J. Vac. Sci. Technol. B 9(6), 2829–2833 (1991). [CrossRef]
2. N. R. Farrar and T. Yen, “The Future of Optical Lithography,” in Photonic Applications Systems Technologies Conference, 2004 OSA Technical Digest Series (Optical Society of America, 2004), paper PThC3.
3. Z. W. Xu, F. Fang, and G. Zeng, Focused Ion Beam Nanofabrication Technology Handbook of Manufacturing Engineering and Technology (Springer, 2015).
4. D. Z. Pan, L. Liebmann, B. Yu, X. Q. Xu, and Y. B. Lin, “Pushing Multiple Patterning in Sub-10nm: Are We Ready?”, in Design Automation Conference (ACM, 2015), paper 197. [CrossRef]
5. Y. W. Chang, R. G. Liu, and S. Y. Fang, “ EUV and e-beam manufacturability: challenges and solutions” in Design Automation Conference (ACM, 2015), paper 198. [CrossRef]
6. H. Tsubaki, W. Nihashi, T. Tsuchihashi, K. Yamamoto, and T. Goto, “Negative-tone imaging with EUV exposure toward 13 nm hp”, in SPIE Advanced Lithography Conference (2016), paper 9776. [CrossRef]
7. J. A. Schuller, E. S. Barnard, W. Cai, Y. C. Jun, J. S. White, and M. L. Brongersma, “Plasmonics for extreme light concentration and manipulation,” Nat. Mater. 9(3), 193–204 (2010). [CrossRef] [PubMed]
8. T. Rindzevicius, Y. Alaverdyan, B. Sepulveda, T. Pakizeh, M. Käll, R. Hillenbrand, J. Aizpurua, and F. J. García de Abajo, “Nanohole Plasmons in Optically Thin Gold Films,” J. Phys. Chem. C 111(3), 1207–1212 (2006). [CrossRef]
9. Y. Zhang, J. Wang, J. Shen, Z. Man, W. Shi, C. Min, G. Yuan, S. Zhu, H. P. Urbach, and X. Yuan, “Plasmonic hybridization induced trapping and manipulation of a single au nanowire on a metallic surface,” Nano Lett. 14(11), 6430–6436 (2014). [CrossRef] [PubMed]
10. H. Ditlbacher, J. R. Krenn, A. Hohenau, A. Leitner, and F. R. Aussenegg, “Efficiency of local light-plasmon coupling,” Appl. Phys. Lett. 83(18), 3665–3667 (2003). [CrossRef]
11. Z. Liu, J. M. Steele, W. Srituravanich, Y. Pikus, C. Sun, and X. Zhang, “Focusing surface plasmons with a plasmonic lens,” Nano Lett. 5(9), 1726–1729 (2005). [CrossRef] [PubMed]
12. W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Surface plasmon subwavelength optics,” Nature 424(6950), 824–830 (2003). [CrossRef] [PubMed]
13. S. A. Maier, Plasmonics: Fundamentals and Applications. Springer, New York (2007).
14. J. Ji, Y. Hu, Y. Meng, J. Zhang, J. Xu, S. Li, and G. Yang, “The steady flying of a plasmonic flying head over a photoresist-coated surface in a near-field photolithography system,” Nanotechnology 27(18), 185303 (2016). [CrossRef] [PubMed]
15. W. Srituravanich, L. Pan, Y. Wang, C. Sun, D. B. Bogy, and X. Zhang, “Flying plasmonic lens in the near field for high-speed nanolithography,” Nat. Nanotechnol. 3(12), 733–737 (2008). [CrossRef] [PubMed]
16. L. Pan, Y. Park, Y. Xiong, E. Ulin-Avila, Y. Wang, L. Zeng, S. Xiong, J. Rho, C. Sun, D. B. Bogy, and X. Zhang, “Maskless plasmonic lithography at 22 nm resolution,” Sci. Rep. 1(11), 116–120 (2011).
17. J. Ji, Y. Meng, and J. Zhang, “Optimization of structure parameters of concentric plasmonic lens for 355 nm radially polarized illumination,” J. Nanophotonics 9(1), 093794 (2015). [CrossRef]
18. A. Farhang, N. Bigler, and O. J. Martin, “Coupling of multiple LSP and SPP resonances: interactions between an elongated nanoparticle and a thin metallic film,” Opt. Lett. 38(22), 4758–4761 (2013). [CrossRef] [PubMed]
19. W. Chen, D. C. Abeysinghe, R. L. Nelson, and Q. Zhan, “Plasmonic lens made of multiple concentric metallic rings under radially polarized illumination,” Nano Lett. 9(12), 4320–4325 (2009). [CrossRef] [PubMed]
20. G. Rui, W. Chen, Y. Lu, P. Wang, H. Ming, and Q. Zhan, “Plasmonic near-field probe using the combination of concentric rings and conical tip under radial polarization illumination,” J. Opt. 12(3), 220–221 (2010).
21. A. Normatov, P. Ginzburg, N. Berkovitch, G. M. Lerman, A. Yanai, U. Levy, and M. Orenstein, “Efficient coupling and field enhancement for the nano-scale: plasmonic needle,” Opt. Express 18(13), 14079–14086 (2010). [CrossRef] [PubMed]
22. R. Peng, X. Li, Z. Zhao, C. Wang, M. Hong, and X. Luo, “Super-Resolution Long-Depth Focusing by Radially Polarized Light Irradiation Through Plasmonic Lens in Optical Meso-field,” Plasmonics 9(1), 55–60 (2014). [CrossRef]
23. G. M. Lerman, A. Yanai, and U. Levy, “Demonstration of Nanofocusing by the use of Plasmonic Lens Illuminated with Radially Polarized Light,” Nano Lett. 9(5), 2139–2143 (2009). [CrossRef] [PubMed]
24. A. Yanai and U. Levy, “Plasmonic focusing with a coaxial structure illuminated by radially polarized light,” Opt. Express 17(2), 924–932 (2009). [CrossRef] [PubMed]
25. J. Ji, Y. Meng, L. Sun, X. Wu, and J. Wang, “Strong focusing of plasmonic lens with nanofinger andmultiple concentric rings under radially polarized illumination,” Plasmonics 11(1), 23–27 (2016). [CrossRef]
26. L. Yin, V. K. Vlasko-Vlasov, A. Rydh, J. Pearson, U. Welp, S. H. Chang, S. K. Gray, G. C. Schatz, D. B. Brown, and C. W. Kimball, “Surface plasmons at single nanoholes in Au films,” Appl. Phys. Lett. 85(3), 467–469 (2004). [CrossRef]
27. L. Chen, J. Xu, H. Yuan, S. Yang, L. Wang, Y. Wu, H. Zhao, M. Chen, H. Liu, S. Li, R. Tai, S. Wang, and G. Yang, “Outgassing analysis of molecular glass photoresists under EUV irradiation,” Sci. China Chem. 57(12), 1746–1750 (2014). [CrossRef]
28. W. A. C. Bauer, C. Neuber, C. K. Ober, and H. W. Schmidt, “Combinatorial optimization of a molecular glass photoresist system for electron beam lithography,” Adv. Mater. 23(45), 5404–5408 (2011). [CrossRef] [PubMed]
29. G. Yang, L. Chen, .J. Xu, S. Wang, and S. Li, Molecular glass photoresists of taking tetraphenyl furan, tetraphenyl pyrrole, tetraphenylthiophene and quinary phenylpyridine as cores China Patent Specification 201210070713.6. 2012–03–16.
30. F. Gibbons, H. M. Zaid, M. Manickam, J. A. Preece, R. E. Palmer, and A. P. G. Robinson, “A chemically amplified fullerene-derivative molecular electron-beam resist,” Small 3(12), 2076–2080 (2007). [CrossRef] [PubMed]
31. J. Dai, S. W. Chang, A. Hamad, D. Yang, N. Felix, and C. K. Ober, “Molecular Glass Resists for High-Resolution Patterning,” Chem. Mater. 18(15), 3404–3411 (2006). [CrossRef]
