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Silicon nanowire grid polarizer for very deep ultraviolet fabricated from a shear-aligned diblock copolymer template Young-Rae Hong,1 Koji Asakawa,2 Douglas H. Adamson,3 Paul M. Chaikin,4 and Richard A. Register4,* 1
Department of Chemical Engineering, Princeton University, Princeton, New Jersey 08544, USA Corporate Research and Development Center, Toshiba Corporation, Kawasaki 212-8582, Japan 3 Princeton Institute for the Science and Technology of Materials, Princeton University, Princeton, New Jersey 08540, USA 4 Department of Physics, New York University, New York, New York 10003, USA *Corresponding author:
[email protected] 2
Received August 1, 2007; revised September 23, 2007; accepted September 23, 2007; posted September 26, 2007 (Doc. ID 85979); published October 22, 2007 A silicon (Si) nanowire grid ultraviolet (UV) transmission polarizer has been fabricated, and its performance was measured over the visible to deep UV range. A cylinder-forming polystyrene-b-poly(hexylmethacrylate) diblock copolymer was coated onto an amorphous Si layer supported on a fused silica substrate, then shear aligned and employed as a mask for reactive-ion etching, resulting in a Si grid of 33 nm period and multicentimeter-squared area. Due to the high plasma frequency and UV reflectance of the deposited Si, this nanowire grid was able to polarize light down into the deep UV, including 193 nm. © 2007 Optical Society of America OCIS codes: 050.2770, 120.7000, 220.4241, 230.5440, 260.5430, 310.6628.
Polarization of light has been of interest for over a century, in applications across many fields of science and technology. While wire grid polarizers (WGPs) have been widely adopted for infrared, far infrared, and recently visible light [1], polarizing ultraviolet (UV) light remains challenging because of the fine grid period required [2]; to obtain good polarization, the wire grid period should be 1 / 3 of the light’s wavelength or less [3,4]. There is strong interest in linear UV polarizers for application in state-of-the-art immersion lithographic instruments that use 193 nm light from ArF excimer lasers. For this purpose, conventional UV polarizers employing a series of Brewster-angle reflection plates or birefringent prisms are too bulky to incorporate into the optical train, with its high numerical aperture. Recently, prototype WGPs with periods as short as 100 nm period have been fabricated using two modern lithographic techniques: UV [4] or extreme ultraviolet [5] interference lithography, and nanoimprint lithography [1,6]. Aluminum (Al) was chosen as the wire material because of its relatively high plasma frequency, and effective polarization down to 300 nm [4,5] and even 266 nm [4] was demonstrated. However, grids with ⬍70 nm period, required for effective polarization at 193 nm or shorter wavelengths, have not been fabricated even by these advanced lithographic processes. Block copolymer lithography (BCL) is an attractive, cost-effective approach for the fabrication of such a fine periodic pattern. The natural periodicity of self-assembling block copolymers is in the required range (typically 10– 100 nm), while recent developments in achieving long-range order of these block copolymer patterns in thin films [7], and transferring these patterns into inorganic materials [8–10], allow 0146-9592/07/213125-3/$15.00
efficient fabrication of WGPs with 10– 100 nm pitch. Moreover, BCL can be employed in the fabrication of stamps [11] or nanoimprint masters for mass production. Recently, our group showed that BCL could be used to fabricate an Al nanowire grid polarizer with a periodicity of 33 nm, which polarized over the visible and near-UV regions [12]. However, its performance below 400 nm was not satisfactory due to its poor reflectance; though pristine Al retains a high reflectance even down into the UV, our Al deposition process unavoidably created a minor fraction of oxide inclusions, which greatly reduced the plasma frequency and thus the reflectance of the composite film. An analytical model developed in the same work [12] anticipated that if a higher plasma frequency material such as pristine Al or pristine silicon (Si) were used for the grids, their performance in the deep UV region would be greatly enhanced. Figure 1 shows the transmittance spectra for thin unpatterned films of as-deposited Si and Al, and indeed, the transmittance below 400 nm is lower (reflectance is higher) for Si than Al. Moreover, the fluorine-based reactiveion etch (RIE) used to transfer the pattern from the block copolymer thin film can also effectively etch Si, so nanowires can be fabricated by directly etching a thin Si layer through a block copolymer mask. By contrast, the Al WGPs employed metal evaporation onto the etched block copolymer film followed by a “lift-off” step, which is often problematic because of the fine features involved. In this Letter, we describe how BCL can fabricate a Si nanowire grid on a fused silica substrate, and we measure the polarization performance from the visible down into the very deep UV. Figure 2(a) shows a schematic of the fabrication process. Amorphous Si (a-Si, 45 nm) was first depos© 2007 Optical Society of America
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Fig. 1. Transmittance of as-deposited Si (open circles) and Al (filled squares) thin films, 25 nm thick, on fused silica. Transmittance between 130 and 300 nm was measured in vacuum, otherwise in air, and corrected for the transmittance of the support.
ited onto a fused silica substrate (10 cm diameter, 0.5 mm thick, AQ, Asahi Glass) by plasma enhanced chemical vapor deposition. A polystyrene-b-poly (n-hexyl methacrylate) (PS–PHMA) diblock copolymer with a molar mass of 21 and 64 kg/ mol for the respective blocks, which forms cylinders of PS in a
Fig. 2. (a) Fabrication process for a Si nanowire grid polarizer using block copolymer lithography. (b) Scanning electron micrograph (SEM) image of the finished Si nanowire grid supported on fused silica. Specimen has been cleaved and tilted at 45° away from the viewer to emphasize the profile of the wires.
PHMA matrix [12], was spin-coated on top of the a-Si at the thickness 共30 nm兲 needed for a monolayer of cylinders. When the sample was quiescently annealed at 150° C, the cylinders formed a typical “fingerprint” pattern with no overall orientation [Fig. 3(a)]. Replicating such a disordered pattern in metal nanowires does not yield an effective polarizer. To align the cylinders, shear stress 共⬃3 kPa兲 was applied to the top of the film through a silicone rubber pad 共2 cm⫻ 2 cm兲 during annealing at 150° C, as described previously [7,12]. This shear-alignment technique generates highly oriented stripe patterns across the entire area under the pad, as shown in Fig. 3(b); the periodicity of the cylinders in this PS– PHMA diblock is 33 nm. This shear-aligned PS– PHMA thin film was then used as a mask for RIE: CF4 or CF4 / SF6 (70/ 30 v / v兲 was employed at a pressure of 15 mTorr, a flow rate of 10 sccm, and a power density of 0.4 W / cm2. Although the etch rate of the PHMA blocks is more than twice as fast as the PS blocks, this contrast was not enough, in our hands, to etch completely through the amorphous Si layer with only the PS–PHMA as a mask. The etch contrast could be increased by staining the block copolymer by 2 min exposure to the vapor from 0.5% aqueous ruthenium tetroxide 共RuO4兲, which selectively reacts with the PS block and increases its etch resistance [13], thus permitting Si nanowires of greater aspect
Fig. 3. Tapping mode atomic force microscopy (TM-AFM) phase images of PS–PHMA thin films on top of an a-Si layer on a fused silica substrate: (a) quiescently annealed, (b) shear aligned. Glassy PS cylinders are shown as light in a dark rubbery PHMA matrix.
November 1, 2007 / Vol. 32, No. 21 / OPTICS LETTERS
ratio to be fabricated. Figure 2(b) shows a SEM of the finished polarizer; the edge-on view of the Si wires reveals a remaining Si thickness of approximately 25 nm after RIE. This image is representative of the entire shear-aligned region, with a few dislocations and some meandering of the wires observed. Images of the stained and unstained block copolymer films suggest that this meandering is induced by uptake of the RuO4 stain. As previously reported, however, these defects do not alter the strong anisotropy of the grid, which is required for it to function as a polarizer [12]. The performance of the finished polarizer was measured by a UV-visible spectrometer (Shimadzu UV3101PC) in air, using a calcite prism to polarize the incident light, which permits measurements down to 250 nm. To extend the measurements into the deep UV 共190– 350 nm兲, two Si WGPs were measured with the wires aligned or crossed, as described previously [12]. The performance is expressed as the polarization efficiency P [12], which is defined as P ⬅ 共Iperp − Ipar兲/共Iperp + Ipar兲,
共1兲
where the subscripts indicate light polarized parallel or perpendicular to the axis of the wires. The absolute value of P 共兩P兩兲 is related to the extinction ratio E as 兩P兩 = 共E − 1兲/共E + 1兲 = 1 − 2/共E + 1兲.
共2兲
Figure 4 compares P for the Si WGP with that for our previously reported Al WGP. As anticipated, the Si WGP performed better below 400 nm than the Al WGP, with the Si grid showing P ⬎ 0.5 near 270 nm. Previously published calculations for grids of varying thickness [12] show that these modest val-
Fig. 4. Polarization efficiency for the Si WGP (open circles) compared with that for an Al WGP (filled squares) previously reported [12].
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ues of P principally reflect the limited height 共25 nm兲 of our Si wires, which in turn is set by the RIE contrast between a-Si and the residual polymer mask (RuO4-stained PS). Altering the RIE conditions or the chemical identity of the residual polymer mask could produce a higher contrast, yielding thicker Si wires and more strongly polarizing grids for the next generation of such devices. The present Si WGP (Fig. 4) shows P ⬎ 0.4 over the important 200– 350 nm range; at 193 nm, P is nearly 0.4, and the transmittance nearly 0.2. In addition, the Si WGP did not show the inversion of P observed for the Al WGP below 300 nm (Fig. 4); as previously described, this crossover results from the inversion of the dielectric function of the grid, which occurs near 300 nm due to the oxide inclusions in Al [12]. For the Si WGP, P decreases below 260 nm, consistent with a crossover 共P = 0兲 near 120 nm as expected from our previously published calculations [12]. We have shown how BCL can be employed to fabricate an ultrafine period Si WGP which can polarize very deep UV light. As the periodic pattern self-assembles and is easily aligned over macroscopic areas by simple shear, while pattern transfer can be achieved with conventional RIE, fabrication of this Si WGP is straightforward and cost effective. This work was generously supported by the National Science Foundation (MRSEC Program) through the Princeton Center for Complex Materials (DMR-0213706), and by Toshiba Corporation. References 1. S. W. Ahn, K. D. Lee, J. S. Kim, S. H. Kim, J. D. Park, S. H. Lee, and P. W. Yoon, Nanotechnology 16, 1874 (2005). 2. J. A. Dobrowolski and A. Waldorf, Appl. Opt. 20, 111 (1981). 3. J. P. Auton, Appl. Opt. 6, 1023 (1967). 4. J. J. Wang, F. Walters, X. Liu, P. Sciortino, and X. Deng, Appl. Phys. Lett. 90, 061104 (2007). 5. Y. Ekinci, H. H. Solak, C. David, and H. Sigg, Opt. Express 14, 2323 (2006). 6. J. S. Kim, K. D. Lee, S. W. Ahn, S. H. Kim, J. D. Park, S. E. Lee, and S. S. Yoon, J. Korean Phys. Soc. 45, S890 (2004). 7. D. E. Angelescu, J. H. Waller, D. H. Adamson, P. Deshpande, S. Y. Chou, R. A. Register, and P. M. Chaikin, Adv. Mater. 16, 1736 (2004). 8. M. Park, C. Harrison, P. M. Chaikin, R. A. Register, and D. H. Adamson, Science 276, 1401 (1997). 9. C. Harrison, M. Park, P. M. Chaikin, R. A. Register, and D. H. Adamson, J. Vac. Sci. Technol. B 16, 544 (1998). 10. M. Park, P. M. Chaikin, R. A. Register, and D. H. Adamson, Appl. Phys. Lett. 79, 257 (2001). 11. D. H. Kim, Z. Q. Lin, H. C. Kim, U. Jeong, and T. P. Russell, Adv. Mater. 15, 811 (2003). 12. V. Pelletier, K. Asakawa, M. S. Wu, D. H. Adamson, R. A. Register, and P. M. Chaikin, Appl. Phys. Lett. 88, 211114 (2006). 13. R. Olayo-Valles, M. S. Lund, C. Leighton, and M. A. Hillmyer, Chem. Mater. 14, 2729 (2004).
