Plasmonic Lithography Utilizing Epsilon Near Zero Hyperbolic

Plasmonic Lithography Utilizing Epsilon Near Zero Hyperbolic Metamaterial. Xi Chen, Cheng Zhang, Fan Yang, Gaofeng Liang, Qiaochu Li, and L. Jay Guo. ...
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Plasmonic Lithography Utilizing Epsilon Near Zero Hyperbolic Metamaterial Xi Chen,† Cheng Zhang,‡ Fan Yang,‡ Gaofeng Liang,‡ Qiaochu Li,‡ and L. Jay Guo*,†,‡ †

Applied Physics and ‡Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan 48109, United States S Supporting Information *

ABSTRACT: In this work, a special hyperbolic metamaterial (HMM) metamaterial is investigated for plasmonic lithography of period reduction patterns. It is a type II HMM (ϵ∥ < 0 and ϵ⊥ > 0) whose tangential component of the permittivity ϵ∥ is close to zero. Due to the high anisotropy of the type II epsilon-near-zero (ENZ) HMM, only one plasmonic mode can propagate horizontally with low loss in a waveguide system with ENZ HMM as its core. This work takes the advantage of a type II ENZ HMM composed of aluminum/aluminum oxide films and the associated unusual mode to expose a photoresist layer in a specially designed lithography system. Periodic patterns with a half pitch of 58.3 nm were achieved due to the interference of third-order diffracted light of the grating. The lines were 1/6 of the mask with a period of 700 nm and ∼1/7 of the wavelength of the incident light. Moreover, the theoretical analyses performed are widely applicable to structures made of different materials such as silver as well as systems working at deep ultraviolet wavelengths including 193, 248, and 365 nm. KEYWORDS: UV lithography, hyperbolic metamaterial, epsilon near zero, nanomanufacturing, spatial filtering, interference

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nanolithography system based on HMMs. To overcome these issues, a system which can create interference patterns while maintaining high light transmission is highly desirable. To this end, we utilized a special epsilon-near-zero (ENZ) HMM to produce subwavelength patterns. ENZ metamaterials19−29 have been reported in many applications, such as tailoring propagation and radiation patterns,19,21,23,24 waveguide coupling,20,21,25 and coherent perfect absorption.26 Among those various structures, a special type of HMM whose tangential component of permittivity almost reaching zero ϵxx → 0 is studied in this work. The ENZ HMM consists of alternating metal and dielectric films which provides a planar structure necessary for constructing a plasmonic lithography system. The effective permittivity of the stacked structure can be calculated using the effective medium theory4−8 as ϵxx = ϵyy = fϵm + (1 − f)ϵd and ϵzz = ϵmϵd/[(1 − f)ϵm + fϵd], where f stands for the fill ratio of the metal f = tm/(tm + td), ϵm (ϵd) is the relative permittivity of the metal (dielectric), and tm (td) are their thicknesses. We use aluminum (Al) and aluminum oxide (Al2O3) to construct a type II ENZ HMM (ϵ∥ → 0 and ϵ⊥ > 0) for nanolithography at a wavelength of 405 nm, that is, the hline in UV lithography. It is worth noting that similar ENZ HMMs can be utilized to realize lithography at other wavelengths including 365 nm (i-line) as well as deep ultraviolet (DUV) 248 and 193 nm (Supporting Information

o date, photolithography is still the most widely used patterning technology in the semiconductor industry. However, the resolution of photolithography is restricted by light diffraction Δ ∼ λ/NA,1−3 where λ is the wavelength of the exposure light and NA is the numerical aperture of the projection system. Hyperbolic metamaterials (HMMs) have been used to improve the resolution, which are described by an effective permittivity4−10 in a tensor form: ⎛ ϵxx 0 0 ⎞ ϵ̅ = ⎜⎜ 0 ϵyy 0 ⎟⎟ with only principal components. The signs ⎝ 0 0 ϵzz ⎠ of the tangential permittivities of HMMs are the same ϵ∥ = ϵxx = ϵyy, but the signs of its tangential and vertical permittivity are opposite ϵ∥·ϵ⊥ = ϵxx·ϵzz < 0, where both ϵ∥ and ϵ⊥ are complex values. HMMs have been widely exploited for various proposes, particularly as a hyperlens in imaging4,11−15 and filters in ultraviolet (UV) lithography.11−16 Using HMMs as a hyperlens, patterns can be replicated with the same size as that of the mask by flat structure (illustrated in Figure 1C1 and Figure S3E1 and 2). While a curved hyperlens can produce patterns smaller than the mask,17 the 1:1 patterning approach is challenging because of the complexities in creating the mask. The approaches using HMMs as filters9,13 produce patterns based on interference effect (Figure 1C3 and Figure S3E4). However, due to the strong attenuation of the light propagating in the HMMs, the field intensity in the light-sensitive photoresist (PR) layer is several orders of magnitude weaker than that of the incident light, which results in extremely long exposure time.13,18 These two characteristics have seriously restricted the practicality of a © 2017 American Chemical Society

Received: May 23, 2017 Accepted: October 2, 2017 Published: October 2, 2017 9863

DOI: 10.1021/acsnano.7b03584 ACS Nano 2017, 11, 9863−9868

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Figure 1. Illustration of how different types of HMM can be applied in lithography. (A) Characteristics of the Al/Al2O3 HMM as a function of wavelength and fill ratio of Al as well as the dispersion relations at different regimes. Column B shows the iso-frequency curve of air (black circle) and the HMM (magenta hyperbola), and column C shows the corresponding electric field distribution log 10(|E|2) in HMM when TM polarized light is incident from air onto two nanoslits right above the HMM: (B1) type I ENZ at the wavelength of 175 nm with fill ratio of f = 16/30 and corresponding (C1) propagating modes in Al/Al2O3 HMM made of 7 layers of 16 nm Al and 14 nm Al2O3 films; (B2, B3) type II ENZ at the wavelength of 405 nm with fill ratios of (B2) f = 6/53 and (B3) f = 12/53, and the corresponding interference pattern formed in the HMM made of (C2) 7 layers of 6 nm Al and 47 nm Al2O3 films and (C3) 7 layers of 12 nm Al and 41 nm Al2O3. Notice the much stronger field at the bottom PR layer in the case of type II ENZ HMM.

kx2/ϵzz + kz2/ϵxx = k02. In Figure 1B, the black circle shows the index ellipsoid of air, and magenta hyperbola depicts the isofrequency curve of HMM as a function of the normalized wavevector kx/k0 and kz/k0. The light is incident downward from air (represented by the upper panels with kz > 0) to the HMM (lower panels with kz < 0), where the conservation of transverse moment is required at the interface of air and PR. When kx and kz have real solutions, the corresponding waves are allowed to propagate in the effective media; otherwise, the waves are forbidden. The white and green regions correspond to the allowed zones for propagating waves, while the gray regions represent the forbidden zones where waves are evanescent in air and the HMMs, respectively. Simulation for the light incident through a double-silt mask is performed to demonstrate the dependence of light propagation on the dispersion relation of HMM. The corresponding electric field distributions |E|2 are shown in Figure 1C. The TM polarized plane wave travels from the positive z-axis in air with zero angle of incidence and transmits through a 150 nm-thick chrome (Cr) photomask. Two slits have a width w of 20 nm and a separation of 700 nm. A poly(methyl methacrylate) (PMMA) layer with a thickness of 50 nm as an index matching layer is placed underneath the photomask, followed by HMM consisting of the 7 layers of alternating stacked Al and Al2O3 films. When the width of the slits is much smaller than the wavelength of incidence, that is, w ≪ λ, the excited wavevector becomes high |kx| ≫ k0. Therefore, high-k eigenmodes can be efficiently excited inside the HMM because of the subwavelength nanoslits located in the near-field proximity of the HMM. Based on the conservation of momentum, the waves in type I HMM are propagating when incident from air, but the waves in type II HMM are evanescent (Figure 1B). At a wavelength of 175 nm (Figure 1B1, C1), the HMM is made of 7 layers of 16 nm Al and 14 nm Al2O3 stacks, and the field propagates directionally in the type I ENZ HMM. This type of ENZ HMM has shown the capability in light confinement, despite the deep subwavelength aperture size. However, it is distinctively different from the type II ENZ HMM composed of 7 layers of 6 nm Al and 47 nm Al2O3 films (Figure 1B2, C2). Since ϵxx → 0, the electric field in the type II ENZ HMM

S1). In addition, the plasmonic material is not limited to Al. A silver (Ag)-based ENZ HMM at wavelength of 405 nm with appropriate fill ratio has the same property as that of Al-based ENZ system (Supporting Information S2). Although a plasmonic lithography system usually relies on the spatial frequency selection principle,13−15 in this work we analyzed the HMMs in terms of mode expansion and wave propagation. By investigating these optical properties of the type II ENZ HMM, we take advantage of the special field distribution inside the HMM to create deep subwavelength patterns in PR layer, with a half-pitch 1/6 of the photomask. The design of period reduction can greatly alleviate the difficulty of making the mask. Furthermore, the light intensity in the PR is comparable with the incident light, which can significantly reduce the exposure time and thus improve the throughput of lithography. First, we discuss how different types of HMM can be applied to lithography applications, by correlating their dispersion properties to the simulated lithographic patterns. For the HMM made of 7 layers of Al/Al2O3 stacks, both the tangential and vertical components ϵxx and ϵzz are frequency dependent, and the signs of ϵxx and ϵzz determine the type of the metamaterials.6−8 As shown in Figure 1A, the type of the metamaterials depends on the light wavelength and the fill ratio of Al.6−8 Specifically, it is a type I HMM (ϵxx > 0 and ϵzz < 0) for shorter wavelength ( 0) at longer wavelength with wide range of fill ratios; the structure becomes an effective dielectric (ϵxx > 0 and ϵzz > 0) when Al fill ratio is small, or an effective metal (ϵxx < 0 and ϵzz < 0) when Al fill ratio is large. To demonstrate how light propagates in HMMs, three typical scenarios with different wavelengths and fill ratios are studied: (1) a type I ENZ HMM (ϵxx → 0 and ϵzz > 0) with ϵxx = −0.008693 − 0.1644i and ϵzz = −25.2214 + 9.9793i for f = 16/30 (Figure 1B1, C1) at the wavelength of 175 nm; (2) a type II HMM ENZ (ϵxx → 0 and ϵzz > 0) with ϵxx = −0.04974 + 0.5014i and ϵzz = 3.2219 + 0.009919i for f = 6/53 (Figure 1B2, C2) at the wavelength of 405 nm; (3) a type II HMM with ϵxx = −3.1372 + 1.0422i and ϵzz = 3.8202 + 0.02898i for f = 12/53 (Figure 1B3, C3) at the wavelength of 405 nm. The transverse magnetic (TM) wave propagating in an HMM is described by the following equation 9864

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ACS Nano remains constant and propagates straight along the vertical direction, but periodic patterns with smaller features are formed inside the HMM along the horizontal direction, which is the result of interference of light diffracted from the two nanoslits. For a type II HMM with 7 layers of 12 nm Al and 41 nm Al2O3 (Figure 1B3, C3), the diffracted light diverges from the slit with a large angle and forms interference patterns. All three scenarios can be used for lithography: B1/C1 can produce 1:1 patterns; while B2/C2 and B3/C3 can both produce patterns with period smaller than that of the mask. Comparing the optical intensity in the PR on the other hand, the field in the type II ENZ HMM (C2) is orders of magnitude stronger than that in a regular type II HMM (C3). Similar discussions of Ag are performed in Supporting Information S2. Indeed, structures like those in 1B1/1C1 and 1B3/1C3 have been exploited for lithography, for example, type I ENZ HMM has been reported as a hyperlens4,5,7,8,30 to obtain subwavelength patterns, and type II HMM has been proposed to produce finer patterns.12,13,18 In comparison, type II ENZ HMM offers the advantages of not only producing features smaller than the patterns on the mask but also transmitting much higher optical intensity to the PR. Therefore, type II ENZ HMM is used in this work for deep subwavelength lithography for these benefits. To understand the unusual field distribution the HMM and utilize it properly in lithography, we considered a waveguide structure with HMM as its core and PR as claddings. Using ϵ1 for PR and ϵ̅ for HMM as an effective medium, by matching the boundary conditions for Hy and Ex at the interface z = 0 with ϵxx·ϵ1 < 0, the corresponding kx with surface wave resonance point ksw can be solved as (Supporting Information S3), ksw = k 0 (ϵ1 − ϵxx)/(ϵ1/ϵzz − ϵxx /ϵ1)

Figure 2. (A) Magnetic field distributions Hy(x, z) in the xz plane (color) and Hy(z) along the z axis (black curve) of distribution of the mode in the waveguide with Al-based type II ENZ metamaterial as core and PR as claddings. The thickness of core is 165 nm. (B) Transmitted intensity |Ht/Hi|2 of 7 layers of 6 nm Al and 47 nm Al2O3 with the TM light illumination at 405 nm wavelength, where the Ht is the transmitted magnetic field and Hi is the incident magnetic field. The diffraction orders of the 700 nm grating are marked by the red arrows.

the process by significantly reducing the PR exposure time. Furthermore, the high k of mode improves the resolution of the lithography. When the cutoff frequency is in the evanescent region, that is, kcutoff > k0, much smaller features can be generated. The resolution limit can be expressed as a function of the cutoff frequency Δ ≈ 4π/kcutoff ≈ 2λ/neff, where neff is the effective mode index determined by β0/k0. In addition, the type II ENZ HMM can also benefit from filter effects15 as illustrated in Figure 2B. In the k space, the diffraction orders of the grating with period of 700 nm are given by the red arrows. The OTF, that is, |Ht/Hi|2 shows a narrow band transmission, where the Ht and Hi are the transmitted and incident magnetic fields, respectively. In this design, only the third diffraction of the grating is selected with its diffracted wavevector coinciding with the resonant peak of the OTF, while the other orders are blocked and cannot reach PR layer. With these characteristics, type II ENZ HMM is an excellent candidate in nanolithography for subwavelength patterns with high field contrast and uniformity. A scheme of the ENZ UV lithography is shown in Figure 3A. One-dimensional (1D) periodic Al grating is used as the photomask, and the type II ENZ HMM is placed in contact with a PMMA spacer as an index-matching layer, followed by the PR layer to record the imaged patterns. Additional Al and Al2O3 layers are placed beneath the PR layer to enhance the field inside the PR by reflection and cavity resonance.15,18,31 The Al grating is used in the mask to increase the field contrast because of the light coupling, and similar results can be obtained experimentally by Cr mask with Al-based ENZ HMM as well.13 In addition, the system is placed on a flexible polymer substrate to ensure intimate contact between the HMM lens and the PR layer.14,15 To make a robust design, we also studied the effects of the geometric parameters in the design including the thickness of each layer, duty cycle, and period of the grating, etc. via simulation, which are detailed in Supporting Information S8. Using the optimized design, the normalized electric field intensity distribution |E|2 of the latent image in the PR layer is shown in Figure 3B for one mask period, illuminated by TM polarized light with the wavelength of 405 nm. The Al grating has thickness of 25 nm, duty cycle of 75%, and period of 700 nm. The PMMA spacer has thickness of 50 nm, which is on top of the HMM made of 7 stacks of 6 nm Al

(1)

The two surface waves at each interface can couple with each other and form a waveguide mode (eq 1). The two surface waves at each interface can couple with each other and form a waveguide mode (Supporting Information S2 and S3). For the waveguide with type II HMM as core and PR as claddings, the solution to TM0 mode is γ0d = 2 arctanh(−α0ϵxx/γ0ϵ1), where the decay wavevector is α0 =

β02 − ϵ1k 02 in the cladding and

γ0 = ϵxxβ02 /ϵzz − ϵxxk 02 in the core made of HMM, where β0 is the tangential wavevector and d is the thickness of the waveguide (Supporting Information S4). Although ϵxx → 0 for both type I and II ENZ HMM, the behaviors are completely different (in Supporting Information S5). Specifically, the magnetic fields Hy(z) and Hy(x, z) in the Al-based type II ENZ HMM are shown in Figure 2A. The fields in the core of the waveguide are uniform, resembling a mode propagating horizontally because Hy(z) = H0 cosh(γ0z) → H0, which is independent of z when γ0 → 0. Since the waveguide mode is in the forbidden zone for both HMM and PR, it is a plasmonic mode resonating at the interface of HMM and PR. The uniform wavefront observed in the HMM (Figure 2A) when treated as an effective medium corresponds to the coupling of long-range surface plasmon (LRSP) of an individual Al2O3-AlAl2O3 waveguide in the actual multilayer structure (Supporting Information S6). Moreover, the loss of the TM0 in the type II ENZHMM is also much lower. As a result, the propagation length and decay length become also much longer (Supporting Information S7). The reduced loss in this case, as compared with typical type II HMM-based lithography, greatly improves 9865

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Figure 3. (A) Schematic of the ENZ lithography design in Cartesian coordinates. (B) Normalized |E|2 distribution in the xz plane with the wavelength illumination for 7 layers of 6 nm Al and 47 nm Al2O3 ENZ HMM, and the white dashed line indicates the middle position in the PR. (C) Normalized electric field distribution |E|2 along horizontal dashed lines at the top, middle, and bottom positions in the PR.

Figure 4. Fabrication process flow of (A−E) masks on transparent glass substrate and (F−G) PR on SU-8 planarized PET flexible substrate. (A) Al sputtered on glass substrate. (B) PMMA patterns after EBL. (C) Al grating after RIE. (D) PMMA coating after planarization. (E) Ebeam evaporation of Al/Al2O3 multilayer films (F) Al2O3-Al-Al2O3 films sputtered on planarized PET with SU-8. (G) PR by spin-coating. (C1, E1, F1) Corresponding SEM images of the processed plotted in (C, E, F). (C1) Top view of the Al grating on glass substrate. (E1) Cross section view of the multilayer structure deposited on Si wafer and (G1) cross section view of the Al and Al2O3 films deposited on Si wafer made by e-beam evaporation.

Figure 5. SEM of the patterns made by the Al/Al2O3 ENZ metamaterials. The (A) top view, (B) angled view, and (C) cross-section view of the patterns with period around 117 nm.

the simulations but also reduce the film roughness in the fabrication. The |E|2 distribution along horizontal lines in the PR is illustrated in Figure 3C, where the amplitude of the periodic distributions at the top, middle, and bottom positions are comparable, which means that the field distribution is uniform along the vertical position. The field contrast can be calculated as (|Emax|2 − |Emin|2)/(|Emax|2 + |Emin|2) ∼ 0.915, which is sufficiently high for exposure and to obtain high contrast ratio in images. The average field distribution |E|2 is around 0.225, which is near a quarter of the incident light set a 1 V/m. It should be noted that the field is several orders of magnitude

and 47 nm Al2O3 layers. The PR has thickness of 100 nm, the bottom Al layer has a thickness of 20 nm, and the two Al2O3 films have the thickness of 47 nm. Within one period, six periodic patterns are created, because the third diffraction of the grating was employed for exposure.15 An additional reduction factor of 2 comes from the interference of two counter-propagating waveguide modes. Comparing the normalized field distribution, the field intensity in the PR is strong, and the aspect ratio of the patterns is high. Moreover, the additional Al and Al2O3 layers underneath the PR not only further improve the contrast and uniformity of the patterns in 9866

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ACS Nano stronger compared with other systems using HMM,13,18 which is due to the low loss of the specially designed waveguide mode supported by ENZ HMM in our system. Moreover, based on our simulations, the dependence of the parameters such as the thickness of each layer is acceptable for the experiments, with a tolerance of a few nanometers, which indicates that our design is quite robust (Supporting Information S8). Due to the low loss and high anisotropy of type II ENZ HMM, the patterns in the PR are much smaller than the photomask while maintaining strong field intensity, high aspect ratio, and good uniformity.

can be extended to different metals and light wavelengths. The ENZ HMM with anisotropy has the potential to be exploited in other applications to create periodic structures with directional propagation, such as in natural HMM,33,34 waveguide system and integrated optics requiring extremely low loss.

METHODS The fabrication process of the mask and substrate of the ENZ lithography is illustrated in Figure 4. Al layer with thickness of 25 nm is deposited via e-beam evaporation on a clean glass substrate as in Figure 4A, followed by the spin coating of a PMMA layer. A grating with period of 700 nm and duty cycle of 75% was obtained by electron beam lithography (EBL), and the Al grating with the same period was achieved by reactive ion etching (RIE, LAM 9400) as shown in Figure 4C. The top view SEM image of the Al grating is shown in Figure 4C1. A thin PMMA layer with thickness of 55 nm was spin-coated on top of Al grating as a spacer layer, which was flattened by thermal nanoimprint14,15,35,36 lithography (NX-2000 Nanonex). After planarization, multilayer structure made of alternating 6 nm Al and 47 nm Al2O3 was deposited on the PMMA spacer by e-beam evaporation, as in Figure 4E. The cross-section SEM image of the multilayer structure on a reference Si sample is given in Figure 4E1. For the preparation of the substrate, regular PET films were planarized by spin coating of a 10 μm thickness SU-8 resist, followed by e-beam evaporation of 47 nm Al2O3, 20 nm Al, and 47 nm Al2O3 films. The PR used in the experiment is 1:1 diluted AR-N 7500.18 positive resist at 405 nm wavelength. The diluted PR layer with 100 nm thickness was spincoated on the Al2O3 layer and soft baked on a hot plate at 85 °C for 1 min. The optical setup built to ensure the conformal contact between the photomask and substrate is the same as shown in the waveguide lithography in our previous work.14,15 The Al/Al2O3 ENZ mask and the PR coated PET substrate were in conformal contact in a specially designed stage during exposure. Collimated 405 nm diode laser (ONDAX x6474) light with TM polarization was incident onto the mask through a circular aperture with diameter of ∼1 cm. The laser intensity was set at 3 mW·cm−2, and the exposure lasts for ∼4 min. Finally, the resist on PET was immersed in 1:1 diluted AR 300−35 developer for 50−55 s, followed by 30 s deionized (DI) water rinse. Simulations in this work were performed in COMSOL 5.1, and calculation was done by MATLAB 2015. All the optical constants used in the simulations are listed in Supporting Information S11.

RESULTS The fabrication process of the plasmonic lithography is shown in Figure 4 and in the Methods section. To verify the theoretical and numerical calculations, we carried out the experiment using the Al-based ENZ metamaterial at 405 nm wavelength, made of 6 nm Al and 47 nm Al2O3. The scanning electron microscopy (SEM) images of the resulting patterns are shown in Figure 5. As shown in the schematic in Figure 3B, the period of the grating mask is 700 nm; however, the patterns made by interference have the period of Λ/6, which is around 117 nm. Subwavelength patterns with half-pitch of 58.5 nm were achieved, which is 1/7 of the light wavelength. The angled and cross-section view of the pattern is illustrated in Figure 5B,C. The height of the PR is 100 nm, which leads to an aspect ratio of the pattern around 2:1. Note that the aspect ratio of the feature is lower than our previous results of waveguide lithography where the PR layer functions as the core of a waveguide.15 The pitch of the patterns can be calculated by the P = Λ/6 ≈ 2λ/neff at 405 nm wavelength, where P is the period of the grating mask. To see the cross-section view, the resist on the substrate was cut manually by a razor blade and deposited with gold/platinum (Au/Pt) alloy before SEM characterization. Along the periodic lines there is tiny roughness, which might result from the deposited metal and cutting during SEM characterization. The roughness can also be attributed to the defects on the mask, for example, the deposited thin Al film as well as the limitation of the sensitivity, resolution, and contrast of the chosen PR at the exposure wavelength.32 The analyses of the roughness of the metal films are performed in the Supporting Information S9. The experimental conditions such as the polarization of the incident light and the bottom reflective layers also affect the experimental results (Supporting Information S10). In addition, by double exposure15 or illumination of circularly polarized light, 13 2D periodic patterns can also be potentially accomplished.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b03584. The file contains (1) lithography systems at the wavelength of 193, 248, and 365 nm; (2) lithography system based on Ag at the wavelength of 405 nm; (3) calculations of the surface wave; (4) analyses of waveguide modes; (5) comparison between type I and II ENZ HMM; (6) mode coupling in the HMM; (7) loss of the waveguide mode in the type II ENZ HMM; (8) parameter sweeping; (9) film roughness analyses; (10) discussion on experimental conditions; and (11) optical constants. Figures S1−11 (PDF)

CONCLUSIONS In conclusion, an HMM composed of Al/Al2O3 multilayer structure is studied to demonstrate a plasmonic lithography system for period reduction patterns. In the waveguide with a type II ENZ HMM as its core, only a single plasmonic mode exists which can function as a high pass filter with low loss. These properties are suitable for the application in UV lithography to produce patterns with high fidelity, high aspect ratios, high field contrast as well as high uniformity. The feature size created in the interference system is much smaller than the period of the photomask and light wavelength. Experimentally, periodic patterns with a half pitch of 58.3 nm were achieved by light exposure with wavelength of 405 nm using the Al/Al2O3 lithography system. Furthermore, applications of the system

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Xi Chen: 0000-0002-3451-7310 Notes

The authors declare no competing financial interest. 9867

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DOI: 10.1021/acsnano.7b03584 ACS Nano 2017, 11, 9863−9868