Large-Area High Aspect Ratio Plasmonic Interference Lithography

Apr 13, 2016 - Plasmonic lithography, which utilizes subwavelength confinement of surface plasmon polartion (SPP) waves, has the capability of breakin...
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Large-Area High Aspect Ratio Plasmonic Interference Lithography Utilizing a Single High‑k Mode Xi Chen,† Fan Yang,‡,§ Cheng Zhang,‡ Jing Zhou,‡ and L. Jay Guo*,†,‡ †

Applied Physics and ‡Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan 48109, United States § Center of Ultra-precision Optoelectronic Instrumentation, Harbin Institute of Technology, Harbin 150080, China S Supporting Information *

ABSTRACT: Plasmonic lithography, which utilizes subwavelength confinement of surface plasmon polartion (SPP) waves, has the capability of breaking the diffraction limit and delivering high resolution. However, all previously reported results suffer from critical issues, such as shallow pattern depth and pattern nonuniformity even over small exposure areas, which limit the application of the technology. In this work, periodic patterns with high aspect ratios and a half-pitch of about 1/6 of the wavelength were achieved with pattern uniformity in square centimeter areas. This was accomplished by designing a special mask and photoresist (PR) system to select a single high spatial frequency mode and incorporating the PR into a waveguide configuration to ensure uniform light exposure over the entire depth of the photoresist layer. In addition to the experimental progress toward large-scale applications of plasmonic interference lithography, the general criteria of designing such an exposure system is also discussed, which can be used for nanoscale fabrication in this fashion for various applications with different requirements for wavelength, pitch, aspect ratio, and structure. KEYWORDS: UV lithography, plasmonics, nanomanufacturing, spatial filtering, optical waveguide, interference, next-generation lithography gratings,27,28 and nanowire-based metamaterials29,30 to image subwavelength objects onto a light-sensitive photoresist (PR). However, almost all of the experimental efforts were only able to show very shallow patterns due to the evanescent nature of the SPP wave, and the periodic patterns could only be achieved in a localized region experimentally.10,11,16,19,27,31 Achieving high aspect ratio patterns and maintaining pattern uniformity over large areas are basic requirements for practical applications. Therefore, in this study, we propose a new design principle to address these problems, which is supported by experimental demonstrations of ∼λ/6 wide gratings with heights up to 200 nm in square centimeter areas. To understand the limitations of the approaches reported in previous literature, we consider a well-known plasmonic lithography scheme,11−14 as displayed in Figure 1A. In this scheme, the photomask is a subwavelength metallic grating, usually made by chromium (Cr). A thin metal film, separated from the mask by a spacer layer, serves as a superlens to image the grating pattern into the PR. Under angular spectrum representation (i.e., Fourier transformation), as illustrated in

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ithography lays the foundation for the semiconductor industry, and photolithography is the most widely used patterning technology today. The resolution of photolithography1−3 is limited by the light diffraction Δ = k1λ/NA, where Δ is the minimum feature size, λ is the wavelength of the exposure light, NA is the numerical aperture of the projection system, and k1 is a coefficient that accounts for various processrelated factors and resolution enhancement. In order to obtain smaller features, various techniques have been developed, such as extreme ultraviolet (UV) lithography,4 by using much shorter wavelengths and nanoimprint lithography5−7 by mechanically deforming the resist. Meanwhile, to break the diffraction limit and achieve subwavelength patterns, other techniques, such as near-field optical lithography2,8 and phase shift lithography,9 have been explored. Plasmonic lithography,10−15 which was recently studied, has been demonstrated to improve the resolution by utilizing surface plasmon polariton (SPP) waves excited at the interface between metal and the dielectric. The surface waves are induced by free electron oscillations at the metal surface and has a wavelength much smaller than that of the excitation light. Over the past decades, several promising simulated and experimental results have been reported, including using thin metal layers,16−20 alternating metal/dielectric stacked metamaterials,21−26 subwavelength © XXXX American Chemical Society

Received: September 30, 2015 Accepted: March 24, 2016

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interface and decay rapidly in the normal direction, seriously limiting the pattern depth. To address the limitations, we exploited two strategies in order to produce large-area, uniform, deep subwavelength patterns with high aspect ratio. First, a specially designed mask provides filtering of spatial frequency, allowing a single high-k mode to reach the PR layer and producing uniform patterns by single-mode interference. Second, to generate high aspect ratio patterns, we incorporate the PR layer into an optical waveguide, which significantly improves the propagation depth of the pattern. If near-UV light is used for exposure (e.g., 405 nm used in this work), a thin aluminum (Al) layer provides a transmission band at the evanescent regime much narrower than that of Ag, which is marked by the white dash-dotted line in Figure 2B. Utilizing this feature, we can achieve the spatial frequency selection of the single k mode using a thin Al layer to block the zero-order direct transmitted light, as well as incorporating the PR in a waveguide structure to only couple to a single high-order diffraction from the grating mask. The diagram of the proposed lithography system is shown in Figure 3, which consists of the mask and the substrate to be

Figure 1. (A) Typical plasmonic lithography scheme in a Cartesian coordinate including a subwavelength grating on a transparent substrate, a polymer spacer, a metal superlens, and PR. (B) Spatial frequency distribution as a function of normalized tangential wavevector; kx is the wavevector along the x direction, and k0 is the wavevector in free space.

Figure 1B, the grating generates a series of diffracted light when excited by the incident light, including both propagating and evanescent waves. In spatial frequency domain (i.e., k-space), the diffracted waves can be expressed as γm = mλ/Λ32−34 for normal incident light, where m stands for the mth diffraction order, λ is the exposure wavelength, and Λ is the period of the grating. The metal superlens supports symmetric and antisymmetric transverse magnetic (TM) modes, and a range of evanescent waves are allowed to transmit through. Therefore, the subwavelength objects can be captured by the PR. Silver (Ag) is usually employed as the superlens material at 365 nm wavelength.10−15 However, the transmission band |Et/ Ei|2 of silver is broad, where Ei is the incident electric field and Et refers to the transmitted electric field. Figure 2A shows the transmitted field amplitude as a function of normalized tangential wavevector kx/k0. If a 365 nm wavelength light is used for the exposure, a wide band of high-k evanescent waves, amplified by the SPP, can transmit through the thin metal (this is indicated by the white dash-dotted line at 365 nm extending from ∼2k0 to 8k0). The term high-k refers to the fact that the SPP evanescent wave has a tangential wavevector larger than that of the free space k0. As a result, multiple diffraction orders of the subwavelength grating can penetrate the thin Ag film and reach the PR, as illustrated in Figure 2B. The transmitted light of different diffraction orders interferes with each other, resulting in modulated patterns in the resist with varied spatial frequencies, inevitably resulting in pattern nonuniformity. In addition, the diffracted evanescent waves propagate along the

Figure 3. Schematics of the design with incident electric field along the x-axis.

exposed. The mask is on a glass substrate, composed of a onedimensional (1-D) Al grating with a 22 nm thickness, a 245 nm period, and 50% duty cycle, followed by a 40 nm poly(methyl methacrylate) (PMMA) spacer and a 10 nm Al layer underneath. The substrate is a polyethylene terephthalate (PET) sheet coated with 15 nm Al, 44 nm SiO2, and 100 nm

Figure 2. Transmitted amplitudes |Et/Ei|2 as a function of normalized tangential wavevector and wavelength, through a (A) 30 nm Ag layer and (B) 15 nm Al layer with a background refractive index of 1.5. The insets show the schematics of the corresponding (A1) dielectric/Ag/ dielectric and (B1) dielectric/Al/dielectric schemes. Red curves refer to an antisymmetric mode and a symmetric mode, both of which exist in the two schemes. B

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Figure 4. (A) Phase matching condition and spatial frequency selection principle, illustrated in normalized k-space. (B) PMMA−Al−resist− SiO2−Al waveguide modes. Red regions corresponds to stronger modes, and the sharp mode with a mode index of ∼1.65 marked by the white dashed curve is selected. (C) Normalized |E|2 map for a 250 nm thick PR. The electric field intensity of the incident light is set at 1 V/m, and the colors with a scale bar on the right represent the field intensity distribution in the xz plane. Brighter colors indicate a stronger field in the map.

Figure 5. COMSOL simulations. (A) Normalized |E|2 map for a 100 nm thick PR. (B) Normalized |E|2 distributions along horizontal lines at z = −80, −125, and −170 nm. Three sinusoidal curves are nearly identical and have a 122.5 nm period.

Supporting Information). When the phase matching condition is reached (i.e., γ1 ∼ kspp/k0 ∼ neff), the high-k resonance is further enhanced while effectively rejecting other diffraction orders from reaching the PR layer. The interference of ±firstorder light or two counter-propagating waveguide modes forms a standing wave with a sufficient intensity contrast and a large pattern depth. The period of the pattern can be calculated by λ/ 2neff, where the effective index, neff, can be regarded as the effective numerical aperture of the system. The refractive index of the PR used is 1.69055 + 0.034625i at 405 nm. The value of neff is around 1.653 in the case of a 245 nm period grating, leading to a deep subwavelength pattern with a period of 122.5 nm, 1/2 the mask period because of the mode interference. Next, we investigate the thickness dependence of the PR pattern to achieve high aspect ratio grating patterns. The effective index of the waveguide modes in the PMMA−Al− PR−SiO2−Al multilayer system as a function of the PR thickness is given in Figure 4B. As the PR thickness varies from 100 to 300 nm, the effective index almost remains constant, which indicates that such a mask design can be used to expose a PR of different thicknesses, thereby creating patterns with desired aspect ratios. An upper limit can be determined to be ∼250 nm for lithography by considering the loss of the metal and the dissipation of the waveguide mode. The simulated normalized |E|2 field distribution maps in the xz plane are given in Figure 4C for PR with a 250 nm thickness and in Figure 5A for PR with a 100 nm thickness. Clearly

PR layer. In our design, the mask and substrate are prepared separately and thus can be optimized individually. During exposure, the mask is in conformal contact with the PR layer on the substrate to ensure the wave coupling. Compared with the previous design shown in Figure 4A, the mask containing the thin Al layer can be reused, while the thin SiO2 and Al layer underneath the PR can be used as a hard mask to help transfer the pattern onto the substrate or a thicker polymer resist layer. When the grating mask is irradiated by a 405 nm (h-line) wavelength TM-polarized light, a series of harmonic modes are excited with the extra momentums provided by the Al grating. In the Fourier space, the SPP wave at the Al and PMMA interface has a wavevector of kspp ∼ 1.62k0 at the 405 nm wavelength (more details can be found in the Supporting Information). When the first-order diffraction of the grating, γ1 = λ/Λ, couples to the surface wave (i.e., γ1 ∼ kspp/k0), it forms a strong high-k resonance. The Al layer filters out the unwanted diffraction light due to its narrow-pass band (Figure 1B), as illustrated in Figure 4A. To achieve high aspect ratio features by increasing the penetration depth of the surface wave, an optical waveguide with an effective mode index of neff35−37 is formed to extract the SPP mode. The optical waveguide consists of the PR as its core, sandwiched between a thin (Al) layer and low index materials as claddings. The effective index neff can be adjusted to satisfy the phase matching condition, that is, tangential momentum conservation, by optimizing the thickness of the PR, SiO2, and the bottom Al layer (derivations shown in the C

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layer, it suffers much less damage and is expected to have a longer lifetime compared with typical contact photolithography approaches such as conventional near-field lithography2,8 and nanoimprint lithography.

uniform patterns with high contrast can be formed at different PR thicknesses. Figure 5B gives the |E|2 distribution for the case of a 100 nm PR along the horizontal cut lines as a function of positions along the x-axis. The field distributions at z = −80, −125, and −170 nm are marked by black, green, and red sinusoidal curves, respectively. Three curves at different z positions have almost the same distributions, with a 2.8 V2/m2 maximum, a 0.1 V2/m2 minimum, and a 122.5 nm periodicity (the incident light E-field is set at 1 V/m). The fact that field distribution is nearly identical at different depths suggests that a vertical sidewall should be possible using this structure and was indeed observed in experiments. The maximum field intensity in the PR is higher than the incident light, indicating that the field is enhanced by the plasmonic resonances. The contrast of the patterns in the PR can be calculated from Figure 5B as (| E|2max − |E|2min)/(|E|2max + |E|2min) = 0.931. The high intensity and contrast in the PR ensure the exposure stability in experiments. As a brief summary for the design criteria, the function of each layer is as follows: (1) the grating provides the needed high-k momentum; (2) the PMMA spacer layer helps form the strong resonance; (3) the Al between serves as a high-pass filter to eliminate the zero, second, and higher diffracted orders while selecting only first-order light for exposure; (4) the SiO2 is a tunable layer for index matching (and can be replaced by other material, such as a polymer layer with an index lower than that of the PR), and the bottom Al layer serves as a reflector in the waveguide to improve the pattern depth. In conventional nearUV photolithography, contact photolithography2,8 can define small patterns due to its near-field nature, but the field decays along the vertical direction so that the fields at different depths of the PR are not uniform (more discussion can be found in the Supporting Information). In addition, off-axis illumination2,9 design can block the zero-order light and employ the higherorder propagating wave for exposure, where the PR layer has to be thin because of the light diffraction. Furthermore, the photolithography techniques that utilize interferometry to generate small features require two or more beam illuminations on the resist, which increases process complexity. In comparison, our design uses the interference of the ±firstorder evanescent wave from the subwavelength grating, which carries high spatial frequency information. A double patterning technique38,39 with alignment can be exploited to further reduce the half-pitch of patterns. The fabrication procedures of the glass mask and PET substrate are illustrated in Figure 6. Once the mask and the substrate were prepared, they were put in conformal contact to ensure the near-field exposure, as shown in Figure 7A,B. Since the Al grating is protected by the PMMA spacer and the Al

RESULTS AND DISCUSSION Figure 8 shows the scanning electron microscopy (SEM) images of the resulting patterns obtained on the 100 nm thick PR. Large-area subwavelength periodic structures were obtained with approximately 55 nm line width, which is less than 1/6 of the light wavelength. The period of the pattern is 122.5 nm, which is 1/2 of the grating mask, and the aspect ratio is around 2:1, significantly improved over previously reported results. Multiple samples were prepared with the same experimental conditions, and good large-area uniformity was achieved consistently. Each sample is 1.5 cm × 1.5 cm in size, while the patterned area has a diameter of ∼1 cm, determined by the laser beam size used in the optical setup; the uniformity is confirmed by SEM and optical microscopy characterizations. Figure 8B is the corresponding cross section view of the pattern, and approximately rectangular profiles were observed, which verified our design principle. The labeled height of the PR pattern was obtained by considering the viewing angle of the SEM to avoid confusion. The SiO2 and Al layer underneath the PR can function as a hard mask if a subsequent etching process is needed to transfer the pattern into features with even higher aspect ratio on the substrate. The flexible substrate was cleaved by a sharp razor blade to ensure a clean cut across the resist patterns as the resist was spontaneously broken by the pressure of the cutting. Along the lines, there is slight roughness, which can be attributed to the defects on the mask, including the roughness along the grating and the deposited thin films (e.g., the thin Al film), as well as the limitation of the sensitivity, resolution, and contrast of the chosen PR at the exposure wavelength (roughness analysis is included in the Supporting Information). In comparison, due to the low intensity contrast, the interference of multiple modes, and the exposure of the shallow surface wave, the previously reported10,11,16,19,26,27,31 patterns made experimentally by plasmonic lithography could only obtain localized uniformity at the micron-scale, and the patterns (e.g., periodic subwavelength patterns usually ∼50 nm deep at 365 nm light illumination) were much shallower than the ones achieved in this work. To study the impact of the resist thickness, a PR with a thickness of 200 nm was also tested, aiming for patterns with even higher aspect ratios. Figure 9 shows the SEM images of the resulting pattern on such a PR. To accommodate the increased thickness, longer exposure time of 40−50 s and develop time of 90−105 s were used, while other experimental conditions remained unchanged. As shown in Figure 9A, uniform patterns with the same half-pitch of 61.25 nm were achieved. Figure 9B is the cross section of the patterns taken at a 30° angle, showing higher aspect ratio patterns. However, the patterns collapsed due to reasons such as the surface tension of the liquid during the development and rinsing, weak mechanical strength of the thicker resist used during cutting, or damage by the electron beam while performing the SEM characterization. The approach described above is general and can be applied to other wavelengths. For example, by using 193 nm light illumination, a half-pitch down to 13 nm can be achieved (simulations given in the Supporting Information). Here, we

Figure 6. Fabrication process flow of masks on a transparent glass substrate (A−G) and PR on a SU-8 planarized PET flexible substrate (H−J). (A) Transfer of patterns from a Si mold to a mr-I resist by nanoimprint lithography; (B) mr-I patterns after reactiveion etching; (C) Ti by shadow evaporation; (D) Al by evaporation; and (E) lift-off. (F) PMMA spin-coating and planarization; (G) Al by sputtering; (H) Al sputtered on planarized PET; (I) SiO2 by evaporation; and (J) PR by spin-coating. D

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Figure 7. (A) Conformal contact of the mask and substrate. (B) Schematic of the optical setup with the electric field of the laser light polarized across the grating.

period of the grating is 264 nm and the photoresist thickness can be as high as 275 nm. Their simulated field distributions in the xz plane are shown in Figure 10A,B. The aspect ratios of the patterns working at 365, 405, and 436 nm are close to 4:1 and nearly identical. The effective indices neff in the three cases are 1.659, 1.653, and 1.6515, respectively, consistent with the dispersion relation of the SPP wave given in Figure 2B. Figure 8. SEM images of 100 nm thick PR. (A) Top view shows large-area uniformity. (B) Cross section. The image was taken at a 30° angle, and the marked dimensions have been converted into the normal view.

CONCLUSIONS In conclusion, we demonstrated a plasmonic interference lithography system by utilizing Al grating on glass as a mask and a PR on stacked metal/dielectric layers on a PET substrate. Our design is based on spatial frequency selection of evanescent waves, so that a single high-k mode is maintained and imaged in the PR, which results in a uniform deep subwavelength periodic pattern. The proposed lithographic approach can be applied to fabricate 61.25 nm half-pitch lines with large-area pattern uniformity and depth exceeding 100 nm. Our approach overcomes two major drawbacks (poor uniformity and limited depth) in previous plasmonic lithography and broadens its applications in nanoscale patterning. In addition, other than simple 1-D grating, two-dimensional (2-D) structures can be generated by multiple exposures in the experiment (results shown in the Supporting Information). The design criteria can also be applied in the noncontact waveguide lithography systems40−42 to improve its pattern depth, pattern profile, and uniformity. It is also worth noting that the proposed plasmonic lithography system can adopt a flexible mask system. Similar to roll-to-roll lithography,43−46 by replacing the glass mask with an Al grating on a flexible substrate, one can envision a roll-based plasmonic interference nanolithography for improved throughput and ability to pattern large-area substrates.

Figure 9. SEM images of the 200 nm thick patterned PR. (A) Top view of the uniform PR patterns. (B) Corresponding cross section view of the patterns from a 30° angle. The special region was selected to see the profile and thickness of the resist.

provide designs for i-line and g-line light sources. Similar to the 405 nm (h-line) case, at 365 nm (i-line) and 436 nm (g-line), periodic patterns can be generated in the PR with single high-k modes and the waveguide coupling strategy. For 365 nm, the mask consists of Al grating with a 220 nm period, and other parameters are the same as those in the 405 nm design. The substrate is composed of 215 nm PR, 50 nm SiO2, and 15 nm Al layers stacked on the PET sheet. For 436 nm, the optimal

Figure 10. Normalized |E|2 maps in the xz plane at wavelengths of (A) 365 nm and (B) 436 nm. Thicknesses of the PR are (A) 215 nm and (B) 275 nm, and the incident electric field is 1 V/m. E

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METHODS Numerical Simulation. Simulations were performed in COMSOL 4.4, and calculation was done by MATLAB 2015. Sample Preparation. The fabrication procedures of the glass mask and PET substrate are illustrated in Figure 6. A 1-D Si grating with a 245 nm period was prepared as a mold to generate mr-I resist patterns by thermal nanoimprint7,24,47 lithography (NX-2000 Nanonex). The residual layer of the imprinted mr-I resist was removed by subsequent reactive-ion etching followed by angled evaporation of 10 nm titanium (Ti) and e-beam evaporation of 22 nm Al. After lift-off in acetone, the Al grating with a 245 nm period (around 50% duty cycle) and a height of 22 nm was obtained on glass. A thin 40 nm PMMA layer was spin-coated on top of the Al grating as a spacer, which was flattened by the imprinter so that a thin and flat 10 nm Al could be sputtered. After the sputtering, mask fabrication was completed. For the preparation of the substrate, regular PET films were planarized by spin-coating of a 10 μm thick SU-8 resist, followed by 15 nm Al sputtering and 44 nm SiO2 evaporation. The PR used in the experiment is a 1:1 diluted AR-N 7500 positive resist at 405 nm wavelength illumination. The diluted PR layer with a 100 nm thickness was spin-coated on the SiO2 layer and soft baked on a hot plate at 85 °C for 1 min. Exposure System. The optical setup built to ensure the conformal contact is shown in Figure 7B. The mask and PET substrate coated with the photoresist were placed between a clean glass slice and a thick polydimethylsiloxane (PDMS) cushion on a flat stage. Conformal contact can be realized since both mask and substrate have a flat surface, while the PET film and PDMS cushion are flexible. Two screws were used to provide pressure when tightened to ensure conformal contact during the light exposure. A collimated 405 nm TM-polarized diode laser (ONDAX x6474) light with its electric field perpendicular to the grating direction was incident to the mask through a circular aperture with diameter of ∼1 cm. The laser intensity was set at 3 mW·cm−2, and the exposure lasted for 15−20 s. Finally, the resist on PET was taken out and immersed in 1:1 diluted AR 30035 developer for 45−50 s, followed by a 30 s deionized water rinse.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b06137. Derivation and detailed explanation for the spatial frequency selection principle, other designs to make extremely small patterns, simulated results of direct contact lithography as a comparison, discussion of the experimental conditions, analyses and effects of film roughness, and Figures S1−S8 (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge the support of this work by the National Science Foundation (CMMI-1537440). REFERENCES (1) Pendry, J. B. Negative Refraction Makes a Perfect Lense. Phys. Rev. Lett. 2000, 85, 3966. (2) McNab, S. J. Evanescent near-Field Optical Lithography: Overcoming the Diffraction Limit. Ph.D. Thesis, University of Canterbury, 2001. F

DOI: 10.1021/acsnano.5b06137 ACS Nano XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsnano.5b06137 ACS Nano XXXX, XXX, XXX−XXX