Letter www.acsami.org
Optically Patternable Metamaterial Below Diffraction Limit Youngseop Lee,†,‡ Sang-Gil Park,†,‡ SeokJae Yoo,§ Minhee Kang,†,‡ Sang Chul Jeon,∥ Young-Su Kim,∥ Q-Han Park,§ and Ki-Hun Jeong*,†,‡ †
Department of Bio and Brain Engineering and ‡KAIST Institute for Health Science and Technology, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea § Department of Physics, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea ∥ National Nanofab Center, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea S Supporting Information *
ABSTRACT: We report an optically patternable metamaterial (OPM) for ultraviolet nanolithography below the diffraction limit. The OPM features monolayered silver nanoislands embedded within a photosensitive polymer by using spin-coating of an ultrathin polymer, oblique angle deposition, and solid-state embedment of silver nanoislands. This unique configuration simultaneously exhibits both negative effective permittivity and high image contrast in the ultraviolet range, which enables the surface plasmon excitation for the clear photolithographic definition of minimum feature size of 70 nm (≲ λ/5) beyond the near-field zone. This new metamaterial provides a new class of photoresist for ultraviolet nanolithography below the diffraction limit.
KEYWORDS: meta-photoresist, surface plasmon excitation, superlens effect, ultraviolet nanolithography, subdiffraction limit
M
through the superlens exponentially decays in a dielectric medium of photoresist. Furthermore, the direct patterning of photoresist film on the substrate of a photomask with superlenses or hyperlenses intrinsically impedes their practical use for nanolithography, as well as the subsequent nanofabrication process. Here we report an optically patternable metamaterial (OPM) comprised of monolayered silver nanoislands (AgNs) embedded within a photosensitive polymer resin for ultraviolet nanolithography (Figure 1). The OPM exhibits the excitation of surface plasmons under UV exposure and defines clear subdiffraction limited image patterns to overcome the physical limit of the CDD. The AgNs also effectively facilitate the rapid removal of unexposed regions during the development process because they allow a developing solution to rapidly permeate through the polymer resin. The surface plasmon excitation is clearly visualized by using the finite difference time domain (FDTD) method (Figure 1b). A chrome (Cr) reticle was prepared with a nanopatterned text spelling “NANO” with a 50 nm line width. For comparison, both the 50 nm thick OPM and a conventional photoresist were placed between the reticle and a silicon substrate. The electric field intensity of ultraviolet light (365 nm) clearly retains the subdiffraction limited image within the OPM, whereas the conventional photoresist shows significantly blurred and highly diffracted images.
etamaterials, i.e., artificial materials for manipulating electromagnetic waves, have become of considerable interest thanks to the miscellaneous and compelling capabilities, including negative index,1−4 bianisotropy,5 chirality,6,7 and zero-index.8,9 In particular, negative index metamaterials overcome the optical diffraction limit by compensating the information loss at the subdiffraction limit scale by the reverse propagation of electromagnetic waves inside a propagating medium.10,11 Negative metamaterials in the ultraviolet range allow optical nanolithographic patterning below the diffraction limit, which is very important for the nanofabrication of a variety of nanoelectronic devices and sensors. Recent metamaterials have demonstrated subdiffraction limit imaging in the ultraviolet (UV) or visible range, based on the superlens effect.11−18 A thin silver film sandwiched between two dielectric spacer layers effectively engraves the nanoscale UV exposure patterns from a far-field chrome mask onto an ultrathin photoresist.12,13 Plasmon-coupled metal thin film and grating structures further transform the near-field image of an object onto the far-field.14 In addition, anisotropic thin multilayers of metal and dielectrics can magnify the subdiffraction limited object by transforming the scattered evanescent waves into propagating waves to project high resolution images onto the far-field.15−17 However, most previous works still have intrinsic limitations when it comes to defining nanoscale patterns on a photoresist that is thicker than the characteristic distance of diffraction (CDD). The CDD determines the range of the near-field zone for a single slit of width D, and is typically given by nD2/λ0. Note that the subdiffraction limited pattern information passing © 2017 American Chemical Society
Received: February 28, 2017 Accepted: May 24, 2017 Published: May 24, 2017 18405
DOI: 10.1021/acsami.7b02940 ACS Appl. Mater. Interfaces 2017, 9, 18405−18409
Letter
ACS Applied Materials & Interfaces
the lithographic contrast C = (IMAX − IMIN)/(IMAX + IMIN), where IMAX and IMIN are the E-field intensities on the substrate at the center of open area and masked area, respectively, was further calculated using FDTD (Figure 2a). The calculated results indicate the OPM has acceptable lithographic contrast of C > 0.5 when the nanoisland widths are relatively larger than the gap of the nanoislands, with 10−16 nm in thickness (Figure 2e). In addition, the lithographic contrast C reaches maximum value when the thickness of the nanoislands and that of the polymer resin have a strong linear relationship with the constant width and gap of the nanoislands (Figure 2f). Ultraviolet nanolithography was experimentally demonstrated by combining the OPM with a conventional contact mask aligner (Figure 3a). The OPM was simply prepared on a 4 in. silicon wafer by using spin-coating of an ultrathin photosensitive polymer (SU-8 2000.5; MicroChem Corp.), oblique angle deposition (OAD) of a thin silver film, and solid-state embedment of AgNs. An ultrathin photosensitive polymer resin was spin-coated on a 4 in. silicon wafer using a dilute SU-8 solution with a cyclopentanone (SU-8 2000 series thinner; MicroChem Corp.) with a dilution ratio of 10:1, which provides an ultrathin SU-8 film thickness ranging from 20 to 180 nm (see Section 2 in the Supporting Information for details). The AgNs were then directly formed on the substrate by thermal evaporation with an oblique angle of 80°.21 Both the ultrathin layers of the SU-8 and the monolayered AgNs were thermally annealed at 110 °C for 10 min (Figure 3d, e). Note that the embedment of the noble metal nanoparticles from the surface inside the polymer or ceramic resins is mainly driven by thermal diffusion.22−24 The embedded depths of the AgNs inside the ultrathin SU-8 depending on the annealing time were experimentally observed by using transmission electron microscopy (TEM) (see Section 3 in the Supporting Information). The embedded depth was also theoretically calculated by using the Stokes−Einstein diffusion model, which mainly depends on annealing time, the size of particles, viscosity of the medium, and physical constants24 (see Section 3 in the Supporting Information). Nanopatterns were photolithographically defined in the OPM by using a conventional UV mask aligner (Karl Suss MA6) (see Section 4 in the Supporting Information for details). A Cr photomask with subdiffraction limited patterns including “NANO’ text and line arrays with different orientations of 0, 45, and 90 deg, and line widths of 70, 100, 120, and 170 nm, was prepared using e-beam lithography and Cr lift-off (see Section 5 in the Supporting Information for details). UV exposure was performed in vacuum contact with the minimum gap spacing between the substrate. The mask and the nanopatterns on the substrate were finally developed using a conventional SU-8 developer (MicroChem Corp.). The experimental results clearly demonstrate that the OPM defines the subdiffraction limited images of the arbitrary text and the line arrays under one-shot UV exposure, whereas the conventional photoresist (SU-8) significantly diffracts and broadens the identical images (Figure 3f, g, see also Figure S5). Based on topographic measurements with the atomic force microscope (AFM), the “NANO” textured patterns of 70 nm (≲ λ/5) line width are clearly defined in the OPM but have significantly broadened up to 240 nm in the SU-8 resist. The quantitative values of the nanolithographic results, such as average critical dimension (CD), CD uniformity (σCD), and line edge roughness (LER), were been statistically evaluated using the AFM measurement results (Table S2). The minimum CD
Figure 1. (a) Schematic illustration of an optically patternable metamaterial (OPM) for ultraviolet nanolithography. The OPM consists of monolayered silver nanoislands sandwiched between two ultrathin photosensitive layers and exhibits the excitation of surface plasmons that preserves ultraviolet image patterns below the diffraction limit. After UV exposure, this unique configuration of embedded nanoislands facilitates the permeation of a developing solution, allowing it to rapidly pass through the top to the bottom of the OPM during the development process. (b) 2D E-field intensities of UV light (λ = 365 nm) on a silicon substrate propagating through the OPM (left) and through a conventional photoresist (right) of the same thickness, for a text Cr pattern of “NANO” with a 50 nm line width. The width, thickness, and gap of the islands are 20, 10, and 4 nm, respectively. Both of the results were calculated using a 3D finite difference time domain (FDTD) method. Unlike the blurred pattern of the photoresist, light diffraction with the OPM was significantly suppressed at the top silicon surface due to the excitation of surface plasmons (scale bar = 200 nm).
The optical properties of the OPM were numerically calculated for different geometric parameters using FDTD (Figure 2). The OPM shows a comparable absolute value for the real part of the permittivity of the neighboring photosensitive resin, but with the opposite sign. Consequently, the high spatial frequency clearly retains the lateral profile without any noticeable diffraction; otherwise it would exponentially decay along the propagation axis. The OPM clearly transfers the subdiffraction limited image with a line width of 50 nm up to farther than the CDD of nD2/λ0, e.g., 11 nm for the 50 nm chrome slit and a photosensitive polymeric medium (SU-8, n = 1.64 at 365 nm19), where D is the aperture size, n is the refractive index of the medium, and λ0 is the wavelength in vacuum (Figure 2b, c). For a unit-cell size below λ/10, the OPM serves as an effective medium of homogeneous permittivity. The OPM also exhibits excitation of surface plasmons that the permittivity of the OPM εM has a negative real part, i.e., εM′ < 0, as well as an absolute value | εM′ | close to the real part of the relative permittivity of the neighboring polymer resin εP.11 Depending on AgNs of different sizes, the real part of the effective permittivity εM′ was first calculated at 365 nm in wavelength, using FDTD and S-parameter retrieval methods20 (see Section 1 in the Supporting Information for details). The calculated results for effective permittivity show the OPM efficiently exhibits the excitation of surface plasmons for nanoisland widths that are relatively larger than the gap spacing (Figure 2d). On the basis of the Michelson equation, 18406
DOI: 10.1021/acsami.7b02940 ACS Appl. Mater. Interfaces 2017, 9, 18405−18409
Letter
ACS Applied Materials & Interfaces
Figure 2. Effective permittivity and lithographic contrast of OPM with different geometric parameters. (a) 2D FDTD model of the intensity distribution of 365 nm UV light passing through Cr slit arrays with a line width of 50 nm and the OPM on a silicon substrate. The geometric parameters of the OPM include the width (w), gap (g), and thickness (ta) of the Ag nanoislands, and the thickness (td) of the dielectric layer. The lithographic contrast C = (IMAX − IMIN)/(IMAX + IMIN), where IMAX and IMIN are the E-field intensities at the center of the open area (point A) and the masked area (point B), respectively. (b) Cross-sectional intensity distribution of UV light (365 nm) propagating through the OPM and dielectric layers of the same thicknesses (50 nm) under Cr slit arrays with 50 nm line widths. The width, thickness, and gap of the Ag nanoislands are 20, 10, and 4 nm, respectively. (c) Cross-sectional E-field intensity at the bottom of the OPM and dielectric shown in Figure 2b. (d) Calculated εM′ values of the OPM depending on w, g, and ta. The OPMs exhibit the excitation of surface plasmons in the regions of negative permittivity (blue regions). (e) Calculated C values depending on the w, g, and ta at td = 20 nm. (f) Calculated C values depending on ta and td at constant w = 20 nm and g = 4 nm. The OPMs with contrast above 0.5 enable nanopatterning of binary line arrays beyond the diffraction limit. The OPMs with width relatively larger than the gap spacing exhibit acceptable lithographic contrast, i.e., C > 0.5, with a negative permittivity (scale bar in b = 50 nm).
Figure 3. OPM based UV nanolithography using a conventional contact printer. (a) Lithography procedures. (i) Ultrathin SU-8 layer was spincoated on a 4 in. silicon wafer using diluted SU-8. (ii) Silver nanoislands were directly formed on the substrate using thermal evaporation with an oblique angle. (iii) Silver nanoislands were diffused through the ultrathin SU-8 layer during low temperature thermal annealing. (iv) UV exposure at 365 nm after a vacuum contact with a 5 in. Cr photomask of subdiffraction limited features. (v) Nanopatterns defined in the developing solution after a postexposure bake. (b) Optical image of the 5 in. Cr photomask with subdiffraction limited features (scale bar = 25 mm). (c) UV nanolithographic results on a 4 in. silicon wafer (scale bar = 20 mm). (d) Measured and calculated embedded depths of silver nanoislands inside an ultrathin SU-8 layer depending on thermal annealing time. (e) Cross-sectional TEM images of an ultrathin SU-8 layer, silver nanoislands on the surface of SU-8, and embedded silver nanoislands in SU-8 after thermal annealing (scale bar = 30 nm). (f) AFM images of text patterns “NANO” and line arrays on the Cr photomask, the OPM, and a conventional photoresist (SU-8) (scale bar = 250 nm). (g) Cross-sectional line widths of the letter “O” on both the OPM and SU-8 patterns defined by the Cr mask with a line width of 70 nm. 18407
DOI: 10.1021/acsami.7b02940 ACS Appl. Mater. Interfaces 2017, 9, 18405−18409
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ACS Applied Materials & Interfaces
dose for the OPM was found to be 1.15 times higher than that in the SU-8 resist due to the UV absorption of AgNs (Figure S6). The optimal UV exposure also significantly depends on the pattern line width (Figure S7). For instance, the optimal exposure dose for the 70 nm line width was 1.33 times higher than that for the 170 nm line width. The nanoisland size substantially affects the nanolithographic results (Figure 4). The OPMs with different AgN sizes were precisely controlled with high uniformity, by changing the deposition thickness of silver thin film (Figure 4a). The average width (w), gap (g), and thickness (ta) of the individual AgNs were 14, 6, and 10 nm for S1, 16, 6, and 12 nm for S2, and 18, 6, and 14 nm for S3 based on the cross-sectional TEM images and the top view SEM images, respectively (Figure 4c). The standard deviations are less than 2 nm. The AgNs can be considered regular arrays, following the numerical analysis in Figure 2. The calculated C and εM′ for each of the OPMs, based on the measured physical parameters, are shown in Table. 1. Table 1. Calculated Contrast (C) and Real Part of Permittivity (εM′) of the OPMs with Different Physical Dimensions C εM′
S1
S2
S3
0.75 −2.33
0.90 −1.42
0.51 −0.34
The S1 and S2 OPMs show high lithographic contrast values above 0.75, and thus clearly define both “NANO” textured patterns and line arrays patterns with a minimum feature size of λ/5. The line arrays of 70 nm become relatively broader on the OPM-S3 due to the unsuitable real part of the permittivity, and the relatively low lithographic contrast, due to the relatively thin top layer of SU-8. Even with variations in the size of the AgNs inside the OPM, stable lithographic results were still obtained. For instance, the OPM still has acceptable lithographic contrast, C > 0.5, when the AgNs sizes vary by ±2 nm (see Figure S8). The OPM-S1 and S2 also clearly defined line arrays with different widths, regardless of their orientations, for different sizes of Ag nanoislands. In conclusion, this work successfully demonstrated an optically patternable metamaterial (OPM) for ultraviolet nanolithography on a wafer level. In conjunction with a conventional contact mask aligner, the OPM with monolayered Ag nanoislands embedded in an ultrathin SU-8 layer effectively served as a new class of photoresist for the ultraviolet nanolithography of arbitrary nanopatterns, with a minimum line width of 70 nm (≲ λ/5), beyond the near-field zone, under one-shot UV exposure. Unlike conventional nanolithographic methods such as electron beam, scanning probe, or nanoimprint lithography, this new material provides a large-area, high-throughput, and low-cost nanopatterning method, and provides new insights for optical nanolithography below the diffraction limit.
Figure 4. UV nanolithographic results with the OPM depending on different dimensions of the Ag nanoislands. (a) Cross-section TEM images of OPMs with different sizes of AgNs (S1−S3). The average dimensions (w, g, ta) of individual AgNs are 14, 6, and 10 nm for S1; 16, 6, and 12 nm for S2; and 18, 6, and 14 nm for S3, respectively. The standard deviations are less than 2 nm. The top black layer indicates a thin platinum (Pt) layer applied for TEM sample preparation. The OPMs with high contrast above 0.5 (S1−S3) clearly define the nanopatterns with a minimum feature size of 70 nm (scale bar = 30 nm). (b) AFM measurement of nanopatterned OPMs for different Cr mask pattern line widths. Both the “NANO” text and line arrays patterns are clearly defined in the OPMs, regardless of their pattern orientations. The OPM-S3 has lower lithographic contrast than other OPMs because the top SU-8 layer of S3 is thinner than that of the others. The line arrays in the S3 OPM are relatively blurred compared to other OPMs due to lower lithographic contrast (scale bar = 500 nm). (c) Extracted effective width and thickness of the Ag nanoislands (AgNs) for OPM S1, S2, and S3. The effective width and thickness of the AgNs were calculated from the TEM images, where the error bar indicates the size distribution (± standard deviation) of the AgNs.
was 86 nm for the 71 nm line patterns on the Cr mask, because additional SU-8 near the exposed region was further crosslinked due to the locally enhanced electric field of the AgNs, as shown in the calculated result in Figure 2b.25 The LER between the OPM and mask patterns exhibited a small difference of 2 nm. The experimental results also showed that the 45-degreestilted line arrays with different widths were clearly defined in the OPM, but merged together in the SU-8 resist. Note that except UV exposure dose, all of the lithographic parameters, including the thickness and the development time, were kept the same for both the OPM and the photoresist. The optimal UV exposure
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02940. Details of the numerical analysis for effective permittivity and lithographic contrast; detailed conditions for the fabrication of ultrathin photoresist, chrome reticle; theoretical modeling for the solid-state embedding of 18408
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silver nanoislands inside the polymer resin; extended data for lithographic results (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
SeokJae Yoo: 0000-0002-6438-7123 Ki-Hun Jeong: 0000-0003-4799-7816 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (2015036205, 2016013061), and a grant of the Korea Health Technology R&D Project through the Korea Health Industry Development Institute (KHDI), funded by the Ministry of Health & Welfare, Republic of Korea (HI13C2181).
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DOI: 10.1021/acsami.7b02940 ACS Appl. Mater. Interfaces 2017, 9, 18405−18409
