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Plasmonic color filters in mass production have been restricted from current fabrication technology, which impede their applications. Soft-X-ray interference lithography (XIL) has recently generated considerable interest as a newly developed technique for the production of periodic nano-structures with resolution theoretically below 4 nm. Here we ameliorate XIL by adding an order sorting aperture and designing the light path properly to achieve perfect-stitching nano-patterns and fast fabrication of large-area color filters. The fill factor of nanostructures prepared on ultrathin Ag films can largely affect the transmission minimum of plasmonic color filters. By changing the fill factor, the color can be controlled flexibly, improving the utilization efficiency of the mask in XIL simultaneously. The calculated data agree well with the experimental results. Finally, an underlying mechanism has been uncovered after systematically analyzing the localized surface plasmon polaritons (LSPPs) coupling in electric field distribution.
© 2016 Optical Society of America
1. Introduction
Metal nanostructures are of substantial interest in the field of industrial and academic research [1–7] due to their unique optical, electronic, and magnetic properties. Considerable efforts have been made for promoting their industrialization process, which focused mainly on reducing structural size to more integrated degree and enabling nanostructure manufacturing into larger scale effectively, i.e., resolution and throughput. Conventional top-down lithography techniques with required resolution, such as electron beam lithography (EBL) [8–10] and focused Ion beam (FIB) [11, 12], are low-throughput, which restrict large-area fabrication for industrial applications. Alternative approaches with high throughput, such as, self-assemble preparation [13], nano-imprint [14, 15], Talbot lithography [16–19] and ultraviolet interference lithography [20–23], are capable of developing large-area fabrication of identical nanopatterns, while they also have obvious limitations. Self-assemble technique suffers from uncontrollable periods. Although nano-imprint is a convenient and robust nanofabrication technique, its performance is then affected by density of particles [24]. Talbot lithography is also an important method for patterning structures of large area. However, it is limited by depth of focus [25]. A highly-accurate technique is required to place the transmission mask and samples extremely close, increasing its fabrication difficulty. Although achromatic Talbot method can improve the distance tolerance, it leads to the overlap of self-images and inevitable un-designed patterns around each of the exposure area [24]. Ultraviolet interference lithography, commonly produced by phase gratings to obtain intensity fringes with low cost, is probably the leading technology for large area manufacturing. However, its resolution is dramatically limited by wavelength of source, such as 248 nm (krypton fluoride lasers) and 193 nm (argon fluoride lasers) [26] and 46.9 nm (hot Ar plasmon column, with low exposure intensity, i.e., high time-consuming) [27], and its mask utilization efficiency is low due to one-to-one correspondence between the pattern and the mask applied. Very recently soft-X-ray interference lithography (XIL) with high resolution, strong exposure intensity and excellent coherence has emerged as a promising technique for manufacturing nanopatterns with both high precision and throughput [28, 29].
Taking advantage of Shanghai Synchrotron Radiation Facility (SSRF), in this work, we develop a XIL-based technology to fabricate periodic large-scale nanostructures, which produces an area of 400 x 400 μm2 in a single exposure only within a few seconds but with high replication and superior uniformity. Meanwhile, the metallic periodic arrays are systematically investigated as candidate to next generation color filters, which are key components for advanced optical instruments, such as image sensors, color printing and display devices [30–35].
2. Experimental design and simulations
Figure 1 shows the schematic layout of the XIL beamline [28, 36]. The high brilliant soft-X-ray with energy of 140 eV is filtered by setting the gap size of elliptically polarized undulator (EPU) to 44.2 mm. A four-knife slit (S1), utilized to define the incident angle to ± 0.04 mrad in horizontal and vertical directions, can absorb most of the heat load and protect the downstream optical elements. To deflect and focus the beam, a cylinder mirror M1 and a toroidal mirror M2 with Au-coated and side water-cooled characters, are placed with designed angles corresponding to the energy required. As all matters absorb soft-X-ray radiation, reflective mirrors rather than refractive ones must be chosen. M1 deflecting the beam horizontally by 1.5° functions to parallel the beams and cuts the higher energy radiation (>2000 eV). M2 deflects the beam by 10° to further eliminate higher harmonic and focus the beam. The focused beam passing through another four-knife slit (S2) possesses the character of high quality spatial coherence, acting as the second light source for XIL. Measured by the photodiode (PD), the exposure intensity on the mask can reach as high as 74.15 mW/cm2, resulting in a very short time for a single exposure.
Four gold gratings were used as the mask as shown in Fig. 2(a). The grating fabrication process is as follows: Gratings on the photoresist layer were first written by EBL (Crestec CABL-9500C) on a low-stress 100 nm Si3N4 transparent membrane. A 70 nm-thick Au was, then, deposited on the photoresist layer and gold gratings were obtained after a lift-off processing. A 10 nm thick Cr layer between the membrane and the gratings served as the adhesion promoter. Outside grating region, a 250 nm thick perm alloy acted as photon stop layer. The backside silicon substrate was removed by a wet-etch process using an aqueous KOH solution at 80 °C. Two dimensional nanodot arrays were realized via the four-beam first order interference. The first order diffracted beams from the four gratings interfere in the center to form a rotated periodic nano-hole array with 45° [37] and the same area as the diffraction gratings on the wafer of polymethyl-methacrylate (PMMA). The scheme for color filters with periodic nano-cylinder array (after deposition and lift-off procedure) is shown in Fig. 2(b).
Fig. 2 (a) Schematic illustration of four-beam interference: four first order beams interfere in the center to form nano-hole array, surrounded by four grating patterns which are from zeroth order diffracted beams. (b) A schematic representation of the color filter. In this work, P is the period, D is the diameter, h is the thickness of ultrathin Ag film.
To achieve multi-patterns by using one mask, here first we systematically carried out three dimensional (3D) simulations for the nano-cylinders with various fill factors (defined as diameter/period) [38]. Transmission spectra were calculated by Lumerical commercial software package (Lumerical Solutions Inc.). The complex dielectric constants, n and k, of the ultrathin Ag film used in simulations are from Palik (0–2 μm) [39]. Symmetric or anti-symmetric boundary conditions were set in x or y direction to reduce time consuming and perfect matched layer (PML) were set in Z direction to prevent unphysical scattering at the edge of the simulation box. The mesh size in calculation was 1, 1, and 2 nm in x, y, and z directions, respectively.
Figure 3 shows the simulated transmission spectra and CIE1931 chromaticity diagrams for three selected periods (180 nm, 230 nm, and 280 nm) nano-cylinder arrays with different fill factors (from 0.3 to 0.7) perforated on a 20 nm-thick Ag film. It is found that periodic nano-cylinders can exhibit fill-factor-dependent transmission spectra. The transmission minimum is characterized by a typical redshift with increasing fill factor. Simultaneously, the full-width at half maximum (FWHM) of the spectrum valley becomes obviously wider, leading to a significant improvement on the color purity [40]. The transmission spectrum is easily modulated by varying the nano-cylinder diameter without changing the period although in our previous work [41], it was found that the transmission minimum exhibits a redshift with increasing period for arrays. Based on these results, we proposed a simple method to obtain different-fill-factor patterns by only tuning exposure time, resulting in a dramatic enhancement to utilization efficiency of a single mask.
Fig. 3 Transmission spectra of nano-cylinder arrays for 20 nm-thick Ag with various fill factors ranging from 0.3 to 0.7 in a step of 0.05 for selected three periods (a) 180 nm, (b) 230 nm and (c) 280 nm. (d)-(f) CIE1931 chromaticity diagrams overlaid with points corresponding to the simulated transmission spectra colors above. The color varies from pink to blue and deviates progressively from the white point (0.31, 0.32) towards the spectral curve of the diagram, suggesting higher color purity.
The large-area manufacturing of nano-cylinder arrays was achieved by stitching multiple exposure fields. We first spin-coated a layer of PMMA-A4 photoresist of 80 nm-thickness on substrate of glass. The coated sample was baked at 180 °C for 90 s. An illustration for exposure is shown in Fig. 4. The first-order diffracted beams from the four gratings in mask interfere in the center to form a square periodic array with a period of [37].
Fig. 4 Schematic diagram of four-beam interference lithography showing cross-section of the shutter, mask, OSA and two interfering beams. Diffracted beams interfere and form periodic nano-cylinder arrays on the PMMA plane. Only first-orders and zeroth-order diffraction are shown for simplicity.
Thanks to the high energy of X-Ray beam at Shanghai synchrotron radiation source, an area as large as 400 x 400 μm2 was obtained in a single exposure with a few seconds, which is only about one several hundredth time needed for EBL or FIB processing. Taking into account image stitching, the zeroth-order elimination is crucial. Otherwise, it will be background field around designed patterns. In order to obstruct the zeroth order, an order sorting aperture (OSA) was placed closely but disconnectedly in front of wafer. Considering practical optical path construction, a tolerant distance () is necessary.
where Lg and LOSA are the side lengths of mask and OSA, respectively. Zs or ZOSA, the distances between sample and mask, or OSA and gratings, respectively. d, the period of phase grating. m, the order of diffraction. θ, the angle of diffraction. λ, the wave length of incident light. Ld, the distance between two vertical or horizontal phase gratings. The maximum tolerance occurs at LOSA = Ld assuming a value of Lg. Ld is limited to the spot size of illuminated source which is inversely proportional to coherence. Hence, by elaborating all parameters, more flexibility in XIL set-ups could improve lithography stitching theoretically without disturbance from zeroth order light. By moving sample platform and blocking light correspondingly with shutter control to accomplish step-and-repeat exposure, a required pattern was prepared on PMMA in large scale. Silver film of 20 nm in thickness was then deposited, performed lift-off in acetone and finally got square periodic cylinders.3. Results and discussion
Figures 5(a)-5(e) show a series of color filters with colors varying from pink to cyan and SEM images, respectively. Optical microscope images of nano-cylinder arrays under incident whitelight illumination were measured using a microscope (Nikon80i). The colors exhibit good contrast and high brightness. Figure 6 shows one filter device, fabricated by XIL stitching, held by fingers. The insert on the up-right corner shows cylinder arrays of 20 nm in height, 0.45 in fill factor and 230 nm in period. The filter with the size of 4 x 4 mm2 was accomplished in less than 30 mins. Except the imperfect stitching step distance leads to the gap between each single exposure, the filter shows good uniformity.
Fig. 5 The SEM images of nano-cylinder arrays on a 20 nm-thick Ag film with period 230 nm, (a) fill factor = 0.25, (b) fill factor = 0.4, (c) fill factor = 0.5, (d) fill factor = 0.6 and (e) fill factor = 0.75. Optical transmission micrographs under white light illumination and measured transmission spectra below the SEM images are consistent with the simulated results in Figs. 3(d)-3(f), realizing color tunability with various fill factors.
Fig. 6 One filter device, fabricated by XIL stitching, held by fingers. The insert on the up-right corner shows cylinder arrays of 20 nm in height, 0.45 in fill factor and 230 nm in period.
Simulated and measured transmission spectra were plotted in Fig. 7. The experimentally observed spectra are consistent with the simulated results. Slight difference between simulation and experimental results could be mainly attributed to nanofabrication defects.
Fig. 7 Simulated and measured transmission spectra of filters with fill factor ranging from 0.3 to 0.7, together with the dashed trend line tracing the location of resonance wavelength.
Furthermore, considering the symmetric shape and arrangement mold of the nano-cylinder, the transmission spectra under both TE or TM polarization illumination are the same [42].
In order to unveil the physical mechanism for above obtained results, 2D transmission spectrum maps with selected parameters (P = 230 nm, h = 20 nm, fill factor = 0.3~0.7) for nano-cylinders are plotted in Fig. 8(a). The short-range surface plasmons polaritons (SRSPPs, black dashed line) and localized SPPs (LSPPs, white solid line) are calculated by the dispersion relations [43] or transmission minimum position for a single Ag nano-cylinder with the same size as that of the nano-cylinder array.
where ,,,and h is the thickness of the metal film. (m = 1,2) are the dielectric constants of glass (m = 1) and air (m = 2), and represents the dielectric constant of ultrathin Ag film.where = = are the reciprocal lattice vectors for a square lattice [44] and, where P is the period, a and b are integers, is the incident angle. For normal incidence, the Eq. (6) can be simplified to Eq. (7).Fig. 8 (a) Two dimensional transmission spectrum maps for 20 nm-thick Ag films composed of cylinder arrays with fill factor from 0.3 to 0.7. The white curve refers to LSPPs. The dashed line represents analytical dispersion relations for SRSPPs modes. (b)-(d) In-plane (xy-cutting plane) views of Electric fields of two nano-cylinder illuminated by an x-polarized plane wave at their resonant wavelengths of 507, 616 and 715 nm with fill factor of 0.25, 0.5 and 0.75, respectively.
In Fig. 8(a), two dimensional transmission spectrum maps show following characteristics: the transmission minimum shifts to longer wavelengths and tends to broaden when increasing the fill factor. Unlike previous work [35] that LSPPs could be the main factor to the variation tendency, here transmission minimum widens obviously in large fill factor, which could be attributed to the hybridization of LSPPs and SRSPPs modes. Additional simulations show the electric field distributions of two nano-cylinder with different fill factors illuminated by an x-polarized plane wave at their corresponding resonant wavelengths in Figs. 8(b)-8(d). As the fill factor increases, the LSPPs becomes weaker as a result of more effective electric coupling between nano-cylinders. Consequently, both contributions of LSPPs and SRSPPs lead to broad dips.
4. Conclusions
In conclusion, we have proposed a design strategy for large scale manufacture of color filtering based on XIL stitching. By controlling the exposure time, we can tune the fill factor to acquire different transmission minimum with the same mask. Various large scale color filters with a series of nano-cylinder arrays on Ag film layers are successfully fabricated. The agreement of experimental results with the simulation ones indicates that this is a fast, up-scalable and economically way for the fabrication of color filters.
Funding
National Natural Science Foundation of China (U1432110, 51371157, U1432105); National Key Basic Research Program of China (2012CB825700); China Scholarship Council (201400260166).
Acknowledgments
The authors thank the support of Soft-X Ray Interference Lithography Beamline (BL08U1B) in SSRF for sample preparation and the calculations performed on computer resources at National Supercomputer Center in Guangzhou and Tianjin and TianHe-1 (A).
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