Article Cite This: ACS Photonics 2018, 5, 1460−1466
Control of Si-Based All-Dielectric Printing Color through Oxidation Yusuke Nagasaki,† Masafumi Suzuki,† Ikuto Hotta,† and Junichi Takahara*,†,‡ †
Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan Photonics Center, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
‡
S Supporting Information *
ABSTRACT: All-dielectric color printing by means of high-index Mie resonators has enabled wider control of reflection colors depending on structural geometry. However, modifying the geometry, including the height, by using conventional fabrication processes remains challenging, and drastic color modification approaches via the addition of a new tuning axis are required to extend color varieties and applications. Here, we demonstrate all-dielectric pixel color control through Si oxidation. Oxidized monocrystalline Si nanostructures exhibit wider tunability of brilliant reflection colors depending on the oxidation reaction. The different color change properties of each nanostructure enable the construction of an “invisible ink” that can hide color information. This approach for controlling printing color could be utilized to further extend color variation and reactive applications. KEYWORDS: all-dielectric, structural color, color printing, Si oxidation, metamaterial, core/shell
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conventional fabrication processes, such as reactive ion etching (RIE), on a single substrate. A new tuning axis is required to further extend the color variation control. Although spontaneous solid-state dewetting,24 which can produce island nanostructures acting as Mie resonators, has emerged as a potential fabrication method for controlling the all-geometric features of color pixels, it is still challenging to fabricate nanostructures with a high aspect ratio and ensure reproducibility in Mie resonators, whose colors are highly sensitive to changes in their structure size. Here, we extended our previous work22 to introduce a color modification method into all-dielectric color printing by focusing on the reactivity of Si with oxygen. Si is a highrefractive-index semiconductor, and its oxidation physics has been thoroughly studied owing to its wide use in massproduction planar processes in the semiconductor industry. Depending on the conditions, an oxide film can be deposited on the surface of a Si antenna, resulting in geometrical changes in every direction. This accumulated knowledge of Si oxidation should facilitate the color generation of very small Si nanostructures that have been difficult to fabricate. In addition, the core/shell nanostructure28−31 of Si surrounded by an oxide film will behave differently from simple Mie resonators. Large changes in the resonance properties through Si oxidation will drastically expand color printing design variation. In this paper, we demonstrate all-dielectric pixel color control through chemical reactions of Mie-resonant building blocks. Simple monocrystalline Si nanopatch arrays generate brilliant reflection colors that can be tuned from red to blue by the oxidation of
reation of various artificial structures driven by the development of microfabrication technology has made a great contribution to the control of optical properties.1,2 Emergence of thin, planar artificial materials (i.e., metasurfaces3−7), which are constructed of nanostructures smaller than the operating wavelengths, has led to the innovative expansion of optical applications such as negative refraction,4,5 anomalous angle reflection/refraction,6 and cloaking,7 which are difficult to realize by only using materials existing in nature. In particular, over the past decade, there has been a focus on metasurfaces using Mie resonances excited in high-refractive-index dielectric materials8−10 as potential minimal photonic devices to replace bulk components. The low inherent losses of these materials at all optical frequencies and their responsiveness to both incident electric and magnetic fields have facilitated various applications, such as phase-control elements,11,12 beam manipulators,13,14 flat lenses,15,16 and perfect mirrors.17 Recently, Mie-resonant all-dielectric color printing,18−24 which generates various colors using small pixel arrays composed of Mie resonators, has been demonstrated and various color generation methods have been examined, including Si-based color printing,18−22 lossless TiO2 printing,23 and high-index SiGe color generation.24 Simultaneous excitement of Mie-resonant electric and magnetic fields in high-index dielectric nanostructures can produce various brilliant colors, depending on the structural geometry. Unlike plasmonic color printing,25−27 where the reflection colors are mainly related to the diameter owing to the use of optical surface waves excited on a metallic surface, the optical properties of Mie-resonant color printing are strongly dependent on not only the diameter but also the antenna height22 owing to the presence of resonant rotating fields inside the resonator. However, it is difficult to modify the entire geometry, including the height, by using © 2018 American Chemical Society
Received: December 2, 2017 Published: January 29, 2018 1460
DOI: 10.1021/acsphotonics.7b01467 ACS Photonics 2018, 5, 1460−1466
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SiO2 layer with thickness t, and 0.45t of the Si domain was consumed35 in both the radial and height directions. To simplify the understanding of how the optical properties varied with Si oxidation, we assumed that the nanostructure surface is uniformly corroded and that oxidation does not occur from the substrate. The relative permittivity of Si was determined from ellipsometry measurements of a silicon on quartz (SOQ) substrate (described later), and the refractive index of quartz was fixed at 1.46. Strong backscattering peaks occur at 650 and 570 nm in the calculated spectra of the nonoxidized Si nanopatch structure (Figure 1b, red line). These peaks originate from the magnetic and electric multipoles simultaneously excited in the Si nanostructure.36,37 The green and blue lines show the cross sections when SiO2 layers of t = 25 and 50 nm were formed on the antenna surface. The peak positions blue-shift considerably as the thickness of the oxidized layer increases. To clarify the contribution of each mode, we performed multipole decomposition calculations using a commercially available finite element method (COMSOL Multiphysics). Analysis of the multipole contributions for a nanopatch structure with initial h and d of 150 nm is shown in Supporting Information S1. The magnetic and electric dipoles mainly contribute to scattering at the longer- and shorterwavelength peaks in Figure 1b, respectively. Thus, nanopatch structures can excite distinct resonance modes that depend on the oxide layer thickness, enabling a wide spectral control. Changes in the resonance peak positions are caused by oxidation. We anticipate that two effects cause resonance shifts: (i) blue-shifted resonance wavelengths that originate from changes in Si antenna geometry (Supporting Information S2); reducing domains in which the displacement current flows leads to shifts of the Mie resonance wavelength; (ii) red-shifted resonance wavelengths that originate from increases in the refractive index outside the antenna (Supporting Information S3). The resonance peak of the electric dipole excited in a core/shell nanostructure tends to be red-shifted and broadened more strongly as compared with that of the magnetic dipole with increasing external refractive index and the outer film thickness, leading to broadening of the apparent scattering cross section. 31 In addition, as shown in Supporting Information S4, the resonance fields of the Si nanopatch structure with a SiO2 layer were confined in the Si domain, but slightly leaked into the SiO2 layer. This effective increase in resonance area causes spectral red-shifts. Competition between these two factors is responsible for shifts in peak position. In this case, the observed spectral blue-shifts indicate a greater contribution from changes in antenna geometry. Thus, the optical response is dramatically modified depending on the Si oxidation. The reflectance is associated with backscattering from each Si nanopatch structure. Therefore, it is possible to control the optical properties of periodic arrays by structural oxidation. Figure 2a and b show a schematic and a scanning ion microscopy image, respectively, of the designed Si nanopatch arranged in a square lattice with a periodicity p of 300 nm. To demonstrate color tuning experimentally, cylindrical nanostructure arrays consisting of nondoped monocrystalline Si (crystal orientation: [100]) with a thickness of 150 nm placed on a quartz substrate were fabricated. Details of the nanofabrication process are described in our previous work.22 The fabricated nanopatch structures were thermally oxidized in a furnace at 750 °C while introducing saturated steam and air at atmospheric pressure to form an oxidized layer on the
each nanostructure. The quantitative evaluation suggested that, as the oxide film thickness increases, the coordinates of oxidized Si nanostructures in the color space largely surround the chromatic point, which indicates a wide color phase control. Furthermore, by applying these color change characteristics to image printing, we demonstrate “invisible ink” that conceals color information. These results could be applied to intriguing photonic devices for authentication security, visualizing external environments using passive sensors, overlaid displays, and information cloaking.
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RESULTS AND DISCUSSION The Mie resonance wavelengths of high-index nanostructures depend on antenna size and the external environment.32−34 In particular, Si can react with oxygen to form a SiO2 surface layer, which is a potential candidate for color modification. Here, we first examined the optical properties of a single surface-oxidized Si nanopatch structure on a substrate. Figure 1 shows
Figure 1. Optical properties of an oxidized Si nanopatch structure. (a) Schematic of the calculation model for Si nanostructure oxidation. (b) Backscattering spectra for antennas with an initial diameter d and height h of 150 nm on a quartz substrate. Linear polarized light was incident on the nanostructure normal to the substrate surface. The oxidized layer thickness t was varied (0, 25, and 50 nm, corresponding to red, green, and blue lines, respectively).
backscattering cross sections of the surface-oxidized Si nanopatch structure as a function of oxide film thickness, as calculated using finite-domain time-domain (FDTD) simulations. Incident light was perpendicularly illuminated on a Si nanopatch structure with an initial height h and diameter d of 150 nm on a quartz substrate. Figure 1a shows the schematics during Si oxidation especially on a Si surface in order to clearly indicate the increase in the SiO2 film thickness. Following oxidation of the Si nanostructure, the surface is replaced by a 1461
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Si/SiO2 interface according to the reactions Si + O2 → SiO2 and Si + 2H2O → SiO2 + 2H2, resulting in an approximately 2.2-fold volume expansion of the oxide film.39 Using ellipsometry, the oxide film thicknesses were determined relative to the thicknesses of SiO2 films formed on Si on quartz and Si substrates with the same surface orientation when heated simultaneously with the target sample. Optical measurements were performed using a confocal reflection microscope with the objective lens connected to a charge-coupled device camera and a spectroscope (Supporting Information S5). Figure 2c shows a bright-field optical image of a fabricated nanopatch array as a function of antenna diameter and total oxidation time under linearly polarized white-light illumination through a 20× objective (numerical aperture (NA): 0.45). The nanostructure was oxidized, the oxide film thickness was measured, the reflection image was recorded, and the spectrum was measured sequentially up to an oxidation time of 90 min. Owing to the sharp reflection peaks based on Mie resonances, nonoxidized Si nanopatch structures exhibit distinct vivid colors, including red, green, and blue, from each area, and the colors are strongly dependent on the structural geometry. As the oxidation time increases, the reflection colors tend to blueshift owing to shifts of the Mie resonance wavelengths by Si oxidation, as discussed above. Thus, tuning of all-dielectric pixel colors is achievable by Si oxidation. Figure 3a shows the variation in the reflection spectra of Si nanopatch arrays with an initial diameter of 170 nm with oxidization time. The experimental spectra (solid black lines) can be tuned across all visible wavelengths. Strong reflection peaks are observed in the spectra, and the peak positions blueshift as the oxidation time increases. These features are attributed to shifts in the Mie resonance wavelength owing to the replacement of high-refractive-index Si with low-refractiveindex SiO2, which decreases the effective optical length and changes the optical properties. The reflection spectra of the nanostructure before and after oxidation do not depend on the incident polarization direction (Supporting Information S6), which suggests that the fabricated nanostructure is symmetric and that nanostructure oxidation occurs symmetrically. To further investigate the reflection properties of the oxidized nanopatch arrays, the experimental reflection spectrum at each oxidation time was compared quantitatively with the corresponding spectrum simulated using three-dimensional rigorous coupled-wave analysis (RCWA) calculations. For these calculations, we assumed that the volume of the nanostructure uniformly increases when a SiO2 layer is formed on the surface, except in the substrate direction. Although the experimental spectrum without thermal oxidation was in good agreement with the calculated spectrum, large deviations were observed as the oxide film thickness increased (Supporting Information S7a). Several factors contribute to this deviation. First, the oxidation rates of the top surface and the wall are not same, as the Si oxidation rate depends on the crystal orientation and the surface state.40 The crystal orientation of the SOQ substrate surface is [100], but after vertically etching the Si layer by RIE to fabricate the nanostructure, the sidewalls exhibit polycrystalline-like surfaces, which are oxidized faster than [100] surfaces. Indeed, from the top SIM image of oxidized nanostructures, the thickness of the oxidation film in the radial direction was greater than twice the predicted film thickness. Second, oxidizing species diffusing into the quartz substrate can react with the bottom of the Si nanostructure.41 An oxide film is formed not only on the top surface of the nanostructure but
Figure 2. Reflection colors of oxidized Si nanopatch arrays. (a) Schematic of a Si nanopatch array with oxide layers on a quartz substrate. Linear polarized white light was incident on the Si nanostructure normal to the substrate surface. (b) Oblique SIM image of a typical oxidized Si nanopatch array with an oxide film thickness of 23.5 nm, an initial diameter d of 150 nm and height h of 150 nm, and a periodicity p of 300 nm. (c) Reflection image of oxidized arrays through a 20× objective (NA: 0.45) irradiated with linear polarized white light. The diameter was systematically changed from 90 nm to 250 nm in 20 nm increments. The vertical axes show total oxidation time and SiO2 layer thickness. Each individual color area is 10 μm × 10 μm. The black frame corresponds to unstructured regions. The structures shown in each row are the same, with the extent of oxidation increasing toward the top of the image. Scale bar, 10 μm.
nanostructure surface. At room temperature, the Si interior is protected by the formation of a native thin oxide film (SiO2), even if exposed to air for a long time.38 However, with a sufficient amount of steam and oxygen at high temperatures, molecules diffuse through the oxide film and react with Si at the 1462
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wavelengths as the oxidation time increases. This unexpected deviation can be experimentally observed for the nanopatch arrays with d = 150 nm in Figure 2c. The color tunes from yellow to blue and finally should disappear. However, the building blocks still generate a blue color, which does not originate from the SiO2 nanostructure arrays alone. Deceleration of this spectral shift arises from an oxidation suppression phenomenon known as the “retarded oxidation effect”.43,44 When a certain degree of oxide film is formed on a Si nanostructure, the remaining Si domain experiences compressive stress from the upward and lateral directions because the SiO2 layer undergoes little viscous flow at an oxidation temperature of 750 °C. This internal stress suppresses reactions between oxide species and Si at the interface, leading to oxidation deceleration.45 The red broken lines in Figure 3 show calculated reflection spectra of oxidized nanostructures with the radial oxide film thicknesses set to values as expected from the experimentally obtained spectra by using the nonuniform model. In other words, calculated data in Figure 3 belong to the nonuniform model, but SiO2 film thicknesses in the calculation were chosen in order to fit the experimental spectra. Estimated spectra fit not only the reflection peak position but also the intensity. Supporting Information S7c represents the comparison between experimentally measured top oxide film thicknesses and those estimated from the fitted spectra. The difference in film thickness increases with increasing the oxidation time. This tendency is similar to that from the previous study,46 which supports the assertion that the retarded oxidation effect decelerates the spectral shift. The retarded oxidation effect is strongly influenced by the initial geometry and could be used to fabricate stable blue resonators, which are difficult to produce conventionally, as smaller nanostructures are required. Si nanostructures with SiO2 layers have other promising properties for printing applications. Supporting Information S8 shows the periodicity dependence of the reflection spectra of nanostructures with different oxide film thicknesses. The positions of the reflection peaks are not affected by the periodicity, and smaller reflection intensity changes are observed for the nanostructures with oxide films. This robustness against periodicity would allow flexible color printing to be realized with color phases and saturation maintained, even on flexible substrates. By tailoring the oxidation of the Si nanostructures, wider color tunability can be achieved. The tunable range of the obtained reflection spectra was quantitatively evaluated by converting to the CIE1931 color space, as shown in Figure 4 (the conversion method is described in Supporting Information S9). Wide-range color tuning over RGB can be achieved by changing the thickness of the oxidized layer for nanostructure arrays with d = 170 nm. Color space coordinates for nanostructures with other d values are shown in Supporting Information S10. Notably, nanopatch structure arrays with oxide layers exhibit brighter color than those without an oxide layer, especially in the shorterwavelength region. This phenomenon is attributed to the fact that the height of the Si domain is decreased by oxidation, and thus a larger diameter is required to produce the same color as before oxidation. This expansion of the physical cross section of the Si domain increases the scattering cross section, resulting in an improved reflection intensity (Supporting Information S11). In addition, as shown in Supporting Information S4, the backscattering cross section increases on addition of an oxide
Figure 3. Experimental and calculated reflection spectra (black solid and red broken lines, respectively) of Si nanopatch arrays at various oxidation times. Insets show corresponding optical microscope images with an area of 5 μm × 5 μm (from Figure 2c).
also on the bottom surface, leading to an increase in the apparent oxidation rate. The oxidation rate of the bottom surface is strongly dependent on the structural geometry owing to the diffusion length of oxidizing species.42 We assumed that the oxidation rates of the bottom and top surfaces were the same, and the oxidation rate in the radial direction is 2.6 times faster than that in the axial direction. Supporting Information S7b presents reflection spectra calculated by considering these two influences. The experimental and calculated spectra agree during the initial stages of oxidation up to 30 min, but the experimental spectra still show deviations toward longer 1463
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yellow, whereas after oxidation forming 30 nm thick oxide films on the nanostructure, the green letters “Si” emerge on a yellow background, as shown in Figure 5b. There is no significant color deviation within the letters or the background. This visual homogeneity resulting from the same degree of saturation allows the letters and background to be clearly distinguished. Our designed nanopatch arrays enable encoding of color information in a hidden area and extraction of the information by structural oxidation. In this study, we have focused on the simple reaction of Si with oxygen and a simple structure using only Si and SiO2. If structural oxidation or film formation can be realized by chemical reaction in liquid or gases at mild temperatures, color tuning would be easier and accelerate reactive all-dielectric color printing applications. In addition, because Si can form other compounds, such as SiN and SiC, on the surface depending on conditions and processes, further expansion of color control is possible. Recently, some researchers have discovered that it is possible to control the optical responses of Mie resonators by thermally modifying their optical characteristics.47,48 When the dielectric constant of material changes due to temperature control, this should cause Mie resonant peak shifts, possibly leading to color changes. Changes in these optical properties in the visible wavelengths are not as large as geometrical changes through oxidation, but combining these affects with our method may lead to greater color change.
Figure 4. Color tunability of the oxidized Si nanopatch arrays in the CIE1931 color space. Black solid circles represent the converted xy values for the reflection spectra in Figure 3 of nanostructure arrays with d = 170 nm. The corresponding oxidation times are shown for each data point.
film, while the Q value is maintained. These factors improve the color purity of nanostructures with oxide films. Thus, oxide films could improve the brightness and color purity for alldielectric printing. The color changes via Si oxidation are dependent on the structural geometry. By applying this characteristic, the nanostructure can act as “invisible ink” to hide color information. To demonstrate the properties of the invisible ink, we arranged an optical image in which letters can appear with oxidation. Figure 5a shows an image through a 20×
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CONCLUSIONS We have experimentally demonstrated all-dielectric pixel color control via structural oxidization. Numerical analysis of the properties of individual oxidized Si nanostructures revealed that color changes mainly originate from changes in antenna geometry, leading to shifts in the Mie resonance wavelengths. The fabricated Si nanopatch structure arrays exhibited distinct brilliant, vivid colors, which were strongly dependent on the oxidation time. Quantitative evaluation of the obtained reflection spectra indicated a wide tunable color range, from blue to red. These elements can be used as invisible ink to hide color information in an area using color shift differences based on oxidization. We believe that controlling the vertical axis by our proposed approach would greatly improve color variation and the performance of all-dielectric color printing, allowing the development of applications such as authenticatable security certification, optical data storage, passive sensors, and information cloaking.
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METHODS Calculations. We performed three-dimensional RCWA simulations (Synopsys, Inc., DiffractMod), FDTD simulations (Lumerical, Inc., FDTD Solutions), and FEM simulations (COMSOL, Inc., COMSOL Multiphysics) using commercially available methods. Total reflection including each diffraction of cylinder Si nanopatch structure arrays with a SiO2 layer was placed on a quartz substrate. For the RCWA simulations, the incident plane wave was irradiated from the Si side under periodic boundary conditions with a period of 300 nm, and the total reflection spectra including the diffracted light were measured. For the FDTD simulations, a pulse plane wave was from the substrate side and the propagating light reaching both ends of the z-boundary was attenuated by perfectly matching layers (PMLs). For the backscattering calculations with FDTD simulations, we set total-field scattered-field sources and a PML
Figure 5. Demonstration of invisible ink. Optical microscope images through a 20× objective (NA: 0.45) (a) before and (b) after oxidization.
objective that consists of two different types of pixels: (i) a mixture of one d = 130 nm and three 170 nm nanopatch structures for the background and (ii) a mixture of two d = 150 nm, one d = 170 nm, and one d = 210 nm nanopatch structure for the emerging letter. The unit cell period of one pixel consisting of four elements is 600 nm. By mixing colors generated from each element, it is possible to create specific colors. The colors of these nanopatch structures were specifically chosen to be of approximately the same saturation before oxidization. In the initial state, the area is uniformly 1464
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Young Scientists. Part of this work was supported by the “Nanotechnology Platform Project (Nanotechnology Open Facilities in Osaka University)” of the Ministry of Education, Culture, Sports, Science and Technology, Japan (Nos. F-17OS-0011 and S-17-OS-0011). We would like to thank ShinEtsu Chemical Co., Ltd. for the donation of high-quality siliconon-quartz substrates.
boundary for each boundary. All backscattering light was detected by a monitor above the quartz substrate. For the multipole decomposition calculations with FEM simulations, a Si nanostructure with a SiO2 layer in a spherical domain surrounded by a PML boundary was placed on a quartz substrate (see Supporting Information S1 for details). Fabrication Procedures. An undoped monocrystalline [100] Si layer with a thickness of 150 nm on quartz (Shin-Etsu Chemical Co., Ltd.) was used as the initial substrate. After coating a chemical resist (ZEP520A, Zeon Corp.) with a thickness of 150 nm on the substrate, the metasurface patterns were produced by 125 kV electron beam lithography (ELS7700T Elionix Inc.). A hard mask Cr layer with a thickness of 30 nm was then deposited on the sample using a heat-resistive evaporator and resist lift-off. The top Si layer was selectively etched by plasma gases in an RIE chamber. Finally, the sample was immersed in a Cr etching solution to remove unnecessary metals. The fabricated Si nanopatch structures were then oxidized in a furnace (Isuzu Seisakusho Co., Ltd.) to form an oxidized layer on the nanostructured surface. The sample was placed on a quartz stage, which was inserted into the center of the furnace after reaching the desired temperature. The substrate temperature was measured with a thermocouple attached to the stage. The nanostructures were heated at 750 °C in air while introducing saturated steam at atmospheric pressure.
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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/acsphotonics.7b01467. Resonance mode analysis; influence of the structural geometry and the surroundings on the scattering cross sections; field distributions; optical setup; confirmation of the symmetric oxidation; comparison between the calculated and the measured reflection spectra; influence of the oxidation on the periodic dependence; CIE conversion method; CIE color space values of the other nanostructures; influence of the nanostructure height on the color saturation (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Junichi Takahara: 0000-0002-3683-626X Author Contributions
Y.N. conceived the idea for color control through oxidation, designed the structures, performed the numerical simulations, analyzed the experimental data, and wrote the initial draft of the manuscript. J.T. supervised the project. M.S., I.H., and Y.N. performed the experiments. All the authors discussed the results and contributed to the writing of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Photonics Center at Osaka University. Y.N. is supported by a Research Fellowship from the Japan Society for the Promotion of Science (JSPS) for 1465
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ACS Photonics
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DOI: 10.1021/acsphotonics.7b01467 ACS Photonics 2018, 5, 1460−1466
