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Abstract
An efficient preparation process for Al hole array structures emitting wavelength-selective thermal radiation that is based on the anisotropic anodic etching of Al was demonstrated. The formation of an ordered hole array was achieved by a masking process prior to the anodic etching. The present process allows the preparation of large samples because the masking of the Al foil has a high throughput owing to the simple printing process using a flexible stamp. The thermal radiation properties of the Al hole array could be controlled by adjusting the depth and aperture size of the holes.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
A surface at a certain temperature emits thermal radiation in accordance with Planck’s law [1,2]. In contrast to smooth surfaces, it is well known that thermal radiation from a surface with specific fine structures can be controlled via the geometrical structure [3–19]. For example, a surface with an ordered array of square holes emits radiation whose wavelength can be selected by adjusting the side length of the squares. Various types of functional optical devices, such as light-emitting devices, thermophotovoltaic cells and cooling elements, have been proposed on the basis of wavelength-selective radiation properties [11–16]. To realize such surfaces with wavelength-selective radiation, precise control of the hole shapes, sizes and positions is essential during their preparation [15,16]. In addition, the formation of holes with sufficient depth is also required for efficient wavelength-selective radiation because of the occurrence of resonance in the cavities of holes [17]. There have been many reports on the preparation of surfaces of metals and semiconductors emitting wavelength-selective radiation. Typically, combined processes involving reactive ion etching (RIE) and photolithography have been employed for the preparation of surfaces emitting wavelength-selective radiation [5,15,18]. In addition, a focused ion beam (FIB) process has also been adopted for the preparation of surfaces emitting wavelength-selective radiation [19]. Although the RIE and FIB processes allow the fabrication of deep holes with a depth of more than 200μm [20], they are unsuitable for the fabrication of fine structures with high-aspect-ratio features due to their low processing speed. Thus, the samples are usually limited to small sizes owing to the low throughput in the fabrication. As an alternative process, a nanoimprinting process using molds has been proposed [18]. In this process, the mechanical imprinting of metals using a mold generates an ordered array of holes. The problem with this process is the difficulty of forming holes with high-aspect-ratio features. In addition, the exhaustion of the mold is a serious problem preventing the efficient production of wavelength-selective surfaces. In the present report, we describe an effective preparation process for surfaces emitting wavelength-selective radiation that is based on the anisotropic anodic etching of Al. In contrast to the previous works, the most important and characteristic point of the present work is that the anisotropic anodic etching of crystal-oriented Al allows the low-cost and high-throughput preparation of wavelength-selective thermal radiation devices with a large sample area. The anisotropic anodic etching of a (100)-oriented Al foil in a Cl--containing electrolyte generates tunnel pits with a square cross section owing to the anisotropic etching in the (100) direction of Al [21,22]. This process is widely used to increase the surface area of Al electrolytic capacitors to optimize their capacitance. In our previous reports, it was shown that the site-controlled formation of tunnel pits can be achieved by a masking process prior to the anodic etching of Al [23–25]. One of the advantages of the present process for the preparation of surfaces emitting wavelength-selective radiation is the capability of preparing holes with high-aspect-ratio features owing to the anisotropic anodic etching. In addition, the square shape of the holes obtained by the anisotropic etching of Al enables the emission of radiation with high wavelength selectivity. The process also allows the effective preparation of large samples because the masking of the Al foil has high throughput owing to the simple printing process using a flexible and reusable stamp. The surfaces prepared by the present process can be applied to various types of functional devices requiring wavelength-selective radiation properties. And, as one of types of end products of the device, flexible and rollable wavelength-selective thermal radiation devices are assumed because a flexible Al foil is used as a starting material.
2. Experimental
Figure 1 shows the fabrication scheme for the preparation of an ideally ordered array of straight holes by anisotropic anodic etching of Al by a masking process [23–25]. A silicone stamp having an ordered array of submicron-scale convexes was prepared [23]. A thin layer of polychloroprene was coated on the stamp by dip-coating. Then, the polychloroprene layer was transferred to the surface of an Al foil by pressing the stamp onto the foil. This stamp could be reused many times. The thicknesses of the Al foil and the mask were 140μm and 150nm, respectively. The Al foil with the polychloroprene mask was heated at 300°C for 3min on a hot plate. Cu was then sputtered on the sample. The thickness of the Cu layer was ca. 20nm. The sample was anodically etched in 7M HCl solution at 40°C for 1-10s. The current density was 1.5A/cm2 and a carbon plate was used for a counter electrode. After the anodic etching, the polychloroprene mask was removed in 1wt% NaOH solution. The mask was disposable. The geometrical structures of the samples were observed using a scanning electron microscope (SEM, JSM-6700F; JEOL). Prior to cross-sectional SEM observations, a cross section of the samples was prepared using ion-milling apparatus (SM09010; JEOL). Thermal radiation properties were evaluated by calculating the emissivity from the regular reflectance measured using a Fourier transform infrared (FTIR) spectrometer (FT/IR-4100; JASCO) equipped with a regular reflection measurement unit (RF-81S; JASCO). The incident angle of the light was 12°. Emissivity was calculated by the relation E = 1-R derived from Kirchhoff’s law, where E and R are the emissivity and reflectance, respectively. The emissivity was analyzed by numerical simulation based on the rigorous coupled-wave analysis (RCWA) method. The simulations were implemented using commercial software (DiffractMOD; RSoft).
Fig. 1 Fabrication process of a straight hole array by anisotropic anodic etching of Al by a masking process. Lx and Ly are the aperture sizes of the hole and Lz is the depth of the hole.
3. Thermal radiation properties of etched Al foil
Figure 2(a) shows a SEM image of a polychloroprene mask prepared on an Al foil. The light gray parts represent the apertures of the mask. The formation of an ordered array of micrometer-size apertures was confirmed. The interval between the apertures and the diameter of the apertures were 5μm and 3μm, respectively. The relative standard deviation (RSD) of the aperture size was 9%. Figure 2(b) shows a SEM image of an anodically etched Al foil. The duration of the anodic etching was 3s. An ordered array of square holes was observed over a 7cm × 7cm area, which was the area of the polychloroprene mask. The square shape of the holes originates from the anisotropic crystallinity of Al. The interval between the square holes and the side length of the holes were 5μm and 3.2μm, respectively. Figure 2(c) shows a cross-sectional SEM image of the etched Al foil. The growth of straight holes perpendicular to the surface of the Al foil was observed. The holes had uniform depths.
Fig. 2 (a) Polychloroprene mask formed on Al surface. The diameter of the apertures and the interval between the apertures in the polychloroprene mask were 3μm and 5μm, respectively. (b) Frontal and (c) cross-sectional views of an etched Al foil. The side length of the square holes and the interval between the holes were 3.2μm and 5μm, respectively. The depth of the holes was 2.5μm.
The emissivity of the straight-hole arrays was evaluated. Figure 3(a) shows the measured emissivity spectrum of the etched Al foil in Figs. 2(b) and 2(c). For comparison, the emissivity spectrum of a smooth Al surface is also shown. In the case of the smooth surface, the emissivity was constant in the measured wavelength range. In contrast, for the etched Al foil, an increase in emissivity was observed at wavelengths of less than 8μm. At wavelengths of longer than 10μm, the emissivity was suppressed to below 0.05. This wavelength selectivity originates from the existence of a cutoff wavelength that depends on the aperture size of the holes (), where x and y are the spatial axes. The oscillation in the emissivity spectra is considered to originate from interference between the incident light and the light reflected from the bottom of the holes. Figure 3(b) shows simulated emissivity spectra. A wavelength-selective increase in emissivity and oscillation of the emissivity were observed, in qualitative agreement with the experimental results. At the wavelength of the peaks of the oscillation in Fig. 3(b), the light resonates in the holes. The resonant wavelengths were calculated using Eq. (1) derived from the theory of cavity resonators [7,17]. The resonant wavelengths are expressed as
with l, m = 0,1,2,3…, n = 0,1,3,5…, Lx and Ly are the aperture sizes in the x- and y-directions, respectively, and Lz is the depth of the hole. The resonant wavelengths calculated using Eq. (1) were marked by gray lines in Fig. 3(b). The gray lines appear to be located at similar wavelengths to the peaks. Figure 4 shows cross-sectional images of the distribution of the time-averaged Ex-field intensity ratio relative to the incident light when the light was irradiated normal to the surface of the sample in Figs. 2(b) and 2(c). The color bar shows the intensity ratio of the Ex-field. The wavelengths of the incident light were (a) 5.39μm and (b) 2.96μm, which were predicted to be resonant wavelengths using Eq. (1). In both Figs. 4(a) and 4(b), the formation of a cavity mode was observed.Fig. 3 (a) Measured and (b) simulated emissivity spectra of the etched Al foil and smooth Al surface. Gray lines show the resonant wavelengths calculated using Eq. (1).
Fig. 4 Cross-sectional images of distribution of time-averaged Ex-field intensity ratio relative to the incident light simulated by the RCWA method at resonant wavelengths of (a) 5.39μm, and (b) 2.96μm.
The aspect ratio (Lz/Lx,y) of the straight holes must be more than 1.5–2.0 for the straight holes to act as a resonator [17]. We controlled the depth of the holes by changing the duration of the anodic etching. Figures 5(a)-5(d) show cross-sectional SEM images of Al foils subjected to different durations of anodic etching. The hole depths were (a) 1.0μm, (b) 2.5μm, (c) 5.4μm and (d) 9.0μm. The depth linearly increased with increasing duration of the anodic etching. The growth rate of the holes was 0.95μm/s, which is over ten times higher than the hole formation speed of a dry-etching process. Figures 5(e)-5(h) show the emissivity spectra of the structures in Figs. 5(a)-5(d), respectively. In each spectrum, the emissivity was increased at wavelengths shorter than 8μm.
Fig. 5 (a)-(d) Cross-sectional SEM images and (e)-(h) emissivity spectra of hole arrays having different hole depths. The hole depths were (a),(e) 1μm, (b),(f) 2.5μm, (c),(g) 5.4μm, (d),(f) 9.0μm. The side length of the square holes and the interval between the holes were 3.2μm and 5μm, respectively. The durations of anodic etching were (a),(e) 1s, (b),(f) 3s, (c),(g) 5s, (d),(h) 10s.
4. Control of thermal radiation wavelength
We controlled the wavelength of thermal radiation by adjusting the side length of the holes and the interval between the holes. The size and the interval of the holes were adjusted by changing the geometrical structures of the etching mask and optimizing the conditions of the anodic etching (Table 1). Figure 6 shows SEM images of etched Al foils having different hole sizes. In all cases, the formation of square holes was confirmed. The side lengths of the square holes were (a) 5.0μm, (b) 2.0μm and (c) 0.7μm. The RSDs of the hole sizes were (a) 8%, (b) 11% and (c) 10%. Hole formation progresses through a combination of the formation of a passivation layer on the hole wall and autocatalytic dissolution at the bottom of the hole. The temperature of the electrolyte controls the balance between the passivation and dissolution phenomena and determines the hole diameter. The temperatures of 40°C and 55°C shown in Table 1 were suitable for fabricating holes having diameters of 2–5μm and less than 1μm, respectively. The size of the holes in the present work is micrometer order because the aim of this work was to control the thermal radiation property in the IR region. This process can be applied to the fabrication of structures with smaller dimensions by employing appropriate anodic etching conditions.
Table 1. Conditions of anodic etching process
Fig. 6 SEM images of ordered hole arrays having different aperture sizes. The side lengths of the holes were (a) 5.0μm, (b) 2.0μm, (c) 0.7μm.
Figure 7(a) shows the emissivity spectra of the structures in Figs. 6(a)-6(c). For comparison, the spectrum of an etched Al foil having an aperture size of 3.2μm is also shown. From Fig. 7(a), wavelength-selective increases in emissivity were confirmed. Figure 7(b) shows the dependence of the wavelength at which the emissivity starts to increase on the hole size. Circles and squares indicate measurement and simulation results, respectively. According to both figures, the wavelength at which the emissivity starts to increase was greater for a larger hole size. It was observed that the measured values were larger than the calculated values for each aperture size. This is considered to be due to the variation in the hole size in the fabricated structures.
Fig. 7 (a) Emissivity spectra of the etched Al foils having different aperture sizes. The aperture size was controlled from 0.7μm to 5.0μm. (b) Dependence of the wavelength at which the emissivity starts to increase on the aperture size of the holes.
An ordered array of holes having a diameter of less than 1μm is expected to be applied to high-temperature uses, such as thermophotovoltaic generation and light bulbs [7,11,16]. Al is unsuitable for high-temperature uses due to its low melting point of 933K. However, for use at high temperatures, a replication technique using the Al hole array as a starting template can be adopted.
5. Summary
Ordered arrays of straight holes with a high aspect ratio were obtained by the anisotropic anodic etching of Al using a masking process, and the obtained structures showed wavelength selectivity of their emissivity spectra. The radiation properties could be controlled by changing the depth and aperture size of the holes. It is thought that the obtained structures can be used for controlling thermal radiation. In addition, the present fabrication process can be used to fabricate various functional optical devices requiring an ordered array of straight holes with a high aspect ratio.
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