Fabrication of Ideally Ordered Anodic Porous Alumina on Glass Substrates by Stamping Process Using Flexible Stamps

Establishing a process to fabricate anodic porous alumina with an ordered array of uniform-sized pores on a substrate is an important challenge of fabricating various functional devices. In this study, ordered anodic porous alumina was fabricated on glass substrates by a stamping process using a flexible stamp. This process enables the formation of a resist mask at a low pressure on the sample surface to pattern the starting point for pore generation during anodization. Therefore, it is possible to pattern samples without breaking the glass substrate, and subsequent anodization can generate ordered anodic porous alumina on the substrate. The stamping process using flexible stamps can be applied regardless of the roughness of the Al film on a substrate because the stamp deforms according to the Al surface shape, even when there is a protruding structure on the Al surface. In addition, it was shown that this process could be used to form ordered anodic porous alumina even on Al surfaces with three-dimensional curvatures, such as convex lenses.

Highly ordered anodic porous alumina, which has a nanohoneycomb structure composed of uniform-sized cylindrical pores, has attracted much attention as a key material for fabricating various functional devices. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] If highly ordered anodic porous alumna can be directly formed on substrates such as Si wafers and indium tin oxide (ITO) -coated glass, it is expected to have various applications in, for example, magnetic recording media, sensors, and templates for preparing one-dimensional nanostructures on the substrates. It has been reported that anodic porous alumina is formed on substrates by forming an Al film by sputtering or thermal evaporation and subsequent anodization in an acidic solution. [15][16][17][18][19][20] However, in most studies, the pore arrangement of the resulting anodic porous alumina was disordered. To fabricate anodic porous alumina with an ordered pore arrangement from the surface to the bottom, it is effective to form an ordered depression pattern on the surface of an Al substrate, which serves as the starting point for pore generation. There are two main methods of preparing ordered depression patterns on the surface of Al substrates. The first method is a self-assembling process. Under optimized anodization conditions, the regularization of pore arrangement in anodic porous alumina proceeds spontaneously. [21][22][23] By removing the resulting oxide film with an etchant, one can form an ordered depression pattern corresponding to the pore arrangement on the porous alumina back surface on the Al surface. 24 The self-assembling process has the advantage of preparing large-area ordered depression patterns in a simple process that does not require expensive equipment. However, since the regularity of pore arrangement at the bottom of anodic porous alumina depends on the film thickness, it is necessary to form a thick anodic oxide layer in order to obtain anodic porous alumina with a long-range ordered arrangement of pores. We showed in our previous reports that the pore size uniformity also depends on the thickness of the anodic porous alumina. This is because the greater the regularity of the pore arrangement, the more uniform the pore diameter. For example, in the case of anodic porous alumina with a 100 nm period, the relative standard deviation, indicating the pore size variation, is less than 5% when the film thickness exceeds 10 μm. 25 Therefore, it is necessary to form an Al thin film with a thickness of 10 μm or more on substrates to prepare ordered depression patterns on the Al film surface by the self-assembling process. The surface roughness of high-purity Al thin films formed on substrates by sputtering or thermal evaporation increases as the film thickness increases because the size of crystal grains increases. 26,27 If the thickness of the Al film formed on substrates exceeds 10 μm, the resulting Al films appear cloudy, and the surface roughness becomes significantly large. Pores in anodic porous alumina grow perpendicular to the surface; thus, a long-range ordered arrangement of pores cannot be obtained on Al surfaces with large surface roughness. For these reasons, it is difficult to form a depression pattern with a long-range ordered arrangement on the surface of an Al thin film formed on substrates by a self-assembling process. The second method of preparing ordered depression patterns on the Al surface is a texturing process using hard molds made of SiC or Ni. 28,29 In this process, a mold with an ordered projection array is pressed directly onto an Al substrate to form an ordered depression pattern on the Al surface. Since depressions can be formed directly on the surface of the Al film, it has the advantage of producing an ordered pattern of depressions on the surface of Al film without forming a thick anodic oxide layer. We previously reported that ordered depression arrays can be fabricated on the surface of an Al thin film formed on a Si substrate by a texturing process using a hard mold, and the subsequent anodization generated ordered anodic porous alumina on the substrate. 27,30 However, to form a depression pattern on an Al thin film using a hard mold, the mold must be pressed against the Al thin film on the substrate at a high pressure of over 2500 kg cm −2 , and the substrate often breaks during the texturing.
In our recent study, we showed a stamping process using flexible stamps made of polydimethylsiloxane (PDMS) as a new method to control the starting point of pore generation in anodic porous alumina. 31 This process is suitable for the formation of large-area ordered anodic porous alumina, because resist masks that control the starting point of pore generation can be formed on Al substrates by pressing the flexible stamp with a finger. In other words, large-area patterning, which conventionally requires high pressure with hard molds, can be achieved at a low pressure using flexible stamps. This method is also promising for the formation of highly ordered anodic porous alumina on substrates without breaking substrates because it does not require high pressure. In this report, we describe the formation of ordered anodic porous alumina on the surface of a glass substrate by a stamping process using a flexible stamp. In addition, the control of the pore period of ordered anodic porous alumina by changing the surface pattern of the flexible stamp was investigated. The obtained ordered anodic porous alumina formed on substrates can be used for various functional devices such as optical devices, electronic devices, and biodevices. z E-mail: yanagish@tmu.ac.jp Experimental Figure 1 shows a schematic of the preparation process for ordered anodic porous alumina on substrates by the stamping process using a flexible stamp. In this study, glass plates (3 × 2 cm 2 ) were used as substrates of an Al thin film formed by a DC sputtering apparatus (SPF-344, SEED Lab., Japan). Al was sputtered at 50 W for 2 h using an Al target (99.99% in purity) to form an Al film with a thickness of 4 μm on a glass substrate. After sputtering, the samples were heat-treated in a vacuum furnace at 150°C-450°C for 7 h. Then, an anodic porous alumina thin film that serves as a mask layer was formed by anodization in 0.3 M oxalic acid at 16°C for 30 min under a constant voltage of 10 V. In our previous report, we showed that forming a resist mask directly on the Al surface results in a pattern with many defects due to polymer reflow during the heat treatment to remove the toluene completely. 31 On the other hand, if a porous alumina layer is formed on the Al surface in advance, pattern deformation during heat treatment can be prevented due to the anchoring effect of the porous surface structure. It has also been found that the smaller the pore size of the pre-formed porous alumina layer, the higher the pattern transfer accuracy. Therefore, even in this study, anodization was performed under a constant voltage of 10 V to produce a porous structure with a small pore size.
To form a resist layer on the sample surface, a PDMS stamp with a hexagonally arranged array of uniform-sized projections, which was fabricated by a molding process using a polymer pattern fabricated by electron beam lithography as a template, was used. The projection period on the PDMS surface was adjusted to 400, 300, and 200 nm by changing the template. The size of the fabricated PDMS stamp and the patterned area were 1 × 1 cm 2 and 5 × 5 mm 2 , respectively. The PDMS stamp was dipped into a toluene solution containing 0.5 wt% polychloroprene and pulled up with a dip coater at a constant speed of 1 mm s −1 to apply a thin chloroprene film, which acts as a resist layer, on the stamp surface. These conditions are the result of optimizing the polychloroprene concentration and pull-up speed so that the polychloroprene film prepared by the evaporation of toluene is uniformly formed only in the recesses of the projection pattern on the PDMS stamp surface. The PDMS stamp with a polychloroprene film on its surface was pressed onto the sample surface with a finger to transfer the resist layer with a hole array pattern. The resist mask can be transferred from the stamp surface to the substrate surface simply by bringing the PDMS stamp into unloaded contact with the substrate surface. In this study, a load of about several g cm −2 was applied using a finger to transfer the pattern. After the formation of the resist mask, the sample was heat treated at 250°C for 3 min to evaporate the toluene completely and adhere the resist to the sample surface. The sample with the resist layer was immersed in a mixture of 1.8 wt% chromic acid and 6 wt% phosphoric acid at 30°C for 13 min to selectively dissolve and remove the porous alumina layer at the resist aperture, forming regular cavity arrays on the sputtered Al surface. After etching, the sample was anodized under conditions matching the cavity array period. The sample with 400 nm period cavity arrays was anodized in a mixture of 0.25 M phosphoric acid and 0.1 M oxalic acid under a constant voltage of 160 V at 0°C for 5 min The sample with 300 nm period cavity arrays was anodized in a mixture of 0.3 M phosphoric acid and 0.25 M oxalic acid under a constant voltage of 120 V at 0°C for 5 min The sample with 200 nm period cavity arrays was anodized in 0.05 M oxalic acid under a constant voltage of 80 V at 16°C for 5 min After the anodization, the cavity layer remaining on the ordered anodic porous alumina surface was removed by etching in 10 wt% phosphoric acid at 30°C for 30 min The obtained samples were observed by scanning electron microscopy (SEM; JSM-7500F, JEOL). Figure 2 shows the effect of heat treatment on the anodization behavior of sputtered Al films. Figure 2a shows the time-voltage curves during anodization for samples before heat treatment and after heat treatment at 150, 250, 350, and 450°C for 7 h. The samples were anodized in 0.1 M phosphoric acid under a constant current density condition of 10 mA cm −2 at 0°C. In this experiment, the phosphoric acid was selected as the electrolyte because it can anodize Al up to high voltages exceeding 200 V. The results of a similar study using an Al plate with a thickness of 0.5 mm are also shown in Fig. 2a for comparison. In all samples, the voltage increased upon the start of anodization and then reached a steady state in which the anodization voltage did not change significantly. The reason for the difference in the rate of voltage increase rate was that masking tape was used to keep the exposed area of the sample, but as anodization progressed, some areas of the tape peeled off, resulting in variations in the area. For the sample without heat treatment, the anodization voltage did not exceed 160 V. However, it was observed that the higher the heat treatment temperature, the higher the anodization voltage was. The time-voltage curve of the sample heat-treated at 450°C showed almost the same behavior as that of the Al plate. From XRD measurements, we confirmed that all supered Al films have nearly the same degree of crystallinity even after heat treatment. However, as shown in Fig. 2a, there are significant differences in anodization behavior at different heat treatment temperatures. Although the details of the cause of this phenomenon have not been clarified, it is assumed that there are crystalline distortions in the sputtered Al film, which adversely affect anodization. Heat treatment is expected to relax the crystalline strain and defect structure, resulting in anodization behavior similar to that of bulk Al. These results indicate that heat treatment is effective for the anodization of Al thin films formed by sputtering on substrates under high voltage conditions. Figure 2b shows surface SEM images of sputtered Al films before and after heat treatment. Although some grain growth was observed as the heat temperature increased, no significant change in surface roughness was observed. In the subsequent experiments, we used samples heat-treated at 450°C

Results and Discussion
, which exhibit almost the same anodization behavior as Al plates with maintained surface smoothness. Figure 3a shows an SEM image of the sample after anodization at 10 V in 0.3 M oxalic acid. It can be seen that fine pores are formed on the surface of the Al thin film on the glass substrate by anodization. Although no gaps were observed at the grain boundary on the sample surface before anodization, narrow gaps were observed at the grain boundaries after anodization. This is considered to be due to the dissolution of grain boundaries containing defects during anodization. However, no significant difference in  surface roughness was observed before and after anodization. Figure 3b shows the surface SEM image of the resist mask formed by the stamping process using a PDMS stamp. In this SEM image, the dark gray area corresponds to the area where the resist film is formed. On the other hand, the areas observed as bright spots correspond to resist openings, where no resist is formed, and the anodic porous alumina is exposed. The SEM image in Fig. 3b confirmed that the bright spots were ideally aligned with a period of 400 nm, and the resist mask corresponding to the surface pattern of the PDMS stamp was transferred to the sample surface. Figure 3c shows the result of etching the resist-formed sample in a mixture of chromic acid and phosphoric acid. From this image, it can be seen that the alumina layer at the resist opening has been dissolved selectively, forming an alumina cavity array with a period of 400 nm. The PDMS stamp can be used repeatedly. In this study, formation of resist film on the surface of PDMS stamp, the PDMS stamp was dipped into a toluene solution containing 0.5 wt% polychloroprene and pulled up with a dip coater at a constant speed of 1 mm s −1 to apply a thin chloroprene film. If the pull-up speed of the dip coating is too fast or the concentration of the polychloroprene is too concentrated, the entire surface of the PDMS stamp will be coated with a polychloroprene film. On the other hand, if the pull-up speed is too slow or the concentration of the polychloroprene is too dilute, a coating film is formed only on the part of the PDMS stamp surface. By optimizing these conditions, including the pull-up speed and the polychloroprene concentration, the polychloroprene film is uniformly formed on the recessed areas of the PDMS stamp surface. This is because the thin liquid film applied to the PDMS stamp surface by dip coating repelled by the convexity and retained only in the concave area around the convexity. Figure 4a shows surface and cross-sectional SEM images of the sample obtained by anodization of the Al film with an alumina cavity array. The images show that pores are generated from the bottom of the cavity during anodization. The pores generated at the surface grew perpendicular to the depth direction, and the ordered   arrangement of pores was maintained in the depth direction. Figure 4b shows SEM images of the anodized sample after the selective removal of the cavity layer by etching. The pore period of the anodic porous alumina in the cavity layer is fine, and the pore wall thickness is thin. Therefore, the cavity layer can be easily removed selectively by etching. The images confirm the ideal arrangement of uniform-sized pores with a period of 400 nm over the sample. These results indicate that anodic porous alumina with regularly arranged pores can be fabricated on glass substrates by this process. Etching to remove the cavity layer increases the pore size of the underlying ordered anodic porous alumina. Therefore, when using ordered anodic porous alumina without increasing the pore size, the cavity layer may be retained. Figure 5 shows the results of the formation of ordered anodic porous alumina with a controlled pore period by the anodization of Al thin films formed on glass substrates. For this experiment, PDMS stamps with ordered projection patterns with periods of 200 and 300 nm were used for the formation of resist masks on the sample surfaces. Figures 5a and 5b show surface and cross-sectional SEM images of ordered anodic porous alumina with periods of 200 and 300 nm, respectively. For both samples, the cavity layer was selectively dissolved by etching before SEM observation. SEM images in Fig. 5 show that anodic porous alumina with an ideal array of uniform-sized holes is obtained in both samples. The results show that ordered anodic porous alumina with a controlled pore period can be formed on a glass substrate by adjusting the pattern of PDMS stamps and the anodization conditions. In this study, PDMS stamps were fabricated from ordered fine patterns obtained by electron beam lithography, which limited the size of the resulting ordered anodic porous alumina. However, if PDMS stamps are fabricated using ordered structures obtained by a self-assembling process, it is expected that large-area samples can also be fabricated.
In the sputtering process, hillocks, which are small mound projections, may be formed on the surface of the deposited Al film owing to the stress gradient caused by heat during sputtering. 32 When forming depressions on a surface of sputtered Al films by a texturing process using a hard mold, if there are hillocks on the Al substrate surface, the mold cannot come in contact with the Al surface to form depressions, as shown in Fig. 6a. On the other hand, when a PDMS stamp is used for patterning, the stamp itself is flexible and can deform according to the surface roughness to form a resist mask around the convexity, as shown in Fig. 6b. The surface pattern period is expected to change when the PDMS stamp is deformed strongly. Therefore, if the surface roughness is too large, the PDMS stamp will expand and contract, resulting in a partial change in the pattern period. However, even if there are convex areas on the Al surface, resist patterns can be formed on areas other than the convex areas without changing the pattern period. On the other hand, when a hard mod is used, if the Al surface has convex, the entire mold surface cannot contact the Al substrate, so pattern transfer is not possible at all. Figure 6c shows the results of resist pattern formation using a PDMS stamp on sputtered Al film with hillocks. It can be observed that the resist pattern is transferred even around a small mound projection. The patterning on uneven surfaces is an advantageous point of flexible stamps only, which cannot be achieved with hard molds. If there is a large projection on the Al surface, it is difficult for the PDMS stamp to come into contact completely with the sample surface, so resist formation around the projection is difficult. However, we believe that the PDMS stamp can contact most parts of the Al surface, allowing resist formation on a large sample area.
Formation of resist masks by the stamping process using the PDMS stamp can be applied to not only flat Al surfaces but also curved Al surfaces. In our previous report, we showed that the stamping process using PDMS stamps could be used to form ordered anodic porous alumina on the curved surfaces of cylindrical Al substrates. 31 Here, we show the results of applying this process to an Al substrate with a convex lens shape. For cylindrical Al substrates, wrapping the PDMS stamp around a curved surface allows the stamp surface to come in contact with the curved sample surface. On the other hand, on a surface with a lens shape, the stamp surface cannot come in contact with the substrate surface without the threedimensional deformation of the PDMS stamp. Figure 7 shows the results of the formation of ordered anodic porous alumina on the surface of a sputtered Al film formed on a glass substrate with a convex lens shape. Figure 7a shows a photograph of the glass substrate before the sputtering of Al. In this experiment, a convexlens-shaped glass substrate with a radius of curvature of 6 cm was used. From the photograph of the sample after anodization at 160 V shown in Fig. 7b, the structural colors associated with the formation of ideally ordered anodic porous alumina with a 400 nm period can be observed at the center of the sample surface with curvature. Ordered anodic porous alumina exhibits structural color from its ordered structure. The sample shown in Fig. 7 does not show a uniform color because the ordered anodic porous alumina is formed on a curved surface, and the angle of incident light varies from place to place. The PDMS stamp was pressed onto the convex-lens-shaped Al substrate with a finger as well as a flat Al substrate. Surface and cross-sectional SEM images in Figs. 7c and 7d also show the formation of anodic porous alumina with an ideally arranged array of uniform-sized holes with a 400 nm period. These results show that this process can be applied not only to simple curved surfaces, such as cylindrical substrates, but also to surfaces with three-dimensional curvatures, such as lens-shaped substrates. In this study, the application of this method to surfaces with curvature other than the sample shown in Fig. 7 has not been investigated. However, in our previous study using PDMS stamps with nano-micro hierarchical patterns as molds for nanoimprinting, we have shown that the PDMS stamp can be applied to convex lens surfaces with curvature radii of 10-52 mm. Therefore, we believe that the PDMS stamps fabricated in this study are also applicable to surfaces with various curvatures. 33

Conclusions
Ordered anodic porous alumina was fabricated on the sputtered Al thin film formed by sputtering on glass substrates by the stamping process using PDMS stamps. Although the as-sputtered Al film on a glass plate could not be anodized at a voltage above 160 V, it was found that heat treatment of the samples improved the quality of the Al film and enabled stable anodization even under high voltage conditions. It was also found that the pore period of ordered anodic porous alumina could be reduced to 200 nm by changing the pattern of the PDMS stamp used for resist formation. The PDMS stamps used in this study are flexible, so they can come in contact with the Al surface even if there are convexities and can form resist patterns successively. It was also shown for the first time that this process could be applied to Al surfaces with three-dimensional curvatures, such as lenses. This process can be used to form ordered anodic porous alumina on Al films formed on substrates of various shapes, and the resulting samples are expected to be used as key materials for fabricating various functional devices.