Color-tuning method by filling porous alumina membrane using atomic layer deposition based on metal–dielectric–metal structure Chenying Yang, Weidong Shen,* Yueguang Zhang, Zhijie Ye, Xing Zhang, Kan Li, Xu Fang, and Xu Liu State Key Laboratory of Modern Optical Instrumentation, Zhejiang University, Hangzhou 310027, China *Corresponding author: [email protected] Received 4 September 2013; revised 4 November 2013; accepted 4 November 2013; posted 5 November 2013 (Doc. ID 196831); published 9 December 2013
A novel and convenient color-tuning method by filling the porous alumina membrane (PAM) based on metal–dielectric–metal structure is proposed. The pore sizes of PAM as well as the thickness of TiO2 films, deposited by atomic layer deposition, causes various reflection colors due to multilayer interference effects. Overlapping films on top and within the PAM structure can be observed with a scanning electron microscope and will be used to show the change of the effective refractive index of the PAM composite layer. The color-tuning method resulting in changing the dynamics of the PAM layer can be widely applied in fields such as micro-optics, microstructures, nanomaterials, and micro/nanotechnology. © 2013 Optical Society of America OCIS codes: (310.1860) Deposition and fabrication; (310.6628) Subwavelength structures, nanostructures; (230.4170) Multilayers. http://dx.doi.org/10.1364/AO.53.00A142
1. Introduction
Nanotechnology has developed so rapidly and universally that structures with the size of several nanometers are successfully fabricated, and more and more applications are becoming involved. In recent years, porous alumina, with all pores regularly hexagonally latticed, has aroused great interest for its simple and efficient preparation process [1]. Due to its unique features, porous alumina has been studied for producing nanotubes, transferring pattern, observing quantum confinement effect, and so on [2–5]. Atomic layer deposition (ALD), as a novel deposition method, has drawn many researches’ attention for its potential applications on optical coatings and microelectronics. The characteristic of completely overlapping the original micro/nanostructures, as the most remarkable advantage, has brought out lots of 1559-128X/14/04A142-06$15.00/0 © 2014 Optical Society of America A142
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attempts with a variety of materials for different structures [6–8]. Since color-shifting metal–dielectric and all dielectric thin-film designs were proposed decades ago [9,10], the study on color tuning has never stopped. For the most popular application of anticounterfeiting, the significant developments on the colortuning method and the visual effect have been achieved [11–14]. In recent years, color tuning based on subwavelength structures has been widely studied owing to the development of numerical simulation algorithms and nano/microfabrication techniques [15–18] for the applications in color displays and imaging devices. However, the preparation of these subwavelength structures is difficult and timeconsuming. Though research in the combination of PAM and ALD has been carried out already [19–21], the color effect resulted from the feature of completely overlapping PAM based on the MDM structure has not been reported yet, as far as we know. In this paper, we introduce a novel and convenient color-tuning method upon the metal–dielectric–metal
structure by filling the porous alumina membrane (PAM), in which various reflection colors are obtained by changing the effective optical thickness of the composite PAM layer. Five PAMs with different pore sizes are applied, and various thickness of TiO2 are deposited by ALD, so as to adjust the effective refractive index. Based on this work, a colorful pattern is fabricated with a lift-off process. 2. Theory and Method
Referring to the widely used reflective interference filter in anticounterfeiting [13], a metal–dielectric– metal structure is proposed to realize color tuning through a simple process with PAM templates. The film stack proposed in our research is Air/Cr/PAM/Al, as shown in Fig. 1(a). Natural light is incident from the air side through the ultrathin metal layer Cr, the modulating layer PAM, and finally reflected on the interface between PAM and Al and presenting a specific color. The ultrathin metal layer, as an absorbing layer, is required to possess the property of n∕k ≈ 1 to achieve the best color saturation [13]. In our work, Cr is adopted as that metal layer. The aluminum layer, on the bottom of the stack, is served to provide high reflectance with a sufficient thickness of more than 100 nm. The optical thickness of dielectric film PAM as a spacer layer determines the central wavelength at peak reflectance. Figure 1(b) presents the calculated spectral reflectance of the structure as Fig. 1(a) without TiO2 [22]. The pore radius ranging from 20 to 50 nm contributes to the changing optical thickness of the PAM layer. Accordingly, the effective optical thickness of the PAM layer changes from 2.7995 to 2.1795 of quarter-wave thickness at λ0 550 nm. As shown in Fig. 1(b), the optical property of the structure is sensitive to the optical thickness of the dielectric layer, so various colors in reflection could be obtained by varying the thickness or the index of the dielectric layer. In our work, the refractive index contributed to the change of effective optical thickness, after filling the PAM layer with different thicknesses of TiO2.
Moreover, the dimension of the pores and periods in the PAM template of about 100 nm is small enough with respect to the wavelength. So, in optics, it can be treated as several uniform layers. Therefore, the PAM could be replaced by three equivalent layers whose refractive indexes depend on the refractive index of the filling material and the volume fraction (shown in Fig. 2). According to the Maxwell–Garnett theory [23], the effective dielectric constant and the effective refractive indexes are defined as below: εeff ;A − εTiO2 εAir − εTiO2 δ1 ; εeff ;A 2εTiO2 εAir 2εTiO2 εeff ;B − εAl2 O3 εAir − εAl2 O3 δ1 Region B εeff ;B 2εAl2 O3 εAir 2εAl2 O3 εTiO2 − εAl2 O3 δ2 ; εTiO2 2εAl2 O3 εeff ;C − εAl2 O3 εTiO2 − εAl2 O3 δ1 δ2 ; Region C εeff ;C 2εAl2 O3 εTiO2 2εAl2 O3 Region A
neff ;A ε2eff ;A ;
neff ;B ε2eff ;B ;
neff ;C ε2eff ;C
t h−t t neff ;A neff ;B n ; ht ht h t eff ;C p where δp 2πr − t2 ∕ 3d2 and δ2 2πr2 − 1 r − t2 ∕ 3d2 are the volume fraction of air and TiO2 in Region B, respectively; t is the thickness of deposited TiO2 , r is the pore radius, d is the length of the adjacent pores, and h is the pore depth. It is not difficult to find that the equivalent refractive index is closely related to the pore radius, the thickness of TiO2 , and the pore depth. As the pore radius decreases or the thickness of TiO2 increases or the pore depth decreases, the equivalent refractive index increases. With the model constructed, the PAM template deposited with TiO2 and Cr could be treated as multilayer film, due to its simple metal–dielectric–metal structure. So it is easy to analyze the reflectance, transmittance, and electrical field of this structure by a transfer matrix method. Total neff
Fig. 1. (a) Schematic diagram of the metal–dielectric–metal reflective color filter. (b) Reflectance of the structure shown in (a) with increasing optical thickness of PAM. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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Fig. 2. Structure of the PAM template after ALD TiO2 process.
3. Experiments
The PAM templates were prepared by two anodization processes originating from the high purity Al foils. By controlling the anodization voltage and the anodization time, the pore size and pore depth could be controlled, respectively. In our experiment, we fabricated five PAM templates with pore radius of 20, 25, 30, 35, and 40 nm. The length of the adjacent pores is kept at about 100 nm. In order to present more colors with the central wavelength covering the entire visible spectrum, considering the limits of the preparation process, we chose 135 nm as the pore depth. Figure 3 shows the top view and crosssectional view of scanning electron microscope (SEM) images of the fabricated PAM template, where the ordered pore arrays can be observed clearly. With the PAM obtained, a high refractive index oxide layer of TiO2 (n 2.35 at 550 nm) was deposited to fill the PAM by ALD. To adjust the effective refractive index, five thicknesses of 20, 25, 30, 35, and 40 nm were respectively deposited on each PAM template with different pore sizes. The ALD experiment of TiO2 was carried out in a TFS 200 reactor made by beneq. The reaction chamber pressure was about 3 mbar, and 120°C was chosen as the deposition temperature. Titanium tetrachloride (TiCl4 ) and deionized water were selected as reactant precursors. The pulsing time was 400 ms for TiCl4 and H2 O, and the purging times were 5 s for both to blow the reaction chamber clean. The uniform and amorphous TiO2 film can be deposited with a growth rate of 0.0596 nm per cycle [24]. Finally, a thinner Cr layer with a thickness of 14 nm was deposited on the TiO2 -filled PAM by e-beam evaporation. During the evaporation, the substrate is kept at room temperature. As for the
Fig. 3. SEM images of a PAM template. (a) Top view. (b) Crosssectional view. A144
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thin layer deposited by e-beam evaporation, we have experiences in the ultrathin Cr deposition in our plant [25]. To produce a complex colorful pattern using this method, filled areas and unfilled areas should be distinguished sharply, so a lift-off technique is employed. First, we used the same steps as above to obtain a piece of PAM with a certain pore size on a 6 mm × 6 mm Al foil. Second, the photoresist pattern coated by spin coating was created with a standard lithography process. Third, TiO2 film with a thickness of 25 nm was deposited by ALD. After removing the photoresist and the overhead TiO2 film by ultrasonic cleaning in acetone, the pattern of the filled and nonfilled areas appeared. Finally, Cr was deposited on the sample by e-beam evaporation to make the gray pattern colorful. 4. Results
Different from other deposition techniques, ALD has the unique advantage of completely overlapping onto/into the nanostructure. Hence, ALD TiO2 on the substrate of PAM result in completely filling the pores, which can be seen in Fig. 4. The ultrathin layer of Cr makes little change on the surface topography. Apparently, the filling shape of the deposition materials is inverted with respect to the pores of the original PAMs, which further confirms the overlapping feature of the ALD technique. Besides, as the TiO2 deposited thickness is just equal to the pore radius, the small dips appear apparently, which could be eliminated gradually by continuous deposition. On the basis of the completely overlapping feature of the ALD technique, the changing refractive index could be received by filling the pores differently. Figures 5(a)–5(e), respectively, show the measured reflectance of the complete MDM structures of five PAM templates for different pore sizes and various TiO2 thicknesses. The equivalent refractive indexes are listed in Table 1. With the pore size fixed, tuning color could be achieved by varying the deposition thickness. As the thickness increases, the reflectance curve moves toward a long wavelength with the displaying color accordingly turned, which could be obviously observed from each figure in Fig. 5. It could be verified as well in Fig. 6 via the variation of the peak wavelength. Generally, the PAM with the larger pore size performs better than that with the smaller
Fig. 4. PAM sample after the processes of ALD TiO2 and deposition Cr. Pore radius and thickness of TiO2 were both 20 nm.
Fig. 5. Reflectance of the five PAM templates of different pore sizes with different thicknesses of TiO2 and the insert displayed the perceived color of these samples with increasing thickness.
one. Because when the pore is larger, the equivalent refractive index changes more apparently so that the reflectance curve moves more. Similarly, with the deposition thickness fixed, tuning color could be achieved by varying the pore size of the PAM, which is verified in Fig. 6. Likewise, if the TiO2 deposited is thicker, the adjustment range of the effective index is wider so that the reflectance curve moves more. If the pore size or the deposition thickness varies gradually, the continuous refractive index could be achieved, which could lead to the color tuning more finely. According to the measured reflectance curve, the chromaticity coordinates are calculated and marked in the CIE 1931 chromaticity diagram (shown in Fig. 7). A large color gamut is presented for these PAMs with a different TiO2 thickness. A particular color in reflection can be realized to select the appropriate pore size and filling factor. Note that the PAM with no TiO2 is distinct from those TiO2 filled ones, producing a large color difference, which is apparent in Fig. 5 as well.
Table 1.
Furthermore, a complex colorful pattern is fabricated using this color-tuning method, as shown in Fig. 8(a). It is a yellow eagle surrounded by blue blank areas. Through SEM, the structure is clear so as to exploit the coloration principle. Figures 8(b), 8(c) and Figs. 8(d), 8(e) are the top view and crosssectional view of the yellow eagle and blue surroundings, respectively. From SEM images, the observed microstructures in both areas can be used to explain the coloration principle. Obviously, TiO2 completely fills the pores of the yellow area, while no TiO2 fills
Effective Refractive Index of All Pam Templates with Different Pore Sizes
Pore Radius/nm Thickness of TiO2 ∕nm 20 25 30 35 40
20
25
30
35
40
1.805 1.825 1.844 1.862 1.879
1.846 1.877 1.894 1.911 1.926
1.874 1.929 1.957 1.972 1.986
1.889 1.970 2.022 2.047 2.059
1.893 1.998 2.076 2.126 2.148
Fig. 6. Peak wavelength of the PAM of different pore sizes with TiO2 deposited thickness t 35 nm and the peak wavelength of the PAM with pore radius r 20 nm for different TiO2 thickness. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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the completely overlapping into/onto the PAM could be observed, which could tailor the effective refractive index precisely. Based on this method, a colorful pattern was fabricated with a lift-off process. The method has a vast array of potential application in the fields of micro-optics, microstructures, nanomaterials, and micro/nanotechnology. It is a pleasure for the authors to acknowledge funding support from the National High Technology Research and Development Program 863 (2012AA040401), the National Natural Science Foundation of China (Nos. 61275161, 61007056), and the Zhejiang Provincial Natural Science Foundation (No. LY13F050001). References
Fig. 7. Calculated chromaticity coordinates of the samples in the CIE 1931 chromaticity diagram.
Fig. 8. (a) Photo of the colorful pattern taken under daylight with yellow eagle and blue background. (b)–(e) SEM images of the two different regions at top view and cross-sectional view. Pore radius of the whole sample and the thickness are both 25 nm.
the pores of the blue area, which makes a large color variation of the pattern. 5. Conclusion
This paper introduces a novel and convenient colortuning method by filling the PAM template in the metal–dielectric–metal structure. The effective refractive index of PAM is adjusted by altering the pore sizes and the thickness of the filling TiO2 layer deposited by ALD. On the basis of the multilayer interference, various colors appear. Through a SEM, A146
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1. O. Jessensky, F. Muller, and U. Gosele, “Self-organized formation of hexagonal pore arrays in anodic alumina,” Appl. Phys. Lett. 72, 1173–1175 (1998). 2. A. Yamaguchi, F. Uejo, T. Yoda, T. Uchida, Y. Tanamura, T. Yamashita, and N. Teramae, “Self-assembly of a silicasurfactant nanocomposite in a porous alumina membrane,” Nat. Mater. 3, 337–341 (2004). 3. Y. Piao, H. Lim, J. Chang, W. Lee, and H. Kim, “Nanostructured materials prepared by use of ordered porous alumina membranes,” Electrochimica Acta 50, 2997–3013 (2005). 4. H. Sai, H. Fujii, K. Arafune, Y. Ohshita, M. Yamaguchi, Y. Kanamori, and H. Yugami, “Antireflective subwavelength structures on crystalline Si fabricated using directly formed anodic porous alumina masks,” Appl. Phys. Lett. 88, 201116 (2006). 5. D. Kim, W. Hwang, H. C. Park, and K. H. Lee, “Superhydrophobic nanostructures based on porous alumina,” Curr. Appl. Phys. 8, 770–773 (2008). 6. V. Brizé, T. Prieur, P. Violet, L. Artaud, G. Berthomé, E. Blanquet, R. Boichot, S. Coindeau, B. Doisneau, A. Farcy, A. Mantoux, I. Nuta, M. Pons, and F. Volpi, “Developments of TaN ALD process for 3D conformal coatings,” Chem. Vapor Depos. 17, 284–295 (2011). 7. U. Huebner, K. Weber, D. Cialla, R. Haehle, H. Schneidewind, M. Zeisberger, R. Mattheis, H. G. Meyer, and J. Popp, “Microfabricated polymer-substrates for SERS,” Microelectron. Eng. 98, 444–447 (2012). 8. Q. Li, C. Dong, A. Nie, J. Liu, W. Zhou, and H. Wang, “Microstructure-dependent conformal atomic layer deposition on 3D nanotopography,” Langmuir 28, 15809–15815 (2012). 9. L. N. Hadley and D. M. Dennison, “Reflection and transmission interference filters,” J. Opt. Soc. Am. 37, 451–453 (1947). 10. A. F. Turner, “Some current developments in multilayer optical films,” J. Phys. Radium 11, 444–460 (1950). 11. J. A. Dobrowolski, K. M. Baird, P. D. Carman, and A. Waldorf, “Optical interference coatings for inhibiting of counterfeiting,” Opt. Acta 20, 925–937 (1973). 12. J. A. Dobrowolski, F. C. Ho, and A. Waldorf, “Research on thin film anticounterfeiting coatings at the National Research Council of Canada,” Appl. Opt. 28, 2702–2717 (1989). 13. R. W. Phillips and A. F. Bleikolm, “Optical coatings for document security,” Appl. Opt. 35, 5529–5534 (1996). 14. B. Baloukasand and L. Martinu, “Metameric interference security image structures,” Appl. Opt. 47, 1585–1593 (2008). 15. K. Kumar, H. G. Duan, R. S. Hegde, S. C. W. Koh, J. N. Wei, and J. K. W. Yang, “Printing colour at the optical diffraction limit,” Nat. Nanotechnol. 7, 557–561 (2012). 16. T. Ellenbogen, K. Seo, and K. B. Crozier, “Chromatic plasmonic polarizers for active visible color filtering and polarimetry,” Nano Lett. 12, 1026–1031 (2012). 17. K. Seo, M. Wober, P. Steinvurzel, E. Schonbrun, Y. Dan, T. Ellenbogen, and K. B. Crozier, “Multicolored vertical silicon nanowires,” Nano Lett. 11, 1851–1856 (2011).
18. Y. Wu, A. Hollowell, C. Zhang, and L. Guo, “Angle-insensitive structural colours based on metallic nanocavities and coloured pixels beyond the diffraction limit,” Science Rep. 3, 1194 (2013). 19. J. W. Elam, D. Routkevitch, P. P. Mardilovich, and S. M. George, “Conformal coating on ultrahigh-aspect-ratio nanopores of anodic alumina by atomic layer deposition,” Chem. Mater. 15, 3507–3517 (2003). 20. G. Xiong, J. W. Elam, H. Feng, C. Y. Han, H. H. Wang, L. E. Iton, L. A. Curtiss, M. J. Pellin, M. Kung, H. Kung, and P. C. Stair, “Effect of atomic layer deposition coatings on the surface structure of anodic aluminum oxide membranes,” J. Phys. Chem. B 109, 14059–14063 (2005).
21. M. Daub, M. Knez, U. Goesele, and K. Nielsch, “Ferromagnetic nanotubes by atomic layer deposition in anodic alumina membranes,” J. Appl. Phys. 101, 09J111 (2007). 22. H. A. Macleod, Thin Film Optical Filters (Institute of Physics Pub, 2001). 23. J. C. Maxwell-Garnett, “Colours in metal glasses and in metallic films,” Philos. Trans. R. Soc. London A 203, 385–420 (1904). 24. Y. Li, W. Shen, Y. Zhang, X. Hao, H. Fan, and X. Liu, “Precise broad-band anti-refection coating fabricated by atomic layer deposition,” Opt. Commun. 292, 31–35 (2013). 25. W. Shen, X. Sun, Y. Zhang, Z. Luo, X. Liu, and P. Gu, “Narrow band filters in both transmission and reflection with metal/ dielectric thin films,” Opt. Commun. 282, 242–246 (2009).
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A novel and convenient color-tuning method by filling the porous alumina membrane (PAM) based on metal–dielectric–metal structure is proposed. The pore sizes of PAM as well as the thickness of TiO2 films, deposited by atomic layer deposition, causes various reflection colors due to multilayer interference effects. Overlapping films on top and within the PAM structure can be observed with a scanning electron microscope and will be used to show the change of the effective refractive index of the PAM composite layer. The color-tuning method resulting in changing the dynamics of the PAM layer can be widely applied in fields such as micro-optics, microstructures, nanomaterials, and micro/nanotechnology. © 2013 Optical Society of America OCIS codes: (310.1860) Deposition and fabrication; (310.6628) Subwavelength structures, nanostructures; (230.4170) Multilayers. http://dx.doi.org/10.1364/AO.53.00A142
1. Introduction
Nanotechnology has developed so rapidly and universally that structures with the size of several nanometers are successfully fabricated, and more and more applications are becoming involved. In recent years, porous alumina, with all pores regularly hexagonally latticed, has aroused great interest for its simple and efficient preparation process [1]. Due to its unique features, porous alumina has been studied for producing nanotubes, transferring pattern, observing quantum confinement effect, and so on [2–5]. Atomic layer deposition (ALD), as a novel deposition method, has drawn many researches’ attention for its potential applications on optical coatings and microelectronics. The characteristic of completely overlapping the original micro/nanostructures, as the most remarkable advantage, has brought out lots of 1559-128X/14/04A142-06$15.00/0 © 2014 Optical Society of America A142
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attempts with a variety of materials for different structures [6–8]. Since color-shifting metal–dielectric and all dielectric thin-film designs were proposed decades ago [9,10], the study on color tuning has never stopped. For the most popular application of anticounterfeiting, the significant developments on the colortuning method and the visual effect have been achieved [11–14]. In recent years, color tuning based on subwavelength structures has been widely studied owing to the development of numerical simulation algorithms and nano/microfabrication techniques [15–18] for the applications in color displays and imaging devices. However, the preparation of these subwavelength structures is difficult and timeconsuming. Though research in the combination of PAM and ALD has been carried out already [19–21], the color effect resulted from the feature of completely overlapping PAM based on the MDM structure has not been reported yet, as far as we know. In this paper, we introduce a novel and convenient color-tuning method upon the metal–dielectric–metal
structure by filling the porous alumina membrane (PAM), in which various reflection colors are obtained by changing the effective optical thickness of the composite PAM layer. Five PAMs with different pore sizes are applied, and various thickness of TiO2 are deposited by ALD, so as to adjust the effective refractive index. Based on this work, a colorful pattern is fabricated with a lift-off process. 2. Theory and Method
Referring to the widely used reflective interference filter in anticounterfeiting [13], a metal–dielectric– metal structure is proposed to realize color tuning through a simple process with PAM templates. The film stack proposed in our research is Air/Cr/PAM/Al, as shown in Fig. 1(a). Natural light is incident from the air side through the ultrathin metal layer Cr, the modulating layer PAM, and finally reflected on the interface between PAM and Al and presenting a specific color. The ultrathin metal layer, as an absorbing layer, is required to possess the property of n∕k ≈ 1 to achieve the best color saturation [13]. In our work, Cr is adopted as that metal layer. The aluminum layer, on the bottom of the stack, is served to provide high reflectance with a sufficient thickness of more than 100 nm. The optical thickness of dielectric film PAM as a spacer layer determines the central wavelength at peak reflectance. Figure 1(b) presents the calculated spectral reflectance of the structure as Fig. 1(a) without TiO2 [22]. The pore radius ranging from 20 to 50 nm contributes to the changing optical thickness of the PAM layer. Accordingly, the effective optical thickness of the PAM layer changes from 2.7995 to 2.1795 of quarter-wave thickness at λ0 550 nm. As shown in Fig. 1(b), the optical property of the structure is sensitive to the optical thickness of the dielectric layer, so various colors in reflection could be obtained by varying the thickness or the index of the dielectric layer. In our work, the refractive index contributed to the change of effective optical thickness, after filling the PAM layer with different thicknesses of TiO2.
Moreover, the dimension of the pores and periods in the PAM template of about 100 nm is small enough with respect to the wavelength. So, in optics, it can be treated as several uniform layers. Therefore, the PAM could be replaced by three equivalent layers whose refractive indexes depend on the refractive index of the filling material and the volume fraction (shown in Fig. 2). According to the Maxwell–Garnett theory [23], the effective dielectric constant and the effective refractive indexes are defined as below: εeff ;A − εTiO2 εAir − εTiO2 δ1 ; εeff ;A 2εTiO2 εAir 2εTiO2 εeff ;B − εAl2 O3 εAir − εAl2 O3 δ1 Region B εeff ;B 2εAl2 O3 εAir 2εAl2 O3 εTiO2 − εAl2 O3 δ2 ; εTiO2 2εAl2 O3 εeff ;C − εAl2 O3 εTiO2 − εAl2 O3 δ1 δ2 ; Region C εeff ;C 2εAl2 O3 εTiO2 2εAl2 O3 Region A
neff ;A ε2eff ;A ;
neff ;B ε2eff ;B ;
neff ;C ε2eff ;C
t h−t t neff ;A neff ;B n ; ht ht h t eff ;C p where δp 2πr − t2 ∕ 3d2 and δ2 2πr2 − 1 r − t2 ∕ 3d2 are the volume fraction of air and TiO2 in Region B, respectively; t is the thickness of deposited TiO2 , r is the pore radius, d is the length of the adjacent pores, and h is the pore depth. It is not difficult to find that the equivalent refractive index is closely related to the pore radius, the thickness of TiO2 , and the pore depth. As the pore radius decreases or the thickness of TiO2 increases or the pore depth decreases, the equivalent refractive index increases. With the model constructed, the PAM template deposited with TiO2 and Cr could be treated as multilayer film, due to its simple metal–dielectric–metal structure. So it is easy to analyze the reflectance, transmittance, and electrical field of this structure by a transfer matrix method. Total neff
Fig. 1. (a) Schematic diagram of the metal–dielectric–metal reflective color filter. (b) Reflectance of the structure shown in (a) with increasing optical thickness of PAM. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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Fig. 2. Structure of the PAM template after ALD TiO2 process.
3. Experiments
The PAM templates were prepared by two anodization processes originating from the high purity Al foils. By controlling the anodization voltage and the anodization time, the pore size and pore depth could be controlled, respectively. In our experiment, we fabricated five PAM templates with pore radius of 20, 25, 30, 35, and 40 nm. The length of the adjacent pores is kept at about 100 nm. In order to present more colors with the central wavelength covering the entire visible spectrum, considering the limits of the preparation process, we chose 135 nm as the pore depth. Figure 3 shows the top view and crosssectional view of scanning electron microscope (SEM) images of the fabricated PAM template, where the ordered pore arrays can be observed clearly. With the PAM obtained, a high refractive index oxide layer of TiO2 (n 2.35 at 550 nm) was deposited to fill the PAM by ALD. To adjust the effective refractive index, five thicknesses of 20, 25, 30, 35, and 40 nm were respectively deposited on each PAM template with different pore sizes. The ALD experiment of TiO2 was carried out in a TFS 200 reactor made by beneq. The reaction chamber pressure was about 3 mbar, and 120°C was chosen as the deposition temperature. Titanium tetrachloride (TiCl4 ) and deionized water were selected as reactant precursors. The pulsing time was 400 ms for TiCl4 and H2 O, and the purging times were 5 s for both to blow the reaction chamber clean. The uniform and amorphous TiO2 film can be deposited with a growth rate of 0.0596 nm per cycle [24]. Finally, a thinner Cr layer with a thickness of 14 nm was deposited on the TiO2 -filled PAM by e-beam evaporation. During the evaporation, the substrate is kept at room temperature. As for the
Fig. 3. SEM images of a PAM template. (a) Top view. (b) Crosssectional view. A144
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thin layer deposited by e-beam evaporation, we have experiences in the ultrathin Cr deposition in our plant [25]. To produce a complex colorful pattern using this method, filled areas and unfilled areas should be distinguished sharply, so a lift-off technique is employed. First, we used the same steps as above to obtain a piece of PAM with a certain pore size on a 6 mm × 6 mm Al foil. Second, the photoresist pattern coated by spin coating was created with a standard lithography process. Third, TiO2 film with a thickness of 25 nm was deposited by ALD. After removing the photoresist and the overhead TiO2 film by ultrasonic cleaning in acetone, the pattern of the filled and nonfilled areas appeared. Finally, Cr was deposited on the sample by e-beam evaporation to make the gray pattern colorful. 4. Results
Different from other deposition techniques, ALD has the unique advantage of completely overlapping onto/into the nanostructure. Hence, ALD TiO2 on the substrate of PAM result in completely filling the pores, which can be seen in Fig. 4. The ultrathin layer of Cr makes little change on the surface topography. Apparently, the filling shape of the deposition materials is inverted with respect to the pores of the original PAMs, which further confirms the overlapping feature of the ALD technique. Besides, as the TiO2 deposited thickness is just equal to the pore radius, the small dips appear apparently, which could be eliminated gradually by continuous deposition. On the basis of the completely overlapping feature of the ALD technique, the changing refractive index could be received by filling the pores differently. Figures 5(a)–5(e), respectively, show the measured reflectance of the complete MDM structures of five PAM templates for different pore sizes and various TiO2 thicknesses. The equivalent refractive indexes are listed in Table 1. With the pore size fixed, tuning color could be achieved by varying the deposition thickness. As the thickness increases, the reflectance curve moves toward a long wavelength with the displaying color accordingly turned, which could be obviously observed from each figure in Fig. 5. It could be verified as well in Fig. 6 via the variation of the peak wavelength. Generally, the PAM with the larger pore size performs better than that with the smaller
Fig. 4. PAM sample after the processes of ALD TiO2 and deposition Cr. Pore radius and thickness of TiO2 were both 20 nm.
Fig. 5. Reflectance of the five PAM templates of different pore sizes with different thicknesses of TiO2 and the insert displayed the perceived color of these samples with increasing thickness.
one. Because when the pore is larger, the equivalent refractive index changes more apparently so that the reflectance curve moves more. Similarly, with the deposition thickness fixed, tuning color could be achieved by varying the pore size of the PAM, which is verified in Fig. 6. Likewise, if the TiO2 deposited is thicker, the adjustment range of the effective index is wider so that the reflectance curve moves more. If the pore size or the deposition thickness varies gradually, the continuous refractive index could be achieved, which could lead to the color tuning more finely. According to the measured reflectance curve, the chromaticity coordinates are calculated and marked in the CIE 1931 chromaticity diagram (shown in Fig. 7). A large color gamut is presented for these PAMs with a different TiO2 thickness. A particular color in reflection can be realized to select the appropriate pore size and filling factor. Note that the PAM with no TiO2 is distinct from those TiO2 filled ones, producing a large color difference, which is apparent in Fig. 5 as well.
Table 1.
Furthermore, a complex colorful pattern is fabricated using this color-tuning method, as shown in Fig. 8(a). It is a yellow eagle surrounded by blue blank areas. Through SEM, the structure is clear so as to exploit the coloration principle. Figures 8(b), 8(c) and Figs. 8(d), 8(e) are the top view and crosssectional view of the yellow eagle and blue surroundings, respectively. From SEM images, the observed microstructures in both areas can be used to explain the coloration principle. Obviously, TiO2 completely fills the pores of the yellow area, while no TiO2 fills
Effective Refractive Index of All Pam Templates with Different Pore Sizes
Pore Radius/nm Thickness of TiO2 ∕nm 20 25 30 35 40
20
25
30
35
40
1.805 1.825 1.844 1.862 1.879
1.846 1.877 1.894 1.911 1.926
1.874 1.929 1.957 1.972 1.986
1.889 1.970 2.022 2.047 2.059
1.893 1.998 2.076 2.126 2.148
Fig. 6. Peak wavelength of the PAM of different pore sizes with TiO2 deposited thickness t 35 nm and the peak wavelength of the PAM with pore radius r 20 nm for different TiO2 thickness. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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the completely overlapping into/onto the PAM could be observed, which could tailor the effective refractive index precisely. Based on this method, a colorful pattern was fabricated with a lift-off process. The method has a vast array of potential application in the fields of micro-optics, microstructures, nanomaterials, and micro/nanotechnology. It is a pleasure for the authors to acknowledge funding support from the National High Technology Research and Development Program 863 (2012AA040401), the National Natural Science Foundation of China (Nos. 61275161, 61007056), and the Zhejiang Provincial Natural Science Foundation (No. LY13F050001). References
Fig. 7. Calculated chromaticity coordinates of the samples in the CIE 1931 chromaticity diagram.
Fig. 8. (a) Photo of the colorful pattern taken under daylight with yellow eagle and blue background. (b)–(e) SEM images of the two different regions at top view and cross-sectional view. Pore radius of the whole sample and the thickness are both 25 nm.
the pores of the blue area, which makes a large color variation of the pattern. 5. Conclusion
This paper introduces a novel and convenient colortuning method by filling the PAM template in the metal–dielectric–metal structure. The effective refractive index of PAM is adjusted by altering the pore sizes and the thickness of the filling TiO2 layer deposited by ALD. On the basis of the multilayer interference, various colors appear. Through a SEM, A146
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1. O. Jessensky, F. Muller, and U. Gosele, “Self-organized formation of hexagonal pore arrays in anodic alumina,” Appl. Phys. Lett. 72, 1173–1175 (1998). 2. A. Yamaguchi, F. Uejo, T. Yoda, T. Uchida, Y. Tanamura, T. Yamashita, and N. Teramae, “Self-assembly of a silicasurfactant nanocomposite in a porous alumina membrane,” Nat. Mater. 3, 337–341 (2004). 3. Y. Piao, H. Lim, J. Chang, W. Lee, and H. Kim, “Nanostructured materials prepared by use of ordered porous alumina membranes,” Electrochimica Acta 50, 2997–3013 (2005). 4. H. Sai, H. Fujii, K. Arafune, Y. Ohshita, M. Yamaguchi, Y. Kanamori, and H. Yugami, “Antireflective subwavelength structures on crystalline Si fabricated using directly formed anodic porous alumina masks,” Appl. Phys. Lett. 88, 201116 (2006). 5. D. Kim, W. Hwang, H. C. Park, and K. H. Lee, “Superhydrophobic nanostructures based on porous alumina,” Curr. Appl. Phys. 8, 770–773 (2008). 6. V. Brizé, T. Prieur, P. Violet, L. Artaud, G. Berthomé, E. Blanquet, R. Boichot, S. Coindeau, B. Doisneau, A. Farcy, A. Mantoux, I. Nuta, M. Pons, and F. Volpi, “Developments of TaN ALD process for 3D conformal coatings,” Chem. Vapor Depos. 17, 284–295 (2011). 7. U. Huebner, K. Weber, D. Cialla, R. Haehle, H. Schneidewind, M. Zeisberger, R. Mattheis, H. G. Meyer, and J. Popp, “Microfabricated polymer-substrates for SERS,” Microelectron. Eng. 98, 444–447 (2012). 8. Q. Li, C. Dong, A. Nie, J. Liu, W. Zhou, and H. Wang, “Microstructure-dependent conformal atomic layer deposition on 3D nanotopography,” Langmuir 28, 15809–15815 (2012). 9. L. N. Hadley and D. M. Dennison, “Reflection and transmission interference filters,” J. Opt. Soc. Am. 37, 451–453 (1947). 10. A. F. Turner, “Some current developments in multilayer optical films,” J. Phys. Radium 11, 444–460 (1950). 11. J. A. Dobrowolski, K. M. Baird, P. D. Carman, and A. Waldorf, “Optical interference coatings for inhibiting of counterfeiting,” Opt. Acta 20, 925–937 (1973). 12. J. A. Dobrowolski, F. C. Ho, and A. Waldorf, “Research on thin film anticounterfeiting coatings at the National Research Council of Canada,” Appl. Opt. 28, 2702–2717 (1989). 13. R. W. Phillips and A. F. Bleikolm, “Optical coatings for document security,” Appl. Opt. 35, 5529–5534 (1996). 14. B. Baloukasand and L. Martinu, “Metameric interference security image structures,” Appl. Opt. 47, 1585–1593 (2008). 15. K. Kumar, H. G. Duan, R. S. Hegde, S. C. W. Koh, J. N. Wei, and J. K. W. Yang, “Printing colour at the optical diffraction limit,” Nat. Nanotechnol. 7, 557–561 (2012). 16. T. Ellenbogen, K. Seo, and K. B. Crozier, “Chromatic plasmonic polarizers for active visible color filtering and polarimetry,” Nano Lett. 12, 1026–1031 (2012). 17. K. Seo, M. Wober, P. Steinvurzel, E. Schonbrun, Y. Dan, T. Ellenbogen, and K. B. Crozier, “Multicolored vertical silicon nanowires,” Nano Lett. 11, 1851–1856 (2011).
18. Y. Wu, A. Hollowell, C. Zhang, and L. Guo, “Angle-insensitive structural colours based on metallic nanocavities and coloured pixels beyond the diffraction limit,” Science Rep. 3, 1194 (2013). 19. J. W. Elam, D. Routkevitch, P. P. Mardilovich, and S. M. George, “Conformal coating on ultrahigh-aspect-ratio nanopores of anodic alumina by atomic layer deposition,” Chem. Mater. 15, 3507–3517 (2003). 20. G. Xiong, J. W. Elam, H. Feng, C. Y. Han, H. H. Wang, L. E. Iton, L. A. Curtiss, M. J. Pellin, M. Kung, H. Kung, and P. C. Stair, “Effect of atomic layer deposition coatings on the surface structure of anodic aluminum oxide membranes,” J. Phys. Chem. B 109, 14059–14063 (2005).
21. M. Daub, M. Knez, U. Goesele, and K. Nielsch, “Ferromagnetic nanotubes by atomic layer deposition in anodic alumina membranes,” J. Appl. Phys. 101, 09J111 (2007). 22. H. A. Macleod, Thin Film Optical Filters (Institute of Physics Pub, 2001). 23. J. C. Maxwell-Garnett, “Colours in metal glasses and in metallic films,” Philos. Trans. R. Soc. London A 203, 385–420 (1904). 24. Y. Li, W. Shen, Y. Zhang, X. Hao, H. Fan, and X. Liu, “Precise broad-band anti-refection coating fabricated by atomic layer deposition,” Opt. Commun. 292, 31–35 (2013). 25. W. Shen, X. Sun, Y. Zhang, Z. Luo, X. Liu, and P. Gu, “Narrow band filters in both transmission and reflection with metal/ dielectric thin films,” Opt. Commun. 282, 242–246 (2009).
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