pubs.acs.org/Langmuir © 2009 American Chemical Society
Simple Approach to Wafer-Scale Self-Cleaning Antireflective Silicon Surfaces Dianpeng Qi,† Nan Lu,*,† Hongbo Xu,† Bingjie Yang,† Chunyu Huang,† Miaojun Xu,† Liguo Gao,† Zhouxiang Wang,† and Lifeng Chi*,†,‡ † State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R. China, and ‡Physikalisches Institut and Center for Nanotechnology (C198eNTech), Westf€ alische Wilhelms-Universit€ at M€ unster, D-48149 M€ unster, Germany
Received April 13, 2009. Revised Manuscript Received May 22, 2009 A simple approach to wafer-scale self-cleaning antireflective hierarchical silicon structures is demonstrated. By employing the KOH etching and silver catalytic etching, pyramidal hierarchical structures were generated on the crystalline silicon wafer, which exhibit strong antireflection and superhydrophobic properties after fluorination. Furthermore, a flexible superhydrophobic substrate was fabricated by transferring the hierarchical Si structure to the NOA 63 film with UV-assisted imprint lithography. This method is of potential application in optical, optoelectronic, and wettability control devices.
Introduction Nanostructured surfaces have attracted increasing attention due to their unique properties, such as superhydrophobicity and antireflection, which have promising applications in industry. For example, they can be applied for contamination prevention, biocompatibility, antioxidation,1 or improving the performance of some optical and optoelectronic devices.2-4 Many efforts have been devoted to achieve the structured superhydrophobic or antireflective surfaces.4-16 From a practical standpoint, the combination of superhydrophobicity and antireflection is more attractive, especially on the silicon surface since they can be integrated with other electronic components; for instance, the multifunctional silicon surfaces can increase the photovoltaic conversion efficiency of solar cells and prevent the device surfaces from being contaminated. So far, several methods have been proposed to generate the multifunctional surfaces, including *Corresponding author. E-mail:
[email protected] (N.L.); chi@ uni-muenster.de (L.C.). (1) Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. Adv. Mater. 1999, 11, 1365. (2) Lee, C.; Bae, S. Y.; Mobasser, S.; Manohara, H. Nano Lett. 2005, 5, 2438. (3) Xu, H. B.; Lu, N.; Qi, D. P.; Hao, J. Y.; Gao, L. G.; Zhang, B.; Chi, L. F. Small 2008, 4, 1972. (4) Lee, Y. J.; Ruby, D. S.; Peters, D. W.; McKenzie, B. B.; Hsu, J.W. P. Nano Lett. 2008, 8, 1501. (5) Feng, L.; Li, S. H.; Li, Y. S.; Li, H. J.; Zhang, L. J.; Zhai, J.; Song, L. Y.; Liu, B. Q.; Jiang, L.; Zhu, D. B. Adv. Mater. 2002, 14, 1857. (6) Zhang, X.; Shi, F.; Yu, X.; Liu, H.; Fu, Y.; Wang, Z. Q.; Jiang, L.; Li, X. Y. J. Am. Chem. Soc. 2004, 126, 3064. (7) Shi, F.; Wang, Z. Q.; Zhang, X. Adv. Mater. 2005, 17, 1005. (8) Fang, W. J.; Mayama, H.; Tsujii, K. J. Phys. Chem. B 2007, 111(3), 564. (9) Shi, F.; Chen, X. X.; Wang, L. Y.; Niu, J.; Yu, J. H.; Wang, Z. Q.; Zhang, X. Chem. Mater. 2005, 17, 6177. (10) Xiu, Y. H.; Zhu, L. B.; Hess, D. W.; Wong, C. P. Langmuir 2006, 22, 9676. (11) Di Mundo, R.; Palumbo, F.; d’Agostino, R. Langmuir 2008, 24, 5044. (12) Xiu, Y. H.; Zhu, L. B.; Hess, D. W.; Wong, C. P. Nano Lett. 2007, 7, 3388. (13) Verplanck, N.; Galopin, E.; Camart, J. C.; Thomy, V.; Coffinier, Y.; Boukherroub, R. Nano Lett. 2007, 7, 813. (14) Nosonovsky, M.; Bhushan, B. Nano Lett. 2007, 7, 2633. (15) Ibn-Elhaj, M.; Schadt, M. Nature 2001, 410, 796. (16) Walheim, S.; Schaffer, E.; Mlynek, J.; Steiner, U. Science 1999, 283, 520. (17) Xiu, Y. H.; Zhang, S.; Yelundur, V.; Rohatgi, A.; Hess, D. W.; Wong, C. P. Langmuir 2008, 24, 10421. (18) Zhang, L. B.; Li, Y.; Sun, J. Q.; Shen, J. C. J. Colloid Interface Sci. 2008, 319, 302.
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hierarchical etching,17 layer by layer,18,19 sol-gel process,20 laser-induced damage of silica surface,21 fluorocarbon plasma nanotexturing of polymer,22 and spin-coating technique.23 However, most of the methods are associated with problems, such as being time-consuming, complicated processes or expensive equipment, which will limit their applications. Therefore, a simple and cost-effective fabrication method for large area multifunctional surfaces is highly desired. In this communication, we demonstrate a simple method for creating hierarchical pyramidal structures on whole silicon wafer using chemical etching. The structured silicon wafer exhibits strong antireflective property in broadband wavelengths, which also shows superhydrophobic behavior after fluorination treatment. Furthermore, a flexible superhydrophobic surface can be fabricated using imprint lithography with the hierarchically structured silicon as a stamp.
Experimental Section Materials. One side polished n-type (100) oriented silicon wafer with a resistivity of 0.008-0.02 Ωcm was purchased from GRINM semiconductor materials Co., Ltd. Beijing, China. AgNO3, KOH, and HF were purchased from a commercial source. Heptadecafluoro-1,1,2,2-tetrahydrodecyl trithoxysilane and 1-dodecanethiol were purchased from Aldrich Chemical Co. Norland Optical Adhesive 63 (NOA 63) was purchased from Norland Products Inc. A kit of a poly(dimethylsiloxane) (PDMS) prepolymer (Sylgard 184 silicone elastomer curing agent) was purchased from Dow Corning Corporation. To remove the native oxide layer, the silicon wafer was submerged in a HF (1% electronic grade) aqueous solution for 30 s. The wafer was then washed in an ultrasonic bath with deionized water for 5 min and dried under nitrogen gas flow before use. (19) Liu, X. M.; He, J. H. J. Phys. Chem. C 2009, 113, 148. (20) Zhang, X. X.; J€arn, M.; Peltonen, J.; Pore, V.; Vuorinen, T.; Lev€anen, E.; M€antyl, T. J. Eur. Ceram. Soc. 2008, 28, 2177. (21) Xu, Y.; Wu, D.; Sun, Y. H.; Huang, Z. X.; Jiang, X. D.; Wei, X. F.; Li, Z. H.; Dong, B. Z.; Wu, Z. H. Appl. Opt. 2005, 44, 527. (22) Mundoa, R. D.; Benedictis, V. D.; Palumbo, F.; d’Agostino, R. Appl. Surf. Sci. 2009, 255, 5461. (23) Min, W. L.; Jiang, B.; Jiang, P. Adv. Mater. 2008, 20, 3914.
Published on Web 06/18/2009
DOI: 10.1021/la9013009
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Fabrication of Superhydrophobic and Antireflective Surface. Pyramidal structures were generated by etching the silicon
wafer in a solution of KOH24,25 (pH=14) for 1-1.25 h at 60 ( 5 °C, and washed in an ultrasonic bath with deionized water for 5 min. To achieve nanostructures on the silicon pyramids, silver nanoparticles were electroless deposited on the surface of silicon pyramids by dipping the pyramidal silicon in HF (4.6M)/AgNO3 (0.01 M) solution,26 and then the silver-nanoparticle-covered silicon wafer was etched in a HF/H2O2 solution (49% HF, 30% H2O2, and H2O with a volume ratio of 1:5:10). At last, the silver nanoparticles were removed by immersing samples in nitric acid and then sonicating them in deionized water for 5 min. Finally, the structured silicon surface was fluorinated by immersing the silicon wafer in a 10 mM solution of fluoroalkylsilane (heptadecafluoro1,1,2,2-tetrahydrodecyl trithoxysilane) in toluene for 30 min, followed by a heat treatment at 150 °C in air for 1 h to complete the hydrophobic surface modification. Characterization. Scanning electron microscopy (SEM) measurements were carried out on an FE-SEM instrument (JSM6700F, JEOL Co., Japan). The contact angles of water droplets were taken on a KRUSS DSA 100 drop shape analysis system. Contact angle hysteresis was measured on s Dataphysics OCA 20 contact angle system. A distilled water droplet of 4 μL was expanded and shrank on the hierarchical silicon substrate. A video system with a CCD camera (CCD resolution 768 576 pixels) and halogen lighting with continuous adjustable intensity without hysteresis for homogeneous back lighting were used to image the whole expanding and shrinking course of the water droplet. Hemispherical reflectance spectra were collected on a spectroscopy meter (Shimadzu UV3600, Shimadzu, Japan). The silver film was deposited on the NOA 63 surface with a commercial thermal evaporation system at a pressure of 5 10-4 Pa (Shenyang City Keyou Institute of Vacuum Technology, China).
Results and Discussion The fabrication procedure for creating hierarchical structures is schematically shown in Figure 1. First, pyramids were fabricated on the silicon surface by anisotropic etching in KOH solution. The SEM image of the created silicon pyramidal structures is shown in Figure 2a, which reveals that the heights of most pyramids range from 3 to 5 μm. The pyramidal structured silicon surface exhibits both antireflective and hydrophobic properties. The contact angle lies on 130°. According to Cassie’s equation, cos θCB ¼ f cos θ þ f -1 θCB is the measured contact angle on a rough surface, and θ is the intrinsic contact angle on a flat surface, where f is the area fraction of the liquid-solid contact. It can be concluded that the surface wettability is dependent on f, which is decided by the surface roughness and porosity that can decrease the solid-liquid contact area.27 In order to gain superhydrophobic property, the value of f should be as small as possible. Therefore, the generation of nanostructures on the microscale silicon pyramids will assist in achieving superhydrophobicity (see Figure S1 in the Supporting Information) and lower reflectivity. Thus, a silver catalytic etching was performed on the pyramidal silicon surface subsequently.28 Silver nanoparticles (about 100 nm in diameter) were electroless deposited on the micrometer-sized pyramid surfaces by (24) Tellier, C. R.; Brahim-Bounab, A. J. Mater. Sci. 1994, 29, 6354. (25) Sato, K.; Shikida, M.; Yamashiro, T; Tsunekawa, M.; Ito, S. Sens. Actuators 1999, A73, 122. (26) Peng, K. Q.; Huang, Z. P.; Zhu, J. Adv. Mater. 2004, 16, 73. (27) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546. (28) Chattopadhyay, S.; Li, X. L.; Bohn, P. W. J. Appl. Phys. 2002, 91, 6134.
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Figure 1. Fabrication procedure for creating hierarchical structures on the silicon surface. (a) Fabrication of pyramidal structures on silicon surface with KOH etching. (b) Electroless deposion of a thin discontinuous layer of Ag nanoparticles on the pyramidal structures in HF/AgNO3 solution. (c) Generation of hierarchical structures with Ag-assisted etching and removal of Ag nanoparticles in nitric acid.
dipping the wafer in HF/AgNO3 solution. A discontinuous film of silver nanoparticles formed on the surface of silicon pyramids, as shown in Figure S2 in the Supporting Information, which will assist to etch the silicon for generating nanostructures. Finally, the silicon wafer with the silver nanoparticles was etched in a HFbased etchant (HF/H2O2). The silver nanoparticles were then removed by immersing the silicon wafer in nitric acid, followed by sonicating in deionized water for 5 min. Figure 2b presents that the silver-assisted etching occurring at the Ag/Si interface produced pit nanostructures directly on the pyramid surfaces. The SEM micrographs and the contact angle measurements show that the deposition duration is not prominent on the superhydrophobic characteristics (see Supporting Information Figures S3 and S4). After taking the silver nanoparticle deposition for 1 min, the duration of silver-assisted etching was optimized. Supporting Information Figure S5a and b reveals that the shapes of the silicon pyramids were kept very well when etching for 15 and 30 s, while the tips of pyramids became blunt with extending the etching time up to 40 s (see Supporting Information Figure S5c and d). As shown in Supporting Information Figure S5e and f, the nanoholes became deeper and some walls of the nanoholes were broken when further extending the etching time to more than 2 min. Figure 2b presents the SEM micrographs of the silicon pyramids, which were obtained by taking silver catalytic etching for 15 s in a HF/H2O2 solution (the deposition duration is 1 min). The modification of the surface with fluoroalkylsilane can yield high contact angles. The water contact angle comparison of the fluorinated hierarchical structures and nanostructures generated on the flat silicon with different silver catalytic etching durations is presented in Figure 3. For the hierarchical samples (9), the contact angle of 169° can be achieved with the etching duration of 15 and 30 s, which decreased with further increase of the etching duration. The optimal etching time is less than 30 s. While the contact angles of the fluorinated flat silicon and pyramid textured silicon surface were only 105° and 130°, respectively, the maximum contact angle of the nanostructured silicon surfaces was 157° (0). The hierarchically structured surface exhibits higher Langmuir 2009, 25(14), 7769–7772
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Figure 2. Cross-sectional SEM photographs of (a) silicon pyramids created with KOH etching and (b) hierarchical structures generated with Ag-assisted etching. Inset: magnified SEM image.
Figure 3. Water contact angles on the fluorinated hierarchically structured silicon surfaces (9) and nanostructured silicon surfaces (0) with different etching durations. Inset: a dynamic water droplet moving on the fluorinated hierarchically structured silicon surface generated by silver-assisted etching for 30 s.
contact angle (169°) and lower sliding angle (less than 3°); the advancing angle and receding angle for etching duration of 15 s is 167° and 165°, respectively. Figure S6 in the Supporting Information shows the micrograph of a moving water droplet on the hierarchical silicon surface. It can be concluded that extending the etching duration can increase the value of f. According to Cassie’s equation, increasing f results in the decrease of the contact angle θCB. It is well-known that the high reflective index of silicon limits the performance of silicon-based optical and optoelectronic devices, such as solar cells, displays, and light sensors. Herein, with the fabricated hierarchical structures, the reflectivity was strongly reduced. The antireflection comparison of the structured and flat silicon wafers is shown in Figure 4. The diffuse reflectivity measurements indicate that up to 40% of the incident light is reflected on the flat silicon wafer (see line a in Figure 4). By creating the structured layers, a refractive index gradient is introduced between air and the silicon wafer. The reflectance can be reduced to 7% and 13% by the microscale pyramids and nanoholes on silicon surfaces, respectively, as shown in Figure 4 (lines b and c), which can be further reduced to less than 4% by constructing the hierarchical structures on the silicon surface (see line d in Figure 4), and it can even be suppressed to 2.8% at the Langmuir 2009, 25(14), 7769–7772
Figure 4. Hemispherical reflectance spectra of flat silicon (a), nanohole textured silicon surface (b), pyramid textured silicon surface (c), and hierarchically structured silicon (d).
Figure 5. Photographs of the polished silicon wafer (left) and the hierarchically structured silicon wafer (right).
wavelength from 800 to 1100 nm. This method allows for a facile wafer-scale fabrication of a superhydrophobic and antireflective silicon surface, as shown in Figure 5. The different antireflective behaviors of the polished silicon wafer (left) and the hierarchically structured silicon wafer (right) can be easily observed from the photographs. The polished silicon wafer is shinning, while the hierarchically structured one shows dark black. The hierarchical structures on silicon substrate can be transferred onto polymers with imprint lithography using a PDMS stamp molded from the structured silicon surface. Supporting DOI: 10.1021/la9013009
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Information Figure S7a shows the SEM photograph of the imprinted hierarchical structures on NOA 63; its contact angle lies on 132° and increases to 155° after depositing silver nanoparticles and modifying it with 1-dodecanethiol. Supporting Information Figure S7b presents the photographs of the superhydrobic NOA 63 film wrapped on the copper stick, and a droplet of water on the NOA 63 film (inset).
Conclusion In summary, we have demonstrated a simple technique for generating a wafer-scale superhydrophobic and antireflective structured silicon surface with chemical etching. After the fluoroalkylsilane treatment, the structured surface exhibits the contact angle of 169° and the sliding angle of less than 3°. The duration of silver-assisted etching plays an important role on the superhydrophobic characteristics, which should be less than 30 s, otherwise the pyramid structure will be damaged and the superhydrophobicity will be degraded. The wafer-scale hierarchically structured silicon surfaces exhibit strong antireflection property, and less than 4% can be obtained in a large wavelength range. By transferring the hierarchical silicon structure to NOA
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63 film, a flexible superhydrophobic substrate was fabricated. It provides a cost-effective and facile approach for producing superhydrophobic and antireflective surfaces, which may have applications in optical and optoelectronic fields, such as prolonging the life of devices by self-cleaning and improving the performance of photon sensitive devices. Acknowledgment. We gratefully acknowledge Fengxia Dong for SEM measurements. Financial support was given by the National Natural Science Foundation of China (20773052, 20373019, and 50520130316), the NCET Program, the National Basic Research Program (2007CB808003 and 2009CB939701), and Project 111. Supporting Information Available: Schematic of the decrease of the solid-liquid contact area after silver catalytic etching, SEM images, plot showing the variation of water contact angle with silver-assisted etching and deposition, and micrograph of moving water droplet on the hierarchically structured silicon surface. This material is available free of charge via the Internet at http://pubs.acs.org.
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