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Langmuir 2008, 24, 10532-10536
Submicron Scale Patterning in Sintered Silica Colloidal Crystal Films Using a Focused Ion Beam Jeffrey L. Moran,† Philip M. Wheat,† and Jonathan D. Posner†,‡,* Department of Mechanical and Aerospace Engineering, Arizona State UniVersity, Tempe, Arizona 85287-6106, Department of Chemical Engineering, Arizona State UniVersity, Tempe, Arizona 85287-6106 ReceiVed June 6, 2008. ReVised Manuscript ReceiVed July 20, 2008 Focused ion beam milling is used to fabricate micron and submicron scale patterns in sintered silica colloidal crystal films. Rectangular cavities with both solid and porous boundaries, fluidic channels, and isolation of a small number of packed spheres are patterned. The ion beam can pattern sintered films of individual submicron size spheres and create patterns that cover up to 40 µm in less than 15 min. The experiments in this work indicate that the amount of redeposited material on the surface of a milled cavity determines whether the surface will be porous or solid. FIB direct patterning has applications in colloidal crystal based lithography, integrated photonic devices, optofluidic devices, and micrototal-analytical systems.
Introduction Colloidal crystal films (CCFs) are highly ordered arrays of microspheres arranged in a close-packed crystal structure. Over the past 15 years, CCFs have been used in a variety of applications across several disciplines, such as photonic crystals,1,2 optical waveguides,3,4 chemical sensors,5,6 and electrophoretic separation media.7 Much of the utility of these crystals stems from their unique periodic structure. Micrometer-scale structural modification of CCFs has been shown to enhance their functionality for various applications. One of the most common and applicable structural modifications to CCFs is line patterning. Ozin et al. have used various microfabrication techniques to create free-standing raised patterns of both CCF structures2 and inverse opals.8 In addition, both Ozin’s group9 and Yan et al.10 have created line defects buried inside layers of CCFs. This is accomplished by printing a sacrificial layer of photoresist in the desired pattern onto a multilayered CCF. Additional layers of spheres are then deposited on top of the pattern of photoresist, which is then removed, leaving a buried line defect in the pattern defined by the photoresist. Yan et al. have also used a similar technique to create buried point defects in CCFs.11 Spheres have also been packed into patterned fluidic channels. Yang et al. packed microfluidic channels with silica colloids * Corresponding author. Fax: (480) 965-1384. E-mail:
[email protected]. † Department of Mechanical and Aerospace Engineering. ‡ Department of Chemical Engineering. (1) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11(8), 2132–2140. (2) Yang, S. M.; Miguez, H.; Ozin, G. A. AdV. Funct. Mater. 2002, 12(6-7), 425–431. (3) Baek, K. H.; Gopinath, A. IEEE Photon. Technol. Lett. 2005, 17(2), 351– 353. (4) Zhao, Y.; Avrutsky, I. Opt. Lett. 1999, 24(12), 817–819. (5) Hu, X. B.; Li, G. T.; Huang, J.; Zhang, D.; Qiu, Y. AdV. Mater. 2007, 19(24), 4327-+. (6) Lee, Y. J.; Braun, P. V. AdV. Mater. 2003, 15(7-8), 563–566. (7) Zheng, S. P.; Ross, E.; Legg, M. A.; Wirth, M. J. J. Am. Chem. Soc. 2006, 128(28), 9016–9017. (8) Tetreault, N.; Miguez, H.; Yang, S. M.; Kitaev, V.; Ozin, G. A. AdV. Mater. 2003, 15(14), 1167. (9) Vekris, E.; Kitaev, V.; von Freymann, G.; Perovic, D. D.; Aitchison, J. S.; Ozin, G. A. AdV. Mater. 2005, 17(10), 1269. (10) Yan, Q. F.; Zhou, Z. C.; Zhao, X. S.; Chua, S. J. AdV. Mater. 2005, 17(15), 1917. (11) Yan, Q. F.; Chen, A.; Chua, S. J.; Zhao, X. S. AdV. Mater. 2005, 17(23), 2849.
using a centrifuge.12 Park et al. patterned channels onto an injection-molded polymer chip and then utilized a self-assembly technique to pack the channels with silica spheres.13 Harrison et al. packed spheres in enclosed micropatterns using electrokinetics.14 These methods do not produce raised CCF patterns, but instead channels packed with colloids. Additionally, Stebe has shown that colloidal particles can be deposited in predefined patterns if the hydrophobicity of the surface is selectively tuned.15,16 Using this technique it is possible to isolate groups of one to four spheres, or to produce stripes of packed spheres. Another method to create line defects in colloidal monolayers is to use high energy electromagnetic fields. Yu et al. used a pulsed Nd:YAG laser to remove single polystyrene spheres at a time in various patterns.17 In this work the spheres were immobilized on the substrate via DNA hybridization, although it is not clear if this is a requisite for patterning. This technique only removed one sphere at a time but by rastering the laser over the substrate it was shown that a line of spheres could be removed. This patterning has a resolution on the order of the diameter of the spheres, which was 1.8 µm. Patterning with light is limited by the diffraction limit governed by the wavelength and numerical aperture of the optics. One instrument that is used frequently for general nanoscale patterning and micromachining is the focused ion beam (FIB).18-20 The FIB instrument is related to the scanning electron microscope in that both are capable of imaging samples with a spatial resolution better than 100 nm. However, unlike the electron beam, the ion beam (usually gallium) is inherently destructive in that it removes material as the massive (relative to an electron) gallium ions collide with and displace the atoms in the sample. (12) Lee, S. K.; Yi, G. R.; Yang, S. M. Lab Chip 2006, 6(9), 1171–1177. (13) Park, J.; Lee, D.; Kim, W.; Horiike, S.; Nishimoto, T.; Lee, S. H.; Ahn, C. H. Anal. Chem. 2007, 79(8), 3214–3219. (14) Oleschuk, R. D.; Shultz-Lockyear, L. L.; Ning, Y. B.; Harrison, D. J. Anal. Chem. 2000, 72(3), 585–590. (15) Fan, F. Q.; Stebe, K. J. Langmuir 2004, 20(8), 3062–3067. (16) Fan, F. Q.; Stebe, K. J. Langmuir 2005, 21(4), 1149–1152. (17) Yu, P.; Kim, S. J.; Marcus, H. L.; Papadimitrakopoulos, F. J. Mater. Sci. 2008, 43, 803–812. (18) Jud, P. P.; Nellen, P. M.; Sennhauser, U. AdV. Eng. Mater. 2005, 7(5), 384–388. (19) Martelli, C.; Olivero, P.; Canning, J.; Groothoff, N.; Gibson, B.; Huntington, S. Opt. Lett. 2007, 32(11), 1575–1577. (20) Young, R. J. Vacuum 1993, 44(3-4), 353–356.
10.1021/la801772q CCC: $40.75 2008 American Chemical Society Published on Web 08/20/2008
FIB Patterning in Colloidal Crystal Films
Therefore, the FIB has both the required precision and power to mill nanometer-scale patterns. Although the milling operations presented here are limited to resolution of a single sphere diameter (∼300 nm), it is possible to create patterns having dimensions smaller than single spheres. The minimum resolution achievable with the FIB is 5 nm and can be considered an absolute lower limit on patterning. The spatial resolution of the FIB is limited primarily by chromatic aberrations caused by the energy spread of the ions, as well as the complex interactions between the incident ions and target atoms. Extended milling times generally cause defects to appear larger due to beam drift. Thus, in theory one could use a finely focused beam to create point defects with diameters on the order of 10 nm. Woldering et al. used FIB to mill cylindrical defects having diameters between 75 and 300 nm into individual spheres in a silica CCF.21 Snoeks et al. demonstrated through various experiments that Xe4+ ion irradiation of silica spheres at 90 K renders them ellipsoidal; the investigation included studies of both free particles and colloidal crystals.22,23 Van Dillen et al. conducted similar studies at 77 K using Au as well as Xe ions.24,25 In this work we used FIB milling for direct patterning in sintered silica colloidal crystal films. The FIB patterning has a resolution on the order of individual spheres, which in this work are between 300 and 500 nm in diameter. In this paper we describe how the beam may be used to create cavities with solid walls that could function as fluid reservoirs or optical lenses. We show how the porosity of these walls can be tuned by adjusting the path of the FIB. We investigate the dependence of the volume of material removed per unit time on beam current. Finally, we demonstrate several prototype structures created using FIB including isolated multisphere packing arrangements, staircase-shaped cavities having variable depth, and cross-shaped fluidic devices. FIB direct patterning also has applications in colloidal crystal based lithography, integrated photonic devices, optofluidic devices, and micrototal-analytical systems.
Experimental Section Materials. We used silica microspheres (Bangs Laboratories, Fishers, IN) with a mean diameter of 305 nm as measured using dynamic light scattering. Prior to CCF synthesis, the spheres were calcined at 300, 450, and 550 °C for 12 h at each temperature. This treatment removes residual ethanol and water from the colloids; CCFs that are sintered without first calcining the spheres are often prone to cracking.26 The substrates for the colloidal crystals were 1 × 1 in.2 quartz microscope slides (GM Associates, Oakland, CA). Synthesis of Colloidal Crystal Films. CCF may be synthesized using a variety of routes, the most common being evaporationinduced self-assembly (EISA),1,26 convective flow coating,27,28 and spin-coating.29-31 In this work we synthesized the films primarily (21) Woldering, L. A.; Otter, A. M.; Husken, B. H.; Vos, W. L. Nanotechnology 2006, 17(23), 5717–5721. (22) Snoeks, E.; van Blaaderen, A.; van Dillen, T.; van Kats, C. M.; Brongersma, M. L.; Polman, A. AdV. Mater. 2000, 12(20), 1511–1514. (23) Snoeks, E.; van Blaaderen, A.; van Dillen, T.; van Kats, C. M.; Velikov, K.; Brongersma, M. L.; Polman, A. Nucl. Instrum. Methods Phys. Res., Sect. B 2001, 178, 62–68. (24) van Dillen, T.; Polman, A.; Fukarek, W.; van Blaaderen, A. Appl. Phys. Lett. 2001, 78(7), 910–912. (25) van Dillen, T.; Snoeks, E.; Fukarek, W.; van Kats, C. M.; Velikov, K. P.; van Blaaderen, A.; Polman, A. Nucl. Instrum. Methods Phys. Res., Sect. B 2001, 175, 350–356. (26) Van Le, T.; Ross, E. E.; Velarde, T. R. C.; Legg, M. A.; Wirth, M. J. Langmuir 2007, 23(16), 8554–8559. (27) Prevo, B. G.; Hwang, Y.; Velev, O. D. Chem. Mater. 2005, 17(14), 3642– 3651. (28) Prevo, B. G.; Velev, O. D. Langmuir 2004, 20(6), 2099–2107. (29) Jiang, P.; McFarland, M. J. J. Am. Chem. Soc. 2004, 126(42), 13778– 13786. (30) Jiang, P.; Prasad, T.; McFarland, M. J.; Colvin, V. L. Appl. Phys. Lett. 2006, 89(1), 011908.
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Figure 1. Reflectance spectra of a colloidal crystal film at an incidence angle of 80°. The film is made up of approximately 10 layers of 444 nm silica spheres. At this angle, the film appears green under white light illumination.
using EISA. Calcined spheres were dispersed in ethanol at a volume fraction of 0.25% (0.1 g silica/20 mL ethanol, assuming a density of silica of 2.04 g/cm3) using an ultrasonic cleaner (Bransonic, Danbury, CT) for at least 2 h. The quartz slides were washed with Tween 20 detergent, rinsed with DI water, and immersed in a Nochromix oxidizer/sulfuric acid solution (Godax Laboratories, Cabin John, MD) overnight. After immersion, slides were thoroughly rinsed with DI water and dried in a stream of nitrogen. They were then placed upright in the colloidal suspension in an incubator at 50 °C for 24 h. The spheres, driven by capillary action, self-assembled into a face-centered cubic colloidal crystal film typically 10-20 layers thick. After deposition, the films were sintered at 900 °C for 12 h.26 After synthesis, the films appear green, red and blue under white light illumination, depending on viewing angle. A typical reflectance spectrum is shown in Figure 1. Prior to SEM and FIB operations, the films were coated with a layer of carbon approximately 300 nm thick to alleviate charging. After the milling experiments, the carbon was removed by annealing in an oxygen-rich furnace at 700 °C for 8 h. Milling Experiments. All milling was performed using a Nova 200 NanoLab dual-beam FIB/SEM system (FEI Company, Hillsboro, OR) consisting of an electron beam and a Ga+ ion beam. The approximate theoretical resolutions of the electron beam and ion beam are, respectively, 1.5 and 7 nm. The milling was performed with the ion beam accelerated at a potential of 30 kV and at various beam currents. The stage was tilted so that the ion beam was normal to the substrate during milling. For all experiments, the dwell time, i.e., the duration the beam stays in a single location, was 1 µs, and the overlap, namely the amount of shared area between adjacent milling locations, was 50% in both the x and y directions (see Figure 2 for coordinate system definition). After milling, the patterns were imaged using the electron beam accelerated at a potential of 5 kV and a beam current of 98 pA.
Results and Discussion Two milling techniques were employed to create the patterns. The first was rectangular milling (RM), which removes successive layers of material parallel to the substrate. The ion beam continually scans a certain number of times in the user-defined pattern, with the number of passes determined by the input depth and the material into which the FIB is configured to mill. The RM technique was used to create the trenches and fluidic channels shown in Figures 3, 4, 6, and 7. The second technique is the cleaning cross-section (CCS), in which the ion beam removes a series of slices perpendicular to the substrate, starting at one end of the pattern and ending at the opposite end. This technique produced the porous walls seen in Figure 4a-d. Schematics of the two patterning techniques are shown in Figure 2a and b. Figure 3 shows an example reservoir and channel created using the RM mode. The reservoir is an 8 µm by 8 µm square, (31) Mihi, A.; Ocana, M.; Miguez, H. AdV. Mater. 2006, 18(17), 2244.
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Figure 2. (a) Schematic of rectangular milling. The beam scans in either a raster or serpentine pattern (serpentine shown) in the user-defined design across the surface to be milled. The beam removes successive x-y layers until a final, user-defined z-depth is reached. (b) Schematic of cleaning cross section. In this case, the beam scans in the x direction at a constant value of y until the final z-depth is attained for that value of y. The beam then commences scanning over the adjacent y-slice. In this figure, the negative y-face of the final rectangular cavity would appear porous because the material removed to make the cavity would be re-deposited on the other three walls. In both figures, the solid black line indicates the path already followed by the ion beam. The dotted black line indicates the future path of the ion beam. The dotted gray lines indicate layers to be removed (the thicknesses of the layers are exaggerated for clarity).
Figure 3. Scanning electron micrograph of fluid reservoir connected to channel. The sides of the cavity are solid. The large square area could be used as a fluid reservoir that opens into the fluidic channel to the left. The channel and reservoir were patterned with a beam current of 3 nA. The reservoir is an 8 µm × 8 µm square. The channel is a rectangle of dimension 3 µm × 30 µm (not all shown). The measured milling depth for both features was approximately 7.9 µm. Total milling time for reservoir was approximately 7 min, and for the channel approximately 11 min.
the channel is 3 µm wide and 30 µm long, and both patterns are 7.9 µm deep. The reservoir required approximately 7 min to mill, and the channel required approximately 11 min. This time could be reduced by using a higher beam current than was used (3 nA). CCFs are close packed porous films. The sintered films are robust and derive their mechanical strength by virtue of the sintering-induced formation of nanoscale silica (SiO2) necks that connect them to their neighboring spheres.26 After sintering the films remain porous. However, as can be seen, the sides of the square trench and channel in Figure 3 are smooth and solid. The solid wall is produced by redeposition of the SiO2 material sputtered from the interior of the cavity. Redeposition is a materialdependent phenomenon that scales with beam current.32 That is, higher beam currents sputter more material from the same milling volume. (32) Prenitzer, B. I.; Urbanik-Shannon, C. A.; Giannuzzi, L. A.; Brown, S. R.; Irwin, R. B.; Shofner, T. L.; Stevie, F. A. Microsc. Microanal. 2003, 9(3), 216– 236.
The fundamental difference between the RM and CCS modes is the path the ion beam follows. As a result, sputtered material redeposits on different faces of the cavity in different amounts depending on the milling mode. In the RM mode, since the beam scans over the entire pattern in each pass, each wall of the cavity is exposed evenly. Since ejected ions sputter isotropically, the four walls are coated evenly with redeposited material. Conversely, in the CCS mode, the beam continually scans over individual lines, milling all the way to the desired z-depth before moving to the next line. Accordingly, the positive y-face in Figure 2b would be the first surface to be completely exposed. Since the negative y-face is not exposed until the end of the milling operation, the SiO2 sputtered from the cavity redeposits on the positive y-face and the parts of the x walls that are exposed. The cleaning cross section thus ensures that the least amount of material is deposited on the negative y-face, leaving it porous. The CCS technique can create cavities with porous walls, i.e., cross sections that are not solid. This is accomplished most easily by first milling a large rectangular trench using RM, and then using CCS to remove layers of material off of one of the faces. Figure 4a and b shows a rectangular trench with a porous niche that was milled after the trench. Figure 4c and d shows two views of a trench with three solid sides and one porous side. Again, the large rectangular trench was milled first using RM and then a CCS operation was performed on the far side, creating a porous boundary. Figure 4a-d demonstrates the ability of the user to control the porosity of a milled surface by choosing the milling mode. To better understand the dependence of material removal rate on beam current, we milled a series of square cavities and recorded the exact volumes removed and the required time. Figure 5 shows a plot of the volume removed per unit time as a function of the beam current. The volume removed by the ion beam per unit time is related to a material-dependent quantity known as the sputtering yield, which is defined as the average number of sample atoms ejected per incident ion.32 Sputtering yield does not depend on beam current. However, the higher the beam current, the more ions bombard the sample and so the rate of milled material increases linearly with beam current. The depth of a milled pattern is controlled by the number of passes of the ion beam. The amount of material removed in each pass is a function of the target material, beam current, pitch, dwell time, and overlap. Control of cavity thickness is achieved by tuning these parameters to the target material. From our experiments we have found that the minimum single pass depth
FIB Patterning in Colloidal Crystal Films
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Figure 4. Scanning electron micrographs of solid and porous walls created with the RM and CCS milling modes. Milling time for all operations was under 10 min. (a) Fluid reservoir (created with RM) with porous section (created with CCS). (b) High magnification of (a). (c) Fluid reservoir with porous boundary. Reservoir and porous boundary were again created with RM and CCS, respectively. (d) High magnification of (c).
Figure 5. Dependence of the mean rate of material removed per unit time (V/t) as a function of the beam current (I). All measurements were taken with the beam at a voltage of 30 kV. Material removal rates were estimated by dividing the total volume of the cavity by the time required to mill. Cavities were created using both RM and CCS modes.
cut of a silica CCF at a low beam current (50 pA), pitch 16.5 nm, 50% overlap in the x and y directions, and dwell time of 1 µs, is approximately 0.365 nm. Below we demonstrate the ability to mill cavities of variable thickness. Figure 6 shows a staircaseshaped cavity with depths of 2, 4, and 6 µm (from left to right). Figure 7 shows the capability of the FIB to produce patterns in CCFs such as microfluidic channels (e.g., cross-shaped channels). All features were created with the RM operation with a beam current of 1 nA. The reservoirs are 5 × 5 µm2 square and all have a depth of 6.5 µm. The north, south, east and west channels are all 15 µm long. Milling time was approximately 4 min for each of the individual reservoirs and approximately
Figure 6. Staircase-shaped cavity created by milling to three different depths. From left to right, the three cavities have depths of 2, 4, and 6 µm, respectively. The unique appearance of the cavity bottoms results from the combined effects of partially milled spheres and redeposited material.
5 min for each channel, again at a beam current of 1 nA. Higher beam currents can decrease milling times considerably. Using the FIB it is possible to mill almost any 2-dimensional pattern the user can design. In addition, by using multiple milling operations it is also possible to form layered 2-dimensional patterns.
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Figure 7. Scanning electron micrograph of a prototype cross-shaped fluidic device milled using FIB. Each reservoir is a 5 × 5 µm2 square. All patterns have depths of approximately 6.5 µm. Milling time was approximately 4 min for each of the individual reservoirs and approximately 5 min for each channel.
Finally, we have also investigated the use of FIB to generate small clusters of spheres. Figure 8 shows a diamond shaped cluster of four spheres that were isolated by carefully removing the surrounding spheres. This specimen was produced using flowcoating, which is a reliable means of quickly generating a monolayer of colloids.28 It can be seen that the spheres have deformed from their initial spherical shape and appear somewhat ellipsoidal. We attribute this to redeposition of milled material on the outward-facing sides of the spheres during milling.
Conclusions In this paper we have demonstrated that FIB milling can be used to create submicron scale patterns in sintered silica colloidal crystal films. We have demonstrated patterning of simple rectangular cavities with both solid and porous boundaries, fluidic channels, and isolation of a small number of packed spheres. The ion beam can pattern sintered films of individual submicron size spheres and create patterns that cover up to 40 µm in less than 15 min. In addition to silica, the methods described in this paper are applicable to any material which can be milled with a FIB, such as silicon, gold, aluminum, ferric oxide, poly(methyl methacrylate), polystyrene,33 gallium arsenide, indium(III) phosphide, silicon nitride, and so on. The experiments in this work indicate that the amount of redeposited material on the surface of a milled cavity determines whether the surface will be porous or solid. The ion beam sputters material off the surface isotropically. If the wall is exposed, material will redeposit on it. Since the rectangular mill exposes (33) White, H.; Pu, Y.; Rafailovich, M.; Sokolov, J.; King, A. H.; Giannuzzi, L. A.; Urbanik-Shannon, C.; Kempshall, B. W.; Eisenberg, A.; Schwarz, S. A.; Strzhemeckny, Y. M. Polymer 2001, 42(4), 1613–1619.
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Figure 8. Scanning electron micrograph of a group of spheres isolated using FIB. The image was created by milling an annular cavity around the four spheres at a beam current of 0.3 nA. Milling time was less than 5 min. The ability to isolate individual spheres on a substrate introduces the possibility of milling arrays of individual spheres or sphere clusters. Note the slightly ellipsoidal shape of the spheres, which is thought to be due to redeposited material.
all four walls of a cavity in a single pass, all four walls are rendered solid. In contrast, the cleaning cross-section mode results in porous walls because it does not expose the final face until it removes the very last slice, meaning that the redeposited material is largely found on the other three walls. The RM and CCS modes can be used in conjunction to pattern nearly any twodimensional structure with either solid or porous walls. Milling operations can be run in series to produce three-dimensional structures as long as they can be accessed from the top surface from which the ion beam originates. Future work will focus on developing optofluidic devices using FIB milling of colloidal crystal films. Acknowledgment. J.L.M. gratefully acknowledges David Wright, Grant Baumgardner, and Karl Weiss at the John M. Cowley Center for High Resolution Electron Microscopy at Arizona State University for assistance with the FIB and useful discussions. The authors also thank Dr. Lucille Giannuzzi of FEI Company for valuable insights on the mechanics of FIB milling. We also appreciate John Lemon, Dr. Suping Zheng, and Tomika R.C. Velarde in Dr. Mary Wirth’s research group at the University of Arizona for their assistance in preparing colloidal crystal films. P.M.W. thanks Dr. Bryan D. Vogt for his assistance with the ellipsometer. This work was sponsored by an NSF CAREER Award (J.D.P., Grant No. CBET-0747917) with William Wendell Schultz as grant monitor and an Arizona Board of Regents Biomedical Collaborative Grant. J.L.M. and P.M.W. were partially supported by NSF Graduate Research Fellowships (NSF GRF). LA801772Q
