Fabrication of Free-Standing, Self-Aligned, High-Aspect-Ratio

Nov 3, 2016 - Free-standing, self-aligned, high-aspect-ratio (length to cross-section, up to 15.5) waveguides that mimic insects' ommatidia are fabric...
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Fabrication of Free-Standing, Self-Aligned, High-Aspect-Ratio Synthetic Ommatidia Brian M. Jun, Francesca Serra, Yu Xia, Hong Suk Kang, and Shu Yang* Department of Materials Science and Engineering, University of Pennsylvania, 3231 Walnut Street, Philadelphia, Pennsylvania 19104, United States S Supporting Information *

ABSTRACT: Free-standing, self-aligned, high-aspect-ratio (length to cross-section, up to 15.5) waveguides that mimic insects’ ommatidia are fabricated. Self-aligned waveguides under the lenses are created after exposing photoresist SU-8 film through the negative polydimethylsiloxane (PDMS) lens array. Instead of drying from the developer, the waveguides are coated with poly(vinyl alcohol) and then immersed into a mixture of PDMS precursor and diethyl ether. The slow drying of diethyl ether, followed by curing and peeling off PDMS, allows for the fabrication of free-standing waveguides without collapse. We show that the synthetic ommatidia can confine light and propagate it all the way to the tips. KEYWORDS: self-aligned, high-aspect-ratio micropillars, free-standing, ommatidia, waveguides nsect compound eyes are known for their wide field-of-view, detection of patterns of polarization, construction of 3D images, and motion sensing with potential applications including display, defense, and medical devices.1−3 This is because the eyes consist of millions of imaging units called ommatidia. Each ommatidium has a micron-sized lens, the cornea, covering the outer surface and a crystalline cone that converges light onto the tip of the rhabdom surrounded by photoreceptor cells and pigment cells. The rhabdom is a rodlike tube, consisting of interdigitating fingerlike microvilli that can detect and guide the light. Longer rhabdoms (∼150 μm in length in many insect vision systems) are able to absorb a larger amount of photons. For example, dragonflies have much longer rhabdoms than Drosophila, enabling them to absorb most of the light while Drosophila eyes can only absorb ∼26%.3 Synthetic ommatidia with self-aligned, high-aspect-ratio (HAR) waveguides have been fabricated with an aspect ratio (AR = length/lateral dimension) up to 100 using microlenses to guide the light profile in photolithography.2,4,5 These HAR waveguides are relatively stable as they are fabricated to be either attached to a flat surface on both ends or embedded in a matrix of less cross-linked photoresist. However, even if both ends of the HAR structures are attached to a flat surface, they are vulnerable to external forces or infiltrating liquids since only a small area of the waveguides is attached to the surface. For waveguides embedded in the polymer matrix, the refractive index contrast between the waveguides and the surrounding medium is small. Thus, light confinement within the waveguides is decreased. It will be highly advantageous that the HAR waveguides are free-standing in air to maximize the refractive index contrast for waveguiding. Liquids of different refractive index can then be introduced to replace air to dynamically tune

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the waveguiding properties.6,7 Typically, free-standing HAR micro- and nanostructures are susceptible to external forces, for example, capillary force when solvent evaporates from the structure, adhesion force between each other, or external mechanical forces,8 which make them bundle up or collapse together. Here, we fabricate free-standing HAR waveguides from SU-8 with AR up to 15.5 by a sequence of surface modification, solvent exchanges, and repeated molding. In the last step of the waveguide fabrication, the SU-8 film is immersed in a solution of uncured poly(dimethylsiloxane) (PDMS) in diethyl ether, followed by slow drying of diethyl ether and curing of PDMS. After peeling off PDMS, the HAR waveguides maintain the mechanical integrity for waveguiding studies. This method avoids capillarity induced pattern collapse in a typical solvent drying process and does not require critical drying. Optical characterization shows that light can be confined and transmitted through the core of the high refractive index SU8 waveguides with most of the light concentrated at the tips. As the waveguide length increases, the overall light intensity coming out of the waveguides decreases. The fabrication of the microlens array is illustrated in Figure S1a by replica molding into SU-8 photoresist, followed by thermal reflow and UV curing using a method similar to the literature9,10 (see details in the Supporting Information). As seen in Figure S1b, an array of hemispherical microlenses with a radius (r) of ∼7.5 μm and height (h) of ∼8 μm was obtained. Although microlenses can be directly fabricated in photoresist, Received: August 15, 2016 Accepted: October 31, 2016

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DOI: 10.1021/acsami.6b10215 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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solvents (e.g., isopropanol, IPA) used in our fabrication processes. To circumvent these issues, we made several modifications in the fabrication process, which are critical to achieve freestanding waveguides, including the surface treatment of the PDMS substrate, submerging the waveguides in PDMS prepolymer in diethyl ether, followed by drying off diethyl ether and curing the PDMS prepolymers to remove the stress built during the typical solvent drying process (see Figure 1). As a result, we successfully fabricated HAR waveguides with aspect ratios ranging from 8 to 15.5 (Figure 2). The aspect ratio was measured as the distance from the focal point of the microlens to the tip (L) divided by the diameter of the waveguide (d). First, we functionalized the negative PDMS microlens template by UV ozonolysis (UVO) treated (for 1 h), followed by immersing in a mixture of poly(vinyl alcohol) (PVA), oxalic acid, and water in a 1:10:500 mass ratio (Figure 1). After drying the microlens array, SU-8 was spin-coated on top of it, followed by soft bake at 55 °C for 2 h and at 95 °C for 15 min to completely remove the solvent, cyclopentanone. We note that it is important to treat the oxidized PDMS negative lens with the aqueous solution of PVA and oxalic acid. Without this surface treatment, the SU-8 photoresist tended to dewet from PDMS, making it difficult to spread SU-8 uniformly. When the surface was functionalized with PVA only, SU-8 could spin coat on PDMS template evenly but would detach from PDMS surface after immersed in the developer, PGMEA, for a few minutes. If PDMS was treated with oxalic acid only, then the surface was not as hydrophilic as PVA-treated surface, and the coating of photoresist was uneven. The coating of the mixture of PVA and oxalic acid (1:10 mass ratio) resulted in both even spin coating and strong adhesion of SU-8 to PDMS. The latter could be attributed to Fischer esterification between hydroxyl groups from SU-8 and oxalic acid. During the UV exposure step, hydroxyl groups are formed because of ringopening reactions of the epoxide groups on SU-8. Meanwhile, oxalic acid groups adsorbed on UVO treated PDMS can form covalent bonds with silanol groups on PDMS activated by UVO treatment.15 With surface treatment with PVA only, hydroxyl groups from SU-8 can form hydrogen bonding with PVA, but they are not as strong as the covalent bonds between oxalic acid and SU-8. Therefore, after immersing in the developer for a prolonged period, SU-8 film would come off from PDMS. Of course, the adsorbed polar groups on PDMS could also improve the adhesion strength. It has been shown that SU-8 can stay on the plasma treated PDMS even after the SU-8 film is immersed in PGMEA and IPA for a long period of time, which normally will delaminate because of mechanical stress built up between the SU-8 layer and the substrate in these liquids.16 After fabrication of the self-aligned waveguides by UV exposure of the SU-8 film through microlenses, we removed the uncured SU-8 by developer propylene glycol monomethyl ether acetate (PGMEA), followed by rinsing in IPA. Instead of drying off the film, which could collapse the HAR waveguides due to capillary force, we replaced IPA with the uncured PDMS/ diethyl ether solution, followed by slow evaporation of diethyl ether. Importantly, diethyl ether does not seem to have swelled the PDMS substrate beneath SU-8, possibly due to the PVA barrier. We then cured the PDMS with the waveguide structures embedded to prevent the undesired surface tension effect during solvent drying process without the use of a critical dryer. We found that higher degree of cross-linking of SU-8 was

followed by thermal reflow, the obtained lenses are typically very thin (e.g., h = 2−3 μm with r = 100 μm11). In contrast, the soft lithography method offers more flexibility to fabricate microstructures of different size and from different materials. For a thin, plano-convex lens, the focal length is determined by f=

n1(r 2 + h2) / 2h , n2 − n1

where n1 is the refractive index of the

surrounding medium, and n2 is the refractive index of the lens. In compound eyes, the diameter of the corneal lenses is typically small, e.g. 25 μm. Therefore, for a fixed r to achieve a larger f, lens with larger h will be preferred. Here, we can finetune both r and h easily in the original PDMS mold to fabricate the desired microlens arrays. We then replicated the SU-8 microlens array to the negative PDMS template, followed by coating of SU-8 photoresist of different thickness to create free-standing, self-aligned waveguides of different length (Figure 1, see experimental details in

Figure 1. Free-standing waveguide fabrication process. (A) Glass slide with an array of negative PDMS lenses. (B) SU-8 is spin-coated on top of the PDMS template. (C) Self-aligning waveguides beneath the lenses are fabricated and embedded in uncured SU-8. (D) Sample is developed in PGMEA. (E) Sample is placed in a mixture of 5% vol. PDMS precursor and curing agent diluted by diethyl ether. (F) Sample embedded in PDMS. (G) Free-standing, self-aligned waveguides.

the Supporting Information). Because of the increased mechanical compliance, HAR structures tend to collapse or bundles on each other under capillary action as the structures dry from a solvent, or by van der Waals interactions between themselves, or under external forces such as mechanical compression.8 To minimize capillary force, the most common method is to dry the structures in supercritical carbon dioxide (SC CO2), which has zero surface tension. However, liquid CO2 is highly soluble in PDMS,12 which is used in our study as substrate for molding, leading to swelling and peeling off of the PDMS from the SU-8 microlens template. Other methods, such as solvent exchange with a low surface tension solvent, e.g., perfluorohexane,13 and optimizing the soft bake conditions and drying under hexane,14 have been suggested. However, perfluorohexane and hexane have nonzero surface tension, and perfluorohexane has a poor miscibility with the polar B

DOI: 10.1021/acsami.6b10215 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. SEM images of waveguiding pillars with different dimensions. (a) L = 45 μm (Sample A), diameter 5 μm, aspect ratio 8. (b) L = 70 μm (Sample B), diameter 6 μm, aspect ratio 11.7. (c−d) L = 85 μm (Sample C), diameter 5.5 μm, aspect ratio 15.5. (d) Side view of c.

the four pillars surrounding it,21 which could collapse the pillars if the capillary force becomes dominant. In our system, the bubbles could escape from the pillar surface easily due to the low viscosity of the PDMS/diethyl ether mixture before curing. It is also noted that we coated SU-8 waveguides with PVA before immersing into PDMS/diethyl ether solution. As seen in Table S2, the RED values between diethyl ether and EPON 1001 are smaller than those between diethyl ether and PVA, thus diethyl ether bubbles are less likely to form or stay on the waveguide surface covered with a PVA layer. We then investigated light coming through the SU-8 waveguides at different focal planes, including near the bottom of the micropillars (H = 0), at the focal points of the lenses (H = f), and at the tip of the micropillars (H = L) (see Figure 3). We tested three different pillar length, L ≈ 45, 70, and 85 μm, respectively, under the same imaging conditions. In all cases, a bright spot appeared at the tip of the waveguide (Figure 3c, e, and g), a much longer distance than the lenses’ focal length, f, which is ∼29 μm (Figure 3b), indicating that the light focused through the lens propagates within the waveguides. The bottom of the waveguides appeared dark (Figure 3a, d, and f). Light intensity profiles across the tips of the synthetic ommatidia are compared in Figure 3h. It can be seen that light is attenuated in the longer pillars (L = 85 μm) with ∼50% loss of the transmitted light coming out of the pillars in comparison with the shorter ones (L = 45 and 70 μm). As seen in Figure 3i, j, the 70 and 85 μm long waveguides appear bright throughout their length all the way to the tips. When light was shone onto the waveguide samples that were tilted nearly parallel to the sample holder on the microscope, the waveguiding effect was more apparent. Last, we examined the light guided through a single synthetic ommatidium under the optical microscopy by sending a white light and a green laser light at the incidence angle of 5° with respect to the waveguide axis (Figure 4). The direction of laser light is marked with a yellow arrow. For light to be confined within the waveguide, the angle of incidence on the wall of the waveguide should be larger than the critical angle, θc, above

not sufficient to prevent collapse of the waveguides. Here, curing liquid PDMS with residual diethyl ether surrounding the SU-8 waveguides seemed not to build up the stress between SU-8 and PDMS. After peeling off PDMS film, we obtained the free-standing HAR waveguides with AR up to 15.5 (diameter 5.5 μm, length 85 μm, see Figure 2c, d). Recently, we have shown to recover clustered elastomeric HAR nanopillars by demolding, where the pull off force can overcome the adhesion between pillars.17 The low affinity between PVA treated SU-8 and the PDMS/diethyl ether solution also helped the separation. Here, we calculated the relative energy difference (RED) using the Hansen solubility parameters to evaluate the affinity between polymers and solvents.18 Since the Hansen solubility parameter of SU-8 is not available, the parameter values of EPON 1001 (Shell Chemical Corp.), a bisphenol A epoxy resin, is used as an approximation of SU-8 as did in literature,19 and the value for PDMS is taken from literature.20 As seen in Tables S1 and S2, the RED value between SU-8 and diethyl ether is 1.3, and that between SU-8 and PDMS is 1.42 (see detailed calculation in the Supporting Information). RED larger than 1.0 indicates low affinity between the pairs, thus PDMS chains will not enter the SU-8 matrix. It is noted that we did not simply take the developed SU-8 sample out of IPA and dipped it into a thick layer of undiluted PDMS, which it would have been very difficult to evaporate the solvents (PGMEA and IPA) remained between SU-8 pillars. Instead, we submerged the wet SU-8 waveguides into 5 vol % PDMS/diethyl ether solution (initial thickness ∼1.5 cm). After evaporation of the diethyl ether, the PDMS layer became ∼300 μm thick. We then cured the PDMS. If PDMS was cured with diethyl ether or the layer of PDMS was too thick, diethyl ether would be trapped at PDMS and SU-8 interface, causing collapse of the SU-8 waveguides upon peeling off the PDMS layer. Defects could also be introduced because of bubble formation during drying of diethyl ether, which has a low boiling point (34.6 °C). The SU-8 waveguides provide larger surface area for heterogeneous nucleation of the diethyl ether bubbles. When the bubble grows larger it becomes pinned by C

DOI: 10.1021/acsami.6b10215 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. Optical images of the artificial ommatidia imaged under an optical microscope at different focal planes. (a−c) L = 45 μm, (d, e) L = 70 μm, (f, g) L = 85 μm. (a) Near the bottom of the micropillars, H = 0. (b) At the focal points of the lenses, H = f = 18 μm. (c) Near the tips of the micropillars, H = 48 μm. (d, e) 70 μm waveguides imaged under the same conditions as a and c. (f, g) 85 μm waveguides imaged under the same conditions as a and c. Insets: illustrations of the locations of the focal planes. (f) Light intensity profile across the tip of two artificial ommatidia seen from c, e, and g. The intensity at the tips of the longer pillars is significantly lower. (i, j) Tilted micropillars with L = 70 and 85 μm, respectively.

Figure 4. (a) Schematic of the optical setup to image the light guiding through a single ommatidium under the optical microscopy. (b) Optical image of light guided through a single waveguide. (c) Green laser light is shown on one waveguide (incident angle of 5° and light intensity of 1 mW/cm2). The light clearly propagates through the waveguide and it is emitted at its tip (brightest spot).

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ACS Applied Materials & Interfaces which total internal reflection occurs. θc is defined as sin θc = n2/n1, where n1 and n2 are the refractive indices of optically more dense and less dense materials, respectively. Due to the relatively large refractive index contrast between SU-8 (n = 1.6) and air (n = 1.0), the critical angle is large, 38.7°. As seen in Figures 4b and 4c, clearly light was confined and guided inside the waveguide, further supporting the observation in Figure 3. The gradient laser light intensity across the cross-section of the waveguide (Figure 4c) showed that light intensity is highest at the core of the pillar but decreased moving away from the center, in agreement with previous studies of synthetic ommatidia.5 In biological compound eyes, an important parameter is the acceptance angle, Δρ, over which each ommatidium accepts light to form an image. It is a measure of the angular resolution of the eye and it should be matched by an optimal packing of the ommatidia. The acceptance angle is dependent on the focal length of the lens and the geometrical parameters of the ommatidium, such as the diameters of the rhabdom and of the lens (d and D, respectively).22 It can be expressed, using certain

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is supported by National Science Foundation (NSF)/INSPIRE grant IOS-1343159 (to S.Y.) and the Berkman Fund for Undergraduate Innovation at Penn Engineering (to B.M.J.) The Laboratory for Research on the Structure of Matter (LRSM), Penn’s NSF/MRSEC, Kathleen Stebe’s lab, and Nanoscale Characterization Facility (NCF) at Singh Center for Nanotechnology are acknowledged for facility usages.

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approximations, as Δρ = (d /f )2 + (λ /D)2 , where λ is the wavelength of the incident light. In our system, the acceptance angle is ∼0.24 rad, i.e., 27°. In diurnal insects, this value is smaller and can be down to 2 o, but in some hexapods, it can be a few tens of degrees.22 The main difference lays in the diameter of the rhabdom, which is smaller in most insects, and the focal length of the lenses, which is longer in insects. On the other hand, rhabdom with a wide acceptance angle is common in dark-adaptive eyes.23 In conclusion, we have successfully fabricated free-standing, self-aligned, high-aspect-ratio (up to 16) waveguides that mimic insects’ ommatidia by combination of soft lithography and UV curing. First, microlens array is fabricated by replica molding to SU-8 photoresist, followed by thermal reflow and UV curing. Self-aligned waveguides under the lenses are created after exposing SU-8 film through the negative PDMS lens template replicated from the SU-8 lens array. Instead of drying the SU-8 waveguides from the developer, we show that treatment of PVA and then immersing the waveguides into a mixture of PDMS and diethyl ether help to release the stress. After curing and peeling off PDMS, free-standing, self-aligned HAR waveguides are obtained and bundling is prevernted. The synthetic ommatidia could confine light within the waveguides with the transmitted light propagated all the way to the tips. The acceptance angle of the artificial ommatidia is relatively large in our system, but is still comparable to that of dark-habitat compound eyes. Because our method is based on soft lithography technique, it can be exploited in the future for creation of artificial ommatidia with higher angular resolution and higher refractive index with smaller d and larger D and f by tuning the geometric parameters in the original mold for the lens fabrication. Higher refractive index contrasts could provide the benefits of minimized crosstalk between the waveguides, allowing for better integration into photonic devices.24 We can also actuate fluid11 of different refractive indices to replace air in the surroundings, thus, dynamically tuning their optical properties that mimics insects’ adaptiveness to their habitat.

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Experimental details, illustration of the microlens fabrication process, Hansen solubility parameter, and calculated relative energy difference (RED) values (PDF)

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b10215. E

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DOI: 10.1021/acsami.6b10215 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX