Electrically Modulated Microtransfer Molding for Fabrication of

Letter pubs.acs.org/Langmuir

Electrically Modulated Microtransfer Molding for Fabrication of Micropillar Arrays with Spatially Varying Heights Xiangming Li, Hongmiao Tian, Jinyou Shao,* Yucheng Ding,* and Hongzhong Liu Micro- and Nano-manufacturing Research Center, State Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China S Supporting Information *

ABSTRACT: The ability to generate a large area micropillar array with spatially varying heights allows for exploring numerous new interesting applications in biotechnology, surface engineering, microfluidics, and so forth. This Letter presents a clever and straightforward method, called electrically modulated microtransfer molding (EM3), for generating such unique microstructures from a silicon mold arrayed with microholes. The key to the process is an application of electrically tunable wettability caused by a spatially modulated voltage, which electrohydrodynamically drives a photocurable and dielectric prepolymer to fill the microholes to a depth depending on the voltage amplitude. Using EM3, micropillar arrays with stepwise or continuously varying heights are successfully fabricated, with the diameter scalable to 1.5 μm and with the maximum height being equal to the depth of the high-aspect-ratio (more than 10:1) microholes.

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molding in capillary,17 and so forth. However, all these processes were intended to fill the materials fully into microor nanocavities in a mold, with attempts made exclusively to eliminate the air trapping for a completely conformal duplication. Since the microstructured molds used for these processes have been generated usually by optical or electronbeam lithography and plasma etching, the depth of the microholes is generally uniform, leading to a uniform height for the duplicated micropillar array. So far, very few attempts have been made to address the issue of economically fabricating a large area micropillar array with spatially varying heights. This Letter presents a method, called electrically modulated microtransfer molding (or EM3 shortly), for generating such unique microstructures from a silicon mold arrayed with microholes of high aspect ratio. The key to EM3 is an application of electrically tunable wettability by a spatially modulated voltage. The applied voltage can electrohydrodynamically drives a dielectric liquid to enter microholes of a silicon mold onto a depth depending on the corresponding voltage amplitude. Therefore, depending on the variation of voltage, a micropillar array with stepwise or continuously varying heights can be produced. Since the silicon mold arrayed with deep microholes can be fabricated by conventional photolithography and plasma etching, EM3 can be implemented economically over a large area substrate.

INTRODUCTION Micropillar array is a typical high-aspect-ratio microstructure with many interesting features, including large surface area, large mechanical compliance, and light or electromagnetic modulability, for instance. Taking advantage of these features, researchers have been exploring various technologically applications, such as force sensing in biomechanics,1−4 selective light trapping or transmission in optics or optoelectronics,5,6 patterned cell culture and particle separation in biomedicine,7,8 tunable wetting and fluid manipulation in surface chemistry and microfluidics,9,10 and so forth. So far, all these explorations were based on the use of micropillar arrays with spatially uniform height. A micropillar array with spatially varying heights (or varying aspect ratio) further allows for the above-mentioned physical, chemical, or mechanical features to be programmed spatially over one single substrate. This unique ability may open a new door for imagination and exploration of numerous new-concept technological applications. For example, a surface with spatially distributed mechanical compliances (due to varying pillar bendability) may be useful in a parallel observation on how the varying physical cues affects the cellular adhesion, migration, or differentiation on one single substrate while maintaining a consistent in vitro environment.4,11 A micropillar array of polymer or other rheological substances (organic hybrids, low temperature superplastic metals or alloys, for instance) can be formed economically by various micro- or nanoscale molding processes, such as imprinting lithography,12,13 hot embossing,13 replica molding,14 microtransfer molding,15 capillary force lithography,16 micro© 2013 American Chemical Society

Received: December 17, 2012 Revised: January 20, 2013 Published: January 24, 2013 1351

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Letter

EXPERIMENTAL DETAILS

voltage, with amplitude varying spatially with the needle’s position, is applied between the needle and the silicon mold. The voltage changes the wettability of the mold surface, leading to a prepolymer filling to a voltage-dependent depth along the needle path. The partially filled mold is then brought onto a transparent substrate (Figure 1b), through which a UV light is used to photocure the prepolymer (Figure 1c). Finally, removal of the mold leads to a micropillar array with spatially varying heights transferred onto the substrate (Figure 1d). It should be indicated that in the EM3 process the partial filling of the microholes is reached with a depth essentially depending on the wettability of the prepolymer to the mold surface, based on the fact that a microhole with better wettability tends to allow for deeper filling.18,19 Since the wettability of a dielectric prepolymer on a solid surface can be manipulated electrically as a liquid-dielectrophoretic behavior,20−22 the filling depth of the prepolymer in a microhole can be modulated by changing the electric field. This reasoning lays a foundation for the ability of EM3 to generate a micropillar array with spatially varying heights. Figure 2 shows how the voltage affects contact angle of the prepolymer droplet on a SiO2/Si substrate (Figure 2a−e), and

The liquid dielectric used in our experiment is an UV-curable acrylicbased component liquid, available from Micro Resist Technology GmbH (with a commercial name Ormostamp), which has a viscosity of 0.41 Pa·s and a dielectric constant of roughly 6.8 at room temperature. The substrate is a slide glass. The injection needle was made of a glass tube which was thermally drawn to a microcapillary with an internal and external diameter of 70 and 200 μm, respectively, and coated with Pt by a Denton Vacuum Explorer14 sputter. The molds are made of n-type doped silicon wafers, which have an electric resistivity of ≈0.005−0.015 Ω·cm, and the arrays in molds are fabricated by standard photolithography and inductively coupled plasma (ICP) etching process. The dielectric SiO2 (about 0.8 μm in thickness) coated on the mold is thermally grown. For easy demolding, the microhole arrayed mold is treated in a 1.0 wt % ethanol solution of heptadecafluorodecyltrimethoxysilane (CF3(CF2)7CH2CH2Si−(OCH3)3, FAS) for 3 h and subsequently baked at 150 °C for 10 h. The voltages are supplied by a function/ arbitrary-waveform generator (AGILENTER 33220A) which is bridged to an amplifier/controller (TREK 610E H.V.) with a response rate of 40 V/ms. The scanning electron microscopy (SEM) images are obtained using a HITACHI S-3000N SEM instrument and SU8010 apparatus. The heights of pillars are measured by laser scanning confocal microscopy (LSCM, Olympus OLS4000) with a resolution of 0.01 μm at the Z axis and 0.12 μm at the X and Y axes. The contact angle was measured on a Dataphysics OCA20 system.

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RESULTS AND DISCUSSION Figure 1 illustrates the process for EM3. An injection needle, made of a glass microcapillary and coated with conductive Pt, continuously supplies a photocurable prepolymer while moving relative to a stage and at a constant clearance over a mold arrayed with microholes of high aspect ratio (Figure 1a). A

Figure 2. Influence of an increasing voltage on the contact angle (a−e) and on the filling depth or pillar height (f−j). Voltage: (a, f) U = 0 V, (b, g) U = 50 Vpp, (c, h) U = 100 Vpp, (d, i) U = 300 Vpp, and (e, j) U = 500 Vpp. The mold used here has holes with diameter of 7.5 μm and depth of roughly 42 μm.

how the voltage determines the filling depth of the prepolymer into microholes of a silicon mold coated with SiO 2 correspondingly (Figure 2f−j, Supporting Information Figure S1). The measured contact angles and pillar heights are also plotted versus the voltage in Figure 2k. Obviously, an increasing voltage leads to a decreasing contact angle and therefore allows for an increasing filling depth or pillar height. It is also worthwhile to note that a voltage higher than 500 Vpp can generate a pillar height of roughly 42 μm, which is equal to the microhole depth of the silicon mold. This implies that the liquid prepolymer can fully fill into the microholes at a sufficiently large voltage. The full filling of the prepolymer can

Figure 1. Schematic illustration of the EM3 process. (a) Electrohydrodynamically filling the prepolymer into the mold’s microholes by applying a voltage between the mold and the microcapillary (the inset image), with the latter connected to a micropump with an volumetric prepolymer supply rate of 0.2 μL/s; (b) removing the microcapillary and bringing a transparent substrate onto the filled mold; (c) curing the prepolymer by UV-exposure; (d) removing the mold from the substrate. A constant clearance of 50 μm is kept between the microcapillary and the mold while the mold-supporting stage is horizontally moving at a speed of 0.1 mm/s relative to the microcapillary; the applied voltage is square-waved, with a frequency of 10 Hz and amplitude modulated spatially with the microcapillary’s position. 1352

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also be proved by the visible hemispheric top of the generated pillars (Figure 2j), which conforms to the usually hemispheric bottom of the mold microholes, typical of a deep microhole fabricated by plasma bulk etching.23 Therefore, we can believe EM3 can produce a micropillar array with maximum height equal to the depth of etched silicon microholes. Based on the preceding reasoning and experimental observations, it can be expected that a micropillar array with well controlled height contour can be produced, depending on variation of the voltage with horizontal position of the microcapillary over the mold. Figure 3 shows micropillar arrays with stepwise varying heights by application of a voltage with stepwise changed

Figure 4. SEM images of micropillar arrays with continuously varying heights generated by application of a continuously declining voltage, with the arrows indicating moving direction of the microcapillary. Voltage decreases (a) from 50 Vpp to 0 V, (b) from 100 Vpp to 0 V, (c) from 200 Vpp to 50 Vpp, and (d) from 300 Vpp to 50 Vpp. Scale bars: 10 μm.

Figure 4c and d also shows pillars with continuously varying heights, generated by applying a voltage varying with different declining rate ranges. Figure 5 shows a micropillar array with stepwise heights, generated at smaller size of 1.5 μm in diameter. The tallest

Figure 3. SEM images of micropillar arrays with stepwise varying heights generated by application of a stepwise declining voltage, with the arrows indicating moving direction of the microcapillary. Voltage steps down (a) from 100 Vpp to 0 V, (b) from 150 Vpp to 50 Vpp, (c) from 300 Vpp to 150 Vpp, and (d) from 500 Vpp to 0 V. (e, f) Micropillar array obtained by two neighboring paths of the microcapillary with different voltage steps at the same position. Scale bars: 10 μm. Figure 5. SEM images of a pillar array of 1.5 μm in diameter. (a) Pillar array with stepwise heights generated by application of a stepwise voltage (from 400 Vpp down to 50 Vpp), with the arrow indicating the moving direction of the microcapillary. (b) Enlarged view for the tallest pillars in the array, where the aspect ratio reaches more than 10:1. The used mold here has holes with diameter of 1.5 μm and depth of roughly 16 μm.

amplitudes. Figure 3a shows a micropillar array with a small step of height, from about 9 μm down to 3 μm, obtained by dropping the voltage from 100 Vpp suddenly down to 0 V while the stage was moving. Figure 3d shows a pillar array with a large step of height over a single substrate, that is, from maximum 42 μm down to 9 μm, obtained by dropping the voltage from 500 Vpp suddenly down to 100 Vpp. Moreover, by precisely controlling the moving path of the microcapillary over the mold, a pillar array with complex step-varying heights can be fabricated (Figure 3e and f). On the other hand, if the applied voltage is continuously varying spatially, the filling depth will also vary continuously; therefore, pillar arrays with continuously varying heights can be fabricated, as shown in Figure 4. Figure 4a shows pillars with heights varying from 6 to 3 μm, corresponding to a voltage continuously varying from 50 Vpp to 0 V and with a small declining rate of 25 V/s. When the declining rate gets larger, from 100 Vpp to 0 V at a declining rate of 50 V/s, for instance, the heights of the pillars decrease from 10 to 3 μm (Figure 4b).

pillars have an aspect ratio of more than 10:1 and a height of 16 μm, which is equal to the depth of the mold microholes, indicating that the prepolymer has fully filled the microholes at 400 Vpp. When the diameter of the microholes scales down to submicrometer, such as 500 or 200 nm with aspect ratios of about 4:1, the prepolymer has fully filled the holes even without application of voltage (Supporting Information Figure S2). That is because the size-dependent capillary force has become so strong at the nanoscale that the prepolymer can easily dissolve almost all of the trapped air with a small volume in a nanohole, which can be explained by Henry’s law.24 However, if 1353

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electrode over the mold, leading to a higher productivity. Therefore, a high voltage and a large clearance can be adequate in most cases as long as the applied voltage is not so high as to electrically break down the prepolymer, the SiO2 coating, or the releasing layer coated on the mold surface, all of which are dielectrics. It has also been found in our experiment that the existence of a dielectric SiO2 layer coated on the mold can also play a significant role in determining the prepolymer’s filling (Supporting Information Figure S3).20

the aspect ratio of the nanoholes reaches 10:1 or higher, the volume of the trapped air inside will become larger and is incompletely dissolved into the prepolymer only by the capillary force. As a result, a space will be left in the bottom of the nanoholes to allow for further filling of prepolymer driven by the electrodynamic force. So, a varying filling-depth in nanoholes with an aspect ratio high enough can be achieved by applying a spatially varying voltage for subsequently transferring a nanopillar array with spatially varying height. For a better understanding of the EM3 process, we performed a numerical simulation for the electrohydrodynamic problem involved, based on an air−liquid phase-field formulation presented in our previous publication.25 Figure 6

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OUTLOOK We believe that EM3 has a number of attractive features as a fabrication process for micropillar arrays, including simplicity (the silicon mold can be easily fabricated), productivity (a micropillar array can be generated potentially over a large area by using a conductive injection blade instead of the needle), and capability of a high aspect ratio (the maximum height of pillar is equal to the depth of microholes of the silicon mold). Most importantly, the height of the micropillar array can also be well modulated electrically into various complicated contours, which is a unique feature of EM3, potentially being able to lead to some new uses of the microstructures. The size of the micropillars may be scalable to the submicrometer level so long as the holes in the mold are sufficiently deep.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 6. Numerical snapshots of the prepolymer filling in a microhole arrayed surface. (a) The prepolymer is flowing into the hole along its sidewall, (b) the hole is sealed by the prepolymer with air trapped inside, (c) the trapped air is being compressed while the prepolymer fills further into the hole electrohydrodynamically, and (d) the voltage steps down, resulting in a smaller filling depth of the prepolymer in the subsequent two holes (e).

Figures S1−S3 and their descriptions. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.S.); [email protected]. cn (Y.D.). Telephone: +862983399529.

shows the numerically obtained snapshots for rheological behaviors of the prepolymer over a microhole arrayed mold. Figure 6a shows that the electric field changes the wettability of a liquid prepolymer over the mold (or see Supporting Information Figure S3), driving the prepolymer to enter a microhole along its sidewall initially,26 but can finally seal the deep microhole with air trapped inside (Figure 6b). Then the trapped air is being compressed while the prepolymer fills further into the hole electrohydrodynamically until the liquiddielectrophoretic force acting on the air−liquid interface equilibrates with resistance by the increasing pressure of the trapped air.27 This equilibrium determines the filling depth of the prepolymer (Figure 6c). Since the liquid-dielectrophoretic force on the prepolymer is proportional to the applied voltage, a variation of the voltage leads to a change of the filling depth (Figure 6 d and e). The clearance between the needle tip and the mold surface was maintained at a constant of 50 μm thoughout the experiment. Since the liquid-dielectrophoretic force is influenced by both the applied voltage and the clearance, a variation of the clearance with horizontal position of the needle electrode can also be expected to change the filling depth of prepolymer theoretically. A smaller clearance will allow for application of a lower voltage to generate the electric field strength or liquiddielectrophoretic force required, but results in smaller electrically infuenced zone (EIZ). A small EIZ tends to lead to low producivity because it would require more scanning paths of the needle electrode over a large area substrate. Furthermore, a higher voltage allows for a higher scanning speed of the needle

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially financed by the Major Research Plan of NSFC on Nanomanufacturing (Grant No. 90923040), the National Basic Research Program of China (Grant No. 2009CB724202), and NSFC (Grant Nos. 51005178, 51175417).

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