Nanostructuring Processes Based on

Feb 6, 2014 - ... the electrically induced micro-/nanostructuring process generally uses a prepatterned electrode to generate a spatially modulated el...
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Electrohydrodynamic Micro-/Nanostructuring Processes Based on Prepatterned Polymer and Prepatterned Template Hongmiao Tian, Jinyou Shao,* Yucheng Ding,* Xiangming Li, and Hong Hu Micro- and Nano-manufacturing Research Center, State Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China ABSTRACT: As widely studied, the electrically induced micro-/ nanostructuring process generally uses a prepatterned electrode to generate a spatially modulated electric field on an initially flat polymer surface to produce a nonuniform polymer deformation, leading to a structure with low aspect ratio, positive to the template pattern. However, a variant to this process that is proposed in this paper, which uses a flat electrode over an initially prepatterned polymer, can also generate the spatially modulated electric field. This paper presents a comparative study on the applicability of these two processes for microstructuring with high aspect ratio based on observations from our electrohydrodynamic simulation and experiment and shows that the proposed approach can provide a stronger electric modulation for the same experimental settings, therefore being more suitable for micro/ nanostructuring with high aspect ratio.

1. INTRODUCTION Liquid dielectrophoresis (L-DEP), as an electrohydrodynamic (EHD) phenomenon, has been widely used for research on the motion of polarizable liquid induced by spatially nonuniform electric field.1,2 The phenomenology of L-DEP may be simply stated: dielectric liquid tends to flow so as to collect in the regions of high electric intensity. L-DEP has been extensively studied for a wide variety of applications, such as microreactor,3 droplet-based lens,4 and mirrors,5 etc. On the basis of this EHD behavior, a UV or thermally curable polymer, as a typical dielectric liquid, can be EHD-deformed into micro/nanostructures with a variety of topologies by a spatially modulated electric field,6−8 which has attracted a lot of attention due to the fundamental and technological implications of this micro/ nanostructuring approach in the liquid crystal devices,9 microlens array,10 and so on. So far, researches on electrically induced structuring (EIS) have been using a prepatterned template as an electrode to generate an spatially modulated electric field on a flat polymer which is coated on a conductive substrate as another electrode,11−14 as shown in Figure 1a, where the dielectric polymer below the template protrusions is affected by a large electrostatic force due to a high electric intensity in this region, and tends to be pulled upward to the protrusion underside, resulting in a polymer micro-/nanostructure positive to the template pattern. However, the EIS with a prepatterned template can only fabricate structures with a low aspect ratio,12,15 depending on the amplitude-modulating intensity of electric field and the air gap between the template and substrate for a fixed polymer thickness. For instance, an increased air gap can provide a sufficient growing space for the polymer, but also tends to lead to a decreased amplitude-modulating intensity, being unable to © 2014 American Chemical Society

Figure 1. Electrically induced structuring with a prepatterned template (a) and with a prepatterned polymer (b).

generate an electrostatic pressure (or Maxwell force) enough to drive the polymer upward by overcoming the surface tension and viscous force and deform it into a structure positive to the template pattern. On the other hand, while the amplitudemodulating intensity of electric field resulting from a small air gap can generate an electrostatic pressure enough for the polymer’s full deformation, the growing space for the polymer is limited. Therefore, in both cases of the air gap the electrically induced structuring with a prepatterned template (EIS-PPT) Received: December 1, 2013 Revised: January 5, 2014 Published: February 6, 2014 1433

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Figure 2. Dynamic evolutions for EIS-PPT (a) and EIS-PPP (b), the spatially modulated electric field on the air−polymer interface on the initiation of microstructuring (c), and the final structure for increased air gap by EIS-PPT (d-i) and by EIS-PPP (d-ii).

constant than the air. This will affect critically the subsequent EHD process as a whole, with the EIS-PPP being able to allow for a large air gap ha at which an enough electrostatic pressure can still be produced to pull the polymer upward the upper flat electrode, or to put it in other words, a large aspect ratio for the generated micro/nanostructure can be expected. The prepatterned polymer can be created by a thermal or UV imprint.20,21 It is true that the approach with prepatterned polymer needs an additional step to fabricate the prepatterned features on the polymer film. However, the combination of EIS with an imprinting process for micro/nanostructuring with a high aspect ratio can be technologically meaningful because only a shallowly prepatterned polymer is needed for the EISPPP, alleviating the problems of the large mechanical pressure and the mold removal for structuring with high aspect ratio which are typical in a conventional imprinting process.22 This paper presents a comparative study on the applicability of these two processes for micro/nanostructuring with high aspect ratio based on observations from our electrohydrodynamic simulation and experiment.

would be difficult to produce a micro/nanostructure with high aspect ratio. Recently some approaches have been exploited to improve the aspect ratio of structures duplicated by EIS-PPT based on the amplitude-modulating intensity and the resistant force, by using an active gap tool to increase the separating clearance progressively in the polymer deformation process16 or by using a template prepatterned with metal protrusions to enhance the amplitude-modulating intensity at a large clearance17 or by using a polymer/polymer bilayer to reduce the surface tension,18,19 for instance. However, the improvement on the aspect ratio is limited, which is usually smaller than 1. The spatially nonuniform electric field over a planar area can also be induced between a flat (or unpatterned) electrode and a prepatterned polymer, as introduced in this paper. Similar to EIS-PPT, the corresponding micro/nanostructuring approach can be called electrically induced structuring with a prepatterned polymer (EIS-PPP), as shown in Figure 1b, where each protrusive underside of the shallowly prepatterned polymer will thermally reflow upward under the electric field to reach into contact finally with the flat electrode when the system is heated, finally generating a polymer structure which is a vertical extension of the initial polymer pattern. At a first glance at Figure 1, it seems that the electrohydrodynamic behavior would be the same for the two EIS processes with an identical air gap ha. In practice, at the electric induced initiation of polymer’s deformation, the electric field intensity at the protrusions of the prepatterned polymer for EIS-PPP can be much higher than that at the polymer surface below the protrusive undersides of the patterned electrode for EIS-PPT, due to the fact that the polymer has a much higher dielectric

2. THEORETICAL ANALYSES The motion of the polymer under an external electric field in the EIS process can be depicted by the Navier−Stokes equations, consisting of momentum conservative and mass conservative equations, with the following fashion:23 1434

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Figure 3. (a) Prepatterned template and (b) polymer structure by the EIS-PPT process and (c) prepatterned polymer and (d) polymer structure by the EIS-PPP process.

⎧ ⎛ ∂u ⃗ ⎞ + u ⃗ ·∇u ⃗⎟ = −∇(p0 + pe + pst ) + η∇2 u ⃗ ⎪ ρ⎜ ⎪ ⎝ ∂t ⎠ ⎨ ⎪ + ρg ⃗ ⎪ ⎩ ∇·u ⃗ = 0

2, the electrostatic pressure is determined by the nonuniform electric field on the air−polymer interface incurred by the prepatterned template, where the larger electrostatic pressure Pt1 corresponds to the protrusions of the template, the lower pressure Pt2 corresponds to the cavities, and their difference generates the discrepancy on the hydrodynamic pressure to guarantee the motion of polymer. Similarly, the nonuniform electric field can also be introduced by the prepatterned polymer with the progress evolution shown in Figure 2b, where the Pp1 on the protrusions of the polymer is larger than Pp2 on the valley due to the different air gap and polymer thickness, thus the polymer protrusions will move upward first, resulting in the deformed structure positive to the initial pattern. Parts a and b of Figure 2 demonstrate that the electrically induced structuring formation, consisting of prepatterned template and prepatterned polymer, can both drive the polymer to move upward to generate microstructure positive to the pattern on the template or the polymer from the viewpoint of the deformed geometry, in which, however, the electrically driving pressure ΔP on the air−polymer interface is largely different with the expression as

(1)

Here ρ is the mass density of the air or polymer, u is the fluid velocity, p0, pe, and pst are the atmospheric pressure, electrostatic pressure and surface tension, respectively, η is the dynamic viscosity coefficient, Here the electrostatic pressure pe on the air−polymer interface can be expressed as 1 pe = − ε0εp(εp − 1)Ep 2 2

(2)

with ε0 the permittivity of the free space, εp the relative permittivity of the polymer, and Ep the electric intensity in the polymer domain on the air−polymer interface which can be deduced by Laplace equation with a assumption that the polymer is a purely dielectric liquid, i.e., no free space charge, followed as:24 ∇·(ε0εr E) = 0

(3)

ΔP = Pmax − Pmin =

where εr is the relative permittivity of the air or polymer, E is the electric field on the problem domain that includes Ea and Ep for air and polymer, respectively. Subsequently, the progress evolution of EIS process (consisting of EIS-PPT and EIS-PPP) is demonstrated in Figure 2 based on the above equations by a present numerical model combing electrohydrodynamics and two phase flow of phase field formulation,25 in which the pattern on the template or polymer both has the feature width of 35 μm, the separation of 35 μm, the depth of 25 μm, with an applied voltage of 500 V, and the air gap of 20 μm for the objective of duplicating structures with an identical height on two EIS processes. Figure 2a demonstrates the progress evolution of EIS-PPT, in which the polymer moves upward to the protrusions on the template due to the locally enhanced electric intensity, leading to a structure positive to the template pattern. According to eq

∝ ΔE2

1 ε0εp(εp − 1)(Emax 2 − Emin 2) 2 (4)

where Pmax and Pmin are the maximum and minimum electrostatic pressure on the air−polymer interface, respectively, corresponding to Pt1, Pt2 in Figure 2a and Pp1 and Pp2 in Figure 2b, Emax and Emin are the electric intensity corresponding to the electrostatic pressure, respectively, and ΔE2 equals of Emax2 − Emin2 that is defined as the amplitude-modulating intensity. Consequently, the performance of the electrically driving pressure can be denoted by the amplitude-modulating intensity, which can be deduced from the electrical distributions on the air−polymer interface in the initial stage of the forming process since the initial electric intensity determines the subsequent polymer behavior.26,27 Figure 3c illustrates the electric field on 1435

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amplitude-modulating intensity cannot pull the polymer to move upward corresponding to the template pattern, leading to the final polymer structure shown in Figure 3b, in which the behavior of polymer acts as evolving with a flat template instead of a prepatterned template and the deformed structure with a periodicity of about 300 μm in accordance with the most unstable wavelength that has been researched in the published literature.28−30 In contrast, the EIS-PPP can still generate a structure positive to the initial morphology with the initially prepatterned polymer shown in Figure 3c and the deformed structure shown in Figure 3d, where the array of pillars has a diameter of 17 μm and a height of 55 μm, leading to a aspect ratio of 3.2 on the structure that has not been fabricated by EIS-PPT process according to the published literatures. The comparison on the deformed structures of EIS-PPT and EIS-PPP validates the superiority of the second approach on duplicating micro/nanostructures with a higher aspect ratio.

the air−polymer interface for the process of EIS-PPT (dashed line) and EIS-PPP (solid line), where the ΔEt2 for prepatterned template with a value of 2.96 × 1012 V2/m2 is smaller than that ΔEp2 for prepatterned polymer with a value of 9.8 × 1012 V2/ m2, implying that EIS-PPP process can duplicate structures more quickly than that of EIS-PPT for an identical height, and also revealing the applicability of the EIS-PPP on fabricating micro/nanostructure with a higher aspect ratio. One interesting phenomenon existing in Figure 2c is the peak on the electrical distributions for the prepatterned polymer, which can be attributed to the corner effect of the electric field due to the initial shape of the polymer structure. Furthermore, the corner effect can be neglected in the case of Figure 2b since the region corresponding to the peak also has extremely large surface tension owing to the extreme curvature, thus leading to the progress evolution shown in Figure 2b. As the air gap is increased to 35 μm with other parameters identical to Figure 2, parts a and b, the deformed structures for EIS-PPT and EIS-PPP are shown in Figure 2d, in which the EIS-PPT process cannot produce sufficient amplitude-modulating intensity to drive the polymer to deform into the structures corresponding to the template pattern, see Figure 2d(i). In contrast, the EIS-PPP process can also generate structures positive to the initial morphology, see Figure 2d(ii). This comparison demonstrates the capability of the EIS-PPP on fabricating micro/nanostructure with a higher aspect ratio from the viewpoint of numerical simulations.

4. DISCUSSIONS In the forming process of EIS-PPT and EIS-PPP, the critical parameters determining the aspect ratio of the deformed structures are the amplitude-modulating intensity that describes the adequacy of the electrostatic force, and the air gap sandwiched between the template and substrate for a fixed polymer thickness that depicts the growing space, in which, furthermore, the amplitude-modulating intensity can act as a criteria to study the two EIS processes for duplicating a structure with an identical air gap. Figure 4 demonstrates the

3. EXPERIMENTAL SECTION Methods. The experimental process of EIS is illustrated in Figure 1. For the EIS-PPT, the polymer is spin-coated into a film of proper thickness on an ITO/glass substrate, and then a micropillar-array silicon template, i.e., prepatterned template, which is doped for a proper electric conductivity and fabricated by conventional photolithography and plasma etching, is placed over the polymer film with an air gap. In our practical experiment, the air gap was adjusted by properly distributed glass spacers, which were sandwiched between the template and the substrate. Once a dc voltage is applied between the template and the substrate with the polymer temperature increased over the glass transition, the polymer can move upward to the template, leading to a micro/nanostructure. For the EIS-PPP, first the prepatterned polymer is obtained on an ITO/glass substrate by imprint lithography with the imprint mold complementary to that for the EIS-PPT. In sequent, a flat ITO/glass template is placed over the prepatterned polymer separated by an air gap which is also guaranteed by distributed glass spacers. As an electric field is exerted with the polymer temperature over the glass transition, the prepatterned polymer can thermally reflow and move upward to the flat template, resulting in a micro/nanostructure. Materials and Equipments. The ITO layer on a glass substrate was sputtered by a Denton Vacuum Explorer14 sputter. The thermally reflow polymer was an acrylic-based compound liquid, available from Micro Resist Technology GmbH (with a commercial name mr-NIL 6000E), a type of curing resist for combined thermal and UV nanoimprint lithography. The dc voltage was supplied by an amplifier/ controller (TREK 610E HV) to the template and ITO substrate. The silicon template (i.e., prepatterned template) was made of a n-typedoped silicon wafer, which has an electric resistivity of 0.005−0.015 Ω· cm. The imprint mold is made of Polydimethylsiloxane (PDMS), duplicated by vacuum-micromolding from the prepatterned template. The SEM images were obtained by using a Hitachi SU8010. Deformed Structures. Figure 3 demonstrates the experimental results for the EIS process, with the process variables listed as follows: the air gap is 30 μm, the features on the template or polymer has the width of 35 μm and depth of 25 μm, and the voltage is 500 V. For the case of prepatterned template in EIS-PPT see Figure 3a, the

Figure 4. Influence of the air gap (a) and applied voltage (b) on the amplitude-modulating intensity, ΔE2, for EIS-PPP and EIS-PPT.

influence of the air gap and the applied voltage on the amplitude-modulating intensity, ΔE2, in which the two parameters are adopted to be discussed due to their prominent role compared to others, polymer permittivity, surface tension coefficient, etc., for instance. Here, the simulated parameters are identical to those in Figure 2 except for the discussed one. 1436

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substrate that depicts the growing space. Consequently, a comparison on the amplitude-modulating intensity between the two EIS processes is performed, focusing on the process variables of the air gap and the applied voltage, which again revels that EIS-PPP process performs a prominent advantage on deforming a structure with a higher aspect ratio especially with a larger air gap and a larger voltage.

Figure 4a demonstrates the influence of the air gap on the amplitude-modulating intensity, in which ΔE2 for two EIS processes are all decreased with the increment of the air gap, i.e., a higher air gap may lead to a smaller amplitude-modulating intensity that is against to improve the electrically driving pressure. However, a higher gap can supply a sufficient growing space for the polymer, beneficial for a structure to be with a high aspect ratio. Thus, for the purpose of obtaining a structure with a higher aspect ratio, an approach is to guarantee a sufficient amplitude-modulating intensity with a precondition of a larger air gap. Opportunely, one important phenomenon in Figure 4a is that ΔE2 for EIS-PPP invariably larger than that for EIS-PPT and the relative difference (ratio of the difference on the value for EIS-PPT) becomes larger and larger as the air gap increases, which implies that improvement on the amplitudemodulating intensity is much more effective for a larger air gap. Thus, with a larger air gap, the EIS-PPP process can generate a structure positive to the initial polymer pattern, however, the EIS-PPT may produce a structures distinguished with the template pattern, which exactly explains the appearance of the numerical simulation in Figure 2d and the experimental results in Figure 3b. The influence of the external voltage on the amplitudemodulating intensity is shown in Figure 4b, where ΔE2 is increased with the increment of the voltage for both two EIS processes, implying a larger voltage is advantageous to improve the electrically driving pressure. In addition, ΔE2 for EIS-PPP is larger than that for EIS-PPP in the variation of the voltage as well as the difference is increased with the increment of the voltage. The appearance on voltage also highlights the superiority of the EIS-PPP on fabricating a micro/nanostructure with a high aspect ratio especially with a larger voltage. Obviously, for reaching a high aspect ratio, a large air gap is needed, which, however, leads to a smaller amplitudemodulating intensity where the surface tension would flatten out the prepatterned polymer. To compensate for this undesirable decrease in amplitude-modulating intensity, a large voltage is preferable for high aspect ratio structuring. In the practical implementation, the maximum voltage allowed will be limited by the physical properties of the polymer which can be electrically broken-down at a high voltage when the polymer structure reaches into contact with the template, corresponding to a timing when a maximum electric intensity is induced in the polymer. Therefore, the limit of aspect ratio which is reachable will be determined mainly by the surface tension and electrical breakdown intensity of the polymer.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: (J.S.) [email protected]. *E-mail: (Y.D.) [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Major Research Program of the NSFC on Nanomanufacturing (Grant No. 91323303), the China “863” High-Tech Program (Grant No. 2012AA041004), and the Funds of NSFC (Grant Nos. 51005178, 51175417, and 51275401).

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REFERENCES

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