Step-Controllable Electric-Field-Assisted Nanoimprint Lithography for

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Step-Controllable Electric-Field-Assisted Nanoimprint Lithography for Uneven Large-Area Substrates Chunhui Wang, Jinyou Shao, Hongmiao Tian, Xiangming Li, Yucheng Ding, and Ben Q Li ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b08032 • Publication Date (Web): 25 Mar 2016 Downloaded from http://pubs.acs.org on March 28, 2016

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nn-2015-08032y.R2

Step-Controllable Electric-Field-Assisted Nanoimprint Lithography for Uneven Large-Area Substrates Chunhui Wang,†§ Jinyou Shao,†* Hongmiao Tian,† Xiangming Li,† Yucheng Ding,† and Ben Q. Li†‡

†Micro- and Nano-technology Research Center, State Key Laboratory for Manufacturing Systems Engineering , Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China §Shaanxi Province Key Laboratory of Thin Film Technology and Optical Test, Xi’an Technological University, Xi’an, Shaanxi 710032, China ‡Department of Mechanical Engineering, College of Engineering and Computer Science, University of Michigan - Dearborn, Dearborn, Michigan 48128, USA *Corresponding authors: [email protected] (Jinyou Shao)

ABSTRACT Large-area nanostructures are widely used in various fields, but fabrication on large-area uneven substrates poses a significant challenge. This study demonstrates a stepcontrollable electric-field-assisted nanoimprint lithography (e-NIL) method that can achieve conformal contact with uneven substrates for high fidelity nano-structuring. Experiments are

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used to demonstrate the method where a substrate coated with liquid resist is brought into contact with a flexible template driven by the applied electric field. Theoretical analysis based on the elasticity theory and electro-hydrodynamic theory is carried out. Effective voltage range and the saturation voltage are also discussed. A step-controllable release of flexible template is proposed and demonstrated to ensure the continuous contact between the template and an uneven substrate. This prevents formation of air traps and allows large area conformal contact to be achieved. A combination of Vacuum-electric field assisted step-controllable e-NIL is implemented in the developed prototype. Finally, photonic crystal nanostructures are successfully fabricated on a 4′′ 158 µm bow gallium nitride light-emitting diode epitaxial wafer using the proposed method, which enhance the light extraction property.

KEYWORDS: nanopatterning ﹒ electric field assisted ﹒ nanoimprint lithography ﹒ photonic crystal﹒LED Large-area nanopatterns have been critical in the recent expansion of research and commercial technologies across various fields such as photonics,1-3 electronics,4,5 biology,6 and food security.7 Development of future applications of the large-area nanopatterns will depend on properties of state-of-the-art lithography techniques and equipment. Traditionally, electron beam lithography8 is the nano-manufacturing method of choice, but the low throughput of this method hinders its large-area application. Laser interference lithography9-11 employs maskless exposure of a photoresist layer with two or more coherent light beams, resulting in a facile large-area nanolithography technique. A drawback of this method is that it is limited to periodic arrayed features only and it is not capable of creating arbitrary-shaped patterns. Nanosphere lithography is an inexpensive and easy-to-implement fabrication method for producing nanostructures with


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controlled shape and size.12-15 Nevertheless, assembly of nanospheres result in defects that decrease nanostructure precision. Over the last two decades, nanoimprint lithography (NIL) has developed as a promising technology for high-throughput, high-resolution, and low-cost nanoscale patterning.16-18 NIL is a convenient method that allows fabrication of nanostructures where a pattern is imprinted via direct contact between the mold and resist. This allows a pattern resolution that exceeds limitations set by methods employing light diffraction or beam scattering in other traditional techniques. As a result, it is considered as a promising method gaining great momentum for numerous applications. Among various NIL processes, step and flash NIL can be used to form nanostrucutres on large-area substrates;19 however, throughput of the current NIL steppers are still too low to satisfy requirements for mass production applications.20 Roll-to-plate NIL/roll-toroll NIL (R2RNIL) provides a solution for high-speed large-area nanoscale patterning with significantly improved throughput. This methods has several advantages due to its lower imprint force and simple machine construction. Unfortunately, variations in residual layer thickness and local defects are frequently observed in R2PNIL, especially in the case of uneven substrates and rigid cylinder molds,21,22 and it is important to note that R2RNIL is only suitable for flexible substrates.23 Substrate conformal imprint lithography is developed by Philips and SUSS GmbH and is demonstrated to achieve wafer-scale conformal contact between the working stamp and substrate as a result of the capillary force.24 However, the design of its mechanical module and adjustment to parameters are correlated to the resist in use. This dependence on the resist can be ascribed to the fact that capillary force is dependent on properties of the resist.25 A new imprint resist (for example, better etching selectivity of substrates for the subsequent transferring process) may indicate a new device structure.


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Except for capillary driving force, the electrostatic force generated by an applied electric field has also been used to deform a liquid polymer and fabricate micro/nano structures, thereby allowing to easily obtain, adjust, and fabricate difficult-to-mold structures.26-31 For example, Chou et al. successfully utilize electrostatic attraction force, equivalent to an external applied mechanical pressure as in the case of a conventional nanoimprint process, to pattern nanogratings with high fidelity and excellent uniformity in a photocurable resist on a 4′′ Si wafer.28 Experimental results show no visible damage to the structures on the substrate. In our previous work, fabrication of high-aspect-ratio (>10:1) microstructures using dielectrophoresiselectrocapillary force-driven UV-imprinting is demonstrated, which prevents drawbacks seen in conventional imprinting lithography such as mechanically induced mold deformation and position shift thereby maximizing pattern uniformity.31 Moreover, a generalized formulation for numerical characterization of electrohydrodynamic patterning processes is presented by coupling liquid dielectrophoresis and the phase field of the air–liquid dual phase.32 Nevertheless, in an uneven large-area substrate, for example, in the case of a curved gallium nitride (GaN) lightemitting diode (LED) epitaxial (epi-) wafer with high stiffness, fabrication of nanostructures with high fidelity still poses a real challenge. In this paper, an external-electrostatic-field-assisted NIL fabrication method is proposed to facilitate the fabrication of nanostructures on large-area uneven substrates, and we coin the term step-controllable electric-field-assisted NIL (e-NIL). Under an electric field, a flexible template released by successively vacuum breaking, is gradually brought into contact with a liquid resist coated on a wafer surface, thereby allowing for transfering the nanostructures. A detailed demonstration of the method is presented in the following sections, and the entire-wafer


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nanostructures (photonic crystals, PhC) fabricated on a 4′′ bow GaN epi-wafer by a unique development prototype are shown at the end. RESULTS AND DISCUSSION Flexible imprinting using an external electric field Electric field assisted contact between flexible template and substrate. Conformal contact between the imprint mold and substrate is critical for a successful and precise imprinting process. A group of experiments are designed to compare effect of contact for a flexible template, flat polydimethylsiloxane (PDMS) template and flexible template with micro-structures, and wafer with or without applied electric field. Templates stacks are 30 µm h-PDMS/100 nm Indium Tin Oxides (ITO)/300 µm Polyethylene terephthalate (PET), while being flexible, conductive, and transparent. For experiments, the wafer sample is coated with 350 nm of liquid imprint resist. Both the wafer and the flexible template are then cut into 60 × 15 mm2 pieces. As shown in Figure 1(a), a 400 µm spacer is placed on the right side of the wafer sample piece, which creats a channel for discharging air in the sandwiched layer. Then, the template piece is placed on the wafer face down without any applied external pressure. For a flat PDMS template, as shown in Figure 1(b), contact area between the template and substrate is so large that hard to be further extended, even if an extra electrostatic potential is imposed. However, the phenomenon becomes totally different for the microstructured template (square holes array, pitch 8 µm, diameter 4 µm) with a higher resist contact angle (CA = 70°), as shown in Figure 1(c). Contact line (from the top view, the contact area expands through a line almost parallel to the initial contact wafer piece edge; therefore, it is defined as the contact line)


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only propagates a short distance for an applied voltage of 0 V. In contrast, the contact line continues to spread at 800 V, leading to a larger contact area in comparison to previous cases. A series of proof-of-principle experiments are carried out to test the ability of the contact front (transient process of contacting) advanced by an external electric field. Different voltages, 100 V, 300 V, 500 V, and 800 V are applied separately to different electrode pairs composed of a micro-structure template and wafer coated with liqud resist. Propagation under the influence of different volatges is recorded on video (30 frames per second). Propagation distance of contact line center from the initial contact edge is defined as the contact length (CL). CL values are extracted using image processing code to the preselected video frames with error < 0.1 mm. Figure 1(d) shows the time varying trajectories of CL at different voltages. The insets in the graph show a time-sequence of contact front propagation at 500 V. Trajectory of the CL at 500 V can be used as an example to illustrate the propagation rule. In the initial 8 s, the contact front is driven rapidly. Subsequently, the movement of contact front slows down, eventually coming to a halt. In the original state, CL stops at ∼ 20 mm, but moves forward with time and eventually reaches to ∼ 42 mm. All curves shown in Figure 1(d) indicate that the applied voltage directly influences the spread of the contact front. In addition, final distance and development rate exhibit significant difference at different voltages. In general, a higher voltage results in a larger spread length and faster response. So, contact front propagation can be precisely controlled by varying a single parameter, i.e., adjusting applied voltage can be used to achieve reasonable spread length and velocity. This is one of the greatest advantages of using electric field assisted contact. Finally, there is an optimized voltage range to advance the propagation of contact front. Trajectory at 100 V is almost horizontal. Whereas, the response at


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higher voltages such as 800 V is more apparent, resulting in the formation of CL, which shows a small increase in comparison to 500 V. Based on these observations, it can be inferred that both high and low voltages are not useful for contact process. Thus, propagation of contact front between a flexible template and substrate can be controlled by the applied voltage with a longer CL and higher efficiency obtained by increasing the voltage.

Figure 1. Proof-of-principle experiments for electric field assisted contact. (a) Schematic diagram for experimental setup. (b) Flat PDMS template in contact with resist coated substrate by liquid-bridging force and applied electric field; contact length improvement is not obvious. (c) Micro-structured template in contact with resist coated substrate by limited and applied electric field; contact length remarkably increases under applied voltage. (d) Trajectory curves for contact front center as a function of the applied voltage at 100 V, 300 V, 500 V, 800 V. Video frame captures at 500 V voltages for 0 s, 3 s, 6 s, and 9 s are shown as insets. Mechanism analysis of electric field assisted contact process. When no applied electric field is applied between the contact pair, capillary force, or call it liquid-bridging force , is the only


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driving force aiding in advancing contact. Topography involving the superstrate-fluid-substrate sandwich has been analyzed in [33]. Liquid-bridging force between the template and liquid resist on the substrate consists of two parts, namely, a surface tension force that resides in the meniscus and the capillary pressure force on the liquid/vapor interface. When the angle formed by the contact pair is very small, liquid-bridging force per unit length given by the Laplace-Young equation = 8 ⁄

(1)

Where, is the initial contact angle, is the interfacial tension between liquid resist and air. In the contact area, is balanced to support the substrate.34 The microstructured template with a larger contact angle than the PDMS template’s (34°) generates a lower drive force than the former and limit advance the contact. On application of an external electric field between the template and resist coated substrate, two main driving electrostatic forces, including electrostatic-liquid-bridging force and electrostatic-attraction pressure , advance the contact front again, as shown in Figure 1(a). On the one hand, the initial contact angle decreases to with the applied voltage according to electrohydrodynamic theory as: = + /2

(2)

Where, is the average dielectric constant of the template and resist layers on the substrate between the two conductive pairs, and the dielectric constant of vacuum. For the microstructured template, resulting electrostatic-liquid-bridging force improves with the contact angle varying with the external voltage. The bridging force reaches a maximum when the contact angle (30°) saturates at about 600 V in our experiments. However, this is not fit for PDMS template since its contact angle has been close to the saturation value. On the other hand,


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an electrostatic-attraction pressure is also generated between the electrode pair for the template and substrate, and is inversely proportional to the square of the distance between the template and the substrate, as shown in eq 3.29 Here, structures depth is neglected because it is negligibly small in comparison to the thickness of the template. =

!

" $

(3)

#

Electrostatic pressure attains a maximum value in the contact area and no longer changes due to a constant distance. So, driving electrostatic force generated from the applied electric field is the resultant of and . Both these forces exist at the same time on application of a voltage between the electrode pair. In the case of contact between the flexible template and substrate, external force overcomes bending stiffness and deforms the template plate. Deformation % of the template under total load & can be depicted by the thin plate elasticity theory, with the following expression: '( ) % = & * + ,-

Where, ' = .(./0

(4) )

is the bending stiffness of the template, 1 2 the Young’s modulus, ℎ the

thickness, and 4 the Poisson’s ratio of the template. Contact process of the PDMS template in Figure 1(b) is limited by the already sufficient deformation of the residual short plate. In this case, application of a voltage has a weak effect on further contact. For the microstructured template shown in Figure 1(c), a large electrostatic drive force is generated to advance the contact front until it is equal to the template bending stiffness. A dynamic simulation is carried out based on discussion above. The simulation is implemented using the COMSOL Multi-physics software to illustrate advance in the contact front for a microstructured template driven by an external electric field. Results in Figure 2(a)


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shows that CL increases following gradual discharge of air between the contact pairs with time. In the simulation, assuming that the resist is incompressible, the initial contact angle of template was 70°, and the applied electric voltage is 500 V. A flexible template of 20 mm in length is tilt to the resist layer with its left side contacting the resist initially. Propagating process from simulation is found to have an identical trend with the experiment results shown in Figure 1(d). However, exact value of the results varies. This can be attributed to deviation in material characteristic such as the dielectric constant of the resist, the Young’s modulus of the template and so on from their ideal values. Besides contact front spreading, the applied electric field is also helpful to operate the liquid resist fill the structures on the template, which is shown in the Figure S1 in Supporting Information. The largest contact length 56 under a given voltage and the effective voltage range to advance the contact is analyzed by balancing drive moment from electrostatic force (both and ) with the resistance moment of template plate deformation, as shown in Figure 2(b). (Moment calculation explained in supporting information). The drive moment due to the applied electric field gradually decreases with the contact front spreading, while demand moment for deformation increases. Intersection of two curves results in the largest contact length at the specific applied voltage. A voltage scan is used to determine an effective voltage range, obtained as 65 V to 850 V shown in inset of Figure 2(b). Saturation voltage where CL changes are negligible at higher voltages is determined to be about 850 V. A lower or higher voltage beyond the effective range does not exert any obvious influence in changing the contact front, which is basically consistent with experimental results. Contact front spreading results in the flexible template increasingly bending to a larger curve. At maximum CL, deformation of flexible template is also largest. Here, vertical deviation '0 of


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the flexible template from the initial plane can be used to characterize the most curved surface on which electric field assisted contact works, as shown in Figure 1(a). For a 4′′ wafer and 400 µm spacer, application of 500 V (56 = 83.3mm) results in a large curved surface '0 reaching to 333.6 µm, which is sufficiently large compared to a vast majority of wafer waviness. Further, adjusting template parameters and the voltage can be employed to obtain a more uneven shape of template.

Figure 2. Simulation results for electric field assisted contact. (a) Contact length (CL) varying with respect to the time at 500 V. Inset shows dynamic evolution of air displacement between flexible template and resist as the contact front spreads. Arrow with triangle tip is the discharge direction. (b) Generated moment with contact front spreading at different applied voltages and demand moment for template plate forming corresponding deformation. Inset shows largest contact length (56 ) changing with the voltage. Effective voltage range is from 65V to 850V. Step-controllable flexible imprinting for a large uneven substrate Conformal contact on an uneven substrate with step-controllable method. Using a flat silicon wafer as the substrate, contact front between a flexible template and substrate is driven without interruption from one side to another at a fixed voltage. However, for an uneven substrate, such


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as the bent surface of a LED epi-wafer, possesses a variety of curvatures. In such a case, conformal contact is a challenge, even though the template is flexible. Figure 3 shows simulation of contact propagation evolution for a flexible template on a warped substrate driven by an applied voltage. The simulation is carried out using COMSOL Multi-physics software. To simplify the simulation, the liquid resist layer used to realize pattern transfer in imprint lithography is not considered in the model. A suppositional warped substrate with 100 µm peakvalley in the full aperture of 45 mm is used. In simulations, the flexible template was set to a thickness of 130 µm and Young’s modulus was 2 GPa, and applied voltage was set to 500 V. Since, silicon has a large Young’s modulus of 150 GPa, deformation of substrate can be neglected. Contact propagation mainly involves deformation of a flexible template under electrostatic-attraction pressure, as described in eq 3. If the template is directly driven from one side to another such that the inclination is against the substrate and left side initially contact with the substrate, an air trap is difficult to avoid as shown in Figure 3(b). While, the contact front moves to a local sunken area, a higher electrostatic-attraction is generated away from the valley, resulting in an air inclusion. This trapping area decreases until there reaches a balance between the inner air pressure and the pressure produced by the applied voltage, and therefore does not completely disappear. A step-controllable release of template is proposed here to avoid air trap in imprinting process. In this method, the template is equally divided into many release regions, which allow the flexible template to accommodate the macro warp substrate. Figure 3(c) shows the simulation results for the step-controllable contact process. The initial distance between the flexible template and warp substrate is set to 400 µm and release region of 15 mm. In the first release


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region, the substrate is flat and conformal contact is successfully realized. In second release region with a large sunken area in substrate, conformal contact in the case is totally attributed to the proposed step release method. Though edge region of sunken area has large electrostaticattraction pressure resulting from the short inter electrode separation, rapid contact between template and edge region A shown in Figure 3(c) is avoided due to the confine of template from the unreleased region, which gives it a chance for the template and sunken area to make contact rather than forming a trapped air bubble. Repeating this process in the subsequent regions resulted in conformal contact across the whole uneven substrate. In this process, size of the release region is a key factor to remove the air completely. Curved area less than the region reached the limit that the process cannot be achieved. A smaller region would allow accommodation of a more uneven substrate. So, the proposed step-controllable e-NIL, in which electric field offers a sufficient driving force and step-control mode ensures continuous contact, may have an ability to fabricate nano-structures on a large area uneven substrate.

Figure 3. Propagation evolution of a warped substrate covers with a flexible template driven by 500 V. Colors signify the total displacement of the template. The applied voltage is 500 V. (a) Initial state. (b) Directly driven contact from one side to another, (i) Variable electrostatic-


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attraction pressure in different locations; (ii) Air inclusion in warp epi-wafer is formed. (c) Step release template accommodates the warped substrate. (i), (ii), and (iii) show the sequential release of template. Vacuum-electric collaborative step-controllable e-NIL. Flexible conductive template stepcontrollably confined by vacuum is designed to achieve the proposed step-controllable e-NIL process, as shown in Figure 4(a). The module for this process is comprised of two key components: a step-distribution plate (SDP) and a template. SDP contains several grooves that can be controlled to switch between vacuum and atmospheric air. The template is flexible, conductive, and transparent, with its backside mount on the SDP, initially using vacuum across all grooves. Therefore, the prepared contact pair composed of flexible template and wafer sample is divided into several rectangular regions by vacuum grooves on the SDP. For experiments, wafer coated with liquid imprint resist is placed parallel to the template, and leave a pre-set gap to form an air discharging channel similar to previous experiments. When a voltage U is applied, a parallel electrode pair sandwiching the air clearance and resist is formed. Starting from one side, vacuum is broken in a groove. Therefore, corresponding part of the template is released and gradually comes closer to the resist. Considering the still fixed two sides, the center first comes in contact with the resist due to the maximum deformation. Under influence of electrostatic force, contact front begins to spread until it is balanced with bending stiffness of template part. Following contact propagation, structures on the template are also filled. Switching to atmospheric pressure in the next groove results in a new part of template following the above process, and expands the contact area. As more of the neighboring grooves are switched with time, contact area spreads across the entire wafer. After the resist is cured, the voltage is switched off. Then, peel off the template from the wafer by sequentially switching the


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grooves back to vacuum from one side to another. This results in the imprinting process being completed for the entire wafer. During the entire process, two sides of the template were held in place by uninterrupted vacuum from the two end grooves in SDP. The Nano e-NIL prototype is developed based on this principle by Micro-and Nano-technology Research Center of Xi’an Jiao Tong University, as shown in Figure 4(b). The flexible template has three composite layers, namely, 30 µm h-PDMS nanostructured layer, 100 nm conductive ITO, and 300 µm back-carrier PET. The SDP is made of optical grade PMMA. To start the process, a wafer in the cassette is handled by a feeder and placed on a vacuum chuck and fixed in place. After the nanostructures are printed to the resist through the step-controllable e-NIL process, the flexible template is gradually peeled off from the cured resist by vacuum. To further improve demolding, surface treatment is recommended, for instance fluorosilane deposition.31 When the wafer covered with the nanostructures is handled to another cassette, the next cycle is run. To complete one round of processing from molding to demolding requires about 42 s. This time can be further reduced by optimizing the experimental parameters.

Figure 4. Scheme of step-controllable e-NIL. (a) As grooves on step-distribution plate (SDP) switching sequentially with time, all parts of template are released successively and achieve the


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conformal contact with resist coated on the wafer driven by the applied . (b) Nano e-NIL prototype and two key components: SDP and flexible template. Fabrication of PhC nanostructures on an uneven GaN epi-wafer Nanostructured anti-reflection surface. Nanostructures have been applied in various fields due to its excellent properties in optical, electronics and biosensing. Here, the optical transmittance of nanostructured or PhC surface is discussed in detail. Nanostructures tested here are hole-arrays on a hexagonal lattice with period of 600 nm, diameter of 300 nm and depth of 500 nm. To investigate optical transmission of the structures, a special configuration of a transparent hemicylindrical PDMS sample is adopted as reference [15]. A 5×5 mm2 area with nanostructures is printed to the center of the flat surface of the hemi-cylinder. Optical transmittance properties depending on the angle of incident light from PDMS to air of two different samples with flat PDMS and nanostructures are observed and measured. In experiments, the hemi-cylinder sample is placed at the center of a rotational stage. A laser beam (with a wavelength of 460 ± 10 nm) vertically passes through the PDMS/air interface and impinges at the center of the nanostructures/flat PMDS with different incident angle. An optical power meter located behind the samples is used to acquire total light transmission every 2°. Figure 5(a) and (b) contrast results of two samples both with a 57° incident angle (larger than total reflection angle 45.3°). Comparing to total internal reflection of flat PDMS/air interface, a major proportion of light is still transmitted into air on the nanostructures sample. Figure 5(c) exhibits the incident angularresolved light transmission for the two samples. The integrated total transmittance for the nanostructured sample increases to 54.6% from 21.7% for flat PDMS. Due to it significantly higher light transmittance, PhC has been extensively applied to improve light extraction in LED


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chips, where low light extraction efficiency is still a major obstacle for high luminance due to the high-refractive-index contrast between the active material and air.35-38

Figure 5. Optical transmission of nanostructures on the hemi-cylinder PDMS sample. (a) Total internal reflection in flat PDMS-air interface. (b) Nanostructures reduce the total reflection; (c) Transmittance measurement curves for flat PDMS and nanostructures respectively, and the inset is the integrated transmittance. PhC fabrication on uneven 4′′ GaN epi-wafer. Although, photonic crystals have been proved to be effective in improving transmittance of LED chips, fabrication of nanostructure on a large GaN epi-wafer without defects is still a major challenge.39,40 Due to the mismatch in lattice constants and thermal expansion coefficients between GaN growth layers and sapphire substrate, GaN epi-wafers are typically bent with various curvatures, and these defects are difficult to counteract when the temperature is necessary to raise as high as ∼1000 °C in the key process of LED wafer fabrication, namely, metal–organic chemical vapor deposition. As a result, ∼ 170 µm deviation can be caused for 10 µm thickness of GaN on a 4′′ wafer.41,42 Imprinting of nanostructures on such an uneven substrate is quite difficult due to the irregular contact of mold and substrate, and large defects that are comparable with the size of LED chips (as shown in


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Figure S3(a) in Supporting Information) influence the chip performance. After dicing the GaN wafer to chips in the follow-up process, these square millimeter-sized LED chips with no PhC structures will have low light extraction efficiency as shown in Figure 5. Herein, the proposed Vacuum-electric collaborative step-controllable e-NIL is used to fabricate large-area PhC nanostructures on such an uneven GaN epi-wafer. Figure 6 shows an image of the PhC nanostructures fabricated using the step-controllable eNIL. The hole-array master mold on silicon wafer and the pillar-array flexible template are respectively shown in Figure 6(a) and (b). Figure 6(c) is an image of PhC nanostructures on a GaN epi-wafer sample whose bow surface measurement result (~158 µm, as measured by Taylor Hobson PGI 3D) is shown in Figure 6(d). Identical interference colors measured at different fabrication stages is one of the powerful macro proofs demonstrating successful transformation process. Figure 6(e) shows the uniform moiré fringes at a magnification of 100 × generated by the superposition between the scanning lines of scanning electron microscope (SEM) and the specimen hole-array, which further demonstrates high fidelity of the structures, because any case of spatial frequency change can cause confusion in the interpretation fringes.43,44 Figure 6(f) shows more details in a magnified SEM image of intact hole-array and cross-section. From micro to macro, replication using step-controllable e-NIL process is demonstrated to be successful. In contrast, imperfect fabrication results without applied electric field are shown in Support Information.


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Figure 6. PhC nanostructure fabrication on 4′′ GaN epi-wafer. (a) Hole-array master mold on silicon wafer, graphical area is 46 × 46 mm2. (b) Flexible template. (c) GaN epi-wafer with PhC hole-array made by step controllable e-NIL. (d) GaN wafers surface profile with bow reaching to 158 µm as measured by Taylor Hobson PGI 3D, A and A′ are the testing endpoints. (e) Uniform SEM moiré of nano hole-array. (f) Magnified SEM image of nano hole-array. Inset shows crosssectional image with a thin and uniform residual resist layer. Thickness uniformity of the residual resist layer determines a reliable pattern transfer from the imprinted features into the substrate, which is critical in post-pattern processing such as inductively couple plasma (ICP) etch. Figure 7 shows five different SEM cross-section views of the above sample. Average residual layer thickness was about 9.3% of the initial resist layer thickness, and difference in the residual layer for different areas was found to be smaller than 8 nm. It is not difficult to understand the high fidelity of duplicated structures and uniformity of the residual layer thickness, because the thickness of both photo-curable resist and h-PDMS layer are certain and homogeneous respectively, so that the driven force generated from the electric field is identical over the entire epi-wafer.


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Figure 7. SEM cross-sectional views in different areas of one epi-wafer sample. Residual layer thicknesses are labeled in corresponding positions. The photoluminescence (PL) spectra45 are used to evaluate the light output intensity of the PhC GaN epi-wafer fabricated by the proposed step-controllable e-NIL. A micro-PL spectra system is used to perform the measurement in the paper. This system adopts a continuous laser as the excitation source and it has a wavelength of 405 nm and a beam spot of about 10 µm. In measurement, nano structures on imprint resist are transferred into GaN surface using ICP etching firstly, and the depth of PhC is controlled to ~100 nm. Figure 8 (a) and (b) show the transferred GaN PhC structures from samples prepared by step-controllable e-NIL and traditional NIL method respectively. It can be found that GaN PhC fabricated by the step-controllable eNIL is quite perfect, and the defects formed in the traditional NIL method shown in Figure S3 are also transferred into the GaN surface faithfully. The perfect GaN PhC structures acquired by the step-controllable e-NIL method (including these perfect regions acquired by traditional NIL) contribute to a large enhancement of PL intensity than these unpatterned samples as shown in Figure 8(c). While the GaN PhC structures with many defects, as shown in Figure 8(b), can only acquire a relative smaller enhancement in PL intensity. It demonstrates that the proposed e-NIL


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method, which can form uniform structures on the mildly curved GaN surface, is important in terms of performance gains in the optical device.

Figure 8. PhC structures on GaN epi-wafer surface and the results of photoluminescence (PL) spectra measurement. (a) SEM image of the perfect GaN PhC structures fabricated by e-NIL; (b) SEM image of the defect GaN PhC structures formed by NIL; (c) PL spectra of different samples, the perfect sample fabricated by e-NIL and the perfect region from NIL sample had an obvious and almost equal enhancement than the unpatterned sample, the defect region of NIL sample had a lower enhancement. CONCLUSION In summary, in this study a nanostructuring approach—step-controllable e-NIL— is developed, which has excellent ability to fabricate nanostructures on uneven large-area substrates. This is achieved by solving problems associated with achieving large-area conformal contact. Driven by the electrostatic force from the application of an electric field, the contact front between the flexible template and sample substrate propagates efficiently in a controlled manner. Based on both the electro-hydrodynamic and elasticity theories, analytic and dynamic evolution of the contact process has been demonstrated, which is validated with experiments. A step-control mode is implemented in the development Nano e-NIL prototype to achieve conformal contact between the flexible template and uneven substrate without air traps. Using the proposed


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fabrication process, PhC nano hole-array is imprinted and transfered with high fidelity onto a 4′′ GaN LED epi-wafer by ICP, which successfully enhanced its light emission efficiency on a large scale. METHODS Fabrication of flexible template. The flexible template used in step-controllable e-NIL is one of the key modules, which employs a three-layer structure composed of a 30 µm h-PDMS/100 nm ITO/300 µm PET. The master mold (available from Eulitha AG) is hole-array on a hexagonal lattice with the period 600 nm, diameter of 300 nm, depth of 500 nm. The pattered area is 46 × 46 mm2 located in the center of a 4′′ Si wafer, produced by PHABLE technology,46 and has an anti-adhesive layer deposited on the surface by chemical vapor deposition.47 To make the template, 100 nm ITO and 20–30 nm SiO2 (only for the transition layer) were sputtered on a 160 × 160 mm2 back-carrier PET, and 30 µm mixing h-PDMS was spin coated on the surface of master mold. Next, the h-PDMS layer was bonded to SiO2 and solidified. Finally, the flexible template is peeled off from the master after the h-PDMS was fully cured after baking for 1 h at 60 °C. SDP parameters. The SDP in NANO e-NIL prototype was fabricated on optical grade polymethyl methacrylate (PMMA). The PMMA was 162 (length) × 162 (width) × 12 (height) mm3 in size with a surface flatness of less than 8 µm after polishing. It was composed of 13 parallel vacuum grooves with a trench depth of 3 mm. The size of middle 11 strips was: length 110 mm, width 1.5 mm, and pitch 10 mm. Other two grooves were symmetrical about the center line with an interval of 70 mm, length of 120 mm, and width of 2.5 mm.


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Experiments on 4′′′ GaN LED wafer. For use in experiments, LED epi-wafers were purchased from Walsin Lihwa Co. The p-GaN layers on the top of the epi-wafer were total 110 ~ 130 nm thickness. Before any fabrication steps were carried out, the epi-wafers were baked at 200 °C and cooled to room temperature before coating. The prepared substrates were spin coated with 8 nm adhesion promoter at 5000 rpm for 60 s and followed by a bake at 150 °C for 60 s on a hot plate. Then, a 350 nm liquid resist layer was spin coated at 2100 rpm for 60 s and baked at 100 °C for 30 s. This improved film quality and guarantee good curing results. Both the adhesion promoter (the commercial name: mr-APS1) and resist (the commercial name: mr-UVCur06) were purchased from Micro Resist Technology, GmbH. The dynamic viscosity of the resist was 14 ± 1 mPa∙s, mass density was 1.021 ± 0.005 g/cm3, and the surface tension was roughly 0.03N/m.

Initial distance from the substrate to the template was 400 µm. This distance was controlled using a precision displacement platform in NANO e-NIL prototype, which has a closed loop resolution of 1 µm, and repeated positioning accuracy was lower than 3 µm. Before imprinting, the resist coated sample was held by a vacuum chuck and aligned to the template. In practice, a 500 V dc voltage supplied by an arbitrary waveform generator (AGILENT 33220A), bridged by an amplifier/controller (TREK610E HV) is applied to the template ITO layer and wafer surface. Switching time between the subsequent grooves from vacuum to air atmosphere was 0.5 s. After the UV-curable resist solidified on exposure to UV irradiation for 10 s at 250mW/cm2, the template was separated from the substrate after a 1.2 s interval for each region. After which, the nanostructures were transferred to the resist on the sample surface. The etching process from the imprint resist to the GaN surface was carried out on the ELEDE ® 330 from NMC Co. The etch gas was Cl2, flow rate of 25 sccm, pressure of 10 mTorr, plasma power of 100 W, and etching


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rate was 8 Å/s. The etch rate selectivity of the using resist to the GaN was about 2.5. The PL spectra tested by QEpro system from Ocean optics Co. Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.*******. Additional numerical simulation, theoretical calculation, and experimental SEM images. Corresponding Author Jinyou Shao, [email protected]; Conflict of Interest: The authors declare no competing financial interest. Acknowledgements. This work is financed by the NSFC Major Research Plan on Nanomanufacturing (grant no. 91323303), NSFC Funds (grant no. 51421004，51522508) and Program for New Century Excellent Talents in University of Ministry of Education of China (N CET-13-0454). Chunhui Wang, Jinyou Shao and Hongmiao Tian contributed equally to this work. REFERENCES AND NOTES 1. Öktem, B.; Pavlov, I.; Ilday, S.; Kalaycıoğlu, H.; Rybak, A.; Yavaş, S.; Erdoğan, M.; Ilday, F. Ö. Nonlinear Laser Lithography for Indefinitely Large Area Nanostructuring with Femtosecond Pulses. Nat. Photonics 2013, 7, 897−901. 2. Lee, S.-M.; Biswas, R.; Li, W.; Kang, D.; Chan, L.; Yoon, J. Printable Nanostructured Silicon Solar Cells for High-Performance, Large-Area Flexible Photovoltaics. ACS Nano 2014, 10, 10507−10516.


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