Wetting Regimes for Residual-Layer-Free Transfer Molding at Micro

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Wetting regimes for residual-layer-free transfer molding at the micro- and nanoscale Michael Deagen, Linda S. Schadler, and Chaitanya K. Ullal ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09402 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 29, 2017

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ACS Applied Materials & Interfaces

Wetting regimes for residual-layer-free transfer molding at the microand nanoscale Michael E. Deagen, Linda S. Schadler, and Chaitanya K. Ullal* Center for Lighting Enabled Systems and Applications, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180 *E-mail: [email protected] Keywords: soft lithography, blade coating, selective wetting, plasma bonding, woodpile

Abstract Transfer molding offers a low-cost approach to large-area fabrication of isolated structures in a variety of materials when recessed features of the open-faced mold are filled without leaving a residual layer on the plateaus of the mold. Considering both macroscale dewetting and microscale capillary flow, a proposed map of wetting regimes for blade meniscus coating provides a guide for achieving discontinuous dewetting at maximum throughput. Dependence of meniscus morphology on the azimuthal orientation of the stamp provides insight into the dominant mechanisms for discontinuous dewetting of 1-D patterns. Critical meniscus velocity is measured and residual-layer-free filling is demonstrated for 1-D patterned soft molds (stamps) with periods ranging from 140 nm to 6 µm. Transfer of isolated lines and a multilayer woodpile structure was achieved through plasma bonding. These results are relevant to other roll-to-roll (R2R) compatible processes for scalable production of high-resolution structures across large areas. 1 ACS Paragon Plus Environment


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Introduction Addressing the need for scalable and economically viable production of high-resolution structures at the device scale requires new processing paradigms that apply a combination of top-down and bottom-up techniques to high-throughput, massively parallel processes such as roll-to-roll (R2R) production.1 Transfer molding is a soft lithographic process in which an openfaced elastomeric mold is filled selectively with a liquid ink, followed by the curing and transfer of the ink onto a flat or curved substrate aided by conformal contact of the soft mold.2 Owing to significant developments in a wide range of functional ink materials,3-5 transfer molding lends itself to the assembly of a variety of materials into isolated arrays over areas several orders of magnitude larger than the nominal feature dimension. Transfer molding may be extended to 3-D structures through iterative layer-by-layer assembly, making the process desirable for a variety of applications in printed electronics and photonics.6-7 A desirable attribute for transfer molding is the residual-layer-free filling of the mold, otherwise the printed features must undergo a post-transfer etching process to remove the residual layer.8 Residual-layer-free filling has been demonstrated by placing either a wetting or non-wetting template against a non-wetting substrate to generate large-area isolated lines and arrays of nanoparticles.9-10 By tuning stamp and ink wettability, Leitgeb et al. created a R2R nanoimprint lithography system capable of single-layer, residual-layer-free patterning with throughput on the order of 10 m/min.11 Recent work by Kothari et al. extended these processes to direct, multilayer patterning by implementing an organic planarizing layer that is burned away during calcination of the inorganic ink.12 Here, we study discontinuous dewetting of an open-faced mold as a viable alternative for direct patterning of multi-layer structures, in particular for materials systems where a planarizing layer cannot be removed without damage to the structures. Discontinuous dewetting, first presented by Jackman et al.,13 achieves residual-free filling by drawing a moving contact line across the surface of a topographically patterned mold. (Figure 2 ACS Paragon Plus Environment

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1a). A qualitative picture of the process is described in which 2-D arrays of micro-wells are selectively filled due to contact line pinning and subsequent rupture of a small volume of ink from the macroscopic coating meniscus into the micro-well (Figure 1b). Discontinuous dewetting of 2-D patterns has since been applied to controlled colloidal assembly,14-15 isolated arrays of printed oxides,16 selective deposition of proteins and cells,17 and synthesis of nanocrystals.18 The concept of discontinuous dewetting has been applied to 1-D patterns in the literature,19-21 but the qualitative picture for the 1-D case remains unclear. Discontinuous dewetting in 1-D is often depicted as a projection of the cross-sectional schematic by Jackman (Figure 1c), however this picture does not capture the mechanism by which the pinned contact line moves from one line to the next, a nuance that could impact the shape and stability of the macro-scale coating meniscus. These considerations become increasingly important when scaling to continuous production processes such as roll-to-roll (R2R) manufacturing. Here, we implement a R2R-compatible blade meniscus coating process, where a blade above the stamp translates a meniscus of ink at a controlled speed, to achieve discontinuous dewetting. An alternative picture for discontinuous dewetting in 1-D, where the meniscus edge travels with a velocity parallel to the channels (Figure 1d), was explored by adjusting the azimuthal orientation of the stamp. The parallel configuration is akin to the “wet-and-drag” approach described by Lee et al. for generating multi-layer transfer molded microstructures.21

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Figure 1. (a) The concept of discontinuous dewetting applied to a 2-D array of micro-wells, (b) a qualitative description of the discontinuous dewetting process as a cross-sectional schematic (adapted from Jackman et al.,13 where A through D represent sequential meniscus profiles), (c) the Jackman model applied to a 1-D array of micro-channels, and (d) discontinuous dewetting of a meniscus traveling parallel to the 1-D channels. Blake and Ruschak have shown that there is a maximum speed with which a contact line may travel along a flat surface.22 Redon et al. demonstrated that this dewetting velocity is proportional to θ 3 up to a contact angle of at least 36°, a relationship that holds universally when the velocity is normalized by viscosity, η, and surface tension, σ, through the capillary number,

η 23

. When dewetting of a partially wetting liquid is forced above a critical capillary number

Ca*, such as rapid withdrawal of a solid plate from a bath, instabilities in the forms of cusps and ridges form, eventually followed by an entrained film described by Landau-Levich dynamics.24-25 Within the context of transfer molding, a Landau-Levich film constitutes a residual film that should be avoided. To achieve discontinuous dewetting in a blade meniscus coating process, we similarly find that a meniscus contact line traveling below Ca* prevents the formation of this film. Filling of the micro-channels may be viewed in terms of the general condition for spontaneous capillary flow. Berthier et al.26 have shown that spontaneous capillary filling occurs when the


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generalized Cassie angle, θ*, is less than 90°, where θ* may be considered an “average” contact angle for the channel defined as cos ∗ cos .

(1)

In this equation, fi is the cross-sectional fraction of the channel occupied by a material with contact angle θi. For an open face, θi is taken as 180°. When applied to an open rectangular micro-channel with homogeneous sidewall composition and an aspect ratio (h/w) equal to a, the critical contact angle, θcrit, for the channel walls may be identified where the generalized Cassie angle θ* = 90°. Applying equation (1) to a homogeneous, open rectangular channel shows that

θcrit is simply a function of the aspect ratio of the channel. 1 cos 2 + 1

(2)

If the contact angle of the liquid falls below θcrit, spontaneous capillary flow should occur. Conversely, if the contact angle exceeds θcrit, one would expect the liquid to recede within the channel. It is important here to note that this condition does not describe the dynamics of capillary flow but rather the condition for advancing or receding of liquid within the channels. In the case of deep and narrow channels, viscosity may determine the extent to which the ink is capable of filling the channels. We hypothesize that, by combining the micro-scale condition for filling of the channels (θ < θcrit) and the macro-scale dewetting dynamics (∗ ), one can identify a regime within which discontinuous dewetting is expected to occur (Figure 2). Here, θd is the dynamic (receding) contact angle of the ink along the stamp surface. Just beyond θcrit, the competing dewetting dynamics within the micro-channels and at the meniscus edge would be expected to determine the degree of filling of the channels.



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Figure 2. Proposed wetting regimes for blade meniscus coating on 1-D patterned microchannels, where a given capillary number, Ca (governed by the blade velocity,

!"#$ ),

may

occupy three distinct wetting regimes depending upon the dynamic contact angle, θd, of the ink: A) residual film, B) discontinuous dewetting, or C) complete dewetting. Here, η and σ represent the viscosity and surface tension of the liquid, respectively. In this work, we explore the regimes for blade meniscus coating by tuning ink viscosity, varying stamp surface energy, and controlling blade velocity to quantify the critical meniscus velocity across 1-D patterns of open rectangular channels with periods ranging from 140 nm to 6 µm. Varying the direction of the channels with respect to the blade velocity suggests that the azimuthal orientation of the stamp impacts the macroscopic meniscus morphology and helps to clarify the qualitative picture of discontinuous dewetting applied to 1-D patterns. The critical dewetting relationship ∗ holds along 1-D patterned surfaces when the contact line velocity is parallel to the channel direction; the value of k is smaller compared to dewetting along a flat surface. Comparison of experimental measurements to the proposed blade meniscus coating regimes provides a guide for selection of stamp and ink materials to achieve residual-layer-free filling at maximum throughput in a continuous production process. Residuallayer-free transfer of isolated 1-D lines, in addition to a multi-layer woodpile structure, demonstrates the applicability of this versatile approach to facile, low-cost nanofabrication. 6 ACS Paragon Plus Environment

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Methods Materials. Stamps. Silicon master patterns (feature periods: 139 nm, 278 nm, 606 nm, and 6.25 µm; duty cycles approximately 50%) were purchased from LightSmyth Technologies (Eugene, OR). Perfluoropolyether (PFPE, Fluorolink MD-700) was purchased from Cornerstone Technology (Newark, DE) and combined with 1 wt.% 2,2-dimethoxy-2-phenylacetophenone photoinitiator from Sigma-Aldrich. Poly(vinyl alcohol) (PVA, Fiberlease) was purchased from Fiberlay (Sarasota, FL). Hard poly(dimethylsiloxane) (h-PDMS) was prepared by combining VDT-731, HMS-301, SIT7900.0 and SIP6831.2 from Gelest (Morrisville, PA) as described by Schmid and Michel27 with 100 ppm [Pt] catalyst with respect to vinyl concentration. Inks. J-91 ink, a UV-curable polyurethane implemented elsewhere for multi-layer transfer molding,21 was purchased from Summers Optical (Hatfield, PA). Divinyl-PDMS (DVPDMS) inks were prepared by combining vinyl-terminated poly(dimethylsiloxane) of various molecular weights (Gelest DMS-V21, -V25, -V31, -V33, or -V35) to provide tunable viscosity with (25-35% methylhydrosilane)-dimethylsiloxane copolymer (Gelest HMS-301) as a crosslinking agent in a 4.6:1 ratio by weight. 5-7 wt.% vinyl modulator (Gelest SIT7900.0) was added to bring the vinyl:SiH ratio to 1:1. Platinum (II) acetylacetonate (Sigma-Aldrich) was added as a catalyst with [Pt] = 100 ppm with respect to the vinyl concentration. Stamp Preparation. PFPE stamps were prepared by drop casting against silicon master patterns and UV-curing in an atmosphere of N2. h-PDMS stamps were prepared by drop casting against cured PFPE molds and thermally curing at 100°C for 2 hours. PVA stamps were prepared by spin casting PVA solution onto PFPE molds at 3500 rpm for 12 seconds, followed by 6000 rpm for 30



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seconds. Flat control samples of each stamp material (PFPE, h-PDMS, and PVA) were prepared by spin casting onto circular glass cover slips. Monitoring Meniscus Velocity. The velocity of a translational stage (range of 1-1,300 µm/s) was adjusted to identify the critical coating velocity of the meniscus, the velocity just above which the meniscus edge can no longer maintain the same velocity as the blade. A razor blade attached at a fixed angle to a vertically mounted 1-axis stage enabled fine height adjustment of the coating blade. A digital microscope (Dino-Lite AM-7013MZTS) was positioned above the angled blade for a top-down view of the ink meniscus. A calibration card with parallel lines of 100 µm width and 200 µm pitch, placed below the stamp, provided a reference point for tracking and quantifying the meniscus position over time. Relatively thin (

Wetting Regimes for Residual-Layer-Free Transfer Molding at Micro

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