Photolithography-Based Patterning of Liquid Metal ... - ACS Publications

Jun 2, 2016 - Department of Advanced Device Technology, Korea University of Science and ..... Communications Technology Promotion (IITP) grant funded...
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Photolithography-Based Patterning of Liquid Metal Interconnects for Monolithically Integrated Stretchable Circuits Chan Woo Park,*,†,‡ Yu Gyeong Moon,†,‡ Hyejeong Seong,§ Soon Won Jung,† Ji-Young Oh,† Bock Soon Na,† Nae-Man Park,†,‡ Sang Seok Lee,† Sung Gap Im,§ and Jae Bon Koo*,† †

Wearable Device Research Section, Electronics and Telecommunications Research Institute (ETRI), 218 Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea ‡ Department of Advanced Device Technology, Korea University of Science and Technology (UST), 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea § Department of Chemical and Biomolecular Engineering & Graphene Research Center KI for Nanocentury, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea S Supporting Information *

ABSTRACT: We demonstrate a new patterning technique for gallium-based liquid metals on flat substrates, which can provide both high pattern resolution (∼20 μm) and alignment precision as required for highly integrated circuits. In a very similar manner as in the patterning of solid metal films by photolithography and lift-off processes, the liquid metal layer painted over the whole substrate area can be selectively removed by dissolving the underlying photoresist layer, leaving behind robust liquid patterns as defined by the photolithography. This quick and simple method makes it possible to integrate fine-scale interconnects with preformed devices precisely, which is indispensable for realizing monolithically integrated stretchable circuits. As a way for constructing stretchable integrated circuits, we propose a hybrid configuration composed of rigid device regions and liquid interconnects, which is constructed on a rigid substrate first but highly stretchable after being transferred onto an elastomeric substrate. This new method can be useful in various applications requiring both high-resolution and precisely aligned patterning of gallium-based liquid metals. KEYWORDS: liquid metal, eutectic gallium−indium, lift-off patterning, stretchable circuit, monolithic integration durability under repetitive deformation.10−12 However, due to some inherent drawbacks of highly viscous and inhomogeneous composites, it is difficult to achieve both high conductivity and narrow pattern sizes comparable to thin metal films.10−12 Recently, as an alternative candidate for stretchable conductors, eutectic gallium-based liquid alloys have been investigated extensively.13−19 As a low-viscosity and highly conductive fluid at room temperature,13 a volume of the liquid metal embedded within an elastomeric matrix can be deformed reversibly in response to an external stress, without any hysteresis or mechanical degradation.14,15 On the basis of such beneficial characteristics as a fluidic conductor, liquid metals have been demonstrated to be useful for various applications such as stretchable interconnects,14,15 antennas and inductors,16,17 or sensors.18,19 However, for more versatile applications of liquid metals as stretchable electronic components, there still remain some critical issues to be resolved, especially for precise and efficient

1. INTRODUCTION Although many approaches have been demonstrated to be successful in realizing stretchable electronic circuits that maintain proper functions even under a large amount of mechanical deformation,1−4 it is still an important issue to find out more practical manufacturing schemes that are highly compatible with conventional flexible technology providing better reproducibility and reliability. In many previous works,5−11 stretchability of an overall circuit has been achieved by connecting active devices with stretchable interconnects, so that the strain of the whole substrate can be mainly accommodated by the elongation of interconnects while the deformation of device area is relatively suppressed. For obtaining such stretchable interconnects, two main approaches have been employed: either by utilizing stretchable configurations such as serpentine,5,6 wavy,7,8 or three-dimensional coil shapes,9 or by using conductive elastomers where metal or carbon-based particles are mixed with elastomeric matrices.10−12 In particular, on the basis of intrinsic stretchability of conducting materials, the latter approach has been demonstrated to be quite useful for those applications requiring relatively large elongation (over several tens of percent) and © XXXX American Chemical Society

Received: February 15, 2016 Accepted: June 2, 2016

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DOI: 10.1021/acsami.6b01896 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces patterning of liquid metals.20 For the past few years, it has been demonstrated that liquid metals can be patterned in various ways such as screen printing,21 contact printing,22 spray painting,23 direct writing,24−30 laser patterning,31 selective surface wetting,32,33 imprint,34 or injection methods. 18 Although these techniques can provide quite uniform, robust, and large-area patternability,24−30 additional capabilities are still required especially for applications in highly integrated circuits. In most approaches for producing liquid metal patterns on a flat surface,21−32 it has been difficult to achieve feature sizes below 80 μm due to unique properties of liquid metals caused by a thin oxide surface layer and high surface tension. 20 Furthermore, due to weak capabilities of precise alignment, most previous works have been focused on manual integration of bulky and discrete device chips with preformed liquid metal interconnects.23−25,31 For utilizing liquid metals as components in integrated circuits, it is necessary that we can produce fine liquid metal patterns over a prefabricated thin-film devices in a well-aligned manner, in similar ways as those employed for solid metal thin films in the conventional microfabrication.20 In the present work, we propose a new technique for patterning gallium-based liquid metals, which can provide both high pattern resolution (∼20 μm) and alignment precision as required for highly integrated circuits. Based on photolithography and lift-off techniques compatible with conventional microfabrication technology, “monolithic” integration between preformed active microdevices and liquid metal interconnects is possible, and such beneficial effects have been demonstrated by the hybrid combination of a rigid thin film transistor (TFT) and liquid metal interconnects within an elastomeric matrix.

for 12 h, the PDMS layer containing liquid metal patterns was released from the Si substrate, and then a backside capping PDMS layer was additionally casted with the same thickness as the top PDMS layer (∼500 μm). 2.3. Fabrication of TFT and Liquid Metal Interconnect on Hybrid Substrate. For forming stiff islands for TFTs, a polyimide solution (VTEC PI-1388 Polyimide Liquid, RPI, Inc.) was spin-coated onto the parylene/PAA double layer on a Si substrate at 2000 rpm for 3 min. After soft bake at 135 °C for 10 min, the polyimide layer was patterned into islands by conventional photolithography and wet etching by AZ 500 MIF developer (AZ Electronic Materials), followed by curing of the polyimide precursor at 200 °C for 1 h. Within the island area, source and drain electrodes of Al (70 nm) and a channel of indium gallium zinc oxide (20 nm) were deposited through metal shadow masks by thermal evaporation and DC sputtering, respectively. After annealing at 200 °C for 1 h, a 160 nm-thick poly(1,3,5-trivinyl1,3,5-trimethyl cyclotrisiloxane) (pV3D3) layer was deposited by initiated chemical vapor deposition (iCVD) as a gate dielectric layer,37 and then a gate electrode of Au was formed by electron beam evaporation. Finally, liquid metal interconnects were formed for connecting the TFT within the island to external pads, and the whole device and interconnecting layer was transferred to a PDMS substrate. 2.4. Electrical Measurement of Stretching Characteristics. Both edges of a rectangular sample sheet to be stretched were first adhered to rigid plastic plates by elastic adhesives (Super X No. 8008, Cemedine), and copper wires were inserted into pads and also fixed by the same adhesives. After the plastic plates were clamped on a housemade automatic stretching equipment, a stretching test was performed under controlled displacement conditions. During the stretching test, the electrical properties of the stretched interconnect or TFT were measured by Keithley 2450 Source Meter or Keithley 4200 parameter analyzer, respectively.

3. RESULTS AND DISCUSSION Figures 1 and S2 demonstrate the overall process for patterning a gallium-based liquid metal on a rigid substrate schematically.

2. EXPERIMENTAL SECTION 2.1. Patterning of Liquid Metal on Rigid Substrates. A negative photoresist (NR9-3000PY, Futurrex) layer was spin-coated at 1000 rpm for 30 s, with a thickness of ∼10 μm. After soft-bake at 110 °C for 90 s on a hot plate, the photoresist was exposed to 365 nm-UV (12 mW/cm2) for 20 s through a soda-lime photomask and baked at 100 °C for 60 s. After development in an AZ 500 MIF developer (AZ Electronic Materials) for 1 min, photoresist patterns with 15 μm-wide undercuts were formed. After oxygen plasma treatment (40 W, 30 sccm, 30 s) on opened surfaces, EGaIn alloy composed of 75.5% Ga and 24.5% In by weight (Sigma-Aldrich, 99.99% purity) was dropped on the substrate and evenly spread by a hand-roller. Finally, the sample was immersed in acetone for 2 h and softly sprayed, by which the liquid metal/photoresist layer was lifted off leaving liquid metal patterns on the substrate. The height profile of the liquid metal patterns was measured by a 3D laser confocal microscope. 2.2. Fabrication of Stretchable Liquid Metal Interconnect. First, an aqueous solution of poly(acrylic acid) (PAA) was spin-coated on a Si substrate at 2000 rpm for 30 s and baked at 100 °C for 60 s, as a sacrificial layer with a thickness of ∼1.5 μm. For obtaining appropriate properties as a water-soluble sacrificial layer, the PAA purchased as a 25% solution (MW: ∼50 000, Polyscience, Inc.) was neutralized with a solution of NaOH until reaching a pH of 7, then diluted with deionized water. Over the PAA sacrificial layer, an 800 nm-thick parylene layer was deposited by thermal evaporation, for protecting the underlying PAA during the following steps for liquid metal patterning. After liquid metal interconnects formed on the parylene layer, a 10:1 mixture (weight ratio) of a polydimethylsiloxane (PDMS) prepolymer and curing agent (Sylgard 184 Silicone Elastomer Kit, Dow Corning) was casted onto the substrate and cured at 60 °C for 2 h in an oven. Before the PDMS was casted on the substrate, the surface of parylene was modified to have silanol moieties (Si−O−H) to enhance the bonding strength between the parylene and PDMS (Figure S1).35,36 By dissolving the PAA layer within water

Figure 1. Schematic illustration of the patterning process of the liquid metal.

First, a 10 μm-thick negative photoresist is spin-coated on the substrate, and then patterned by conventional photolithography so that the area to be wetted by the liquid metal is opened. Then a eutectic gallium−indium (EGaIn) liquid alloy layer is casted onto the surface of the photoresist layer, and subsequently by applying a normal force with a roller, the openings within the photoresist layer are filled with the liquid metal. At this step, as sidewalls of the photoresist patterns have B

DOI: 10.1021/acsami.6b01896 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 2. (a) Line-and-space and mesh patterns of the liquid metal formed on a Si substrate. (b) Height profiles of the liquid metal patterns measured by a confocal microscope.

a negative slop (with a 5−15 μm undercut); it is expected that wetting of the liquid metal on the sidewall can be suppressed efficiently. Finally, by dissolving the photoresist layer in acetone, the liquid metal patterns are obtained as intended. Figure 2a shows liquid metal line patterns produced on a Si substrate, where the patterns have sharp edges with the minimum line-width as small as ∼20 μm. Such results demonstrate that the present method can provide a pattern resolution much smaller than those obtained by direct writing or printing techniques, which are larger than 80 μm.21−31 Although it was demonstrated that line-widths of a few micrometers can be achieved by imprint34 or injection18 methods, they are less applicable to the patterning on flat and rigid substrates as patterns can be obtained only in the form of microfluidic channels filled with the liquid metal. For the injection method,18 it might be possible to place an elastomeric mold directly on the rigid substrate, but the liquid metal needs to be injected into each isolated channel. Recently, it was reported that the resolution below 10 μm can also be achieved by selective wetting of the liquid metal33 or mechanical sintering of a liquid-metal nanoparticle layer.38 However, for utilizing these techniques in forming liquid metal interconnects on a prefabricated device layer, some issues need to be resolved. For example, for selective wetting of the liquid metal, it is required that whole surface except the interconnecting region is nonwettable,32,33 which is hard to achieve for the substrate containing prefabricated devices. It is also difficult to obtain high uniformity in the feature height, as it highly depends on the pattern size.32,33 In the mechanical sintering technique,38 conductive traces are surrounded by the nanoparticle region, which is readily transformable into a conductive area even by minute external pressure. In the present technique, as the area not to be coated by the liquid metal is completely masked by the photoresist layer, we can produce liquid metal patterns even on an entirely wettable surface. By adjusting the thickness of the photoresist layer, we can also control the height of liquid metal patterns more easily. Unlike ordinary liquid fluids, gallium-based liquid alloys have a thin oxide skin on its surface in air at ambient conditions,13,20 which mechanically stabilizes the liquid droplet below a critical yield surface stress. Despite its high surface tension, the liquid metal in air can maintain nonequilibrium contact achieved by external forces.24 For example, it was reported that the liquid metal injected or imprinted into microchannels can remain inside the channel even when the pressure is removed.18,34 Our

results also show that the liquid metal pressed into the openings of the photoresist layer can withstand the external forces imposed by the lift-off of surrounding structures, maintaining robust adhesion to the underlying substrate. Figure 2b shows representative profiles of the liquid metal line with various widths, where the line patterns have rounded shapes with heights close to the thickness of the photoresist layer (∼10 μm). As shown in Figure S3, the variation in the height and width were measured to be less than 10% along 30 mm-long traces. The root mean squared (RMS) surface roughness on the top of a 100 μm-wide trace was measured to be ∼0.61 μm, which was resulted from rough surface morphology of the roller head.39 The height of the finest trace (∼20 μm-wide) was observed to be relatively lower (∼7 μm), which is thought to be caused by partial loss of the top liquid metal near the boundary region during the lift-off process. The electrical connectivity of the traces was confirmed by linear current−voltage characteristics, as demonstrated in Figure S4. Figure 3 shows some examples of liquid metal patterns aligned to prefabricated micropatterns as defined by photolithography. Figure 2 and 3 demonstrate together that the present technique can provide both high resolution and alignment accuracy as required for monolithically integrated

Figure 3. Examples of liquid metal patterns aligned to prefabricated micropatterns. C

DOI: 10.1021/acsami.6b01896 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. (a) Schematic illustration of the fabrication process of the stretchable liquid metal interconnect. (b) Transfer of liquid metal interconnects from a Si wafer to a stretchable PDMS substrate. (c) Liquid metal interconnect loaded on the stretching stage.

Figure 5. (a) Representative variation of the electrical resistance of a liquid metal interconnect with the tensile strain under single stretching from 0% to 60%, and (b) under cyclic stretching of 40%.

metal interconnect was measured with increasing tensile strain on an automatic stretching stage (Figure 4c). As demonstrated in Figure 5a, the resistance gradually increased with the strain but the electrical connectivity was maintained up to at least 60% strain, over which the PDMS sheet was broken at the vicinity of the edge of fixing plates due to local stress concentration. The increase of the electrical resistance should be caused by both the increased length and reduced cross section area of the liquid metal line, as well as the variation of the contact resistance of the liquid metal pads to copper wires inserted for the measurement. On the basis of the dimensions of the liquid metal interconnect (width = 0.8 mm, length = 10 mm, thickness = 10 μm) and the electrical conductivity of the EGaIn (σ = 3.4 × 106 S m−1),32 the electrical resistance of the liquid metal interconnect is estimated to be ∼0.4 Ω. Therefore, a large portion of the measured resistance (∼1.87 Ω) should be originated from the contact resistance between the copper wires and the liquid pads, which implies that a better strategy for achieving perfect contact is still need to be developed. However, our observation demonstrates that the external strain is well accommodated by the internal reflow of the liquid metal. As shown in Figure 5b, the resistance of a liquid metal interconnect recovers its initial resistance even after 2000 cycles of 40% repeated stretching, which shows that

circuits. In previous works employing masked printing techniques,21,23 shadow masks were prepared by laser-assisted or mechanical cutting of metal21 or polymer sheets,23 and needed to be manually positioned on the substrate to be processed. In those cases, alignment accuracy is much poorer than those obtainable by photolithography, and only a limited range of circuit geometries can be produced because all structures need to be connected within a stencil mask.21−23 Although the photolithography-based process is more expensive and slower than direct writing or printing techniques,24−30 the present method is expected to provide a useful way for fabricating small-area but highly integrated stretchable circuits. For demonstrating the applicability of the new technique as a way for constructing stretchable circuits, we first fabricated stretchable liquid metal interconnects embedded within an elastomeric polydimethylsiloxane (PDMS) matrix by a transfer method. As shown in Figure 4a,b, liquid metal lines were patterned first on a thin parylene film (∼800 nm thick) over a water-soluble sacrificial layer (poly(acrylic acid)), and transferred to the stretchable matrix by casting a PDMS layer (∼500 μm thick) over the liquid metal patterns and then dissolving the underlying sacrificial layer. The electrical resistance of a liquid D

DOI: 10.1021/acsami.6b01896 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 6. (a) Schematic illustration of the fabrication process of the combination of a TFT on a stiff polyimide island and liquid metal interconnects. (b) Shape of a TFT and liquid metal interconnects transferred to a PDMS substrate.

Figure 7. (a) Representative evolution of drain current (Id)−gate voltage (Vg) characteristics of a TFT as the tensile strain of the substrate is increased from 0% to 50%, and the variation of (b) threshold voltage, (c) mobility, and (d) subthreshold swing with the applied strain.

polyimide islands and surrounding elastomer. In another previous work,42 similar hybrid structures have been achieved also by using various high-modulus island materials such as silicon oxide, silicon nitride, and diamond-like carbon. In those approaches,40−42 however, the maximum process temperature and pattern density were quite limited, as the whole processes for producing islands and active devices had to be performed directly on an elastomeric substrate. In addition, as heterogeneous islands tend to delaminate or slip against the soft matrix under highly stretched conditions,43,44 it is very

the liquid metal can provide high stability against fatigue failures commonly observed for solid metal interconnects.6 As a strategy for integrating stretchable liquid metal interconnects with “nonstretchable” thin film active devices, we employed a hybrid substrate structure where discrete polyimide islands are embedded within an elastomeric matrix. As demonstrated in our earlier works,40,41 polyimide islands within an elastomeric matrix can provide robust and stable sites for active thin film transistor (TFT) devices, as they are hardly deformed even under a large amount of the overall stretching due to the large difference in elastic moduli between the E

DOI: 10.1021/acsami.6b01896 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces likely that solid metal interconnects would be broken along the boundary of islands. In the present work, we fabricated all circuit components (including polyimide islands, oxide TFTs, and liquid interconnects) on a rigid substrate first, and then transferred them onto a stretchable substrate (Figure 6a). In this scheme, as no soft elastomer layer is engaged during the device fabrication, the process temperature can be elevated high (∼200 °C) enough to achieve proper TFT characteristics as well as full curing of the polyimide precursor. As shown in Figure 6b, it was also observed that the sidewalls of polyimide islands (6 μmthick) were completely buried under the thicker liquid metal lines crossing with them. Thus, even if some minor fractures are formed along the boundary of islands during stretching, they are expected to be healed by reflow of the liquid metal. Figure 7a shows the evolution of drain current (Id)−gate voltage (Vg) characteristics of a TFT, as the tensile strain of the hybrid substrate was gradually increased. As shown in Figure 7b−d, the combination of a rigid TFT and liquid metal interconnects maintained stable operations under the strain up to at least 50%, with little variation in the threshold voltage, mobility, and subthreshold swing. Such results demonstrate that, combined with the hybrid substrate structure, the new patterning technique can be a useful tool for manufacturing high-performance and high-density stretchable circuits. As the whole processes are performed by means of conventional technologies based on photolithography, it would be possible to integrate liquid metal interconnects monolithically with various thin film devices at the scale of tens of micrometers, which has been nearly impossible with previous patterning techniques.18,21−34

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AUTHOR INFORMATION

Corresponding Authors

*Chan Woo Park. E-mail: [email protected]. *Jae Bon Koo. E-mail: [email protected]. Author Contributions

The paper was written through contributions of all authors. All authors have given approval to the final version of the paper. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Institute for Information & Communications Technology Promotion (IITP) grant funded by the Korea government (MSIP) (B0101-16-0133, the core technology development of light and space adaptable energysaving I/O platform for future advertising service).

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REFERENCES

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4. CONCLUSIONS We have demonstrated a new patterning technique for galliumbased liquid metals, which is based on conventional photolithography and lift-off processes. In a very similar manner as in the patterning of solid metal films, the liquid metal layer painted over the whole substrate area could be selectively removed by dissolving the underlying photoresist layer, leaving behind robust liquid patterns as defined by the photolithography. This quick and simple method makes it possible to integrate fine-scale interconnects with preformed devices precisely, which is indispensable for realizing monolithically integrated stretchable circuits. As a way for constructing stretchable integrated circuits, we proposed a hybrid configuration composed of rigid device regions and liquid interconnects, which is constructed on a rigid substrate first but highly stretchable after being transferred onto an elastomeric substrate. Although some process parameters still need to be optimized further, this new method could be useful in various applications requiring both high-resolution and precisely aligned patterning of gallium-based liquid metals.

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current−voltage characteristics of liquid metal traces on Si with widths of 20, 35, and 75 μm (Figure S4) (PDF).

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b01896. Schematic illustration of the surface modification process for the parylene layer (Figure S1); patterning process of the liquid metal on a Si substrate (Figure S2); height profiles of the liquid metal patterns at three representative points across 30 mm-long traces (Figure S3); F

DOI: 10.1021/acsami.6b01896 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.6b01896 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX