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This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.JOURNAL OF MICROELECTROMECHANICAL SYSTEMS1Laser Shock-Induced Conformal Transferring ofFunctional Devices on 3-D Stretchable SubstratesHuang Gao, Rui Tang, Teng Ma, Hanqing Jiang, Hongyu Yu, and Gary J. ChengAbstract— This paper discussed a top–down integrationmethod to achieve the three-dimensional (3-D) microscale conformal transferring of functional devices on flexible elastomericsubstrates at ambient conditions. By the tunable laser-inducedpressure, the functional device inherits the microscale wrinklelike patterns, without compromising functions. The functionalmaterials are encapsulated in the biocompatible parylene layersto avoid the drastic plastic deformations in functional layers. Theelectrical resistivity of functional device increases marginally withthe applied laser intensity, aspect ratios of microscale features,and overall tensile strain applied to the whole flexible assembly.The stretchability of the transferred functional devices was studied by measuring the electrical property as function of bendingand tensile strains. It shows that the device can sustain more than40% strain in the stretchable substrate. It is demonstrated thatthe process can achieve the flexible and stretchable functionalintegration conformal to 3-D micrometer-patterns in a fast andscalable way.[2013-0365]Index Terms— Laser shock, transfer, functional devices,stretchable substrates.I. I NTRODUCTIONAS THE building blocks of modern instruments andequipments for sensing, computation, display and communication, electronic semiconductor components are revolutionized nowadays by two distinct strategies: miniaturizationand stretchability/flexibility. While miniaturization improvesthe computation power in the limited space, systems thatare highly bendable, stretchable, conformable to any surfacetopology, and mechanically robust would greatly expand thehorizon of applications. The diversified applications, such asflexible solar cells [1], flexible displays [2], flexible stressManuscript received November 27, 2013; revised May 29, 2014; acceptedJune 14, 2014. The work of G. J. Cheng was supported in part by the NationalScience Foundation (NSF) CAREER Award CMMI-0547636, and in part bythe NSF through the Materials Processing and Manufacturing Program underGrant CMMI-0928752. The work of H. Jiang was supported in part by theNSF under Grant CMMI-0700440. Subject Editor H. Fujita. (Correspondingauthor: Gary J. Cheng.)H. Gao and G. J. Cheng are with the School of Industrial Engineering,Purdue University, West Lafayette, IN 47907 USA (e-mail:; Tang is with the School of Electrical, Computer, and EnergyEngineering, Arizona State University, Tempe, AZ 85287 USA ( Ma and H. Jiang are with the School for Engineering of Matter,Transport, and Energy, Arizona State University, Tempe, AZ 85287 USA(e-mail:; Yu is with the School of Electrical, Computer, and Energy Engineering,Arizona State University, Tempe, AZ 85287 USA; and is also with the Schoolof Earth and Space Exploration, Arizona State University, Tempe, AZ 85287USA (e-mail: versions of one or more of the figures in this paper are availableonline at Object Identifier 10.1109/JMEMS.2014.2332512sensors [3], wearable electronics [4], as well as medicalimplants for health monitoring and disease diagnostics [5],have motivated much progress in various fabrication methodsof stretchable conductors and devices.Most of the fabrication methods are bottom-up and categorized into two major strategies, either “stretchable materials”or “stretchable structures” [6]. In the first strategy, the conductive material, such as metals and carbon nano-material,is either dispersed into an elastic matrix to form elasticconductive composites [7] or deposited on the substrate surface [8]. The elastic conductor is readily capable of enduringtensile strain more than 100%, but its conductivity changesor even diminishes when defects and delamination occur athigh tensile strains or after fatigue testing. Kim et al., [9]demonstrated stretchable conductors of polyurethane containing spherical nanoparticles deposited by either layer-by-layerassembly or vacuum-assisted flocculation. High conductivityand stretchability were observed in both composites despite theminimal aspect ratio of the nanoparticles. These materials alsodemonstrate the electronic tunability of mechanical properties,which arise from the dynamic self-organization of the nanoparticles under stress. Our new strategy can directly transferfunctional devices conformally onto 3D surfaces of flexibleand stretchable substrate, and allow the assembly to inheritstretchability simultaneously. In the second strategy, the brittledevices are bonded onto elastomeric substrates and interconnected by stretchable spring-like ribbons of highly conductivemetal or alloy [10]. The ribbons are selectively bonded withelastomeric substrates by surface energy differentiation toform either in-plane serpentines or out-of-plane buckles [11].The ribbons deform to experience most of stretching andbending so that the brittle devices are almost immune toexcessive strains and failure. This strategy requires speciallycustomized microfabrication and metal evaporation technologies, and relatively complicated multiple transfer processes [8].The maximum strain is usually limited to the range of20% to 70% due to adhesive failure and interconnectionredundancy. In addition, this method can only be applied tomaterials that could generate buckling under compression. Itis still an open topic to demonstrate how to assemble morecomplicated multilayer devices on microscale 3D-patternedsurfaces.II. P ROCESS S CHEMEThe top-down method proposed in this study, laser shockinduced transferring (LST), provides an alternative to selectively fabricate stretchable functional structures conformal to1057-7157 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See standards/publications/rights/index.html for more information.

This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.2Fig. 1.JOURNAL OF MICROELECTROMECHANICAL SYSTEMSThe process scheme of flexible assembly by LST.3D microscale surface of the underlying elastomeric substrate.Fig. 1 illustrates the fabrication process schematically. Thelaser-induced shockwave propagates through to-be-deformedlaminated functional materials and provides sufficient momentum to achieve the conformal assembly with the underlyingμ-patterned flexible substrates. The shockwave arises whenthe incoming laser pulse irradiates an ablation layer abovefunctional materials and turns it into a plasma plume. Theμ-patterns on the flexible substrates could be either replicatedfrom rigid substrates multiple times by Polydimethylsiloxane(PDMS) replica molding process [12] or induced by prestraining and oxygen plasma treatments [13]. The patternedmicroscale features are designed to mimic surface wrinkling inpractical surface conditions during biological and biomedicaltransfer, which have been widely applied to tunable adhesionand wetting [14], tunable open-channel microfluidics [15],and stretchable electronics [16]. To minimize the mechanicaldamage to functional materials during deformation, they areencapsulated by symmetrical parylene layers and stay atmechanical neutral plane. The parylene layer not only absorbsmost of shockwave energy, but also provides compliant mediabetween functional layer and sharp microfeatures to mitigatethe localized intensive deformation. A uniform nanolayer ofadhesive (SU-8-2000, MicroChem ) will be spin-coated onthe surface of encapsulated functional materials interfacingflexible substrates to improve the bonding strength duringfurther transfers requiring stretching and bending. Anotherstrategy to preserve the assembly functions is to differentiateplastic strains in ductile and brittle functional materials bypatterned 3D surfaces. It is expected that metallic connectionsaligned to wavy regions will experience moderate plasticdeformation, while the devices aligned with flat regions willhave minimal plastic strains.LST is a flexible and fast-shaping process suitable for mesomicroscale 3D integration. The transferring process is ultrafastdue to the nature of short pulse laser material interaction.During each pulse of laser beam (diameter 1–10 mm),the functional material could be faithfully transferred to3D microscale surface of the underlying substrate withinnanoseconds. In order to integrate large areas, laser beamcould be operated in scanning mode. Given a specific configuration of confining media and ablation material, the laser pulse intensity, i.e. pulse energy averaged over beam size, determines the shockwave pressure. The laser intensity could be programmed to changeinstantaneously for different 3D microscale features andfunctional materials. The coded laser intensity also ensureshigh repeatability over multiple tests. The forming area, pending on laser beam size, could be changed in the range of fewmicrometers to millimeters. The laser pulse could readily scanover wafer-scale area or irradiate through mask to generatepatterned shaping. The forming process could be consideredto be athermal owing to the ablation and sacrifice layersabove functional materials, which minimize thermal damageto functional materials. Compared with other stage-transfertechniques, it requires no complementary mold and reducesprocess complexity and cost. As the non-contact driving force,the shockwave has demonstrated its advantages in consistentmesoscale, microscale to nanoscale deformations conformalto the underlying 3D surfaces [17]. The conformal assembly of functional materials and the 3D microscale patterns(μ-patterns) has great potential to realize electronic systemsmore sensitive and responsive to environmental change.III. E XPERIMENTAL M ETHODSFig. 2 illustrates the experimental setup of LST to selectively fabricate stretchable functional structures conformal to3D μ-patterns on the underlying elastomeric substrate. Thelaser pulse is provided by a Q-switched Nd:YAG laser inTEM00 mode and uniformed by a beam diffuser. It transmitsthrough the top glass sample holder and irradiates the ablation

This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.GAO et al.: LASER SHOCK-INDUCED CONFORMAL TRANSFERRING OF FUNCTIONAL DEVICESFig. 2.3The experimental setup of conformal flexible assembly by LST.Fig. 3. Fabrication process of the shear stress sensor. (a) Thermal growth of SiO2 and deposition of sacrificial Si layer (1 μm). (b) Deposition and patterningof Ti/Pt layers (0.12 μm /0.02 μm) for the sensing element. (c) Deposition of Parylene-C (9 μm). (d) Deposition and patterning of a metal layer of Cr/Aufor electrode leads (0.6 μm). (e) Deposition and patterning of a thick layer of Parylene-C (12 μm) to form the device structure. (f) Etching the underneathSi sacrificial layer leading to the final device.layer (graphite coating) into the plasma instantaneously. Confined by the sample holder, the plasma generates a transientshockwave which propagates through the aluminum sacrificelayer and the laminated functional materials and shapes theminto structures conformal to the underlying 3D surfaces. Thegraphite ablation was sprayed onto the upper surface ofaluminum sacrifice layer while the bottom surface interfaceswith the laminated thin film, in order to prevent the graphitepollution and direct laser irradiation. The laminated thin filmand 3D surface are bonded simultaneously by spin-coatedadhesive on the interface. The magnitude of shockwave pressure is in the range of hundreds of MPa to several GPa,determined by laser intensity, shock impedance of confiningmedia and ablation coating, as well as absorption coefficientof laser energy in plasma generation. In this study, shockwavepressure can be controlled by laser intensity solely to preservethe proper function of fabricated devices. The flexible PDMSsubstrates replicate μ-patterns in Si master molds faithfully bythe method of mixing and curing the Dow Corning’s Sylgard184 elastromer kit at 80 for 90 mins before being peeled offfrom Si molds.The functional laminates, such as shear stress sensor, wasfabricated by surface micromachining of polymer Parylene-C.Parylene refers to a variety of chemical vapor deposited poly(p-xylene) polymers functioning as moisture and dielectricbarriers. Among them, Parylene-C is most widely used considering its advantages in barrier properties, cost, and processingmethods. In Fig. 3, the fabrication process includes: 1) drythermal grow 0.3 μm thick SiO2 on Si wafer, followedby deposition of 1 μm sacrificial silicon layer (polysilicon)

This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.4JOURNAL OF MICROELECTROMECHANICAL SYSTEMSFig. 4. Schematic of the shear stress sensor. (a) Sensing element; (b) Bond pads. The 3 lines bridging two electrodes are the sensing element, Ti/Pt layerswith thickness of 0.12 μm/0.02 μm and patterned by lift-off to form a 2–3 μm wide metal traces [18].through LPCVD. This layer will be etched completely during sensor releasing. Then, dry thermal grow approximately0.2 μm SiO2 ; 2) deposit Ti/Pt layers with thickness of0.12 μm/0.02 μm for the sensing element with e-beam evaporator, and pattern these layers by lift-off to form a 2 μmto 3 μm wide metal traces; 3) deposit about 9 μm ParyleneC layer in the Parylene vacuum coating system and patternthis layer by oxygen plasma; 4) deposit and pattern a metallayer of Cr/Au for electrode leads (0.02 μm/0.6 μm); 5)deposit another layer of Parylene-C (12 μm) and patternboth Parylene-C layers to form the device structure; 6) etchthe underneath silicon sacrificial layer through XeF2 dryetching system, resulting in the final device. The overall sensordimensions were 4 cm in length, 320 μm in width, and 21 μmin thickness, as illustrated in Fig. 4. We can also start fromSOI wafers and skip the first two steps, but it may furtherincrease the cost due to the higher unit price of SOI wafers.IV. R ESULTS AND DISCUSSIONFig. 5(a) shows the bending of a conformal assembly of alaminated shear stress sensor with a flexible PDMS mold, bythe laser intensity of 0.25 GW/cm2 , with the zoom-in viewin Fig. 5(b). In Fig. 5(c) and (d), it shows the images beforeand after LST of a laminated shear stress sensor on a flexibleμ-patterned PDMS mold. In Fig. 5(d), the μ-patterns on thesurface of PDMS mold, replicated from a master Si micromold, were faithfully transferred to the ductile connection ofthe sensor. The μ-pattern is 5 mm square in area, 160 μmin pitch width and 50 μm in depth. The sensor resistivitychanged from 1.215K to 1.292K after LST and Temperature Coefficient of Resistance (TCR) was kept unchangedat approximately 0.11%/ C. The electrical resistance R ofthe metallic connection is determined by its length l, itscross-sectional area S, and the intrinsic resistivity ρ of thematerial: R ρlS . The slight increase of sensor resistivityresults from the longitudinal extension and the cross-sectiondecrease of ductile metallic connections arising from thelocalized plastic deformation conformal to the μ-patterns.The deformation-induced microstructural defects, such asdislocations, will also contribute to the resistivity increase. Theapplied laser intensities and the aspect ratios of μ-patternsneed to be controlled to avoid the fracture and failure ofmetallic connection. This method assumes that the parylenelayer absorbs most of shockwave energy while providing acompliant soft substrate to the encapsulated functional layer.To validate this assumption, the laminated sensor was replacedby a 5 μm thick laminated thin film. Its cross section wasfabricated by Focused Ion Beam (FIB) and characterized inScanning Electron Microscope (SEM) in Fig. 5(e), showing itssymmetric structures deposited on Si wafer. The thin film wasdeformed on another PDMS mold with wavy surface featuresand peeled off from PDMS substrate and characterized individually upside down in FIB in Fig. 5(f), since non-conductivePDMS substrate would deteriorate local ion beam cuttingand observation if otherwise. Its cross section shows that,after laser-induced shock and deformation, the sandwichedAu layer keeps its thickness almost constant along the wavewhile parylene is subject to violent thickness changes, esp. inthe layer interfacing PDMS mold. At the wave valley withsharp curvature, parylene thickness increases from 2.4 μm to7.86 μm. The experimental observation reveals two underlyingmechanisms coexisting to achieve uniform and significant

This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.GAO et al.: LASER SHOCK-INDUCED CONFORMAL TRANSFERRING OF FUNCTIONAL DEVICES5Fig. 5. (a) Bending of conformal assembly of a laminated shear stress sensor with a flexible PDMS mold, by the laser intensity of 0.25 GW/cm2 . (b) Thezoom-in view of (a) demonstrates the structure after the sensor is transferred to the microscale pattern on the surface of PDMS mold. (c) The optical imagesshows the integration of sensor and PDMS micromold before LST (d) after transferring of a laminated shear stress sensor to a flexible μ-patterned PDMS mold.The laser intensity applied to make this assembly is 0.25 GW/cm2 . The SEM images of conformal assembly of laminated thin film with a wavy PDMS moldand the cross sectional view of the parylene layer and metallic connection layer (e) before and (f) after LST. After LST, the laminated thin film was peeledoff from PDMS mold, then separately cut and characterized its convex “Lower Side” upside down in FIB/SEM. The applied laser intensity is 0.30 GW/cm2 .plastic strain in functional layer. First, the encapsulation layernot only allows functional material to sustain large plasticstrain, but also provides compliant media between functionallayer and sharp microfeatures to mitigate the localized intensive deformation. More importantly, the permanent plasticdeformation of parylene layer absorbs most of laser-inducedshockwave energy. Second, the through-thickness compressiveshockwave suppresses the debonding between parylene andAu layer so as to reduce the localized necking. It has beenproved experimentally and analytically that the well-bondedmetallic interconnections could sustain strains up to a fewtens of percent without appreciable cracks since the localized necking could be suppressed by the adherent flexiblesubstrate [19].

This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.6JOURNAL OF MICROELECTROMECHANICAL SYSTEMSFig. 6. The resistivity changes of shear stress sensors in the flexible assemblies: (a) due to the increasing laser intensities and aspect ratios of μ-patterns;(b) due to uniaxial stretching; (c) due to pure bending. The samples in (b) were fabricated by laser intensity: 0.25 GW/cm2 in Group (a), and those in (c) bylaser intensity: 0.13 GW/cm2 in Group (a). In (d), d is the thickness of flexible assembly, which is approximately equal to the thickness of PDMS substratesince that of shear stress sensor is only 20 μm. r is the radius of cylindrical surface.TABLE IT HE D IMENSIONS OF M ICRO -PATTERNS R EPLICATED I NTOPDMS S UBSTRATEThe applied laser intensities and the dimensions or aspectratios of μ-patterns need to be controlled to avoid in

laser-induced shockwave propagates through to-be-deformed laminated functional materials and provides sufficient momen-tum to achieve the conformal assembly with the underlying μ-patterned flexible substrates. The shockwave arises when the incoming laser pulse irradiates an ablation layer above functional

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