Direct Laser Writing-Based Programmable Transfer Printing via

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Direct Laser-Writing Based Programmable Transfer Printing via Bio-Inspired Shape Memory Reversible Adhesive Yin Huang, Ning Zheng, Zhiqiang Cheng, Ying Chen, Bingwei Lu, Tao Xie, and Xue Feng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11696 • Publication Date (Web): 29 Nov 2016 Downloaded from http://pubs.acs.org on December 1, 2016

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ACS Applied Materials & Interfaces

Direct Laser-Writing Based Programmable Transfer Printing via Bio-Inspired Shape Memory Reversible Adhesive Yin Huang1,2, Ning Zheng3, Zhiqiang Cheng1,2, Ying Chen1,2, Bingwei Lu1,2, Tao Xie3* and Xue Feng1,2* 1

AML, Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China

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Center for Mechanics and Materials, Tsinghua University, Beijing 100084, China

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State Key Laboratory of Chemical Engineering, College of Chemical and Biological

Engineering, Zhejiang University, 38 Zheda Road, Hangzhou, 310027, P. R. China Keywords: Programmable transfer printing, direct laser-writing, bio-inspired, shape memory polymers, micropatterns

Abstract: Flexible and stretchable electronics offer a wide range of unprecedented opportunities beyond conventional rigid electronics. Despite the vast promise, a significant bottleneck lies in the availability of a transfer printing technique to manufacture such devices in a highly controllable and scalable manner. Current technologies rely usually on manual stick-and-place and do not offer feasible mechanisms for precise and quantitative process control especially when scalability is taken into account. Here we demonstrate a spatio-selective and programmable

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transfer strategy to print electronic micro-elements onto a soft substrate. The method takes advantage of automated direct laser-writing to trigger localized heating of a micro-patterned shape memory polymer adhesive stamp, allowing highly controlled and spatio-selective switching of the interfacial adhesion. This, coupled to the proper tuning of the stamp properties, enables printing with perfect yield. The wide range adhesion switchability further allows printing of hybrid electronic elements, which is otherwise challenging given the complex interfacial manipulation involved. Our temperature-controlled transfer printing technique shows its critical importance and obvious advantages in the potential scale-up device manufacturing. Our strategy opens a route to manufacturing flexible electronics with exceptional versatility and potential scalability. Introduction Flexible and stretchable electronics represent an ongoing technological revolution in the electronics industry.1-4 Their conformal and adaptive nature offers unique advantages over conventional rigid electronics. Amongst numerous possibilities, for instance, flexible electronics is emerging as an ideal physical bridge for the mechanically incompatible interface between human (soft) and machine (rigid).5-7 Inorganic flexible and stretchable electronics, in particular, have attracted significant attention owing to the high performance inherent in inorganic electronic components.5, 8, 9 The intrinsic rigid brittle nature at the individual component level can be successfully translated to the flexibility (even stretchability) at the whole device level via component miniaturization along with delicate mechanical designs.2 From the device growth standpoint, such a strategy takes full advantages of the mature practices in the inorganic semiconductor industry. The critical challenge lies in forming a device on a flexible plastic substrate with the appropriate mechanical layout. The high temperature and severe chemical environment


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typically encountered in the fabrication of inorganic electronic components make it impossible for direct device growth on most plastic substrates.10 Transfer printing has emerged as the most promising solution in which devices are printed under a very mild condition onto a flexible substrate (receiver) after their fabrication on a donor substrate (e.g. silicon wafer).10-12 Typically, it involves the use of an elastomeric stamp (a soft adhesive) to pick up discrete micro-/nano-scale inorganic devices from a donor substrate and subsequently print onto a receiver substrate as spatially organized functional arrays. While seemingly simple, achieving complete transfer and accurate positioning of the devices is pivotal and an uneasy task, especially when process scalability is taken into account. The challenge here is to manipulate the interfacial adhesion between the stamp and inorganic devices in an active and robust manner. Kinetically controlled transfer printing adopts a viscoelastic stamp to retrieve the micro-devices rapidly from the donor and print them slowly onto the receiver.12 However, the tunability of the interfacial adhesion based solely on the rate-dependent adhesion of elastomers is quite limited. The adhesion switchability, reflected in the ratio between the strong and the weak adhesion, can be greatly improved via the introduction of relief micro-patterns on the surface of the elastomeric stamp.13 While promising, how to control the process reliably in a potentially more scalable manner remains an unresolved challenge. An interesting attempt is to use a laser pulse to control the interfacial adhesion. Such a laser-driven transfer printing relies on interfacial thermal mismatch (thus stress) due to laser heating to facilitate the release of the micro-devices.14 The approach, however, has an intrinsic limitation in that reaching interfacial stress sufficient for device release requires very high temperatures (e.g. 300 ºC) that are detrimental to the plastic substrates. In addition, the relatively high powered laser required is a significant safety concern for actual manufacturing.


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An ideal transfer printing technique should offer easily controlled adhesion switchability. The ongoing pursue of such stands in contrast to various examples of reversible (switchable) adhesion found in nature. In particular, the adhesion mechanism behind the nano-structured gecko footpads15 has stimulated significant effort in controlling the adhesion of elastomers via surface micro-patterns.16-20 While interesting, typical gecko-inspired surface patterned adhesives rely on external mechanical forces for adhesion switching, somewhat similar to the non-ideal practices in transfer printing. In contrast, a gecko uses an internal mechanism, the “peeling” action by its own toes, for active adhesion release triggered by its nerve signals. Recognizing this critical difference, we have previously developed a shape memory polymer (SMP) based reversible adhesive.21,

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The shape changing nature of SMP,23,

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as typically triggered by

temperature variation, leads to a new generation of actively releasable adhesive with a much easier controlling mechanism compared to the mechanical adhesion switching. The SMP deforms to a temporary shape in response to external pressure above the shape memory transition temperature Tg. Cooling down to a temperature below Tg, the SMP will maintain its deformed, temporary shape even after the external pressure is removed. If the temperature is increased above Tg again, the SMP will recover to its original undeformed shape due to the shape-memory effect. Introduction of surface micro-patterns allows further control, as demonstrated by transferring of free-standing silicon inks.25 For actual transfer printing, however, an ideal SMP adhesive should possess the right mechanical and adhesive properties to accommodate diverse devices with various brittleness and interfacial properties. In addition, fully exploring the versatile shape changing characteristics of SMP may present new opportunities in its use for transfer printing. Such efforts are described hereafter.


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Laser-induced forward transfer (LIFT) is one of the important direct-write techniques for printing solid pixels or liquid droplets with high-resolution.26-27 For the transfer of solid pixels, irradiating a thin layer with a pulse laser to induce a light-matter interaction at the interface and generate a strong increase of the local pressure. As a result, a small pixel of the thin film is ejected from the donor to the target substrate. This single-step process does not necessarily require a special vacuum or any shadow masks. More importantly, this procedure allows performing surface micro-patterning without altering the transferred material. Despite all the above merits, the LIFT process relies on mechanical forces to break the thin film and is not readily suitable for printing brittle materials widely used in inorganic electronics, i.e. Si and GaAs. With the above considerations in mind, we propose an ultra-low power direct laserwriting strategy to transfer print inorganic materials by taking advantage of the active shape changing characteristics of SMPs. Results and Discussion Micro-patterned SMP stamps with cone arrays were fabricated using lithographically defined silicon molds. A representative scanning electron microscope (SEM) image of such a stamp is shown in Supplementary Figure S1. The programmable nature of SMP opens up a rather unique opportunity for spatio-selective printing. The concept of the laser-triggered programmable transfer printing is illustrated in Figure 1a. The process consists of two major steps: pick up and programmable printing. For pick up, the SMP stamp is first uniformly heated above its glass transition temperature (Tg) and pressed against micro-devices on the donor substrate. The pressing force (preload) leads to the deformation of the cone tips. Cooling the SMP stamps to the room temperature under the preload fixes the temporary configuration of the cones owing to their shape memory characteristics. This establishes robust interfacial contact and most critically


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strong adhesion with the devices. The SMP stamp is then lifted vertically away from the donor substrate and, importantly, the micro-devices remain firmly attached to the stamp due to the strong stamp-device adhesion. This completes the “pick-up” process. In the next step (i.e. programmable printing), the SMP stamp with the devices is first moved to a target substrate. Applying a gentle load again ensures that the devices come into full contact with the target substrate. An intended area of the SMP stamp is then selectively heated above its Tg via a laser beam. Subsequent release of the load causes the deformed cones in the heated area to recover to their original configuration. This leaves the devices in this area in firm contact with the target substrate, importantly minimal contact with the stamp through its recovered sharp tips. Retracting the SMP stamp selectively leaves the devices in the heated area on the target substrate, completing the programmable “printing” process. The same stamp can be repeatedly used for multiple cycles of transfer printing due to its recovery capability. The above process utilizes the shape memory characteristics of the stamp to modulate the interfacial contact (thus adhesion) by controlling the temperature. The method is largely independent of the retraction velocity and thus more suitable for scale-up operation. While the process is conceptually feasible, its actual fait towards transfer printing depends critically on the availability of a suitable SMP. Besides excellent shape memory performances, the SMP should exhibit strong adhesion for pick up. The residual adhesion for device release should also be minimized, but this is determined mostly by the geometric design of the arrays (shape tips) and much less by the SMP material. At first, an epoxy SMP adhesive developed by us21-22 and showed promise in printing thick silicon disks25 appeared to meet the above requirements.

Our initial attempts using this SMP to print ultrathin

silicon ribbons yielded rather unfavorable results with low transfer yields and damaged ribbons (Supplementary Figure S2). We suspected that this was likely related to the poor deformability


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of the SMP. The epoxy SMP used in our initial trials was heavily crosslinked, leading to its high rubbery modulus (ca. 10 MPa) and low breaking strain (200%).29 Presumably, the excellent deformability (softness and high strain) should favor transfer printing. Indeed, this was confirmed by the results presented below with this newly developed epoxy SMP (see method section).


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Figure 1. Programmable transfer printing process via laser writing. (a) Schematic illustration of the printing process via automated laser-writing on a micro-patterned shape memory polymer stamp. (b) Diagram of the automated printing setup. Herein, the spatio-selective shape change capability as triggered by laser was explored. An array of silicon squares was first picked up by a SMP stamp. The stamp was then placed onto a PDMS sitting on a custom-built motorized translation stage (Figure 1b). The setup allows focusing a laser beam onto the SMP/Si/PDMS printing assembly with a high precision. The laser beam can scan through the intended printing area(s), induce localized heating and tip recovery, eventually spatio-selective device printing. Figure 2a shows experimentally the actual geometrical change of the cone tips in the heated region and unheated regions, suggesting that laser writing can indeed be used for selective debonding. The geometrical change of the cone tips


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is indicated by their color transformation, which will be discussed in more detail later in this paper. In principle, such a process should allow silicon squares to be printed at any arbitrary layout on a PDMS substrate. Figure 2b~d shows the printed silicon squares in representative line-shape, cross-shape and T-shape layouts, confirming such an unusual capability. Of importance here is that a low power blue laser (1 W) is sufficient to enable such an operation due to the mild temperature increase needed to induce the SMP recovery. The process is further facilitated by the low light absorption by the SMP and PDMS, which ensures that most of the laser energy is absorbed by the exposed micro-devices, importantly with the heat confined to their immediate nearby areas due to the low thermal conductivities of the SMP and PDMS. The overall printing resolution is thus largely determined by the diameter of the laser, which is approximately 800 µm in the current case.


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Figure 2. Experimental transfer printing results via laser writing. (a) Spatio-selective debonding via laser writing. The red lines illustrate the heated region. Cones in the heated region become dark whereas those in the unheated region remains light colored, indicating their recovered nonadhesive state and flattened adhesive state, respectively. (b~d) Printed patterns on PDMS in line, cross, and T-shape, respectively. (scale bars: 100 µm) Cone arrays were chosen as the SMP micro-patterns for this study although we note that many other geometries may also be suitable provided that the deformation of the micro-patterns should lead to a larger surface contact (adhesion) and, upon recovery, return to the minimum contact (adhesion) state. Several cone geometric parameters may impact the interfacial contact and adhesion including the bottom radius R and the height H for each cone and center to center distance d between two cones. Figure 3a~d show the evolution of geometric change occurred to a cone array (R=4.5 µm, H=4.1 µm, d=20 µm) during a transfer printing process. The as fabricated cones have sharp tips (Figure 3a). With a preload and temperature changes needed for pick up (Figure 1), the tips are deformed, leading to the temporarily fixed flattened tops. The degree of flattening increases with the preload (Figure 3b and 3c), with the preload of 22 µN per cone corresponding to completely flattened tops (Figure 3c). Importantly, upon heating under a stress free condition, the deformed cones recover to their original state (Figure 3d) as is required for the printing step. Supplementary Figure S4 shows the corresponding 2D geometrical evolution of the cone array. For two separate sets of cone arrays (R=4.5 µm, H=4.1 µm, d=20 µm and R=9.0 µm, H=7.1 µm, d=30 µm), the interfacial contact area (Figure 3e) increases linearly with the preload per cone until it reaches the completely flattened maximum contact area state. In further consideration of reliable delamination from silicon wafers, two sets of cone arrays (R=9.0 µm, H=7.1 µm, d=20 µm or 30 µm) were used to conduct adhesion test. For both


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arrays, the apparent adhesion (Figure 3f) initially increases linearly with the preload per cone. When the cones are completely flattened, the apparent adhesion increases abruptly (marked by the dash line) due to the dramatic increase of the contact area and keeps almost constant after that. Hereafter, we used these two sets of cone arrays for further transfer printing studies.

Figure 3. Evolution of surface topography, interfacial contact, and adhesion for an SMP stamp. 3D laser microscopic images (Keyence, VK-X200) of a cone array during transfer printing. (a) Original configuration. (b) Under the preload of 11 µN for each cone. (c) Under the preload of 22 µN for each cone. (d) Recovered configuration. (e) Dependence of the interfacial contact area


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on the preload for each cone. (f) Variation of the apparent adhesion with the preload for each cone (R=9.0 µm, H=7.1 µm). Si ribbons and Si squares were picked up from the donor substrate by the SMP stamp and then transferred to a PDMS elastomer (Sylgard 184 cured at 60 ºC for 4 hours) to investigate quantitatively the transfer printing yields. Figure 4a shows that the silicon squares were perfectly transferred onto the SMP stamp. Thicker Si squares (2 µm) and silicon ribbons (thickness: 200 nm) were similarly transfer printed onto PDMS (Supplementary Figure S6). Herein, the shape and thickness variations of these Si components indicate different mechanics (e.g. component fragility and interfacial adhesion) for transfer printing. Despite such, the transfer printing yield was 100% in all cases and no component damage was observed. Again, this was due to the use of an advanced epoxy SMP stamp with favorable properties and geometric design. The importance of the latter is reflected in the much less satisfactory transfer printing yields (2050%) obtained using SMP stamps with a much larger center to center spacing of 50 µm. Besides the transfer printing yield, a bigger challenge lies in simultaneous transfer printing of devices made from different substrates, which we call hybrid transfer printing. Below, we attempt to achieve hybrid transfer printing of Si and GaAs utilizing the aforementioned micropatterned shape memory polymer stamps. Herein, Si is a semiconductor whereas GaAs is a photoelectric semiconductor material. Thin devices of these two materials require different deposition methods, interfacial layers, and growth substrate (Figure 4b). The drastically different interfacial adhesion as well as component fragility makes the hybrid printing particularly challenging, yet such a versatility is extremely desirable from the standpoints of device manufacturing. Gratifyingly, we found the robustness of our transfer printing method can


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be extended for hybrid transfer printing, as evidenced in Figure 4c showing printed Si and GaAs ribbons in parallel alignment on a PDMS substrate, which is fully consistent with the intended layout (Figure 4b). In addition, no defect can be identified and the printed GaAs ribbons remain fully functional, as evidenced by the appearance of the normalized intensity peaks in the photoluminescence (PL) spectra (Figure 4d) obtained under illumination of 1% and 10% of the total laser power, respectively. The above study clearly showed that the challengeable hybrid transfer printing can be achieved by our versatile technique with perfect transfer printing yields.

Figure 4. Hybrid transfer printing. (a) Transfer printing of Si squares (thickness: 200 nm): Si squares were first transferred onto the SMP stamp with deformed micro-patterns (top), the light colored cones reflected that they were in the flattened adhesive state; The micro-patterns then recovered to their original configuration when heated (middle), the dark color of the cones indicated that they recovered to their sharp geometry (non-adhesive state); Finally, Si squares were printed onto PDMS substrate (bottom). (b) Comparison of interfaces and growth substrates


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for Si and GaAs and the intended hybrid printing layout. (c) Experimental printed Si and GaAs ribbons on PDMS. (d) Photoluminescence spectra of the GaAs ribbons on PDMS. (Scale bars: 100 µm). Conclusion In summary, we demonstrate a highly efficient and versatile transfer printing technique using a micro-patterned SMP adhesive as the transfer stamp. With the critically important advance in SMP with favorable properties, transfer printing of diverse silicon components is achieved with perfect yields with no device damage. The active nature of the SMP stamp allows controlling the process via temperature instead of retraction velocity for conventional passive transfer printing. The quantitative, synchronous temperature control on the full-wafer scale makes our technique uniquely advantageous in the potential scale-up (e.g. full wafer) device manufacturing. The robustness of our technique further allows hybrid printing of devices of drastically different materials. Mostly importantly, the programmable transfer printing using a direct laser-writing technique represents an unprecedented versatility. Overall, the laser-writing based SMP transfer printing strategy opens up the door for scalable integration of sophisticated electronic devices. Experimental Section Fabrication of silicon micro-molds. A thin layer of silicon dioxide was first deposited on a clean Si (100) wafer. With a photoresist mask, this layer of silicon dioxide was defined to a circular pattern by etching in a diluted hydrofluoric solution (9%). The photoresist was washed away with acetone. The Si wafer was further etched in a KOH solution (250 g of KOH, 250 g of IPA, 800 g of H2O) at 80 ºC using the patterned silicon dioxide layer as the protective mask.


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Afterwards, the wafer was immersed into the diluted hydrofluoric solution again to remove the silicon dioxide layer. In the final step, the silicon mold was treated with 1H,1H,2H,2Hperfluorooctyltriethoxysilane in a vacuum chamber for 15 hours. Preparation of SMP stamps. SMP stamps were prepared using a classical molding technique. An epoxy monomer E44 (China petrochemical corporation) and the curing agent poly(propylene glycol)bis(2-aminopropyl) ether (Jeffamine D230, Sigma-Aldrich) were first mixed at a weight ratio of 45:23. The mixture was poured onto a silicon mold in an aluminum pan. The epoxy was cured at 100 ºC for 2 hours followed by postcuring at 130 ºC for another 1 hours. Finally, the SMP stamp was carefully peeled away from the mold. Adhesion measurement. An SMP stamp above its Tg was first pressed against a thick silicon square (10 mm × 10 mm × 1 mm). Cooling under the preload led to adhesion between the SMP stamp and the Si square. Then, one side of the SMP/Si system was bonded to a fixed stainless steel plate supported by blocks at two ends, while the other side was bonded to a custom-made thick glass slide with a hook. During the adhesion test, a plastic holder was hung on the hook and weights were progressively added to the holder until the silicon wafer separated from the SMP stamp. The maximum force per unit area the SMP/Si system was designated as the adhesion. Fabrication of Si ribbons and squares. Silicon-on-insulator (SOI) wafers with thickness of 200 nm or 2 µm were used as the source of Si. The Si ribbons and squares were defined through a photoresist mask by dry etching with inductively coupled plasma (ICP). The underlying SiO2 layer was etched away in a dilute hydrofluoric acid solution (HF, ∼12%) to yield the ribbons and squares. Importantly, it was necessary to anchor the ribbons and squares before the SiO2 undercut. In the case of ribbons, this was done with extra square pads at both ends of ribbons. In


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the case of squares, the four sides of each square were fixed onto the substrate using a photoresist layer in a strip shape. Fabrication of GaAs ribbons. A top layer of GaAs and a buried sacrificial layer were deposited on a GaAs wafer using an epitaxial growth method to yield a multilayer structure (GaAs/Al0.9Ga0.1As/GaAs, 600nm/500nm/500µm). A patterned photoresist layer was coated onto the wafer using a mask. Etching in a H2O2: H3PO4:H2O=1:1:20 solution for about 150 seconds led to a patterned multilayer structure in dogbone-shapes with ribbons (50 µm x 400 µm) anchored by square pads (100 µm x 100 µm) at both ends of each ribbon. Finally, the sacrificial Al0.9Ga0.1As layer was etched away in a dilute hydrofluoric solution (5%) for 9 minutes. Importantly, the photoresist layer was preserved to throughout the process to protect the GaAs film. It was only removed after it was transferred onto a PDMS substrate at a later stage. ASSOCIATED CONTENT Supporting Information. Additional information on the SMP array, the rubbery modulus for SMP stamp, 2D geometrical evolution of the cone array, deformation for a cone with the preload and transfer printing of silicon components. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions


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X.F. and T.X. conceived the concept and directed the project. Y.H. conducted the transfer printing experiments. N.Z. developed the SMP. Y.H., T.X. and X.F. wrote the paper. All authors analyzed and interpreted data. ACKNOWLEDGMENT This research was supported by the following programs: the National Basic Research Program of China (Grant No. 2015CB351900), National Natural Science Foundation of China (Grant Nos. 11222220, 11320101001, 21474084) and Tsinghua University Initiative Scientific Research Program; T.X. thanks the Chinese central government’s Recruitment Program of Global Experts and 985 program for the startup funding. REFERENCES 1. Kaltenbrunner, M.; Sekitani, T.; Reeder, J.; Yokota, T.; Kuribara, K.; Tokuhara, T.; Drack, M.; Schwodiauer, R.; Graz, I.; Bauer-Gogonea, S.; Bauer, S.; Someya, T., An Ultra-Lightweight Design for Imperceptible Plastic Electronics. Nature 2013, 499, 458-463. 2. Kim, D. H.; Xiao, J. L.; Song, J. Z.; Huang, Y. G.; Rogers, J. A., Stretchable, Curvilinear Electronics Based on Inorganic Materials. Adv. Mater. 2010, 22, 2108-2124. 3. Sekitani, T.; Nakajima, H.; Maeda, H.; Fukushima, T.; Aida, T.; Hata, K.; Someya, T., Stretchable Active-Matrix Organic Light-Emitting Diode Display Using Printable Elastic Conductors. Nat. Mater. 2009, 8, 494-499. 4. Diao, Y.; Tee, B. C. K.; Giri, G.; Xu, J.; Kim, D. H.; Becerril, H. A.; Stoltenberg, R. M.; Lee, T. H.; Xue, G.; Mannsfeld, S. C. B.; Bao, Z., Solution Coating of Large-Area Organic Semiconductor Thin Films with Aligned Single-Crystalline Domains. Nat. Mater. 2013, 12, 665671.


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Direct Laser Writing-Based Programmable Transfer Printing via

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