Research Article www.acsami.org
Ultimate Control of Rate-Dependent Adhesion for Reversible Transfer Process via a Thin Elastomeric Layer Chan Kim,†,‡ Min-Ah Yoon,†,‡ Bongkyun Jang,‡ Jae-Hyun Kim,†,‡ Hak-Joo Lee,†,§ and Kwang-Seop Kim*,†,‡ †
Department of Nano-Mechatronics, Korea University of Science & Technology (UST), 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea ‡ Nano-Convergence Mechanical Systems Research Division, Korea Institute of Machinery & Materials (KIMM), 156 Gajeongbuk-ro, Yuseong-gu, Daejeon 34103, Republic of Korea § Center for Advanced Meta-Materials (CAMM), 156 Gajeongbuk-ro, Yuseong-gu, Daejeon 34103, Republic of Korea S Supporting Information *
ABSTRACT: Adhesion between a stamp with an elastomeric layer and various devices or substrates is crucial to successfully fabricate flexible electronics using a transfer process. Although various transfer processes using stamps with different adhesion strengths have been suggested, the controllable range of adhesion is still limited to a narrow range. To precisely transfer devices onto selected substrates, however, the difference in adhesion between the picking and placing processes should be large enough to achieve a high yield. Herein, we report a simple way to extend the controllable adhesion range of stamps, which can be achieved by adjusting the thickness of the elastomeric layer and the separation velocity. The adhesion strength increased with decreasing layer thickness on the stamp due to a magnification of the confinement and ratedependent effects on the adhesion. This enabled the controllable range of the adhesion strength for a 15 μm-thick elastomeric layer to be extended up to 12 times that of the bulk under the same separation conditions. The strategy of designing stamps using simple adhesion tests is also introduced, and the reversible transfer of thin Si chips was successfully demonstrated. Tuning and optimizing the adhesion strength of a stamp according to the design process suggested here can be applied to various materials for the selective transfer and replacement of individual devices. KEYWORDS: adhesion, thin elastomeric film, confinement effect, rate-dependent adhesion, reversible transfer process
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INTRODUCTION Flexible and wearable electronic devices are of growing importance. In such devices, polymer substrates such as a polyethylene terephthalate (PET), polydimethylsiloxane (PDMS), and polyimide (PI) are typically used due to their flexibilities and transparencies.1−4 The fabrication processes for high-performance electronic devices such as light-emitting diodes (LEDs), thin-film solar cells (TFSCs), and thin-film transistors (TFTs) usually include high-temperature processes in which the polymer substrates can degrade. To overcome this problem, a transfer process was suggested where highperformance electronic devices could be fabricated on rigid semiconductor substrates using conventional semiconductor processes and transferred onto flexible substrates using stamps with thin elastomeric layers at room temperature.5 In the transfer process, adhesion between the stamp and the device is crucial for the successful transfer of the device. In the picking step, when the stamp picks up a target device on a donor substrate, the adhesion between the stamp and the device © XXXX American Chemical Society
should be higher than that between the device and the donor substrate. On the other hand, in the placing step, when the stamp places a device onto a flexible substrate, the adhesion between the stamp and the target device should be lower than that between the device and the flexible substrate. In most cases, because the adhesion strength between the stamp and the device is determined by the contacting materials, a sticky adhesive layer is coated on the flexible target substrate to increase the adhesion between the device and the substrate in the placing step. However, using sticky adhesives limits the rearrangement and replacement of individual devices after the placing step. Therefore, instead of sticky adhesives, advanced methods to reversibly control the adhesion of stamps are needed to ensure flexibility of the process. Received: February 15, 2017 Accepted: March 24, 2017 Published: March 24, 2017 A
DOI: 10.1021/acsami.7b02214 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Microscope images of PDMS/SiO2 samples and their water contact angles. (b) Schematic diagram of the home-built adhesion tester. (c) A representative load−time graph of the adhesion test.
millimeter scale; this methodology is suitable for roll-to-roll material transfer processes.14 As the thickness of the stamp relative to the lateral dimension of the devices decreases, the adhesion between the stamp and devices increases. This is because the adhesion is governed by the geometric confinement of the interface. The confinement effects were analyzed using contact mechanics in previous research.15−18 Recently, it was reported that the adhesion of an elastomeric stamp could be reversibly controlled by the bending radius of the stamp.19 Even if the methodologies are simple and effective to reversibly control the adhesion of an elastomeric stamp, the controllable adhesion range of the stamp is still limited to a narrow range. In this study, a simple and straightforward method to extend the controllable range of adhesion was identified for the versatile handling of devices in the transfer process. The design strategy of the elastomeric stamp combined the confinement effects with kinetic control. The adhesion of the stamp with a thickness of a few tens of micrometers showed a controllable range about 12 times larger than that of the bulk layer. A simple adhesion test was used in the design process to choose the adhesion range of a stamp depending on the target substrate. The reversible picking and placing of Si chips, which were arranged on a flexible substrate, were successfully demonstrated using a stamp with a thin elastomeric layer.
Various techniques such as surface treatments, use of the rate-dependent characteristic of elastomers, and surface patterning have been suggested to control the adhesiveness of a stamp.6−13 Chemical surface treatments such as plasma and ultraviolet (UV) treatments are typically used due to their simplicity and compatibility for large-area treatments.6,7 However, the adhesion of a stamp is irreversibly modified by such treatments, rendering them unusable for repeated picking and placing.8 One way to reversibly control adhesion is to use the viscoelastic response of an elastomeric stamp, i.e., its ratedependent adhesion.9−11 The adhesion energy could be much larger than the thermodynamic work of adhesion at high separation velocity because the energy dissipation due to viscoelastic loss increases with the separation velocity.12 When pulling the stamp away from the device at a sufficiently high separation velocity, the adhesion between the stamp and device is strong enough to detach the device from the donor substrate. In contrast, when removing the stamp at a sufficiently low separation velocity, the adhesion is weaker, and the device is preferentially adhered to the target substrate. Such reversible kinetic control of adhesion could be used in both picking and placing processes without any surface treatments or adhesives. However, the controllable range of adhesion is limited to a narrow range and, in most cases, the adhesives need to be applied to the target substrates. Surface patterning is a recent way to reversibly control the adhesion of a stamp. In particular, for picking and placing processes, pressure-modulated switchable adhesion was demonstrated in which the contact area between the stamp with microtips and devices was switched by the contact pressure; the adhesion strength could be changed by up to three orders of magnitude.13 Although the stamp with microtips showed a distinct difference in adhesion between the picking and placing processes and could be reversibly changed, it required that the devices were sufficiently flat and stiff to endure high, nonuniform contact pressures. Additionally, the microtip geometry required optimization based on the geometry of the target devices and the adhesion between the devices and counterpart substrates. The adhesion of an elastomeric stamp can be also tuned by its thickness at the
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EXPERIMENTAL DETAILS
Materials and Fabrication of Samples. Laser-quality fused-silica plano-convex lenses (PLCX-10.0−25.8-UV, CVI Melles Griot, United States) were used as a counterpart material in the adhesion test. Before the adhesion test, the lenses were cleaned in piranha solution for 20 min. Subsequently, the lenses were washed in deionized water and dried in a flow of nitrogen gas. The curvature of the lenses was 25.8 mm. Polydimethylsiloxane (PDMS, Sylgard 184, Dow Corning, United States) was used as an adhesive layer on a SiO2 wafer. PDMS was prepared by mixing the liquid prepolymer (Sylgard 184A, Dow Corning) and the curing agent (Sylgard 184B, Dow Corning) at a ratio of 10:1. The mixture was deaerated in a vacuum chamber for 1 h. Subsequently, the mixture was used to coat 15, 30, 80, and 200 μmthick layers onto the SiO2 wafer. Then, the PDMS/SiO2 samples were baked for 12 h in an oven at 65 °C to remove residual solvent. The B
DOI: 10.1021/acsami.7b02214 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 2. Pull-off force (Fpull‑off) and critical contact radius (ac) for different PDMS layer thicknesses as a function of the contact load (L) and separation velocity (Vsep). (a and b) Pull-off force (Fpull‑off) for different PDMS layer thicknesses for separation velocities of 2 and 500 μm/s, respectively. (c and d) Critical contact radii for PDMS layers of different thicknesses corresponding to the pull-off forces in panels a and b, respectively.
Figure 3. Variation of load and contact radius with time and the corresponding microscope images of the contact area during the separation process. The thickness of the PDMS layer was greater than (a) 5 mm and (b) 15 μm. The numbers on the microscope images correspond to the numbers in the contact radius−time graph. measured water contact angle of the elastomeric layer was 113 ± 1°, regardless of the thickness (Figure 1a). Adhesion Test. Figure 1b shows a schematic diagram of the homebuilt adhesion tester designed for the adhesion test.20 The adhesion tester was located on an antivibration table in an environmental chamber maintained at 21 ± 1 °C to prevent the surface of the samples from dust contamination. The lens was fixed onto the upper part of the tester. A sample holder on a load cell (LTS-100GA, Kyowa, Japan) with a load capacity of 1 N was installed on a precise XYZ stage (M111.1DG, PI, Germany). The PDMS/SiO2 samples were fixed on the sample holder and moved upward to contact the lens. The sample was pressed until the load reached a predetermined contact load, and then the contact was maintained for 100 s. After the dwell time, the
PDMS/SiO2 samples were pulled away from the lens at a prescribed separation velocity. The contact load was ranged from 10 to 800 mN, and the separation velocities ranged from 2 to 500 μm/s. Figure 1c shows a representative load−time graph recorded during the test; the maximum tensile load measured during the separation process was defined as the pull-off force. The change of contact area during the adhesion test was also recorded using an optical microscope and a high-speed charged-coupled device (CCD) camera at a sampling rate of 500 frame/s. The critical contact radius was measured from the image of the contact area when the maximum tensile force was applied. Reversible Transfer of Si Chips. The Si chip array on a siliconon-insulator (SOI) wafer was fabricated by conventional photolithography and chemical etching processes and transferred onto a C
DOI: 10.1021/acsami.7b02214 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces flexible polymeric substrate. Commercial screen protective films (TANK Protection, UMT Laboratories, Korea) were used as the flexible substrate, which is composed of thin silicone layer coated on a polyethylene terephthalate (PET) substrate. The thicknesses of the adhesive layer and PET base substrate are 40 and 128 μm, respectively. The surface energy of the flexible substrate is 15 mJ/m2, comparable to that of PDMS layers (see Figure S2 in the Supporting Information). The Si chip size was 700 × 700 μm2, and the thickness was 6 μm. To achieve reversible detachment and attachment of the chips on the flexible substrate, the adhesion strength of the stamp should be switched higher or lower than that of the substrate depending on the processes. The adhesion strength of the substrate was characterized under various loads, and separation velocities through the adhesion test and the maximum and minimum values of the adhesion strength were measured. When the test results were compared to the adhesion strengths of thin PDMS layers, the thickness of the PDMS layer necessary to reversibly transfer the chip was determined. This constituted the design process of the stamp. To fabricate the stamp, the fused-silica lens was spin-coated with a PDMS layer as much as the designed thickness. (see Figure S3 in the Supporting Information). The adhesion tester was used for the reversible transfer of Si chips. The transfer procedure was recorded using an optical microscope and a high-speed CCD camera at a sampling rate of 60 frame/s (Movie S1). The separation velocities for the picking and placing processes were 500 and 2 μm/s, respectively. The contact load was 800 mN.
velocity and contact load on the increase in pull-off force were amplified when the thickness of the elastomeric layer was less than 80 μm. Figures 2c and d show the variation of the critical contact radius obtained by optical microscope images during the separation process. Generally, as the thickness of the layer decreased, the critical contact area also decreased due to the increase in effective stiffness of the layer. For the separation velocity of 2 μm/s (Figure 2c), the critical contact radius of a layer with a thickness less than 80 μm abruptly increased with the contact load, similar to that observed for the pull-off force (Figure 2a). When the separation velocity was 500 μm/s, the critical contact radius also increased with the contact load (Figure 2d). The contact load effect on the increase of the critical contact area for each layer was analogous to that of the increase of the pull-off force. The results show that the pull-off force of the layer was strongly related to the critical contact area (πac2) and depended on the thickness, contact load, and separation velocity. Optical microscope images of the contact area and the corresponding tensile loads during the separation process were analyzed to investigate the changes in the pull-off force and critical contact area as a function of the thickness of the elastomeric layer (Figure 3). The contact area gradually decreased as the tensile load increased for elastomeric layers with a thickness of 5 mm (i.e., the bulk) (Figure 3a); the number on the image corresponds to the number in the contact radius−time graph.) In contrast, for an elastomeric layer with a thickness of 15 μm (Figure 3b), the contact area remained nearly constant before complete separation; the tensile load increased linearly before the separation and decreased rapidly after the separation. The thickness of the elastomeric layer for a given contact radius significantly influenced the distribution of the normal stress in the layer; this is called the confinement effect.21,22 In a spherical contact geometry, for a thick elastomeric layer, the stress is more concentrated at the edges of the contact area; a crack can easily propagate inward of the contact area, even under a low tensile load. For an infinitely thin elastomeric layer, the maximum stress occurs in the center of the contact so that the contact area can be maintained at a higher tensile load and the fingering instability could be observed right before the separation. As a result, the pull-off force (maximum tensile load) for the thin elastomeric layer is greater than that for the bulk layer, although the adhesion energy for the bulk could be greater than that for the thin adhesive layer. Therefore, the adhesion strength (pull-off force divided by the critical contact area) for the thin adhesive layer could be much greater than that for the bulk adhesive. In our case, the confinement effect began to influence the adhesion strength for a layer with a thickness less than 80 μm. The normalized pull-off force is widely used in the field of thin-film adhesion as the measure of adhesion strength.14,23 To quantitatively describe the adhesion behavior of thin elastomeric layers, the normalized pull-off force and energy release rate can be calculated using empirical equations from the compliance method.24 When the lens and the substrate are assumed to be rigid, the pull-off force for the thin elastomeric layer on the rigid substrate is related by the compliance of the layer as described by eq 1:25,26
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RESULTS AND DISCUSSION Adhesion tests were performed for layers of different thickness on the SiO2 substrate to investigate the adhesion properties of
Figure 4. Normalized pull-off force (Fpull‑off/ac3/2) of the PDMS layer as a function of confinement ratio (ac/t) for different separation velocities.
the thin elastomeric layers. Figure 2 shows the measured pulloff force (Fpull‑off) and critical contact radius (ac) between the lenses and elastomeric layer as a function of layer thickness (t). The separation velocities (Vsep) were 2 μm/s for slow separation and 500 μm/s for fast separation, and the contact load (L) was varied from 10 to 800 mN. As shown in Figures 2a and b, the pull-off force increased with decreasing layer thickness. When the separation velocity was 2 μm/s (Figure 2a), an abrupt increase in the pull-off force occurred for samples thicknesses less than 80 μm, and the rate of the pull-off force increased with increasing contact load. For samples with thicknesses greater than 80 μm, the pull-off forces were almost the same as that of the bulk sample regardless of the contact load. With increasing separation velocity, the pull-off force increased with increasing contact load. Figure 2b shows that when the separation velocity was 500 μm/s, a significant increase in the pull-off force was observed for samples having thicknesses less than 80 μm. The effects of the separation
Fpull ‐ off ∼ D
G
A C
(1) DOI: 10.1021/acsami.7b02214 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 5. Design strategy for a stamp with a thin elastomeric layer and demonstration of the reversible transfer of Si chips using the designed stamp. (a) Controllable range of the normalized pull-off force (Fpull‑off/ac3/2) of the PDMS layer for different thicknesses and a flexible substrate. The blue box represents the range of the normalized pull-off force of the flexible substrate when the separation velocity was changed from 2 to 500 μm/s. (b) Schematic diagram for the reversible picking and placing of a Si chip onto a flexible substrate using the stamp with a 15 μm-thick PDMS layer. (c) Demonstration of the selective transfer and rearrangement of Si chips using the stamp.
where G is the energy release rate of the interface, A is the critical contact area (πac2, where ac is the critical contact radius), and C is the compliance of the layer. The compliance and contact area change according to the contact geometry of the contacting materials. For the contact geometry between a spherical lens and a flat, thin elastomeric layer, the compliance of the layer can be determined as a function of the confinement ratio (ac/t) according to eq 2:22 5/2 ⎡ 1 9π ⎜⎛ aC ⎟⎞ 9π ⎜⎛ aC ⎟⎞ ⎤ ⎥ = 2.67aCE⎢1 + + ⎢⎣ C 16 ⎝ t ⎠ 32 ⎝ t ⎠ ⎥⎦
effective stiffness of the layer increases due to the confinement effect. When the compliance of eq 1 is substituted with eq 2, the normalized pull-off force (Fpull‑off/ac3/2) can be expressed as eq 3: Fpull ‐ off aC3/2
=
5/2 ⎡ 9π ⎜⎛ aC ⎟⎞ 9π ⎜⎛ aC ⎟⎞ ⎤ ⎥ 2.67πEG⎢1 + + ⎢⎣ 16 ⎝ t ⎠ 32 ⎝ t ⎠ ⎥⎦
(3)
Figure 4 shows that the normalized pull-off force increased with the confinement ratio due to the enhanced effective stiffness of the layer and the change in the normal stress distribution in the layer. Notably, higher separation velocities exaggerated the confinement effect. The energy release rate, G, for each separation velocity condition was calculated by fitting the experimental results using eq 3. As the separation velocity increased from 2 to 500 μm/s, the energy release rate increased
(2)
where E is the elastic modulus of the elastomeric layer and has a value of 2 MPa for PDMS.11 The relationship between the compliance and the ratio ac/t in eq 2 shows that an increase in ac/t reduces the compliance of the layer, which means that the E
DOI: 10.1021/acsami.7b02214 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
ACS Applied Materials & Interfaces
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from 314 to 5050 mJ/m2. This indicated that the adhesion strength of the thin elastomeric layer could be readily tuned by adjusting the confinement ratio and the separation velocity. When the separation velocity was 2 μm/s, the energy release rate was 314 mJ/m2, which is comparable to values reported for bulk samples.11,27 Knowing the rate-dependent characteristics and the confinement effect of the thin elastomeric layer, a stamp with a thin elastomeric layer could be designed by considering the adhesion properties of the target substrate. To verify the design process of the stamp, we evaluated the adhesion strength of a target flexible substrate and demonstrated the reversible transfer of Si chips onto the flexible substrate using the designed stamp. Using the same experimental apparatus, the pull-off force and critical contact area on the flexible substrate were measured, and the normalized pull-off force for given separation conditions was compared for elastomeric layers of various thicknesses. Figure 5a shows that the normalized pulloff force of the flexible substrate ranged from 3.2 N/mm3/2 for the separation velocity of 2 μm/s to 5.1 N/mm3/2 for the separation velocity of 500 μm/s. These results indicate that the stamp should be designed with a normalized pull-off force greater than 5.1 N/mm3/2 for the picking process and less than 3.2 N/mm3/2 for the placing process. The thickness of the elastomeric layer on the stamp should be less than 30 μm for the reversible transfer because the normalized pull-off force of the elastomeric layer with a thickness of 30 μm was comparable to that of the flexible substrate under the separation velocity of 500 μm/s; a stamp coated with a 30 μm-thick elastomeric layer could not pick up a Si chip off the flexible substrate. Figure 5b and Movie S1 show that the stamp with a 15 μm-thick elastomeric layer had more extended adhesion strength, and each individual Si chip could be readily attached to or detached from the flexible substrate only by adjusting the separation velocity. Consequently, the array of Si chips could be easily rearranged as desired (Figure 5c). From a practical perspective, this technique could be used to replace defective devices with more suitable ones and extend the usefulness of the transfer process for the entire device array over large areas. One note of caution is that at high confinement ratio fingering instability occurs right before separation, as shown in Figures 3b and S4, and the adhesion is locally changed in a moment. This change in adhesion can cause the fracture of thin devices.
Research Article
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02214. Variation of load and displacement with time during selective transfer process (Figure S1); XPS spectra, surface topography, surface energy, and cross-section view of the flexible substrate (Figure S2); thickness and surface morphology of the stamp coated with a PDMS layer (Figure S3); and critical contact area of PDMS layers with different thickness (Figure S4) (PDF) Movie clip showing the demonstration of reversible transfer of the Si chip (AVI)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Kwang-Seop Kim: 0000-0003-4939-1973 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Center for Advanced MetaMaterials (CAMM) funded by the Ministry of Science, ICT and Future Planning as a Global Frontier Project (CAMM nos. 2014063701 and 2014063700) and by an internal research program of the Korean Institute of Machinery and Materials (SC1240).
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REFERENCES
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CONCLUSIONS We demonstrated that the controllable range of adhesion of thin elastomeric layers could be extended up to 12 times that of the bulk when rate-dependent effects on adhesion combined with confinement effects. As the confinement ratio (ac/t) increased, the adhesion strength of thin elastomeric layers increased, and the rate-dependent effect on the adhesion was also magnified. On the basis of an adhesion test, we suggested the design strategy of a stamp with a thin elastomeric layer for the reversible transfer of Si-based devices onto a target flexible substrate. Through an evaluation of the adhesion strength of the flexible substrate, we chose the optimal thickness of the elastomeric layer having the desired extended controllable adhesion strength. Reversible detachment and attachment of Sibased devices onto the flexible substrate was successfully demonstrated. This technique is expected to be broadly useful for the replacement of defective individual devices and applicable to large-area roll-to-roll transfer of electronic devices in the high-yield fabrication of flexible electronics. F
DOI: 10.1021/acsami.7b02214 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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DOI: 10.1021/acsami.7b02214 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
