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IEEE ELECTRON DEVICE LETTERS, VOL. 35, NO. 7, JULY 2014 ... Index Terms—Stretchable electronics, organic thin-film tran- sistor (OTFT), solution process, ...

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IEEE ELECTRON DEVICE LETTERS, VOL. 35, NO. 7, JULY 2014

Stretchable Organic Thin-Film Transistors Fabricated on Wavy-Dimensional Elastomer Substrates Using Stiff-Island Structures Jeong-Seon Choi, Chan Woo Park, Bock Soon Na, Sang Chul Lim, Sang Seok Lee, Kyoung-Ik Cho, Hye Yong Chu, Jae Bon Koo, Soon-Won Jung, and Sung-Min Yoon Abstract— Stretchable organic thin-film transistors (OTFTs) were fabricated on the polydimethysiloxane (PDMS) elastomer substrates by employing the wavy-dimensional and polyimide stiff-island structures. A low-temperature solution process was also designed to obtain high strain profiles. The endurable maximum strains were estimated to be 2.28, 9.70, and 9.32% for the OTFTs formed on the flat, 1D-, and 2D-wavy PDMS elastomers, respectively. The field-effect mobilities were obtained to be 5 ∼ 7 × 10−4 cm2 V−1 s−1 for all devices and they did not exhibit any degradation under the stretchable conditions before the fracture. The results suggest that the proposed methodologies were quite suitable for high-performance stretchable OTFTs. Index Terms— Stretchable electronics, organic thin-film transistor (OTFT), solution process, polydimethysiloxane elastomer, wavy-dimensional structure.

I. I NTRODUCTION LASTOMERS have been used in the fields of stretchable electronics, such as rollable touch screens, electronic eyes, human skins, and stretchable RF electronic devices. Various fabrication methodologies have been proposed for the stretchable electronic devices, which include the transfer techniques [1]–[3], hybrid processes [4]–[5], and direct fabrications onto the elastic substrates [6]–[8]. A wavy-dimensional structure is one of the promising methods to realize the stretchable electronic devices, which can be built between the stiff-island structures formed on elastic substrate. However, the devices such as thin-film transistors (TFTs) are generally composed of gate stacks using metals and oxides, which are extremely brittle and easily fractured upon small

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Manuscript received April 15, 2014; revised April 29, 2014 and May 7, 2014; accepted May 9, 2014. Date of publication June 4, 2014; date of current version June 24, 2014. This work was supported by the IT R&D program of MSIP (Ministry of Science, ICT & Future Planning)/KEIT (Grant 10041416; the core technology development of light and space adaptable new mode display for energy saving on 7inch and 2 W). The review of this letter was arranged by Editor B.-L. Lee. J.-S. Choi is with the Components and Materials Research Laboratory, Electronics and Telecommunications Research Institute, Daejeon 305-700, Korea, and also with the Department of Advanced Materials Engineering for Information and Electronics, Kyung Hee University, Gyeonggi-do 446-701, Korea. C. W. Park, B. S. Na, S. C. Lim, S. S. Lee, K.-I. Cho, H. Y. Chu, J. B. Koo, and S.-W. Jung are with the Components and Materials Research Laboratory, Electronics and Telecommunications Research Institute, Daejeon 305-700, Korea (e-mail: [email protected]). S.-M. Yoon is with the Department of Advanced Materials Engineering for Information and Electronics, Kyung Hee University, Gyeonggi-do 446-701, Korea (e-mail: [email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2014.2324559

Fig. 1. (a) Schematic illustrations of OTFT full structure fabricated between the stiff-islands formed on the PDMS elastomers and magnified schematic views of stiff-islands and electrodes formed on (b) flat, (c) 1D-wavy, and (d) 2D-wavy PDMS elastomer structures.

mechanical deformation. The polydimethysiloxane (PDMS) elastomers have been extensively studied and developed to obtain high strain profiles in using the wavy structures for the fabrication of stretchable electronic devices. Conventionally, the wavy structures using the PDMS elastomers can be formed by exploiting the ripple structures generated during the film transfer process [1]–[3] or the pre-strain techniques [9]–[10]. However, these approaches are difficult to control the direction and amplitude of wavy structures. In this work, the stretchable organic TFTs (OTFTs) were fabricated onto the stiff-island structures built on the elastomer substrates [11]–[12] by using low-temperature solution process to effectively protect the devices. Poly(9,9-dioctylfuorene-co-bithiophene)(F8T2) was used as a semiconducting polymer because it is one of the most suitable materials for evaluating the strain evolution under atmospheric conditions. And, we focused on the strain profiles of wavy structures by using stiff-island rather than the improvements in OTFT characteristics. A PDMS mold wafer was used to obtain uniform wavy structures by conventional photolithography process [13]. It has advantages to provide various types of wavy profiles with arbitrarily designed sizes and orientations and to form the wavy structures within only the selected regions. Both 1-dimensional (1D) and 2-dimensional (2D) wavy structures could be prepared. Thanks to the combined strategies, the fabricated stretchable OTFTs were confirmed to exhibit the strain profile as high as approximately 10% without degrading the field-effect mobility.

0741-3106 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

CHOI et al.: STRETCHABLE OTFTs FABRICATED ON WAVY-DIMENSIONAL ELASTOMER SUBSTRATES

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Fig. 2. Sets of transfer characteristics for the OTFTs fabricated on (a) flat, (b) 1D-wavy, and (c) 2D-wavy PDMS elastomer structures. (d) Variations in µsat for all devices as a function of applied strain amplitude.

Thus, this work provides useful insights in realizing highperformance stretchable OTFTs. II. D EVICE FABRICATION Figure 1(a) shows a schematic illustration of OTFT fabricated between the polyimide (PI) stiff-island structures formed on the PDMS elastomers. 1-mm-thick substrates of PDMS elastomers were prepared by mixing dimethylsiloxane prepolymer and curing agent with a weight ratio of 10:1 and curing at 60 o C for 2 h in an oven. After the PDMS elastomers were transferred onto the Si wafer, plasma-assisted surface treatments were performed by using oxygen plasma on the PDMS elastomers to improve the adhesion between the PDMS elastomers and PI stiff-island structures, in which the plasma power, oxygen partial pressure, and process time were 50 W, 30 sccm, and 3 min, respectively. Then, stiff-island structures were patterned into the sizes of 200 × 200 and 260 × 610 µm2 by means of conventional photolithography process. The thickness of the stiff-island structures was measured to be approximately 4.5 µm. To prevent the PDMS films from swelling, the devices were annealed at 200 °C for 1 h by rapid thermal process. Source and drain electrodes were formed on the stiffisland structures by e-beam evaporation of Au/Ti (70/10 nm) films using a metal shadow mask. A semiconducting polymer, F8T2 (50 nm) and polymer dielectrics, the mixture blending P(VDF-TrFE) and PMMA (300 nm) with a 7:3 ratio were successively spin-coated and annealed at 100 °C for 30 min and 80 °C for 30 min in N2 glove box, respectively [14]. Gate electrodes of Au (100 nm) were evaporated on polymer dielectrics using a metal shadow mask. Finally, the polymer gate stacks composed of F8T2 and P(VDF-TrFE)/PMMA were removed away by oxygen plasma except for the gate area using the Au gate electrodes as a hard mask. The plasma power, oxygen partial pressure, and process time were chosen

as 100 W, 100 sccm, and 3 min, respectively, which were optimized to minimize the crack formation into the PDMS. Figures 1(b), (c), and (d) illustrate the regions of adjacent PI stiff-island structures and Au/Ti electrodes formed on flat, 1D-, and 2D-wavy PDMS elastomer structures, respectively. 1D- and 2D-wavy PDMS elastomer structures were very important to realize the stretchable OTFTs, which were prepared with Si wafer mold having local wavy structures. The details for fabricating the wavy structures can be referred to our previous publication [13]. The amplitudes and wavelengths of wavy-dimensional structures were approximately 3 and 10 µm, respectively. III. R ESULTS AND D ISCUSSION Figures 2(a), (b), and (c) show the drain current (ID ) – gate voltage (VG ) characteristics of the flat, 1D-, and 2D-wavy PDMS OTFTs, respectively, when the mechanical strains of given amplitudes were applied to the substrates. The transfer characteristics were evaluated until the device operations failed owing to the introduction of excessive strain. The maximum strains at which devices could exhibit stable operations were estimated to be 2.28, 9.70, and 9.32% for the flat, 1D-, and 2D-wavy PDMS OTFTs, respectively. When the devices were excessively stretched, PDMS elastomers were broken and the device could not show TFT operations any more. It was impressive that the PDMS OTFTs fabricated with wavy-dimensional structures exhibited enhanced stretchable characteristics compared to the flat structures. Note that similar characteristics in strain tests were obtained between the 1D- and 2D-wavy structures. The channel width and length of the fabricated devices were defined to be 20 and 20 µm. The threshold voltage (Vth ) and on/off ratio for the fabricated devices were estimated to be approximately −7 V and 105 , respectively, irrespective of the device structures.

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IEEE ELECTRON DEVICE LETTERS, VOL. 35, NO. 7, JULY 2014

might weaken the initial state of mechanical strength of the fabricated devices. Another issue was related to the adhesion between PDMS elastomers and PI stiff-island structures. To solve these problems, the capping layers can be introduced onto the OTFTs [13], with which some devices were reported to have higher stretchable properties even with a strain larger than 20%. As future works, the device structures will be modified to have capping layers for the PDMS OTFTs having higher stretchable characteristics.

Fig. 3. (Color online) (a) Microscopic photo images of the OTFTs fabricated on the PI stiff-islands. The magnified views of the interconnection areas of the OTFT on flat PDMS at (b) initial, (c) 2.28%-strained, and (d) over-strained situations, and on 1D-wavy PDMS at (e) initial, (f) 9.70%-strained, and (g) over-strained situations, and on 2D-wavy PDMS at (h) initial, (j) 9.32%strained, and (i) over-strained situations.

The field-effect mobility at saturation region (µsat ) and subthreshold swing were calculated to be in the ranges of 5 ∼ 7 × 10−4 cm2 V−1 s−1 and 2 ∼ 7 V/decade, respectively. Considering that the most OTFTs using an F8T2 channel were reported to have a µsat of 1∼4 × 10−4 cm2 V−1 s−1 [15], the device characteristics obtained from the stretchable OTFTs were sufficiently encouraging. Figure 2(d) shows the variations in µsat as a function of applied strain amplitude for all devices, in which the fluctuation margin in µsat was as small as 1 × 10−4 cm2 V−1 s−1 . The µsat did not experience marked changes under the stretchable conditions with given strains. Figure 3(a) shows a microscopic photo image of the OTFTs fabricated using the PI stiff-island structures formed on the PDMS elastomers. With stretching the OTFTs, the elastic moduli for the employed organic semiconducting materials did not make any difference in their stretchable characteristics, because the OTFTs were formed on the PI stiff-island structures. So, we focused entirely on comparing the interconnection areas. Figure 3(b)-(d) compares the magnified views of the interconnection areas connecting the gate and source electrodes on the flat PDMS at initial, 2.28%-strained, and over-strained situations, respectively. Mechanical cracks in the interconnection areas were clearly observed when the PDMS elastomers were stretched with a strain larger than 2.28%. Similarly, the same parts of the OTFTs fabricated on the 1D- and 2D-wavy PDMS substrates were shown in Figs. 3(e)-(g) and in Figs. 3(h)-(j), respectively. As can be seen in Figs. 3(g) and (j), the interconnection areas were broken at the application of strain larger than 9.70 and 9.32%, respectively. The obtained results suggested that the 1D-wavy PDMS structures showed highly stretchable characteristics than the flat PDMS structures. While the 1D-wavy PDMS structures can be amenable to only parallel direction along the S/D electrodes, the 2D-wavy PDMS structures can provide advantages for stretching along the biaxial directions. Although the proposed PDMS OTFTs formed on wavydimensional structures were successfully confirmed to realize the stretchable characteristics, the endurable strain amplitude should be more extended. The first concern was that the delamination of PDMS elastomers from the Si wafer, which was performed as a final process for the stretchable OTFTs,

IV. C ONCLUSIONS We proposed the wavy elastomer structures and fabricated the stretchable OTFTs on the PDMS substrates to obtain effective strain profiles. Active devices were uniquely featured to be isolated by forming the stiff-island structures for preventing the metal cracks under the stretchable conditions. The endurable maximum strain was much enhanced from 2.28 to 9.70% when the PDMS elastomer substrate was modified from flat to 1D-wavy PDMS structures. It was also noticeable that the values of µsat for the fabricated stretchable OTFTs were reasonably obtained to be 5∼7 × 10−4 cm2 V−1 s−1 , and that they did not experience any marked changes under the stretchable conditions with given strains. We can conclude that the proposed wavy-dimensional and stiff-island structures are very promising methodologies to realize high-performance OTFTs on the stretchable substrates. R EFERENCES [1] J. Pu et al., “Fabrication of stretchable MoS2 thin-film transistors using elastic ion-gel gate dielectrics,” Appl. Phys. Lett., vol. 103, no. 2, pp. 023505-1–023505-4, 2013. [2] K. Park et al., “Stretchable, transparent zinc oxide thin film transistors,” Adv. Funct. Mater., vol. 20, no. 20, pp. 3577–3582, 2010. [3] S. H. Chae et al., “Transferred wrinkled Al2 O3 for highly stretchable and transparent graphene–carbon nanotube transistors,” Nature Mater., vol. 12, pp. 403–409, May 2013. [4] J. S. Bendall, I. Graz, and S. P. Lacour, “Zinc oxide nanowire rigid platforms on elastomeric substrates,” ACS Appl. Mater. Interf., vol. 3, no. 8, pp. 3162–3166, 2011. [5] R. M. Erb et al., “Locally reinforced polymer-based composites for elastic electronics,” ACS Appl. Mater. Interf., vol. 4, no. 6, pp. 2860– 2864, 2012. [6] I. M. Graz and S. P. Lacour, “Flexible pentacene organic thin film transistor circuits fabricated directly onto elastic silicone membranes,” Appl. Phys. Lett., vol. 95, no. 24, pp. 243305-1–243305-3, Dec. 2009. [7] A. N. Sokolov et al., “Mechanistic considerations of bending-strain effects within organic semiconductors on polymer dielectrics,” Adv. Funct. Mater., vol. 22, no. 1, pp. 175–183, 2012. [8] I. M. Graz and S. P. Lacour, “Complementary organic thin film transistor circuits fabricated directly on silicone substrates,” Organ. Electron., vol. 11, no. 11, pp. 1815–1820, 2010. [9] H. Wu et al., “Topographic substrates as strain relief features in stretchable organic thin film transistors,” Organ. Electron., vol. 14, no. 6, pp. 1636–1642, 2013. [10] S. P. Lacour et al., “Stretchable interconnects for elastic electronic surfaces,” Proc. IEEE, vol. 93, no. 8, pp. 1459–1467, Aug. 2005. [11] I. M. Graz et al., “Silicone substrate with in situ strain relief for stretchable thin-film transistors,” Appl. Phys. Lett., vol. 98, no. 12, pp. 124101-1–124101-3, Mar. 2011. [12] S. P. Lacour et al., “Stiff subcircuit islands of diamondlike carbon for stretchable electronics,” J. Appl. Phys., vol. 100, no. 1, pp. 014913-1–014913-6, Jul. 2006. [13] C. W. Park et al., “Fabrication of well-controlled wavy metal interconnect structures on stress-free elastomeric substrates,” Microelectron. Eng., vol. 113, pp. 55–60, Jan. 2014. [14] K.-J. Baeg et al., “Low-voltage, high speed inkjet-printed flexible complementary polymer electronic circuits,” Organ. Electron., vol. 14, no. 5, pp. 1407–1418, 2013. [15] S.-W. Jung et al., “Low-voltage-operated top-gate polymer thinfilm transistors with high-capacitance P(VDF-TrFE)/PVDF-blended dielectrics,” Current Appl. Phys., vol. 11, no. 3, pp. S213–S218, 2011.

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