Impact of Polyimide Film Thickness for Improving the Mechanical

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Functional Inorganic Materials and Devices

Impact of Polyimide Film Thickness for Improving the Mechanical Robustness of Stretchable InGaZnO Thin Film Transistors Prepared on Wavy-Dimensional Elastomer Substrates Hye-Won Jang, SangKyun Kim, and Sung Min Yoon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08902 • Publication Date (Web): 23 Aug 2019 Downloaded from pubs.acs.org on August 25, 2019

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ACS Applied Materials & Interfaces

Impact of Polyimide Film Thickness for Improving the Mechanical Robustness of Stretchable InGaZnO Thin Film Transistors Prepared on WavyDimensional Elastomer Substrates Hye-Won Jang1, SangKyun Kim2, and Sung-Min Yoon1* 1

Department of Advanced Materials Engineering for Information and Electronics, Kyung Hee

University, Yongin, Gyeonggi 17104, South Korea 2

Kolon Industries, Inc., Seoul 07793, Korea

E-mail: [email protected] and [email protected]

Keywords: amorphous indium−gallium−zinc oxide (a-IGZO); thin film transistor (TFT); Polyimide; Stretchable electronics; Wavy-dimensional structure

ACS Paragon Plus Environment

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Abstract

We report on the In-Ga-Zn-O TFTs with outstanding mechanical stretchability, which were fabricated on ultra-thin polyimide (PI) film/pre-strained elastomer with wavy-dimensional structure. The device characteristics of the fabricated devices were evaluated under mechanically strained conditions with various strains. The operational reliabilities against the bias-stress conditions and during the cyclic stretching tests were also carefully examined. The stretchable IGZO TFTs exhibited good device operations without any marked degradation under stretching/compressed conditions with a strain of 40 %. Under positive bias stress with a prestrain of 50 %, the turn-on voltage instabilities for the TFTs prepared on 0.9- and 2.0-μm-thick PI films were estimated to be 1.5 and 3.9 V, respectively. During the cyclic stretching tests with a strain of 50 %, the device operations failed after 20,000 and 100,000 stretching cycles for the TFTs fabricated on 2.0- and 0.9-μm-thick PI films, respectively. As results, the IGZO TFTs fabricated on thinner PI film presented more reliable operations after the repeated stretching events. The robust mechanical stretchability dependent on the PI film thickness was suggested to be due to the difference in critical values of bending radii and the influence of the local strain induced by the spatial fluctuations of the wavy structures.


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1. INTRODUCTION

Beyond flexible electronics, the stretchable platform is emerging as a breakthrough for the nextgeneration future consumer electronics such as wearable electronic skin, stretchable display, and health care devices.1-6 The strategies for realizing these stretchable devices can be classified into material engineering and structural approaches. First of all, from a material engineering perspective, suitable designs of actual stretchable materials have been focused so that all the circuit components of the stretchable systems can be operated even under the stretched conditions.7-9 However, these materials are difficult to be prepared for manufacturing the highlyfunctional devices because they have various limitations to realize sufficient electrical performance of the devices. Alternatively, from the viewpoint of structural strategies, various methodologies have been developed to provide the stretchability by means of properly designing the mechanical structure of the devices composed of conventional brittle materials such as oxides and metals. These specified structural designs include an island interconnection configuration, a mesh structure, a textile structure, a wave structure configuration, and a kirigami structure.10-14 However, we have to remind that considerable amounts of mechanical strain would be induced into the systems and/or devices, resulting in the substantial stretching events accompanied by structural deformation. In this work, we introduced two technical approaches to realize the oxide TFTs with excellent device characteristics, mechanical stretchability, and operation stability. One is to employ a wavy-dimensional structure, which is one of the most promising methods to fabricate a stretchable devices using conventional inorganic materials. The devices are prepared on the pre-


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stretched elastomer substrate by transferring method. After removing the stretching force applied to the substrate, the waves are formed on the surface of the thin film due to the differences in mechanical properties between the thin film and the substrate.15 When the devices are mechanically stretched, the deformation induced to the device plane can be reduced by releasing the waves, allowing it to be reliably operated against a higher degree of stretching strain. Unlike other methods, such as a solution or a printing process, fabricated stretchable devices have excellent reproducibility and a high yield due to the simple transfer process of conventional oxide TFTs. However, most devices fabricated with wavy structures have been reported to experience critical deterioration at mechanical strains less than 20% and/or after repetitive stretching cycles of less than 1000 times.16-17 Therefore, to improve the elasticity and durability of the stretchable devices, the introduction of an ultra-thin PI film is our second approach. This can minimize the amount of mechanical stress applied to the device by reducing the overall thickness of the device. Since the mechanical stress is induced by the curvature of wavy structures formed on the surface during unstretched conditions, the thickness of flexible substrate should be carefully chosen for this method. Generally, the magnitudes of the stress induced on the device plane under the bending conditions are sensitively influenced by the substrate thickness.18 In other words, the critical bending radius can be reduced to several tens of micrometers with enhancing the mechanical stretchability by employing a-few-µm-thick PI island. In order to investigate the influence of the PI thickness on the stretchability of the devices, the degradation behaviors of the devices prepared on the PI films with different thicknesses were evaluated and analyzed. Alternatively, the device operation stability is also important to be necessarily secured with improvements in mechanical properties for various practical applications. Thus, the device


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reliability under the bias stress and mechanical reliability under repetitive stretching stress were extensively investigated for the first time. The target device was designed as In-Ga-Zn-O (IGZO) TFTs on wavy structural PI/elastomer substrates. The TFTs prepared on ultra-thin PI films were attached onto pre-strained elastomer. The degradations in device characteristics under mechanically stretching conditions were found to be affected by magnitude of applied strain. The effects of PI island thickness were analyzed under the static compression, bias stress, and dynamic stretching cycles. The IGZO TFTs fabricated on 0.9-μm-thick PI film showed successful operations even after the repeated stretching events with 105 cycles at a stretching ratio of 50 %.


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2. EXPERIMENTAL DETAILS

2.1. Substrate Preparation and TFT Fabrication Figure 1(a) and (b) show the process flow for the TFT fabrications and the schematic crosssectional view of the fabricated flexible/stretchable IGZO TFT with a top-gate bottom-contact structure on a CPI substrate. To control the film thickness of the coated PI, the CPI solution was spin-coated on carrier glass substrate at 2000, 3000, and 4000 rpm for 120 s, resulting in the film thicknesses of approximately 2.0, 1.3, and 0.9 μm, respectively. The coated films were thermally cured at 80 oC for 30 min in a hot plate and at 290 oC for 30 min in a furnace. For providing the stabilities and reliabilities of the fabricated devices, a 50-nm-thick Al2O3 buffer layer was prepared on the PI surface by atomic layer deposition (ALD) at 150 oC. A 150-nm-thick ITO was deposited by means of dc sputtering and patterned into source/drain (S/D) electrodes by wet etching. A 20-nm-thick IGZO was deposited by rf sputtering method at room temperature (RT) with a single IGZO (In:Ga:Zn=2:1:2) target as an active channel layer. The patterning process of active area was followed by the formation of Al2O3 (9 nm) protection layer prepared by ALD at 150 oC. Then, a 100-nm-thick Al2O3 gate insulator was deposited by ALD at 150 oC. Finally, Al thin films were deposited by thermal evaporation and patterned into gate electrode and S/D pads for interconnections. After the device fabrication, post annealing process was performed at 150 oC

for 1 h in an oxygen ambient.

2.2. Transfer process on elastomeric film


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We employed a poly(dimethylsiloxane) (PDMS) as a transfer stamp, which is a bi-component polymer mixed with dimethylsiloxane prepolymer and curing agent with different capabilities, accordingly. The adhesion between the PDMS stamp and the device plane was optimized by controlling the mixing ratio.(19-20) In this work, we prepared the PDMS stamp with a weight ratio of 10:1 (prepolymer : curing agent) and dried at RT for 72 h. For the transfer process, the upper surface of the TFTs were adhered onto the PDMS stamps. The fabricated TFTs could be picked up from the carrier glass with the aid of PDMS stamps after the delamination performed by laser lift-off (LLO) process. The elastomeric tape (VHB4905 produced by 3M) was mounted on a custom-made stretching jig between two clamps. The clamp was displaced to stretch the elastomer to the desired length. Then, the PDMS/TFTs/PI stack was transferred to the prestretched elastomer and then the PDMS stamp was peeled away. The PDMS stamp forms weak Van der Waal’s bonds with the device plane, which is sufficient to pick up the TFTs from the carrier glass, but not strong enough to stick the functional layer. Therefore, the transfer printing process could be successfully performed without any critical problems in this work. Furthermore, the wavy structures were formed by moving the clamp close to the other clamp to release the pre-strained elastomer. Fig. 1(c) illustrates this forming process of an out-of-plane design with wavy dimensional structures for providing the mechanical stretchability to the device plane. Under stretching or compression, the fabricated devices can achieve given stretchability by changing the wavy structures, as shown in Fig. 1(d).

2.3. Stretching tests and electrical evaluations


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Cyclic stretching was tested with a customized stretching machine, as shown in Figure S1 (b) (SI). The translational stage moves along the x-axis, with which a tensile or compressive strains can be continuously applied. The number of cycles and the stretching speed can be adjusted at a control box. The device characteristics including the bias-stress stability of the fabricated TFTs were evaluated using a semiconductor parameter analyzer (Keithley 4200A-SCS) with whisker tips in a dark box at RT. The channel width (W) and length (L) of the evaluated TFTs were 40 and 20 µm, respectively.


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3. RESULTS AND DISCUSSIONS

Figure 2(a) and (b) show photo images and microscopic views of the device plane transferred onto a pre-strained elastomer when a tensile strain increased to 50 % and then released without strain, in which stretching/compression direction was set to be parallel to the current flow. The fully-compressed state and the maximum tensile strain corresponded to the initial edge length (L) of the elastomer without a tensile stain and the pre-strained elastomer length, respectively. At each stage of stretching and compression, the L was regarded as the magnitude of strain, which was calculated by ΔL/L. Releasing the pre-strained elastomer induced considerable amount of compressive stress into the devices, and hence, a wavy-dimensional structure was formed on the surface of the devices by locally folding the PI film on the elastomer. As the compressive stress increased, the L decreased in the direction of channel width and returned to the pre-strained state when it was stretched again, as shown in Fig. 2(a). These behaviors were also well observed in Fig. 2(b). The distances between the peaks of the waves were continuously getting closer at each stage of compression. In this geometry, by means of the changes in wavelengths and amplitudes of the waves, complete stretchability and compressibility can be reversibly established for the devices without marked amounts of deformation. In other words, the devices can be easily restretched with the same value of the specified strains, and thus the surface deformation such as cracking can be effectively avoided.

The critical mechanical strain for the fabricated TFT is mainly influenced by the pre-strain of the elastomer rather than the mechanical properties of each layer.21 Therefore, the degradation


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behaviors of the TFTs were analyzed according to the magnitude of the pre-strain. Figure 3(a) and (b) shows the variations in device parameters as a function of assigned pre-strain value, such as on current (ION), subthreshold swing (SS), hysteresis width in transfer curve, and the shifts of turn-on voltage (ΔVON), in which the relative ratio of estimated values to the initially obtained values were provided from the transfer curves shown in Figure S2. The ION was defined as the drain current obtained when the gate voltage of 20 V was applied to the gate terminal. The VON was determined as the gate voltage where the drain current was amounted to 10-10 A in each transfer curve. The hysteresis width was also defined as the shift of VON (∆VON) between forward and reverse sweeps in gate voltage. The device evaluations were performed after keeping the TFTs in a released state (with a tensile strain of 0 %) for 3 days, during which the devices locally suffered from compressive strain. In these evaluations, the devices fabricated on the thinnest PI film (0.9 µm) were firstly characterized to examine the effect of applied prestrain. The ION and SS tend to monotonously increase with increasing the pre-strain, as shown in Fig. 3(a). While there were very slight variations within 2% under the stretching conditions with a strain of less than 30 %, when the strain increased from 30 to 60 %, the ION and SS started to remarkably increase and their relative ratios were estimated to be 1.36 and 1.29, respectively. As can be seen in Fig. 3(b), the hysteresis width and ΔVON also showed similar behaviors. Until the magnitude of strain amounted to 40 %, there was no hysteresis and the ΔVON remained to be as small as -0.3 V. Whereas, owing to a large strain of 60%, the hysteresis width and the ΔVON increased to 0.6 V and -1.2 V, respectively. As the pre-strain of the elastomer increases, larger mechanical stress is applied to the TFT. The variations in device parameters (ION, SS, VON) as a function of pre-strain value can be explained by following two feasible scenarios: (1) the changes in Fermi energy level for the IGZO active layer was suggested to be the first consideration.


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When the tensile strain is applied to the region of circular arc within the wavy structure, the interatomic distance in the IGZO thin film might be varied, and hence, the difference between the energy levels splitting with the bonding and anti-bonding orbitals between the atoms is effectively reduced.22-23 As a result, the Fermi level can be shifted toward the conduction band edge, resulting in a reduced bandgap. Therefore, a larger number of electrons are excited to the conduction band, and hence, the density of conduction carriers increases so that the ION also increases and VON is negatively shifted owing to increase in channel conductivity 18,24-26; (2) the defect creation in the IGZO active channel can be the second scenario. Structural defects have been suggested to be generated in the IGZO active channel near the strained region with increasing the strain even without marked crack development into the GI layer.27 The mechanical strain-induced defects increase the carrier concentration by means of the increase in density of donor-like states. In other words, deep-state defects such as oxygen vacancies in IGZO bulk and at IGZO/GI interface regions increase the interface states and trap densities, which deteriorates the values of SS and hysteresis width.24,28-31

From these obtained results, it was expected to be interesting and informative to investigate the effects of PI film thickness on the deterioration behavior of the devices under the static compressive stress conditions. Figure 3(c) and 3(d) show the comparisons in the variations in ION and ΔVON of the devices evaluated with pre-strain of 50 %, respectively, when the PI film thickness was varied to 0.9, 1.3, and 2.0 μm, which were termed TFT1, TFT2, and TFT3, respectively, for conveniences. The prepared PI film substrates had been verified to be very uniform in thickness and to have a very smooth surface by an optical profilometer and an atomic force microscopy (AFM) in Figure. S3. Thus, the comparisons in operational characteristics


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among the TFTs fabricated on PI films with different thicknesses could be suggested to be sufficiently reliable. It was noteworthy that the degrees of variations in ION and ΔVON did not exhibit any marked differences among the devices, which were estimated to be as small as 1.2 and -0.5 V for all the devices, respectively. Therefore, in order to investigate the influence of PI film thickness more carefully and to figure out design strategies for securing the devices with more stable operations even under the stretching/compression conditions, the device operational reliabilities against the bias-stresses and cyclic stretching events were extensively evaluated and compared among the devices.

Positive bias stress (PBS) stabilities of the TFT1, TFT2, and TFT3 fabricated on PI film with different thickness were examined during the stress time in static stretching conditions with given strains, as shown in Figs. 4(a)-4(c), respectively. The pre-strain was fixed at 50 % and the elastomer was kept at the initial position for 3 days to induce the compressive strains. Figure S4 shows the variations in transfer curves with a lapse of stress time for the TFT1 with given prestrains of 10 to 60 %. A bias stress of +30 V was continuously applied to the gate terminal for 104 s and the drain bias was fixed at 10.5 V. For all the TFTs, the VON’s were positively shifted without the degradation of SS. The ΔVON's of the TFT1, TFT2, and TFT3 were estimated to be 1.5, 1.8 V, and 3.9 V, respectively. TFT1 and TFT2 exhibited stable characteristics even under harsh stretching conditions with a strain of 50 % and a bias stress of 30 V. Alternatively, the TFT3 on a thicker PI film showed a relatively large ΔVON. Figure 4(d) shows the variations in ΔVON for each TFT as a function of PI thickness after the PBS test. The positive shift in VON without a marked degradation of the SS under PBS stress can be generally explained by the simple electron trapping at the bulk region of the gate insulator and/or interface between the


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gate insulator and active channel layers.32-34 As mentioned above, the mechanical stress induced by the compressed state can create additional defect sites in the IGZO active layer, which influenced on PBS instabilities. Furthermore, the increase in trap sites at the interface may greatly affect the charge-trapping events due to easily activated excess electrons.35-37 It was noticeable that the PBS stability was deteriorated with increasing the PI film thickness, which was suggested from the fact that the magnitude of mechanical strain induced into the device plane is closely related to the PI film thickness. Since smaller radii of curvature can be imposed on the devices fabricated on thicker PI film, the TFT3 is subject to greater local stress at compressed states, which will be discussed below in a more detailed way. Thus, the VON of the TFT3 was more positively shifted under PBS condition owing to the generation of a larger number of defect states. As results, even under the same stretching conditions, the TFT3 on thicker PI film was found to be most vulnerable to mechanical stress. Consequently, the suitable choice of the PI film thickness has an important impact on the bias-stress stability under mechanically-strained conditions.

Next, the cyclic stretching test was carried out for the TFT1, TFT2 and TFT3 to further confirm the effects of PI film thickness on the operation stability at repeated stretching events. The magnitude of pre-strain was also fixed at 50 % and the tests were conducted using the custommade cyclic stretching jig, with which the clamp moved repeatedly at a speed of 10 cm/s from the unstrained position to the pre-strained position and paused for 0.2 s with a fully stretched state with a strain of 50 %. Figure S5 shows the sets of transfer curves measured during the cyclic stretching events with a pre-strain of 50 % for the TFT1, TFT2 and TFT3. Figures 5(a) and (b) show the variations in the ΔVON and the field-effect mobility at the saturation region


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(μsat) estimated with every 10,000 cycles as a function of stretching cycles. In these measurements, device-to-device uniformity was also carefully examined, which was indicated as error bars in data plots, because the mechanical stresses caused by wavy structures formed on the device plane were supposed to have considerable fluctuations among the devices during the cyclic stretching tests. Notwithstanding, the obtained behaviors of variations in the ΔVON and the μsat can be regarded to show reliable trend, since all the devices exhibited the similar tendencies of deviations and degradation behaviors to device failures. In other words, the VON was estimated to be continuously shifted in a negative direction for three devices with the evolution of cyclic numbers. These degradation behaviors originated from the increase in the conduction carries generated by the defects during the repeated application of the mechanical strain.23

It also was noticeable from the obtained results that the evaluated three devices exhibited completely different device failure operations with increasing the number of stretching cycles. The μsat values for the TFT3, TFT2 and TFT1 showed initial increasing trends up to 10,000, 30,000 and 60,000 cycles, respectively, and then sharply decreased. As results, the IDS’s estimated after the repeated stretching events with 20,000, 70,000 and 100,000 cycles, respectively, were estimated to have lower values by 2-orders-of magnitude compared with those initially obtained states. The origin of the initial increase in the ION is due to the increase in conduction carrier density due to the changes in the Fermi level under the mechanically strained conditions, as mentioned above. Contrarily, with increasing the bending cycles, the values of ION and μsat abruptly decreased and then, all the devices finally exhibited complete failures. These fatigue characteristics might be supposed to result from the increase in contact resistance owing to the local delamination of the ITO electrodes and/or the formation of microcracks.37-39


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Therefore, the these obtained results implies that there are no marked generations of cracks against the mechanical strain during the repeated stretching tests with 60,000 cycles. This superior mechanical stability can be attributed to the thin film thickness of the PI, which induces lower strain into the device plane during the stretching test.

To investigate the fundamental origins for the PBS and cyclic stress instabilities influenced by the PI film thickness under the mechanically stretching states, it is useful to discuss the feasible shapes and distributions of the waves formed onto the device plane in the wavy-dimensional structures generated on PI films. Generally, a physical model has been mainly investigated for a single-layered metal thin film in previous publications, 40-41 in which a sinusoidal wave was implemented and a certain radius of curvature was assigned at peaks and valleys of the waves, as shown in Fig. 6(a). In these wavy-dimensional structures, the amplitude and wavelength of the generated wavy forms proportionally increased with the thickness of the substrate on the whole area of the substrate.40 As results, with increasing the amplitude and the wavelength of wavy dimensional structure, the radius of curvature has a larger value inducing a relatively smaller mechanical strain at a curved region. However, in practical cases, the wavy forms cannot be ideally generated for the devices composed of multiple functional layers with patterned shapes when the elastomer substrate was stretched and compressed. In other words, the waves on the device plane are arbitrarily generated with losing the regular forms. Figure 6(b) schematically illustrates a typical situation of random variations in formed waves onto the PI film,21,42 which can be appropriately imagined by the microscopic views of the device planes on PI at a compressed state. Therefore, the locally formed curvature of the PI film was not precisely determined and have some spatial fluctuations, as can be seen in the circled area of Fig. 6(c). A


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very small radius of curvature formed by a sharp peak of wave has critical impacts on the device characteristics. Therefore, the degree of random fluctuations in the generated wavy forms at mechanically compressed situations has more important roles in determining the degradation mechanism for the stretchable IGZO TFTs on the PI film substrate with different thickness values. Consequently, the PI film thickness should be reduced so that the magnitude of mechanical strain can be sufficiently alleviated on the device plane to avoid the device failures. The surface bending strain (εsur) induced into the devices fabricated on PI film substrate can be estimated by the Eq. (1) as below, 𝑑𝑓 + 𝑑𝑠 (1 + 2η + χη2) 2𝑅 )(1 + η)(1 + χη)

𝜀sur = (

(1)

where the ds and df correspond to the thicknesses of the flexible PI substrate and the film deposited on the substrate, respectively. R is the radius of curvature. Parameters of χ and η are defined by Yf / Ys and df / ds, respectively, in which Ys and Yf can be regarded as Young’s moduli of the Al2O3 gate insulator (180 GPa) and the PI film substrate (6.3 GPa), respectively. Alternatively, the elastomer has a much smaller Young's modulus of 1.8 MPa. Due to the difference in Young's moduli at the interfaces between the PI and the elastomer films, the influence of the elastomer on the mechanical strain induced onto the device plane can be negligible.43 Thus, the mechanical stretchability of the fabricated devices can be expected to be much more sensitively influenced by the thickness of thinner PI films, irrespective of the elastomer thickness as thick as 0.5 mm. According to Eq. (1), with an assumption that other parameters have the same values, the value of εsur induced to the TFTs decreases with reducing the PI film thickness. Figure 6(d) shows the variations in the εsur calculated for the TFT1 and TFT3 as a function of R formed by the wavy structures, which were prepared on 0.9 and 2.0-µm-


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thick PI films, respectively. Assuming that the critical strain value for the device failures is in the range from 0.5 to 1%, the critical value of R (Rc) for the TFT1 and TFT3 can be estimated to have approximate ranges from 25 to 50 μm and from 38 to 75 μm, respectively. The allowable values of Rc’s between the TFT1 and TFT3 might be not so critical to cause any degradations in device characteristics under the mechanical stress merely induced in a single mode, as shown in Figs.3(c)-3(d). However, these variations in Rc’s were examined to have marked impacts on the device characteristics under harsh complexed stress conditions, in detail, bias/mechanical stresses and repeatedly induced mechanical stress. As results, the TFT1 can be more intrinsically robust to the mechanical deformation during the compressed conditions than other devices of TFT2 and TFT3 owing to the introduction of smaller degree of strain. Furthermore, the thinner PI film thickness was also desirable to effectively suppress the range of random fluctuations in the curvature values locally formed by wavy-dimensional structures. From the obtained results, it was suggested that mechanically stretchable TFTs have to be designed so that the geometric factors can be properly determined by considering various mechanical properties of oxide thin films composing the devices. The stretchable IGZO TFTs fabricated in this work correspond to one of the thinnest device reported so far. With a total thickness of only 1.5 µm, extremely mechanical flexibility and stretchability can be accomplished with bending radii of 25 to 50 µm, which can be regarded as sufficiently small radius of curvature for various practical mobile applications.


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4. CONCLUSIONS

We fabricated and characterized the stretchable IGZO TFTs on ultra-thin PI/elastomer with a pre-strained wavy structure. The devices fabricated on 0.9-µm-thick PI film exhibited sound device operations even at a strain of 40 %, excellent PBS stability, and robust operation during the repeated stretching with 105 cycles at a stretching ratio of 50 %. The device characteristics and their degradation behaviors were extensively investigated under the mechanically stretched and compressed conditions when the PI film thickness was varied for the stretchable IGZO TFTs with wavy dimensions. From the obtained results of PBS and cyclic stress instabilities, the strain induced during the mechanical deformation was verified to sensitively be influenced by the variations in PI film thickness, resulting in important impact on the device performance. These results originated from the fact that a thinner PI film can reduce the random fluctuations in bending curvatures and the magnitude of mechanical strain induced into the devices during the stretching and compressions states. Therefore, the mechanical stretchability of the oxide TFTs with intrinsic brittle natures could be far enhanced by the choice of ultra-thin PI film substrates and the wavy-dimensional structures formed on the elastomers. Consequently, the obtained results and related analysis provided useful guidelines for properly designing the stretchable TFTs for various applications.


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Figures

Figure. 1. (a) Schematic illustration of the process flow for the device fabrications and (b) crosssectional view of the fabricated IGZO TFTs on polyimide film substrates. (c) Schematic illustrations of the formation process of wavy dimensional structures for the stretchable TFTs and (d) the wavy structures formed on the device plane under stretched and compressed conditions.


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Figure. 2. (a) Photo images and (b) optical microscopic views of the TFT device plane at each strain conditions.


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Figure. 3. Variations in the (a) drain current obtained at a VGS of +20 V (ION), SS, (b) hysteresis width in transfer curve, and the ΔVON as a function of pre-strain value, in which the relative ratio of estimated values to the initially obtained values were plotted. Comparisons in the variations of (c) ION and (d) ΔVON of the devices evaluated with pre-strain of 50 %, respectively, when the PI film thickness was varied to 0.9, 1.3, and 2.0 μm.


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Figure. 4. Variations in transfer curves during the PBS tests with a lapse of stress time for (a) TFT1, (b) TFT2, and (c) TFT3 with a pre-strain of 50%. (d) Comparisons in ΔVON measured under the PBS as a function of PI film thickness.


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Figure. 5. (a) Variations in the μsat and (b) the ΔVON for the TFT1, TFT2, and TFT3 as a function of the number of stretching cycles.


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Figure. 6. Schematic illustrations of (a) an ideal waveform model with uniform and constant R and (b) a real waveform model with random fluctuations in R under compressed conditions. (c) Microscopic photo image of the stretchable TFTs with randomly formed wavy configurations. (d) Variations in the εsur calculated for the TFT1 and TFT3 as a function of R formed by the wavy structures.


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ACKNOWLEDGMENT This work was partly supported by the Korea Evaluation Institute of Industrial Technology through the Korean Government (10079974, Development of core technologies on materials, devices, and processes for TFT backplane and light emitting frontplane with enhanced stretchability above 20%, with application to stretchable display), and by the Kyung Hee University–Samsung Electronics Research and Development Program entitled Flexible Flash Memory Device Technologies for Next-Gen Consumer Electronics.

ASSOCIATED CONTENT Supporting Information Photo images of (a) the measurement configuration for the electrical evaluations of the fabricated stretchable IGZO TFTs and (b) a customized stretching cycle machine (Figure S1); Comparisons in the transfer characteristics between the pristine and the strained states for the stretchable IGZO TFTs with given pre-strains (Figure S2); (a) A profile data measured by optical profilometer and (b) an AFM image representing the surface morphology of the coated PI film with a thickness of 2 µm (Figure S3); Variations in transfer curves with a lapse of stress time under positive bias stress of +30 V for the TFT1 with given pre-strain (Figure S4); Variations in transfer characteristics measured during the cyclic stretching events with a pre-strain of 50 % for (a) the TFT1 and (b) the TFT3 (Figure S5).


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43) Park, S.; Cho, k.; Oh, H.; Kim, S.; Electrical and Mechanical Characteristics of Fully Transparent IZO Thin-Film Transistors on Stress-Relieving Bendable Substrates Appl. Phys. Lett. 2016, 109, 143504


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Impact of Polyimide Film Thickness for Improving the Mechanical

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