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Functional Inorganic Materials and Devices

Low-Temperature Fabrication of Robust, Transparent, and Flexible Thin Film Transistor with Nano-Laminated Insulator. Jeong Hyun Kwon, Jun Hong Park, Myung Keun Lee, Jeong Woo Park, Yongmin Jeon, Jeong Bin Shin, Minwoo Nam, Choong-Ki Kim, Yang-Kyu Choi, and Kyung Cheol Choi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01438 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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ACS Applied Materials & Interfaces

Low-Temperature Fabrication of Robust, Transparent, and Flexible Thin Film Transistor with Nano-Laminated Insulator †

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Jeong Hyun Kwon , Jun Hong Park , Myung Keun Lee , Jeong Woo Park , Yongmin Jeon , Jeong Bin Shin†, Minwoo Nam†, Choong-Ki Kim†, Yang-Kyu Choi† and Kyung Cheol Choi*,†

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School of Electrical Engineering, KAIST, Daejeon 34141, Republic of Korea

Keywords. Thin film transistor; dielectric-metal-dielectric (DMD); Gate insulator; Transparent and flexible; Nanolamiante; Environmental stability;

ACS Paragon Plus Environment

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Abstract The lack of reliable, transparent, and flexible electrodes and insulators for application in TFTs makes it difficult to commercialize transparent, flexible thin film transistors (TF-TFTs). More specifically, conventional high process temperatures and the brittleness of these elements have been hurdles in developing flexible substrates vulnerable to heat. Here, we propose electrode and insulator fabrication techniques considering process temperature, transmittance, flexibility, and environmental stability. A transparent and flexible ITO/Ag/ITO (IAI) electrode and an Al2O3/MgO (AM) laminated insulator were optimized at the low temperature of 70 °C for the fabrication of TF-TFTs on a PET substrate. The optimized IAI electrode with a sheet resistance of 7 Ω/sq exhibited the luminous transmittance of 85.17% and, maintained its electrical conductivity after exposure to damp heat conditions due to an environmentally stable ITO capping layer. In addition, the electrical conductivity of IAI was maintained after 10,000 bending cycles with a tensile strain of 3% due to the ductile Ag film. In the metal/insulator/metal structure, the insulating and mechanical properties of the optimized AM laminated film deposited at 70 °C were significantly improved due to the highly dense nanolaminate system, compared to those of Al2O3 film deposited at 70 °C. In addition, the amorphous indiumgallium-zinc oxide (a-IGZO) was used as the active channel for TF-TFTs due to its excellent chemical stability. In environmental stability test, the ITO, a-IGZO, and AM-laminated films showed the excellent environmental stability. Therefore, our IGZO-based TFT with IAI electrodes and the 70 °C AM laminated insulator was fabricated both robustness, transparency, flexibility, and process temperature, resulting in transfer characteristics comparable to those of an IGZO-based TFT with a 150 °C Al2O3 insulator. 1. INTRODUCTION There has been growing interest in the development of thin film transistors (TFT) as well as organic-based electronic devices for flexible and wearable display applications.1–9 Among various TFTs, such as a-Si TFTs, poly-Si TFTs10, organic TFTs11, and oxide TFTs7,9, and so forth, oxide TFTs are considered particularly promising due to their high mobility, high Ion/Ioff ratio, high channel transparency, and comparatively low temperature process to produce transparent, flexible TFTs (TF-TFTs). However, there have been many technical issues in commercializing TF-TFTs. First, the individual gate, source, and drain (GSD) electrodes in previous TFTs were mostly composed of brittle indium tin oxide (ITO) and opaque metal film.4,12,13 To replace these limited electrodes, alternatives such as graphene14, carbon nanotubes (CNTs)15, metal nanowires16, conducting polymers16, and so forth were proposed for transparent, flexible electrodes. However, the trade-off between electrical conductivity and optical transmittance, the complex and non-uniform process, and environmental instability have made it difficult to use these materials as electrodes for TF-TFTs. Therefore, environmentally stable, transparent and flexible electrodes are required that can be fabricated through simple deposition process. Second, although the oxide TFT process has the advantage that thermal annealing is conducted at a relatively low temperature, gate insulators, electrodes, and flexible substrates based on fabrics and polymer, exhibit poor thermal stability and cannot withstand such a temperature process in terms of the coefficient of thermal expansion (CTE) and glass transition temperature (Tg).17 The CTE is the degree of deformation of the material with increasing temperature as a representative measure of thermal stability against temperature change. The lower the CTE value is, the more stable the material is. The Tg value, which indicates the maximum temperature


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that the substrate can withstand, is also an important measure of the thermal stability of materials. On the existing glass substrate, the process temperature (< 350 °C) can be raised sufficiently, and the characteristics of the device necessary for driving can be easily obtained. However, in the case of temperature-vulnerable flexible substrates, the previous temperature process is impossible due to poor thermal stability. For example, PET substrates exhibit a glass transition temperature and a crystallizing temperature of 76 °C and 120 °C, respectively, resulting in more thermal damage with increasing process temperature.17 In particular, the process temperature issue is more critical as the thickness of the substrate becomes thinner. Therefore, the development of a low-temperature process to enable the commercialization of fabric-based and plastic substrate-based flexible displays is important. To overcome the above-mentioned limitations for the realization of TF-TFTs, we introduce low-temperature processed dielectric/metal/dielectric (DMD) electrodes and a laminated film that can be applied as GSD electrodes and an insulator, respectively. The DMD electrodes, which exhibit high electrical conductivity and high flexibility of the metal film, have guaranteed high transparency due to the light interference effect between the high-refractive-index dielectric/Ag film and the anti-reflection achieved by optimization of the dielectric thickness in the DMD structure although a metal film is included in the multilayer structure.18–20 So far, various DMD-based electrodes have been reported for application in organic light-emitting diodes and TFTs (e.g. ZnO/Ag/ZnO, AZO/Ag/AZO, WoO3/Ag/WoO3, etc.). However, although these DMD-based structure are transparent and conductive, environmental stability is not guaranteed in harsh environments.21, 22 Environmental stability means that the main properties of the thin films are well maintained under damp heat conditions, where chemical stability in such an extreme environment is scarcely considered. Andreas et al. reported that the conductivity of ZnO and AZO rapidly degraded within a few minutes at 85 °C/85% RH due to oxygen chemisorption and the formation of –OH groups.21 Therefore, an environmentally reliable dielectric layer is required for the reliability of the DMD structure. We aimed to optimize the ITO/Ag/ITO (IAI) multilayered electrode for use as transparent and flexible GSD electrodes. The IAI electrode suitable for TF-TFTs was fabricated by combining transparent and chemically stable ITO and highly conductive flexible Ag film. In addition to the GSD electrodes, the gate insulator is the key element for the reliability of TFTs. A high temperature process is required for electrically and mechanically stable insulators with leakage current insulating properties, high capacitance, a high breakdown field, and superior flexibility. For high-quality insulators, oxides and nitrides are mainly applied by plasma-enhanced chemical vapor deposition (PECVD) and atomic layer deposition (ALD). Si-based materials (SiO2 and SiNx) deposited by PECVD have shown limited reliability due to their low dielectric constant below 4, brittleness, and high fabrication temperature (> 300 °C).23 On the other hand, various oxides grown by ALD, such as Al2O3, TiO2, HfO2, and ZrO2 have been reported as high-k insulators of TFTs.23–25 Among various high-k dielectric materials, Al2O3 ALD film is promising as a gate insulator in TFTs, showing a stable deposition process and reliability as an insulator. However, Al2O3 gate insulators have some disadvantages, such as a high temperature process, low chemical stability, and brittleness.22, 26, 27

More specifically, Al2O3 gate insulators are deposited at high temperatures above 100 °C for the insulating

properties. In addition, the neat Al2O3 film is easily corroded upon exposure to accelerated environments due to hydrolysis.26,28 Finally, Al2O3 ALD film exhibits poor mechanical properties under bending strain of less than 1%.26,29 Therefore, to overcome the limitations of Al2O3 film, we fabricated a gate insulator using a nanolaminate system in which ultra-thin Al2O3 layers are laminated with other ultra-thin layers. The use of



inorganic nanolaminate structures (e.g. Al2O3/ZnO30, Al2O3/TiO231, and Al2O3/ZrO232, etc.), stacked with another functional ultra-thin layer has been a good approach to fabricate highly dense, corrosion-resistant and flexible thin film. In particular, the formation of aluminate phases in laminated structures help produce films that are chemically stable and highly dense. Therefore, we developed an Al2O3/MgO (AM) laminated film that can be fabricated at a low temperature of 70 °C, which is lower than the Tg of a PET substrate. MgO, which is a potential candidate as a gate dielectric, has a wide band gap (7.8 eV) and a dielectric constant of 9.8 for bulk material.33 In addition, MgO film, which works as a water absorber, shows good compatibility with Al2O3 film for the formation of aluminate phases.28 These reactions make Al2O3 film more reliable. Therefore, the AM nanolaminate makes low-temperature films that are dense, water-resistant and flexible. In this paper, we present a low-temperature-processed oxide TFT fabricated with an AM laminated insulator and IAI electrodes on a 125-µm-PET. The IAI electrodes with a sheet resistance of approximately 7 Ω/sq were used as GSD electrodes for the TFT. The IAI electrodes showed almost no deterioration in the conductivity under damp heat conditions due to the environmentally stable ITO, whereas the conductivity of ZnO and Ag films deteriorates within a few minutes. As an insulator, the AM film deposited at a chamber temperature of 70℃ was used for highly flexible, electrically insulating, and superior barrier performance. The leakage-current insulating properties and capacitance of the 40-nm-thick AM insulator deposited at 70 °C are comparable to those of an Al2O3 insulator deposited at 150 °C, showing a leakage current density less than 10-9 A/cm2. Finally, the amorphous indium-gallium-zinc oxide (a-IGZO) TFT fabricated with an AM insulator exhibits stable electrical characteristics corresponding to the TFT fabricated with 150 °C Al2O3, showing the mobility of 11.35 cm2/(V-s). Furthermore, the AM insulator works as a gas diffusion barrier to protect the channel, source, and drain agasinst water vapor and oxygen. The measured water vapor transmission rates (WVTRs) of 70 °C AM are equivalent to that of 150 °C Al2O3 despite the presence of many defects with fabrication at a low temperature. Therefore, a transparent and flexible oxide TFT was fabricated with an AM insulator, and it demonstrated good potential for application as a gate insulator at low-temperatures. 2. Results and Discussion 2.1. Functional design of transparent and flexible a-IGZO TFT.


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Figure 1. a) Optimized ITO/Ag/ITO electrode for the gate, source and drain electrodes. b) Al2O3/MgO laminated film for the gate dielectric. c) Fabrication process of transparent and flexible oxide TFT. For transparent and flexible TFFs, transparent and flexible multilayer structures were introduced as GSD electrodes and insulators, respectively. First, prior to TFT fabrication, the 125-µm-thick PET substrate was preannealed at 150 °C for one hour to sufficiently shrink the PET. This pretreatment on the PET substrate helps reduce the misalignment between the source-drain and gate and residual stress buildup in TFTs. The IAI multilayer structure was optimized based on an optical simulation and experiments. As shown in Figure 1a, the thicknesses of the bottom/top ITO layers were respectively 51 and 45 nm using a radio-frequency (RF) sputtering system. We fabricated highly robust, transparent, and flexible electrodes with insert of a thermally evaporated Ag thin film with a thickness of about 12 nm between the ITO films. The sheet resistances of the ITO (51 nm)/Ag (12 nm)/ITO (45 nm) and the 100-nm-thick ITO electrodes was 7.2 and 48.9 Ω/sq and the luminous transmittance of the IAI electrode and the 100-nm-thick ITO electrodes was 85.79 and 82.21 %, respectively (see Supporting Information, Figure S1). The electrical conductivity and optical transmittance of the IAI electrode were significantly better than those of the ITO film as a result of the structure optimization, despite the insertion of the Ag film. In addtion, we fabricated a highly dense and flexible AM laminated film of 40 nm thickness by alternatively stacking 1-nm-thick sublayers as a gate insulator (Figure 1b). After optimization of the electrodes and insulator for the TF-TFT, we fabricated a staggered-type TFT structure to evaluate the electrical characterisitics of TFTs in which the channel layer, source, and drain electrodes are protected by the insulator. The rough surface of the PET substrate with high roughness and peak-to-valley values was planarized though sequential coating of an organic layer and an Al2O3 ALD film, resulting in a significant reduction in the peak-to-valley value from 57.367 to 0.307 nm (see SSupporting Information, Figure S2). After the PET surface planarization, total TF-TFT fabrication was conducted (Figure 1c). The structure of the TFTs was IAI /a-IGZO (20 nm)/ Al2O3 or AM insulator (40 nm)/IAI, and the electrical characteristics of the TFTs with different kinds of insulators were compared.



2.2. Chemical and mechanical stability of multilayer electrodes.

Figure 2. a) Normalized conductance of Ag, ITO, ZnO, and ITO films at 85 °C and 85% RH. (Inset : schematic of the stability testing structure for the measurement of conductivity) b) Optical transmittance of the IGZO film after 0, 1, 4, and 7 days under storage at 85 °C and 85% RH. c) Sheet resistance of ITO and IAI electrodes vs. tensile strain (Inset: schematic of bending test for electrodes and insulators on PET substrates). The degree of change in sheet resistance (R) relative to the initial resistance value (R0) was obtained. d) Change in sheet resistance of IAI electrodes vs. bending cycle at a bending strain of 3.1 %. Recently, many transparent conductive channel and electrode layers based on oxides (e.g. ZnO, AZO, and WO3, etc.) and nanostructured materials (e.g. nanowire, CNTs, graphene and molybdenum disulfide, etc.) have been proposed for transparent, flexible devices.34–37 However, these materials exhibit rapid degradation of their electrical properties within a few minutes after exposure to damp heat conditions of 85 °C/85%. In particular, the electrical conductivity of ZnO-based films degraded due to oxygen chemisorption within an hour of exposure to a harsh environment. In addition, bare Ag or Cu films, which are mainly used as metal thin films in DMD multilayer structures, degrade rapidly due to their high reactivity with oxygen.21 Therefore, environmentally stable thin films should be used to ensure the reliability of devices. Among various transparent and conductive oxides, indium-based oxide films have shown excellent environmental stability.38 We conducted a stability test to assess the environmental stability of oxide and metal films. In addition, graphene film, which is often used as the active channel layer and electrode, was evaluated to compare its environmental stability with other films.14,15,39 The changes in the electrical conductivity of ZnO, ITO, Ag, and graphene films and changes in the optical transmittance of the IGZO film under a damp heat condition of 85 °C/85% are shown in Figures


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2a and b. The ZnO, Ag, and graphene films showed rapid degradation in conductivity due to their material and structural limitations, whereas the conductivity and transmittance of ITO and IGZO films remained almost unchanged after 7 days due to the indium-based chemical stability.38 The excellent environmental robustness of indium-based ITO and IGZO films lends the TFT greater reliability and stability, and it thereby shows much better environmental stability than carbon-based and metallic nanostructures. In addition to environmental stability, the mechanical properties of the IAI electrode were evaluated through a bending test. For the realization of TF-TFTs, the mechanical properties of GSD electrodes are very important. The previous transparent and conductive oxides were very brittle, resulting in bending-induced cracks and poor electrical conductivity at a bending strain of less than 1%.21,39 However, the DMD structure demonstrated significantly improved flexibility due to the ductility of the Ag film in comparison to the ITO electrode, which is an important advantage of DMD structures. As shown in Figure 2c, the IAI electrode maintained its initial sheet resistance after 1000 bending iterations at a tensile strain of 3%, while the ITO electrode started to show increased sheet resistance at a tensile strain of approximately 0.4 %. In addition, we evaluated the mechanical properties of the IAI electrode according to the number of bending cycles in terms of fatigue stress. The sheet resistance of the IAI electrode remained almost unchanged even after 10000 bending cycles at a tensile strain of 3.1 %. The environmental stability and bending test results demonstrated that the IAI electrode shows sufficient reliability for application in various devices. 2.3. Optimized desigin and mechanical properties of Al2O3/MgO-laminated insulators

Figure 3. a) Sequential formation of each AM film according to the sublayer thickness. Elemental analysis



results of each AM film by EDS for b) Al and c) Mg. d) Cross-sectional TEM and FFT diffraction pattern images of the AM insulators according to the sublayer thickness. The AM-laminated structure was optimized through an analysis of TEM cross-section images of various AM laminated film structures with various sublayer thicknesses. Alternately stacked quasi-perfect sublayers form a multi-interfacial layer-by-layer structure, which makes the AM film highly dense and flexible due to the defectdecoupling system and aluminate phases at the interfaces.26 AM lamianted film structures with various sublayer thicknesses were deposited on Si wafers. We verified that each AM laminated film had the expected thickness according to the cyclic ratio for Al2O3 and MgO deposition in TEM images (Figure 3a). In addition, Figures 3b and c show two-dimentional (2D) mapping results of Al and Mg in the TEM images, which demonstrate that all of the AM laminated film stuructures contain both Al and Mg elements. However, the multi-interfacial system, which shows clear layer separation between sublayers, is very important for the formation of a highly dense and effective nanolaminate structure. In the AM laminated films with sublayer thicknesses of less than 1 nm, the TEM cross-sectional images showed unclear interfaces with an unclear blended membrane structure with structural imperfections, whereas the AM laminated film with a sublayer thickness of 1 to 1.5 nm starts to display a layer-by-layer structure with clear interfaces (Figure 3d). In addition, film growth was clearly explained by the fast Fourier transform (FFT) pattern, which was employed to investigate the crystalline nature of AM laminated films from the localized regions in TEM images. The diffraction pattern in the FFT images was clear because of the quasi-perfect layer formation and crystal growth of MgO sublayers. As shown in the FFT images presented in Figure 2d, the images become clear ring patterns with increasing thickness of the MgO sublayer in the AM laminated film. These structural imperfections in the blended membrane structure tend to generate defects in films and interfaces, facilitating the diffusion of water vapor and oxygen inside the film. These defects appear due to the mismatch of the lattice parameters between Al and Mg atoms.28 In staggeredtype TFTs, the channel, source, and drain are surrounded by gate insulators (Figrure 1c). In other words, gate insulators protect the channel, souce, and drain against water vapor and oxygen and help maintain the reliaibility of the TFTs. Therefore, insulators should be optimized in terms of gas diffusion barrier properties. Our multilayered stacks consisting of alternatively grown ultra-thin sublayers can effectively restrict the size and growth of defects and separate the positions of defects, creating complex and tortuous diffusion paths for the permeation of water vapor and oxygen into a thin film (Figure 4a).


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Figure 4. a) Schematic of the complicated defect-coupling system exhibited by the nanolaminate structure. b) Schematic of the permeation sensor for the Ca test. (Inset: Photograph of Ca test cell before and after evaluation of barrier film) c) WVTR values of 40-nm-thick AM laminated films with various sublayer thicknesses according to tensile strain. d) WVTR values of 40-nm-thick ALD films according to tensile strain. To optimize the sublayer thickness in the AM laminated film in terms of barrier properties, additional experiments to assess the WVTR were conducted through an electrical calcium (Ca) test.40 WVTR measurements of thin films were conducted under accelerated conditions of 30°C/90% RH. The optimization of the AM laminated film was conducted through Ca tests to assess the barrier and mechanical properties. The electrical Ca test measures the gas diffusion barrier properties of thin films by measuring the oxidation degree of the Ca layer reactive with water vapor and oxygen that penetrated the barrier films, as shown in Figure 4b. First, we simultaneously evaluated the barrier and mechanical properties of the AM laminated films in relation to the sublayer thickness. The mechanical properties of the insulator were evaluated indirectly through a comparison of WVTR values after bending tests, unlike the electrodes, which were compared in terms of resistance change. An increase in the WVTR after bending tests means that water vapor passes through the film more easily due to cracking in the films resulting from the application of tensile strain. Figure 4c shows the measured change of WVTR values before and after bending according to sublayer thicknesses of 0.5, 1.5, 3, 5 nm. As seen in the TEM images, the Ca test results showed the expected trends. The AM laminated film with a sublayer thickness of 1.5 nm exhibited the lowest WVTR value. The reason for the lowest WVTR value appears be the formation of the first quasi-perfect sublayers and many interfaces between sublayers. The AM laminated film with a sublayer thickness of 0.5 nm showed an unclear blended membrane structure without any interface between sublayers. The film structure without an interface allows water vapor and oxygen to diffuse easily. Also, when



the thickness of the AM laminated film was fixed, as the sublayer thickness was increased to more than 1.5 nm, the number of interfaces in the film was reduced. AM laminated films with sublayer thicknesses of 3 and 5 nm exhibited slightly increased WVTR values. The WVTR changes under a tensile strain of 1.25 % were noteworthy. In the case of the laminated film with 1.5 nm-thick sublayers, the lowest WVTR increase was observed. This appears to be due to crack deflection and crack arresting in the nanolaminate structure consisting of quasi-perfect sublayers. Also, the stress concentration effect was weakened by reducing the number and size of defects in the film. Based on the results of the Ca test and TEM analysis, the AM laminated film was optimized to have a sublayer thickness of ~1.5 nm. In addition, the optimized AM laminated film exhibited significantly improved barrier and mechanical properties in comparison to the Al2O3 and MgO single layers. The WVTR values of 40-nm-thick Al2O3 and MgO and the AM laminated films deposited at 70 °C were 7.58 × 10−4, 3.54 × 10−2, and 1.12 × 10−4 g/m2/day, where the WVTR difference indicates a significant improvement in the film quality. In general, as the deposition temperature for the ALD film was increased, the defects and impurities in the films were gradually reduced. In fact, the 150 °C Al2O3 exhibited a WVTR of 9.75 × 10−5 g/m2/day. Therefore, the high temperature in the process of device fabrication was required to achieve high film quality. In addition, the MgO ALD film started to show a crystalline state when its thickness increased beyond a certain value. The XRD analysis revealed that the MgO thin film exhibited crystal growth for thicknesses from 12 to 15 nm (see Supporting Information, Figure S3). As a result, the crystalline MgO film exhibited poor barrier and mechanical properties. The 70 °C MgO was easily broken at a tensile strain of approximately 0.1% due to its crystalline structure. On the other hand, although Al2O3 films deposited at 70 °C and 150 °C showed amorphous structures, they exhibited a large difference in film quality according to the deposition temperature. The high deposition temperature of 150 °C for the Al2O3 film resulted in a meaningful improvement in its barrier performance and mechanical properties. However, as could be seen from the WVTR values related to film density and the number of defects and cracks, the structural and material design makes the film highly dense, overcoming the limitations of the lowtemperature process. Therefore, our optimized AM laminated film deposited at 70 °C exhibited a film quality similar to that of the Al2O3 film deposited at 150 °C in terms of barrier performance and mechanical properties. To summarize again, the high density and reliability of the AM laminated film is attributed to the following three factors. First, the MgO thin film starts to show crystalline growth at a thickness of approximately 15 nm. Note that an MgO single layer is ineffective as a flexible barrier film or insulator due to its crystallization which allows gas permeation thorough grain boundaries. However, amorphous AM nanolaminates inhibit MgO growth and thus provide excellent barrier and mechanical properties. Second, the formation of aluminate phases at the interfaces between Al2O3 and MgO sublayers results in high packing density of the AM film.


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Figure 5. X-ray photoelectron spectra of AM, Al2O3, and MgO of a) Al 2p and b) Mg 1s. c) Formation of the AlMg-O phase at the interface of Al2O3/MgO. Aluminate phases are formed in the process of exchanging electrons due to the difference in the electronegativities of Al (1.61) and Mg (1.31), as shown in Figure 5.26,41 The core-level peaks of Al 2p and Mg 1s in MgO, Al2O3, and the AM nannolaminate were compared by XPS analysis (Figures 5a and b). The binding energy values of Al 2p and Mg 1s core levels in the AM laminated film changed slightly in comparison to those in the Al2O3 and MgO films. In other words, in the case of the AM laminated film, the binding energy of the Al 2p core level film is lower (74.03 (Al2O3) 73.58 (AM) eV) and the binding energy of the Mg 1s is higher (1302 (MgO) 1303.23 (AM) eV) in the process of gaining and donating electrons at the Al2O3/MgO interfaces by the difference in the electronegativities between Al (1.61) and Mg (1.31), respectiviely (Figure 5c). These thermodynamically stable aluminate formations have been observed for various nanolaminate structures (e.g Al2O3/TiO231, Al2O3/ZrO241, etc.), resulting in highly dense, corrosion-resistant composite films. This chemical bonding improved the environmental reliability of the AM laminated film, overcoming the corrosion problem of the neat Al2O3 film under harsh environmental conditions. Third, the locations of defects and pinholes in the sublayers are separated by the defect-decoupling system. Although the single layer become thicker, the initial defects and pinholes appearing in a single layer grow long.42 However, the nanolaminate system could prevent the growth and occurence of statistical defects and pinholes. As a result, these effects make the film electrically and physically dense. Therefore, the AM laminated film is clearly suitable for application as a gas diffusion barrier film and an insulator requiring excellent film quality for organic electronics and field effect transistors.



2.4. Insulating properties and mechanical stability of AM laminated film and Al2O3 film

Figure 6. a) Schematic of the ITO/insulator/Al MIM structure. Leakage current density of b) single layers and c) AM laminated layers according to the deposition temperature in ITO/insulator/Al structure. d) Comparison of the leakage current density of the 70 °C AM, Al2O3, MgO and 150 °C Al2O3 films in the MIM structure. e) Capacitance versus voltage of Al2O3, MgO, and AM films at 100 kHz. f) Leakage current densities versus electric field of the MIM structure after storage at room temperature for 300 days. Table 1. Insulating properties and capacitance of gate insulators.

To investigate the insulating properties and reliability of the AM laminated film, different oxide films fabricated at various temperatures were tested as an inulator in the metal/insulator/metal (MIM) structure with an ITO/insulator/Al structure (Figure 6a). From evaluations of the insulator in the MIM structure, single layers, namely, Al2O3 and MgO, exhibited differences in insulating properties in relation to process temperature, while the AM laminated films exhibited similar insulating properties regardless of the process temperature (Figures


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6b and c). The 70 °C AM laminated film showed excellent insulating properties with leakage current density lower than 10-9 A/cm2, which is comparable to that of the 150 °C Al2O3 layer (Figure 6d). In addition, the 70 °C AM laminated film exhibited a high capacitance of approximately 137 nF/cm2, which is slightly lower than that of the 150 °C Al2O3 film, as shown in Figure 6e and Table 1. On the other hand, the Al2O3 and MgO single layers grown at 70 °C did not exhibit sufficient insulating properites and capacitance. Poor insulating properties in low-temperature single layers appear to be due to -OH radicals, which act as impurities at the interface.43 Although the AM laminated film was deposited at a low temperature of 70 °C, it exhibits superior insulating properties and a high dielectric constant of 6.2 suitable for a gate insulator achieved through structural and material design. In other words, the formation of the aluminate phases at the Al2O3/MgO interfaces prevents the formation of -OH radicals in laminated films. In addition, this effectively improves the breakdown voltage. Figure 6f shows the current density versus the electric field of the MIM structures, which were stored for 300 days at room temperature. The results of MIM tests showed that the MIM device with the 70 °C AM film exhibited stable operation up to 3 MV/cm; however, it started to exhibit soft breakdown at ~3.2 MV/cm. The MIM with the 150 °C Al2O3 film also exhibited a similar trend; however, it experienced a hard breakdown at 6 MV/cm due to the dissociation of -OH radicals. Note that the MIM with the 150 °C AM laminated film remained stable with changes in the current density within a magnitude of one order up to 10 MV/cm. Therefore, much lower defect density in the AM laminated films facilitates the improvement of the insulating properties and capacitance, demonstrating the superiority of the materially and structurally designed AM nanolaminate structure. The residual stress of a thin film is the stress present in the thin film immediately after it is formed, and this is calculated as the sum of thermal stress and intrinsic stress.26,44,45 Intrinsic stress is generated by various factors during film formation, such as coalescence, growth mechanism, and so forth. Thermal stress is generated due to the different coefficients of thermal expansion (CTE) of the substrate and deposited thin films during heating and cooling. When tensile stress is applied to a thin film, it becomes concave upward, causing pores and crazing in the thin film, and when the stress becomes more intense, cracks are generated and the thin film is broken. Therefore, stress management in thin films is important to ensure the mechanical reliability of TFTs. In general, it is well-known that high-temperature ALD deposition causes high residual stress in ALD thin films due to thermal stress induced by the different coefficients of thermal expansion (CTE) and ALD growth mechanism. The thermal-induced stress (

) was calculated by mulipliaction of the elastic modulus (E), the CTE difference between the substrate and film ( ∆α) , and the temperature difference between deposition and measurement (∆T), ℎ = ∆∆. Therefore, because the CTE values of the Al2O3, MgO, and Si wafer were 4.2, 13.5, and 2.6 ppm/K near room temperature, respectively, we measured the residual stress in various 40-nm-thick ALD films using the wafer curvature method, where the residual stress value is calculated by using Stoney’s equation after measuring the change in the wafer curvature before and after formation of the thin film on the bare Si wafer (Figure S4a). As shown in Figure S4b, the stress values in the ALD-deposited 70 °C Al2O3 and MgO single layers exhibited residual tensile stress on the order of 200 to 300 MPa. Changes of residual stress in ALD films can be affected by temperature, thickness, and kinds of precursors and reactants, but appear to be largely due to the ALD film growth mechanism. However, the AM laminated film exhibited a significantly reduced tensile stress of 92.83 MPa. Considering the thermal stress resulting from different



coefficients of thermal expansion between Al2O3 and MgO sublayers (σthermal stress= EAM 70°C × ∆α × ∆T =110 × 9.3 × 50/1000 = 51.15 MPa), thermal stress induced by CTE differences did not play a role in the resulting residual stress. We could not find reasonable references to support our measurement results. However, in a previous report on an Al2O3/TiO2 (AT) nanolaminate structure, similar results where the residual stress of the AT nanolaminate was lower than that of Al2O3 and TiO2 single layers were obtained.26,44 As a result of various reports and our experimental results, the nanolamate system helps reduce the residual stress in comparison to the Al2O3 single layer by inhibiting the ALD growth mechanism which causes stress build-up. However, the hightemperature deposition process can cause high thermal stress in the AM laminated film due to the increased elastic modulus and ∆T (σthermal stress= EAM 150°C × ∆α × ∆T =165 × 9.3 × 130/1000 = 199.485 MPa). In other words, the significant stress relaxation observed in the AM nanolaminate structure is likely due to the formation of dense and stable aluminate phases at the Al2O3/MgO interfaces, as observed in ALD-deposited AT nanolaminate structures forming bonds at the interfaces between sublayers. On the other hand, the 150 °C Al2O3 film exhibited residual tensile stress of approximately 300 MPa, due to the ALD growth mechanism and thermal-induced stress. Therefore, the low-temperature AM laminated insulator can work effectively as a flexible insulator for TF-TFTs in comparison to the high-temperature Al2O3 layer in terms of mechanical reliability in relation to flexibility and residual stress. 2.5. Electrical and bending characteristics of TFTs with IAI electrodes and AM laminated insulator

Figure 7. a) Schematic, b) cross-sectional TEM image, and EDS analysis of transparent and flexible a-IGZO thin film transistor structure with IAI electrodes and AM insulator. c) Transfer characteristic of a-IGZO TFTs according to gate insulator and GSD electrodes. d) Measured transmittance of various film structures. e) Photograph of various film structures with letters in the background. f) Photograph of the TF-TFTs fabricated on


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a PET substrate and a microscope image of the fabricated TFT device. To prove the feasibility of an a-IGZO TFT with IAI electrodes and an AM insulator, we fabricated a-IGZO TFTs with GSD electrodes and a gate insulator. The TF-TFTs were fabricated with a staggered structure of IAI (51/12/45 nm) /a-IGZO (20 nm)/AM or Al2O3 (40 nm)/IAI (51/12/45 nm) or ITO (100 nm) after PET surface planarization through coating of sequential organic layers and Al2O3 thin films as outlined in the experimental section (Figure 7a). Figure 7b shows cross-sectional TEM image and elemental analysis of a multi-stack of all films constituting the TF-TFT with the AM laminated insulator, including planarization layers. The rough surface of the PET substrate resulted in unstable operation of the device formed thereon. Therefore, sequential 400-nm-thick silane-based organic layers and 30-nm-thick Al2O3 films were coated by spin-coating and ALD, respectively. To ensure the reliability and stability of the TFTs, post-annealing at 140 °C was conducted for 2 h in a furnace. TFTs with the 70 °C AM laminated film and the 150 °C Al2O3 film were compared to verify the effectiveness of the low-temperature AM laminated insulator. The transfer characteristics of the TFTs were measured at a constant drain volage of 10.1 V (Figure 7c). The electrical characteristics of the 70 °C AMinserted TFT with IAI electrodes were comparable to those of the 150 °C Al2O3-inserted TFT with IAI electrodes. As seen in Table S1, there were no significant differences in electrical performance. The AM nanolaminate-based TFT showed a Vth of approximately 2.3 V, an Ion/Ioff ratio of >108, a subthreshold swing of 139.27 mV/dec, and a saturation mobility of 11.35 cm2/(V-s). The fully transparent flexible TF-TFT with IAI electrodes and AM laminated insulator exhibited an average transmittance of 71% in the wavelength range of 400 to 720 nm (Figure 7d). Its high luminous transmittance of 75.56% at the wavelength of 550 nm makes the TF-TFT more transparent. A photograph of the fabricated TF-TFT is shown in Figure 7f.

Figure 8. a) Transfer curves and gate leakage for the a-IGZO TFT with 150 °C Al2O3 insulator as a function of tensile strain. b) The leakage current density of the 40-nm-thick 70 °C AM and 150 °C Al2O3 insulators in the PET/MIM structure as a function of tensile stress. (Inset: the structure of a PET/planarization layer/Al/insulator/Al MIM device).

Furthermore, we measured the bending characteristics of TFTs as a function of tensile strain to show the



practical applicability of the fabricated TF-TFT to flexible devices (Figure 8a). As the bending strain applied to the TFTs increases, their transfer curves began to exhibit a slightly positive shift but remained stable until a strain of 0.833 %. However, when bending strain of 1 % was applied, the TFT showed performance degradation in the on current and mobility without change of the off current. The TFT performance degradation is likely due to the increased oxygen vacancies and the degradation of the metal-oxygen bonds induced by mechanical stress in the brittle IGZO channel layer.23 Therefore, our a-IGZO TFTs have limitations in measuring the critical bending strain of the insulating layers due to the channel layer. To investigate the bending characteristics of the insulating layers, we additionally fabricated MIM devices with an Al/insulator/Al structure. The Al/insulator/Al structures were fabricated after surface planarization due to the unstable operation caused by the rough PET surface. As shown in Figure 8b, the 150 °C Al2O3 and 70 °C AM insulators maintained their insulating properties until strain of 1 % without change. However, when bending strain of 1.25 % was applied to the insulators, the Al2O3 completely lost its insulating properties due to stress-induced cracks. On the other hand, although the AM film exhibited reduced insulating properties due to the cracking, its insulating properties were completely degraded at a tensile strain of 1.5 %. Therefore, the low-temperature ALD nanolaminate structure will be used as an insulator for flexible TFTs due to its insulating and mechanical properties corresponding to high-temperature ALD film quality. Overall, the potential of the IAI structure for application to transparent flexible electrodes and an AM laminated structure as a transparent flexible insulator was investigated through various reliability experiments and device applications. The low-temperature processed, multilayer-based electrode and insulator showed excellent reliability and it was verified that they are suitable for application to TF-TFTs; their application can be instrumental in the realization of TF-TFTs on various plastics substrates. However, as the TF-TFTs shown in this work are based on channel and insulator layers composed of brittle oxides, there are limitations in realizing real flexible TFTs for flexible electronic applications. In addition, for a real low-temperature process at less than 100 °C for stable device operation, special environments such as plasma and ultraviolet rays may be necessary. Therefore, for realization of highly reliable and flexible TFTs, the constituent components such as the channel, insulator, electrodes, and passivation should be formed using organic or nanostructured materials with extremely mechanical flexibility and environmentally chemical stability to pave the way for scalable manufacturing of real flexible, low-cost TFTs. Furthermore, the mechanical characteristics of real TF-TFTs will be meaningfully improved to a more meaningful level by controlling the neutral axis and reducing the substrate PET thickness. 3. CONCLUSION In summary, we proposed electrode and insulator fabrication techniques considering process temperature, transmittance, flexibility, and environmental stability. The low-temperature–processed IAI electrode and AM laminated insulator were functionally designed to improve the reliability and stability of TF-TFTs. As a GSD electrode, the IAI electrode provides a high luminous transmittance of 85% as well as excellent flexibility and environmental stability without changes in conductivity upon exposure to damp heat conditions through the combination of ITO and Ag films. For a highly robust and flexible insulator, the AM laminated film deposited at 70 °C was used with repetitively stacked ultra-thin Al2O3 and MgO, resulting in high packing density and robustness due to the aluminate phase at the Al2O3/MgO interface. In addition, its insulating properties and


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capacitance were comparable to those of Al2O3 deposited at 150 °C. A fully transparent flexible TF-TFT with IAI electrodes and an AM laminated insulator was fabricated at a temperature below 70 °C, except for the indispensable 140 °C annealing step for densification of the a-IGZO channel; it exhibited an average transmittance of 71% in the wavelength range of 400 to 720 nm and the electrical characteristics of Vth of approximately 2.3 V, an Ion/Ioff ratio of >108, and μ of 11.35 cm2/(V-s). Finally, our IGZO-based TFT with IAI electrodes and the 70 ℃ AM laminated insulator was fabricated to evaluate robustness, transparency, flexibility, and process temperature, and the feasibility of the proposed electrodes and insulator for TF-TFTs was verified. We presented the possibility of fabricating TFTs considering not only the device performance but also reliability and stability. We believe that this research will help pave the way to produce real transparent flexible TFTs by ensuring that all elements of TFTs will be flexible and reliable through the use of DMD and nanolaminate structures in the near future. 4. Experimental Section ALD Thin Film Deposition. Al2O3 and MgO thin films were grown using H2O as a reactant, where their metal sources were trimethylaluminum (TMA) and bis-ethyl-cyclopentadienyl-magnesium (Mg(EtCp)2), respectively, using thermal ALD (Lucida D100 from NCD). Prior to ALD deposition, the TMA and H2O sources were cooled to 10 °C, while Mg(EtCp)2 was heated to 90 °C. In general, one deposition cycle for the ALD film comprised sequences of ts1, tp1, ts2, and tp2, where ts1 and ts2 are the expoure times of the metal source and reactant, respectively, and tp2 and tP2 are the purging times for N2 flow. The exposure times of the Al2O3 thin films were 0.25 s, 10 s, 0.25 s, and 10 s, and the exposure times of the MgO thin films were 0.75 s, 15 s, 0.25 s, and 10 s at a base pressure of 3.1 × 10−1 torr, resulting in 0.83 Å and 1.03 Å per cycle in an ALD chamber temperature of 70 °C, respectively. Organic/Inorganic Hybrid Layer Prepared by Spin-Coating. The organic-inorganic hybrid layer based on tetraethoxy silane (TEOS) was synthesized by a sol-gel method. After spin-coating the organic-inorganic hybrid layer on a bare PET substrate, the spin-coated layer was thermally cured forming a three-dimensional SiO2embedded organic layer. The thermal annealing was carried out at 70 °C under 5 × 10-2 Torr for 10 min. The 500-nm-thick planarization layer was effective to planarize the rough PET surface, enabling the fabricated devices to operate stably. Device fabrication and characterization. The staggered a-IGZO TFT structure was fabricated in the following order: IAI electrode, a-IGZO channel, AM laminated insulator, and IAI electrode. Prior to TFT fabrication, the PET substrate was thermally stabilized by pre-shrinking the substrate at 150 °C in order to minimize the thermal damage to the PET substrate during ALD deposition and the annealing process. For the IAI electrodes used as source and drain electrodes, the bottom 51-nm-thick ITO layer was first fabricated by radio frequency (RF) sputtering on a planarized PET substrate. The RF power, total pressure, and oxygen partial pressure were 100 W, 5 m Torr, and 5%, respectively. Next, the 12-nm-thick Ag film was deposited by thermal evaporation. Finally, the top 45-nm-thick ITO layer was fabricated on the ITO/Ag. The IAI electrodes were defined by photoresist (PR) patterning and wet etching using ITO etchant. A 20-nm-thick a-IGZO layer was deposited by RF sputtering with total pressure and oxygen of 3%, where other sputtering conditions were the same. The active region was defined by PR patterning and wet etching with a diluted hydrofluoric acid. For the



gate insulator, 40-nm-thick ALD films were deposited following the above-described ALD process. The first annealing step was carried out again at 140 °C for 2 h in air. The gate electrode was defined by a lift-off process followed by the deposition of a 100 nm ITO layer. Finally, the second annealing step was carried out at 140 °C for 2 h. The channel length and width of the fabricated TFTs are 50 and 500 µm, respectively. All TFT characteristics were measured with a Keithley 4200-SCS semiconductor characterization system in air. Method for measuring barrier properties and optical transmittance. For measurement of WVTR values, the thin films to be measured are attached to the Ca sensor using the UV-curable sealant (Figure 4b). The optical transmittance of electrodes and insulators was measured using a UV-VIS spectrophotometer (UV2550, SHIMADZU) in the wavelength range of 400-800 nm. Cross-sectional TEM and elemental analysis. TEM cross-sectional images were analyzed using cs-corrected Scanning Transmission Electron (JEM-ARM200F, JEOL) and element analysis in thin films were analyzed by EDS (Quantax 400, Bruker), respectively. XPS (Sigma probe, Thermo VG Scientific) analysis was conducted using a micro-focused X-ray monochromator in an ultra-high vacuum of 10-9 torr. Stress measurement in thin film calculated by wafer curvature method. The residual stress in the thin film was determined by measuring the curvature change in the wafer before and after the film deposition on a wafer (Figure S4a). The film stress related to the radius of curvature induced in a wafer is derived using the Stoney formalism. 5. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Optical transmittance of IAI and ITO films with the simulation results; Atomic force microscope images of the PET surface before and after surface planarization using the sequential organic layer and Al2O3 ALD film; XRD patterns of 12-nm-thick MgO film, 15-nm-thick MgO film and 40-nm-thick MgO, Al2O3, and AM nanolaminate films; Schematic of the wafer curvature method for the measurement of residual stress in ALD films; Residual stress of various ALD films; a-IGZO TFT performance with Al2O3 and AM laminated gate insulators; 6. AUTHOR INFORMATION Corresponding Author K.C. Choi.* Author is with the School of Electrical Engineering, the Korea Advanced Institute of Science and Technology, Yuseong-gu, Daejeon 34141, Republic of Korea (corresponding author phone: +82-42-350-3482; fax: +82-42-350-8082; e-mail: kyungcc@ kaist.ac.kr). ORCID Kyung Cheol Choi: 0000-0001-6483-9516 Notes The authors declare no competing financial interest. 7. ACKNOWLEDGEMENT


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This work was supported by the Engineering Research Center of Excellence (ERC) Program (Grant No. NRF2017R1A5A1014708)

and

Nano·Material

Technology

Development

Program

(Grant

No.

NRF-

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