Stretchable Oxide TFT for Wearable Electronics

Stretchable Oxide TFT for Wearable Electronics

Oxide thin-film transistors (TFTs) in a neutral plane are shown to be robust under mechanical bending and thus can also be suitable for TFT backplanes for stretchable electronics. The oxide TFTs when built on a PI substrate can then be transferred onto a PDMS substrate and applied to stretchable electronics. The PDMS regions on the TFT parts are UV/O3 treated for stiff PDMS and then the TFTs are transferred onto this region. This substrate with TFTs applied can then be stretched up to 50 percent without significant performance degradation. Therefore, stretchable oxide TFT arrays can be used for stretchable displays and sensors.

by Xiuling Li and Jin Jang

Stretchable electronics can enable innovative applications such as electronic textiles, wearable displays, and sensors. One of the key challenges of this technology is to design a device/system that tolerates high levels of strain (>>1 percent) without degradation of performance. This challenge entails numerous research issues covering a broad range of fields, including materials, device architectures, mechanics, and fabrication methods.1

A stretchable thin-film transistor (TFT) is a basic building block for electronic circuits, displays, and sensors. Table 1 presents a brief summary of stretchable TFTs reported in the literature. Although mechanically robust materials and components have been improved recently, fabrication processes with high yield and device stability under mechanical strain remain challenging issues in terms of commercialization.2–6 Of note are some efforts in ruggedization of devices that show excellent performance but less “softness.” Wavy, coiled, net-shaped, or spring-like structures have also been used to accommodate large mechanical deformations.7–10 In addition, active devices on stiff islands that are interconnected with stretchable conductors have proven successful in achieving highly stretchable electronics.10–14

Concurrently, oxide semiconductor TFTs, such as amorphous-indium-gallium-zinc-oxide (a-IGZO) TFTs, have drawn considerable attention due to their combined merits of high transparency, large field-effect mobility (>10 cm2/V.s), good uniformity, and excellent electrical stability, even when deposited at room temperature.15

We have recently reported highly robust neutral plane (NP) a-IGZO TFTs that can undergo bending with a 0.25-mm radius. The combination of a thin substrate and TFTs located near the neutral bending plane results in highly stable oxide TFTs under mechanical strain.16 The TFT can be in a neutral plane if there is no strain to it, even though it is bent. This is possible when it is in between two polyimide (PI) layers with the same thickness. In this article, we report on the integration of the neutral plane oxide TFTs onto selectively modified polydimethylsiloxane (PDMS) substrate material and evaluate the performance of the stretchable devices16 resulting from this architecture. The PDMS substrate in this case was selectively treated by UV/O3 to form a stiff silica-like layer in specific regions of the surface. These regions become rigid, while the untreated surrounding areas remain highly stretchable.17 The oxide TFTs on the stiff regions, which you might think of as islands within a stretchable sea, can tolerate as much as 50 percent mechanical stretching of the surrounding substrate and still perform well after repeated stretching and relaxing. We attribute this advantage to the reduced strain on these hardened islands,17 which in turn preserve the structural integrity of the TFTs. This technology provides a promising approach to wearable electronics using oxide TFT technology. It is based on papers recently published on NP a-IGZO TFTs16 and stretchable TFTs.18

Oxide TFTs on a Neutral Plane

The path to a stretchable backplane system begins with a very thin PI substrate selected for its extreme bending capability. On this substrate we fabricate a-IGZO TFTs. Once the TFT fabrication is complete, we deposit a second, very thin PI layer on top of the TFT devices to ensure that the devices are located close to the neutral bending plane between the two PI layers, for minimum strain exposure. As the resulting construction bends, the least amount of stress is experienced by the TFTs in the sandwich between the two PI layers. Also, a mixture of graphene oxide (GO) and carbon nanotubes (CNTs) is applied to the bottom surface of this substrate for mechanical support and reduction of electrostatic discharge (ESD) damage.

The process to create this architecture is illustrated in Fig. 1, beginning with the CNT/GO layer, which is first deposited by spin coating onto a carrier glass as illustrated in Fig. 1(a). In Fig. 1(b), we show the PI layer also being deposited by spin-coating onto the carrier. Next, the PI layer with CNT/GO is covered by SiO2 and SiNX as shown in Fig. 1(c). The purpose of this SiO2 and SiNX layer is to act as a gas barrier. The prepared PI substrate with gas barrier is now ready for TFT fabrication and that step is illustrated in Fig. 1(d). The second PI layer (1.5 μm) previously discussed is then deposited on top of the devices also using spin coating, as shown in Fig. 1(e). In Fig. 1(f), detachment of the complete stack-up from the carrier glass occurs using a detachment machine {Fig. 1(g)] resulting in the complete structure of a-IGZO TFTs fabricated on the PI substrate without [Fig. 1(h)] and with [Fig. 1(i)] the top PI layer. Once the structure is detached from the glass, it yields a freestanding flexible device. We produced a number (more than 10) of these devices with the goal of comparing the performance of devices with and without the top PI layer by performing prolonged mechanical stress tests under extreme bending conditions.

Fig. 1:  The fabrication process flow of neutral-plane TFTs on a 1.5-μm PI substrate is shown above. See text below for explanations.

To evaluate the bending stability of the a-IGZO TFTs, the samples were wound onto a cylinder of decreasing radius and tested while bending. Starting from a cylinder with a radius of 3 mm all the way down to a cylinder with radius of 0.25 mm, devices with and without the top PI layer showed negligible changes in operation for all bending radii. Figure 2(a) shows an image of a sample wound to a cylinder of a radius of 0.25 mm.

The TFTs were then exposed to repeated bending cycles down to a radius of 0.25 mm using a special bending machine [Fig. 2(b)]. The placement of the sample on the bending machine was such that the direction of the bending stress was perpendicular to the TFT current flow. The TFTs without the top PI layer achieved up to 2,000 cycles before breaking down [Fig. 2(c)], while those with the top PI layer (resulting in an NP TFT structure) remained operational even after 20,000 cycles [Fig. 2(d)]. Clearly, this shows that the NP structure is superior for producing bendable a-IGZO TFT devices.

Fig. 2:  In these bending test figures, (a) shows an optical micrograph of a sample wound on the cylinder with a radius of 0.25 mm; (b) is a photograph of the extreme bending machine; (c) shows transfer characteristics as a function of the bending cycle of a-IGZO TFTs without and (d), with, the top PI layer. All TFTs have a channel width (W) = 50 μm and a channel length (L) = 8 μm.16

Stretchable Oxide TFTs

The next step in this investigation was to develop a stretchable system utilizing these NP oxide TFTs on PI substrate. We did this by cutting the NP oxide TFTs on PI substrate into small squares of 3 mm × 3 mm using a CO2 laser. The squares were then transferred to specific regions of a PDMS substrate that had been previously treated by UV/O3 to form a stiff, silica-like surface layer in those locations.

This process is shown below in Fig. 3 beginning with the TFT arrays on the PI substrate ready for laser cutting and transfer in Fig. 3(a). The preparation of the PDMS stretchable substrate as shown in Fig. 3(b) was achieved by spin-coating PDMS onto the carrier glass, followed by a subsequent baking cycle at 150 °C for 15 minutes. The stand-alone stretchable PDMS film was then separated by peeling it from the carrier glass. To selectively modify its surface, the PDMS substrate was covered with a square-shaped photo-mask and then exposed to UV/O3 for 2 hours. The mask pattern was 3 mm × 3 mm squares at intervals of 3 mm. The surface of PDMS in the regions exposed to UV/O3 forms a thin, stiff, silica-like layer with a gradient depth profile, while the unexposed regions remain completely soft.17 Finally, the island-shaped TFTs arrays cut from the PI substrate were transferred to the UV/O3-treated PDMS regions. Figure 3(c) shows the optical micrograph of the NP oxide TFT arrays on UV/O3-treated islands of the PDMS substrate.

Fig. 3:  The design concept for stretchable oxide TFTs includes (a) neutral-plane TFT arrays on PI substrates for cutting by a laser and transfer onto PDMS substrate; (b) spin coating and detachment of PDMS from release layer on glass; and (c) an optical image of TFT arrays on PDMS islands selectively treated by UV/O3 (islands: 3 × 3 mm2, intervals: 3 mm).18

Once this fabrication process was completed, we measured the relevant performance characteristics of the a-IGZO TFTs. Figures 4(a) and 4(b) show the transfer and output characteristics respectively in the relaxed state after transfer onto a PDMS substrate with a channel width (W) of 6 µm and a channel length (L) of 10 µm. The devices exhibited a field-effect mobility (μFE) of 13.8 cm2/V.s, turn-on voltage (VON) of 0.1 V, and sub-threshold swing (SS) of 0.18 V/dec, indicating they had been successfully transferred onto the PDMS substrate. The field-effect mobility (µFE) is derived from the transconductance gM = ∂IDS/∂VGS, with VDS = 0.1 V. VON is taken as the gate voltage (VGS) at which IDS starts to monotonically increase, and SS is taken as (d log (IDS)/d VGS)−1 of the range 10 pA ≤ IDS ≤ 100 pA, with VDS = 0.1 V.

Fig. 4:  Performance of oxide TFTs on the UV/O3-modified region of the PDMS substrate is shown in (a) transfer and (b) output curves of a typical oxide TFT with W of 6 µm and L of 10 µm. The TFT exhibits a field-effect mobility of 13.8 cm2/V.s, threshold voltage of ~0.1 V, and subthreshold swing of 0.18 V/dec.18

To evaluate the stretchability of this construction and the resulting performance of the TFTs, the substrate was put into a stretch machine that we designed. When a sample, which is initially 21 mm in length, is stretched up to 31.5 mm, the resulting elastic elongation is 50 percent. We used multiple 21-mm samples and measured the performance of the TFTs in the relaxed state after being repeatedly stretched and relaxed at various elongations from 20 percent to 50 percent. Figure 5 shows the evolution of transfer characteristics as a function of stretching strain. The performance of oxide TFTs remained almost unchanged, and stable even after the sample was repeatedly stretched up to 50 percent and relaxed 10 times. The μFE is 13.8 ± 1.2 cm2/V.s, VON of 0.1 ± 0.2 V, and SS of 0.18 ± 0.05 V/dec. The stable operation indicates that UV/O3-treated islands are effective in releasing mechanical strains.

Fig. 5:  The stretchability of the oxide TFTs on PDMS substrate is shown above, with the evolution of transfer curves as a function of stretching strain for the oxide TFT. The TFT performance is measured in the relaxed condition after being stretched 10 times for each strain..18

We believe this methodology was successful because the oxide TFTs were transferred onto the modified PDMS substrate in the form of islands that were hardened, so that the mechanical stretching of the material occurred mainly in the untreated PDMS regions. The elastic modulus of the stiff, silica-like layer was reported to increase over 10 times compared to the untreated PDMS, while the total thickness is ~5 µm with a gradient depth profile.17 Since the islands are stiffer than the surrounding PDMS regions, the components on the islands experience very little strain when the sample is macroscopically stretched. Furthermore, the NP oxide TFTs have proven to be more mechanically stable than TFTs that are not located in the neutral bending plane.16 By combining the modified PDMS substrate and the oxide TFTs located in the neutral bending plane, highly stretchable oxide TFT structures can be achieved.

Enabling Stretchable, Wearable Displays

We have thus reported the results of a simple integration of neutral-plane oxide TFTs onto a selectively modified PDMS substrate to achieve stretchable properties. The oxide TFTs described in this article show µFE ~14 cm2/V.s and change <8 percent during repeated stretching up to 50 percent. The stretchability can be further improved by optimizing the materials and properly designing the structures and transfer methods. The robustness of the stretchable devices can be enhanced by reducing the overall thickness, inserting adhesive layers between TFT membrane and PDMS substrate, or employing an encapsulation layer (i.e., elastic PDMS) on the top. Looking forward, we are now developing an AMOLED on a stretchable substrate (PDMS) using an oxide TFT backplane on a PI substrate. Thermal evaporations of organic semiconductors are used for the OLED-on-TFT array, and then the thin-film encapsulation process is carried out. The AMOLED is cut into small slots and then transferred into the PDMS after being cut.

Combining high mobility, excellent uniformity, and stability with the mechanical robustness of stretchable substrates, oxide TFT technology can provide exciting opportunities in wearable electronics, including stretchable display and skin-like sensors.


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Xiuling Li received her MS degree in information display engineering from Kyung Hee University, Seoul, Korea, in 2014, where she is currently pursuing a Ph.D. in information display technology. Her current research interests include thin-film transistors and circuits for use in display applications. Jin Jang received a Ph.D. in physics from the Korea Advanced Institute of Science and Technology. He is currently a director of the Advanced Display Research Center in Kyung Hee University, Seoul, Korea. His current interests include flexible and stretchable electronics using oxide and LTPS TFTs. He can be reached at