Yung-Hui Yeh, and Bo-Cheng Kung Display Technology Center (DTC), Industrial Technology Research Institute, Hsinchu 310, Taiwan

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1 Amorphous In 2 O 3 -Ga 2 O 3 -ZnO Thin Film Transistors and Integrated Circuits on Flexible and Colorless Polyimide Substrates Hsing-Hung Hsieh, and Chung-Chih Wu* Graduate Institute of Electronics Engineering, Graduate Institute of Photonics and Optoelectronics, and Department of Electrical Engineering, National Taiwan University, Taipei 106, Taiwan Tel: , Fax: , * chungwu@cc.ee.ntu.edu.tw Yung-Hui Yeh, and Bo-Cheng Kung Display Technology Center (DTC), Industrial Technology Research Institute, Hsinchu 310, Taiwan Horng-Long Tyan Material and Chemical Research Laboratories (MCL), Industrial Technology Research Institute, Hsinchu 310, Taiwan Abstract A process was developed for fine fabrication of amorphous IGZO TFTs and integrated circuits on flexible and colorless polyimide substrates. TFTs with field-effect mobilities of ~10 cm 2 /Vs and ring oscillators with propagation delay of 0.35 µs per stage were achieved on the polyimide substrates. Keywords -- amorphous IGZO, thin film transistors, integrated circuits, flexible, polyimide. This paper is not intended for applications sessions. Symposium topics: Active-Matrix Devices Oral/Poster Preference: Oral is preferred. The presenter is currently a student. Materials in this summary have not been published. 1 SID 08 Paper 425 Page 1

2 Technical Summary (1) Objective and Background Over the past few years, studies on flexible electronics and displays have increased rapidly. Flexible electronics and displays have some potential advantages such as thin profiles, lightweight, and the ability to form conformable shapes. To realize flexible electronics and displays, suitable active materials and the flexible substrates are two of the most important issues. Amorphous silicon and organic semiconductors are two major candidates for the active materials in flexible electronics up to date. However, these materials usually have limited performances such as low mobilities at or below ~1cm 2 /Vs, limited on currents, and large operation voltages. On the other hand, plastic substrates are one of the major options for the flexible substrates. However, most plastic materials have certain disadvantages like low glass transition temperature (T g ), large coefficient of thermal expansion (CTE), or poor optical transparency. In this work, we investigated amorphous In 2 O 3 -Ga 2 O 3 -ZnO (a-igzo) as the active layer for fabrication of thin film transistors (TFTs) and integrated circuits on transparent plastic substrates. Oxide semiconductors composed of heavy-metal cations with (n-1)d 10 ns 0 (n 4) electronic configurations have been widely investigated recently due to several merits such as high mobilities (e.g. ~10cm 2 /Vs), high transparencies, and low processing temperatures [1-5]. Particularly, oxide-semiconductor-based TFTs have shown promising characteristics as strong candidates for the display backplanes [6]. Although there were few reports of oxide TFTs on flexible substrates [7-8], they were either fabricated on steel foils or fabricated on plastics substrates with rather primitive technologies (e.g. shadow masking), not readily applicable for general display uses. In this work, we developed a complete process for fine fabrication and integration of amorphous IGZO TFTs on flexible and colorless polyimide substrates, readily rendering possible further integration and applications, e.g. integrated circuits on plastics. (2) Results Although oxide TFTs can be fabricated completely at room temperature, it has been shown that a high temperature annealing (e.g. 250~350 o C) would improve device performance, particularly stability [9]. Thus in this work, we had chosen the high T g plastics as substrates from the beginning. The polyimides we used was developed by ITRI (of Taiwan) and have the features of high T g (~350 o C), high transmittance in the visible range (~90 %, Fig. 1), and moderate CTE (~40 ppm/ o C) [10-11]. Fig. 2 shows the device structure and the microfabrication process flow for the top-gate a-igzo TFTs on flexible substrates in this study. Glass was used as the carrier substrate on which the colorless polyimide was directly coated without using the glue, rendering no issues of glue residues during the fabrication. Then top-gate a-igzo TFTs and circuits were fabricated on the polyimide with only 4 masks and a maximum processing temperature of 260 o C. First, multilayer metals were deposited and patterned as the source/drain electrodes. Then the IGZO channel layer was deposited by RF sputtering in the Ar/O 2 environment, followed by a SiN x layer deposited by using plasma enhanced chemical vapor deposition (PECVD). Next, SiN x was etched by reactive ion etching (RIE) to serve as the etching mask. Using the SiN x etching mask, IGZO was patterned by wet etching. Another layer of SiN x was then deposited to complete the gate insulator and etched to 2 SID 08 Paper 425 Page 2

3 expose the source/drain contact holes. Finally multilayer metals were deposited and patterned as the gate electrode and the interconnection for circuits. After the TFT fabrication, a post annealing at 260 o C was performed to improve TFT performances. The IGZO thin films were examined to be amorphous and no crystalline features were revealed by XRD, SEM, and AFM studies. In addition to the discrete devices, integrated circuits such as inverters and ring oscillators were also fabricated on the same substrate to test validity of the complete integration process. The polyimide substrate with fabricated TFTs and circuits can be easily de-bonded from the glass carrier. Fig. 3 (a) shows the photo of the a-igzo TFTs and integrated circuits on the free-standing flexible and colorless polyimide. Various conducting materials had been tested as the electrodes and the interconnection bus lines on the flexible polyimide substrate, because the release of internal stress usually results in cracking during the fabrication process. For example, indium tin oxide (ITO) and chromium (Cr) both cracked on the flexible polyimide substrate as shown in Fig. 4. Therefore we had used the multilayer metals to retain the integrity of the electrodes and intercconection. Fig. 5 (a) shows the typical output characteristics of the top-gate amorphous IGZO TFTs with the channel width and channel length of 50 µm and 10 µm, respectively. Fig. 5 (b) shows the corresponding transfer characteristics of the top-gate a-igzo TFTs (V DS = 10.1 V). The a-igzo TFTs operated in the n-type enhancement mode. From I 1/2 D -V GS, a saturation mobility (µ) of ~10 cm 2 /Vs and a threshold voltage (V t ) of 3.03 V are extracted using a saturation current equation. From logi D -V GS, the subthreshold slope and the on/off current ratio are estimated to be 0.41 V/decade and 2.9x10 7, respectively. The high mobility from such amorphous ionic oxide semiconductor is perhaps associated with the fact that their conduction bands are derived from the large, spherical, and symmetrical ns orbitals of metal cations, rendering carrier transport very efficient and less sensitive to disorder. In addition, using TCAD modeling techniques Hsieh et al. [12] also extracted the subgap states (tail and deep gap states) in amorphous oxide semiconductors to be generally 2-3 orders of magnitudes lower than those in usual amorphous covalent semiconductor such as degenerated amorphous silicon (a-si:h), which means it is much easier for carriers in amorphous oxide semiconductors to reach band-like conduction and high mobility. To investigate the effects of bending on the flexible a-igzo TFTs, a-igzo TFTs on the plastic substrates were bended in a curved surface with a curvature radius of 30 mm (Fig. 3 (b)) and probed. Their typical output and transfer characteristics under bending are shown in Fig. 6. The overall performance of flexible a-igzo TFTs does not change much (except the on current is slightly decreased) and can be repeated after bending tests for times. Fig. 7 (a) shows the circuit diagram of the inverter consisted of two n-enhancement-type amorphous IGZO TFTs. Both the gate and the drain of the load transistor are connected to V DD. For the load transistor, the channel width and channel length are W load = 5 µm and L load = 10 µm. For the drive transistor, the channel width and channel length are W drive = 50 µm and L drive = 10 µm. The geometrical beta ratio, (W drive /L drive )/(W load /L load ), is 10. Fig. 7 (b) shows the transfer characteristics of such an inverter. With the applied voltage V DD of 20 V, the voltage gain is ~2.5. The voltage gain, dv out /dv in, is an important parameter for subsequent stage switching. The magnitude of the gain is affected by many factors such as device geometry, carrier mobility, and bias condition, etc., and a gain of at least one is needed for signal propagation. 3 SID 08 Paper 425 Page 3

Fig. 8 shows the layout of a five-stage ring oscillator consisting of the above inverters. The typical output characteristics of the ring oscillator with a voltage supply V DD of 20 V is shown in Fig.

4 Fig. 8 shows the layout of a five-stage ring oscillator consisting of the above inverters. The typical output characteristics of the ring oscillator with a voltage supply V DD of 20 V is shown in Fig. 9 (a), which has an oscillation frequency (f osc ) of ~182 khz (corresponds to a propagation delay ( t) of 0.55 µs per stage) and clear demonstrates that a-igzo TFTs can be operated normally in the continuous charging and discharging processes. The f osc increases roughly linearly with V DD, and reaches 286 khz ( t of 0.35 µs per stage) at V DD of 30V (Fig. 9 (b)). Such operation speed and frequencies indeed are enough for some circuit applications such as integrated scan drivers for high-performance display panels, in addition to use in pixels. The experimentally observed voltage swings and oscillation frequencies in general are consistent with our SPICE simulations using NMOS models. It is worthy to point out that the work here represents the first report of ring oscillators of amorphous oxide TFTs on plastic substrates, and the speed/frequencies achieved is already comparable to the best results on glass substrates [13]. (3) Impact Oxide-semiconductor-based TFTs have advanced remarkably in recent years, yet the demonstration of oxide TFTs and applications on flexible substrates (particularly plastics) is still rare. In this research, using a proprietarily developed high-temperature and colorless polyimide substrates, we successfully developed a process for fine fabrication of a-igzo TFTs and integrated circuits on flexible and transparent plastic substrates. The fabricated a-igzo TFTs operated in the n-type enhancement mode with decent mobilities, subthreshold swings, and on/off ratios. In addition, this process has been successfully used to implement integrated circuits such as inverters and ring oscillators on the flexible plastic substrates. Through these results, it is believed that oxide semiconductors are getting readily applicable for the flexible electronic/display applications. (4) References [1] K. Nomura et al., Nature, vol. 432, pp. 488 (2004). [2] E. Fortunato et al., Advanced Materials, vol. 17, pp. 590 (2005). [3] D. Hong et al., Thin Solid Films, vol. 515, pp (2006). [4], Applied Physics Letters, vol. 89, pp (2006). [5], Applied Physics Letters, vol. 91, pp (2007). [6] H.-N. Lee et al., SID 07 Technical Digest, pp.1826 (2007). [7] M.-C. Sung et al., IMID 07 Technical Digest, pp. 133 (2007) [8] I.-D. Kim et al., Applied Physics Letters, vol. 87, pp (2005) [9] R. Hayashi et al., Journal of SID, vol. 15, pp. 915 (2007). [10] M.-H. Lee et al., IEEE IEDM 2006, (2006). [11] Y.-H. Yeh et al., SID 07 Technical Digest, pp.1677 (2007). [12] H.-H. Hsieh, T. Kamiya, K. Nomura, H. Hosono, C.-C. Wu, (submitted to SID 2008) [13] M. Ofuji et al., IEEE Electron Device Letters, vol. 28, pp. 273 (2007). Fig. 1. (a) The commercial polyimide (brown color), and (b) polyimide developed by ITRI (colorless). 4 SID 08 Paper 425 Page 4

Fig. 2. Device structure and the process flow of the top gate a-igzo TFTs and integrated circuits on flexible polyimide. Fig. 6.

(a) Photo of the flexible a-igzo TFTs and integrated circuits on free-standing polyimide. (b) Photo of devices measured at a curved surface. Fig. 7.

Optical micrograph of (a) ITO, (b) Cr, and (c) multilayer metals on the flexible polyimide substrate. Fig. 8.

5 Fig. 2. Device structure and the process flow of the top gate a-igzo TFTs and integrated circuits on flexible polyimide. Fig. 6. The typical (a) output and (b) transfer characteristics (V DS = 10.1 V) of the a-igzo TFT measured on a curved surface. Fig. 3. (a) Photo of the flexible a-igzo TFTs and integrated circuits on free-standing polyimide. (b) Photo of devices measured at a curved surface. Fig. 7. (a) Circuit diagram, and (b) transfer characteristics of an a-igzo inverter at V DD =20V. Fig. 4. Optical micrograph of (a) ITO, (b) Cr, and (c) multilayer metals on the flexible polyimide substrate. Fig. 8. Photo of an a-igzo five-stage ring oscillator. Fig. 5. The typical (a) output and (b) transfer characteristics of the a-igzo TFT. Fig. 9. (a) Output characteristics of a five-stage ring oscillator at V DD = 20 V, and (b) oscillation frequency as a function of V DD. 5 SID 08 Paper 425 Page 5

Low contact resistance a-igzo TFT based on. Copper-Molybdenum Source/Drain electrode

Low contact resistance a-igzo TFT based on Copper-Molybdenum Source/Drain electrode Shi-Ben Hu 1,Hong-Long Ning 1,2, Feng Zhu 1,Rui-QiangTao 1,Xian-Zhe Liu 1, Yong Zeng 1, Ri-Hui Yao 1, Lei Wang 1, Lin-Feng