Low-temperature and Low-Voltage, Solution-Processed Metal Oxide n-TFTs and Flexible Circuitry on Large-Area Polyimide Foil Maarten Rockelé*’** , Duy-Vu Pham***, Jürgen Steiger***, Silviu Botnaras***, Dennis Weber***, Jan Vanfleteren*’****, Tom Sterken*’****, Dieter Cuypers*’****, Soeren Steudel*, Kris Myny*’**, Sarah Schols*, Bas van der Putten*****, Jan Genoe* and Paul Heremans*’** * imec, 3001 Leuven, Belgium ** Katholieke Universiteit Leuven, 3001 Leuven, Belgium *** Evonik Degussa GmbH, 45772 Marl, Germany **** Universiteit Gent, 9052 Gent, Belgium ***** Holst Centre, 5656 AE Eindhoven, The Netherlands ABSTRACT In this article, we report on high-performance solution-based n-type metal oxide TFTs processed directly on polyimide foil and annealed at 250 °C. Saturation mobilities exceeding 2 cm²/(Vs) and Ion/Ioff ratios beyond 10 8 have been achieved. Using these oxide n-TFTs, fast and low-voltage flexible circuitry is presented. Furthermore, a complete 8-bit RFID transponder chip, containing 294 oxide n-TFTs has been fabricated. Both high-speed and low-voltage operation makes the presented oxide n-TFT technology suited for both the pixel driving and embedded line-drive circuitry at the borders of flexible AMOLED displays. 1. INTRODUCTION The main driving forces behind flexible electronics are the potential applications, such as large-area roll-up AMOLED displays. This perspective involves the realization of a fast, stable and low-power thin-film transistor technology on plastic foil suited for driving the display pixels on the active-matrix backplane. The display costs can be even further reduced by integrating the display drive circuitry (such as the line drivers) directly on the flexible substrate. For this the circuit technology needs both high speed operation and low-power driving. High-performance n-type metal oxide TFTs (with electron mobilities exceeding 10 cm²/(Vs)) on plastic foil, are currently only achieved using zinc- and indium-based oxides deposited by vacuum techniques such as sputtering [1,2,8]. Recently, increasing attention is given to metal oxide semiconductors processed from solution. Advanced solution-processing techniques, such as printing, will eventually allow a simple and high throughput fabrication of inexpensive (i.e. low cost per unit area) and large-area electronics on a flexible substrate. Unfortunately, to date the annealing temperature to convert soluble precursors into semiconductor oxide films lies mostly in the range of 300 – 500 °C and is hence difficult to process on foil [3,4]. Here, we show high-performance indium-based n-type metal oxide TFTs processed from solution and annealed at a temperature down to 250 °C. Moreover, the oxide TFTs and corresponding circuitry were processed directly on top of a flexible substrate, i.e. polyimide foil. The metal oxide precursor formulation was developed by Evonik Industries [5] and the polyimide foil was fabricated with a unique spin-on process directly on a 6 inch carrier wafer [6]. 2. RESULTS AND DISCUSSION 2.1 Solution-based oxide n-TFTs on polyimide The n-type indium-based oxide transistors were photolithographically fabricated in bottom Au gate – top Ti S/D contact geometry with channel lengths down to 2–10 μm and 100 nm of high-k Al2O3 acting as the gate dielectric, as shown in the inset of Figure 1. The oxide active layer is deposited by spin-coating and subsequently baked in air at a temperature as low as 250 °C. Figure 1 shows the distribution of the average linear mobility values of the oxide n-TFTs over five successive experiments [7]. A steady increase of the electron mobility is observed as the baking temperature increases from 250 °C to 330 °C. Above 330 °C, the mobility plateaus around 15 cm²/(Vs), a performance similar to state-of-the-art sputtered oxide TFTs [8]. Excellent transistor performances have been achieved on polyimide foil (at 250 °C) with saturation mobilities exceeding 2 cm²/(Vs), leakage currents below 1 pA and Ion/Ioff ratios up to 10 8 for a W/L equal to 140/6 μm/μm, as shown in Figure 2. Only a limited amount of hysteresis is observed in their transfer characteristics. The current drive of these small transistors (20 μA at gate voltages as low as 5 V) clearly exceeds the demands for AMOLED pixels on foil, typically 2 μA for a 100 dpi resolution [9]. Moreover, this current is constant for SD voltages from 5 to 15 V. The drive current spread over a complete 6 inch substrate, containing 216 identical n-TFTs, was 14.4%. The output and on-state resistance of the oxide transistors are respectively 190 FLX1/AMD2 - 2 ISSN-L 1883-2490/18/1267 © 2011 ITE and SID IDW ’11 1267