Low Temperature Zinc Indium Oxide Backplane Development for Flexible OLED Displays in a Manufacturing Pilot Line Environ
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Low Temperature Zinc Indium Oxide Backplane Development for Flexible OLED Displays in a Manufacturing Pilot Line Environment Michael A. Marrs1, Sameer M. Venugopal1, Curtis D. Moyer1, Edward J. Bawolek1, Dirk Bottesch1, Barry P. O’Brien1, Rita J. Cordova1, Jovan Trujillo1, Cynthia S. Bell1, Douglas E. Loy1, Gregory B. Raupp1, David R. Allee1 1
Flexible Display Center at Arizona State University, 7700 S. River Pkwy, Tempe, AZ 85284
ABSTRACT A low temperature amorphous zinc indium oxide (ZIO) thin film transistor (TFT) backplane technology for high information content flexible organic light emitting diode (OLED) displays has been developed. We have fabricated 4.1-in. diagonal OLED backplanes on the Flexible Display Center’s six-inch wafer-scale pilot line using ZIO as the active layer. The ZIO based TFTs exhibited an effective saturation mobility of 18.6 cm2/V-s and a threshold voltage shift of 2.2 Volts or less under positive and negative gate bias DC stress for 10000 seconds. We report on the critical steps in the evolution of the backplane process: the qualification of the low temperature (200°C) ZIO process, the stability of the devices under forward and reverse bias stress, the transfer of the process to flexible plastic substrates, and the fabrication of white organic light emitting diode (OLED) displays. INTRODUCTION Transparent oxide semiconductors have drawn considerable attention due to their unique electrical and optical properties [1,2,3]. Thin film transistors fabricated with a transparent oxide channel have shown higher saturation mobility [4,5] and improved threshold voltage stability [6,7] in comparison to amorphous silicon (a-Si:H), which is currently used in the fabrication of LCD displays [8]. Additionally, these oxides can be deposited using conventional RF [9] or DC [10] sputtering techniques near room temperature and are optically transparent [11]. We are developing a low temperature (200 °C) Zinc indium oxide (ZIO) process compatible with flexible substrates, such as heat-stabilized PEN and stainless steel foils. The main focus of this paper is to demonstrate the use of ZIO TFTs for active matrix backplanes on flexible substrates. We have designed and fabricated multiple 4.1-in diagonal Organic Light Emitting Diode (OLED) displays on PEN substrates using ZIO TFTs. EXPERIMENT The TFTs for OLED backplanes were fabricated with a bottom gate, inverted, staggered design and feature two transistors and one capacitor for each pixel. The transistors function as ntype devices in enhancement mode. The devices are fabricated entirely within the pilot line at the Flexible Display Center. The backplane TFT structure was described previously by Venugopal et al. [12] and is shown in Figure 1.
Figure 1: Backplane Pixel Structure The ZIO layer was deposited by reactive DC sputtering using a ceramic target with a composition of 60% zinc oxide and 40% indium oxide. The process features PECVD silicon oxide for the passivation and gate dielectric. The temperature of all PECVD steps does not exceed 180 °C. The ma
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