High-performance hydrogenated amorphous silicon TFT on flexible metal foil with polyimide planarization Se Hwan Kim, Jun Hyuk Cheon, Eung Bum Kim, Jung Ho Bae, Ji Ho Hur, Jin Jang * Department of Information Display and Advanced Display Research Center, Kyung Hee University, Seoul 130-701, Republic of Korea Available online 28 January 2008 Abstract We have studied the fabrication of hydrogenated amorphous silicon (a-Si:H) thin-film transistor (TFT) on flexible metal foil with polyimide planarization. The metal foil was coated with polyimide (PI) two times for palanarization with the total thickness of 2.6 lm. The PI was chosen because of its superior planarization capability, easy spin-on process and relatively high temperature process. To coat a PI layer two step process was carried out; room temperature coating and annealing at 180 °C for 1 h and then 300 °C curing for 1 h. The RMS surface roughness was changed from 663 to 20.6 A ˚ by two times coatings. The a-Si:H TFT on the PI planarized metal foil exhibited the field-effect mobility of 1.47 cm 2 /V s and a threshold voltage of 1.8 V. The flexibility of the high-performance TFT was stud- ied for AMOLED backplane application. Ó 2008 Elsevier B.V. All rights reserved. PACS: 81.05.Gc; 62.20.x; 07.10.Lw; 07.10.Pz Keywords: Thin-film transistors; Spin coating; Mechanical, stress relaxation; Atomic force and scanning tunneling microscopy 1. Introduction Portable communication devices are becoming ubiqui- tous as their functionality and convenience continue to improve. A key component of such devices is the display on which information is communicated. Most are made of glass and are therefore breakable. The future of displays for portable applications is flexible (or conformable) and rugged. Over the last few years there has been much work on fabricating OLED displays on flexible substrates such as plastic and metal foil [1,2]. The dominant technologies for portable displays are active-matrix liquid crystal displays (AMLCDs) and active-matrix organic light emitting diode (AMOLED), using the hydrogenated amorphous silicon TFT prepared on a glass substrate. However, AMLCDs and AMOLEDs on glass have the disadvantages of being fragile and heavy. Therefore, new substrates as replace- ments have been strongly required to make a flexible, rug- ged, and lightweight flat panel display [3,4]. As alternate substrates, metal foil, flexible glass, and plastic substrates have been proposed [5–7]. To ensure high-performance backplanes to drive OLEDs, the TFT fabrication (whether amorphous Si or poly-Si) must be performed at around 300 °C or above. This makes the use of low cost, thin metal substrates very attractive for the fabrication of flexible AMOLED displays. In addition, the a-Si:H TFTs are widely used as switching devices for active-matrix liquid- crystal displays and can be also used for the driving of AMOLED and flat panel image sensors. The a-Si:H TFT has advantages of low cost and good uniformity as com- pared with low-temperature poly-silicon (LTPS) TFT. The stainless steel foil has advantage for the substrate of AMOLED because of its high heat capacity and extremely low gas transmission rate. One key issue for the fabrication of AMOLED displays on stainless steel substrates is ensur- ing very smooth surfaces for TFT fabrication and subse- quent OLED growth [8–10]. In this paper, we present the results showing that using a 2.6 lm organic planarization layer surface roughness of the 0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.09.034 * Corresponding author. Tel.: +822 961 0270; fax: +822 961 9154. E-mail address: jjang@khu.ac.kr (J. Jang). www.elsevier.com/locate/jnoncrysol Available online at www.sciencedirect.com Journal of Non-Crystalline Solids 354 (2008) 2529–2533