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Illumination & Displays

High-performance, highly rollable oxide thin-film transistors

Stacked titanium and indium zinc oxide electrodes enable the fabrication of amorphous indium gallium zinc oxide thin-film transistors on transparent, pliable substrates.
20 January 2012, SPIE Newsroom. DOI: 10.1117/2.1201112.004010

Flexible electronics and displays have attracted attention due to merits such as light weight, thin profiles, portability, and the ability to form conformable shapes, all of which allow novel applications. Lightweight, bendable thin-film transistors (TFTs) are a must for the circuitry that drives such displays. Emerging TFT technology based on oxide semiconductors is promising for this application due to advantages such as high mobility in the amorphous phase, low processing temperatures, and relative compatibility with existing fabrication methods.1–4

Most reports on flexible oxide TFTs involve the use of steel foil substrates or of fabrication techniques such as lift-off and shadow masking. These processes are relatively primitive and cannot be easily applied for mass production. Steel foils, meanwhile, suffer limited flexibility and issues of parasitic capacitance. Recently, there have been some reports of oxide TFTs on polyimide substrates, which are compatible with higher processing temperatures compared with other common plastic substrates. Although polyimides can, in general, withstand higher processing temperatures, to date most of these materials are tinted or not transparent, limiting applications in displays and other flexible electronics.

We have used fully colorless and transparent polyimide-based nanocomposite substrates to make high-performance, highly flexible amorphous indium gallium zinc oxide (a-IGZO) TFTs: see Figure 1.5 The lithographic and etching processes we use are compatible with existing TFT fabrication technologies.

Figure 1. (a) Flexible amorphous indium gallium zinc oxide thin-film transistors (a-IGZO TFTs) using a titanium/indium zinc oxide (Ti/ZnO) stacked electrode on a freestanding nanocomposite substrate. (b) Structure of the a-IGZO TFT. SiNx: Silicon nitride.

The first step is to coat smooth polyimide films tens of microns thick on glass substrates to which they adhere strongly. To easily separate the flexible sheet from the glass after TFT fabrication, the glass surface is selectively coated with a release layer to which the polyimide material adheres more weakly. The sheet could be easily lifted after device processing by simply cutting around the edges of the release-layer region, giving highly transparent freestanding flexible films. This technique does not require the use of glue, eliminating the issue of glue residues. The films reveal a high level of transparency over the whole visible spectrum both when coated on the glass substrates and when freestanding: see Figure 2.

Figure 2. (a) Freestanding transparent polyimide-based nanocomposite film. (b) Transmission spectra of the film on a glass substrate and that of a freestanding film.

To maintain a smooth substrate surface after the source and drain electrodes are patterned, we use a stacked architecture containing a 20nm-thick layer of conductive indium zinc oxide (IZO) below a 70nm-thick titanium layer. Dry etching and weak acid were used to etch the titanium layer and the thin IZO, respectively. The dry etching of titanium does not affect IZO, which serves as an etch-stop layer during the process, effectively protecting the nanocomposite substrate below. Furthermore, the weak acid does not attack the nanocomposite substrate. An atomic force microscope (AFM) image of the pristine substrate surface shows a reasonable measure of roughness of about 0.68nm. An AFM image of the surface after the source/drain electrodes are patterned by dry and wet etching shows that it maintains a smoothness similar to that of the pristine substrate with a roughness of 0.89nm.

The adoption of stacked titanium/IZO as the source, drain, and gate electrodes significantly improves etching compatibility with other material layers. This in turn enables the successful implementation of a-IGZO TFTs onto the transparent nanocomposite substrates by conventional lithographic and etching processes (using a maximal processing temperature <250°C). The flexible devices exhibit decent mobility and mechanical bending capability. A field-effect mobility of up to 15.9cm2/V·s, sub-threshold swing of 0.4V/decade, and an on/off ratio >108 have been extracted from the TFT characteristics: see Figure 3. The devices remain normally functional when bent down to a radius of curvature of 3mm: see Figure 4.

Figure 3. (a) Output and (b) transfer (and extracted mobility) characteristics of a-IGZO TFTs on nanocomposite substrates. ID: Drain current. VDS: Drain-source voltage. VGS: Gate-source voltage.

Figure 4. ION, IOFF, and IGS as a function of the bending radius for the Ti/IZO devices. ION: On current. IOFF: Off current. IGS: Gate-source current.

We plan to develop dielectric materials and oxide thin film processing for low-temperature fabrication of flexible oxide TFTs, which would facilitate their use in flexible displays and electronics.

Chih-Wei Chien, Cheng-Han Wu, Hsing-Hung Hsieh, Chung-Chih Wu
National Taiwan University
Taipei, Taiwan

Chih-Wei Chien received a BS in electrical engineering from National Tsing Hua University, Hsinchu, Taiwan (2007), and an MS in electronics engineering from National Taiwan University (2009). His research interests include oxide thin-film transistors and flexible electronics/displays.

Cheng-Han Wu received a BS in electrical engineering from National Tsing Hua University (2004) and a PhD in electronics engineering from National Taiwan University (2011). His research interests include oxide semiconductors and thin-film transistor technologies.

Hsing-Hung Hsieh received a BS in electrical engineering and a PhD in electronics engineering from National Taiwan University (2001 and 2008, respectively). He has studied oxide electronics since 2003. He is currently a manager at AU Optronics Corporation, Hsinchu, Taiwan, developing oxide thin-film transistors and active-matrix organic LED technologies. His current research interests include oxide semiconductors for electronic devices and their application to flat panel displays and transparent and flexible electronics.

Chung-Chih Wu received a BS in electrical engineering from National Taiwan University (1990) and a PhD in electrical engineering from Princeton University, NJ (1997). He was a display researcher at the Industry Technology Research Institute, Hsinchu, Taiwan (1997–1998), and then joined the faculty of National Taiwan University, where he is currently a full professor. His research interests include organic semiconductors for optoelectronic and electronic devices, flat panel displays, oxide semiconductors, and devices for displays and transparent and flexible electronics.

Yung-Hui Yeh
Industrial Technology Research Institute (ITRI)
Hsinchu, Taiwan

Yung-Hui Yeh received a PhD in electrical engineering from National Tsing Hua University (1998). That same year, he joined ITRI, where he is currently the deputy director with the Flexible Electronics Division. His research interests include silicon-based thin-film transistors, active-matrix organic LED displays, flexible electronics, and displays.

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