Laser-Crystallized High Quality ITO on Plastic Substrates for Flexible Displays
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Laser-Crystallized High Quality ITO on Plastic Substrates for Flexible Displays
Wonsuk Chung, Paul Wickboldt, Daniel Toet, and Paul G. Carey FlexICs, Inc., 165 Topaz Street, Milpitas, CA 95035 Michael O. Thompson Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853
ABSTRACT In the study presented here, we successfully demonstrated that high quality ITO films could be obtained on plastic substrates using Excimer laser crystallization. ITO films were first deposited at 10 °C on PEN substrates by DC magnetron sputtering, and then irradiated using a homogenized pulsed XeCl excimer laser beam (308 nm, 35 ns pulse duration) in a vacuum chamber. It was possible to reliably attain Type I ITO films with sheet resistances down to 35 Ω/ٱ, combined with 80 % optical transmittance in visible range. Well defined 2 µm lines could be obtained using simple HCl etchant at room temperature. We also developed Type II ITO films with a sheet resistance of 15 Ω/ ٱand an optical transmittance of 80 % by means of laser annealing on plastic substrates, although these materials were found inferior to Type I in etching properties. INTRODUCTION Due to their unique properties, transparent conducting oxides (TCOs) have been drawing significant interest ever since cadmium oxide (CdO) thin films were first reported in 1907 [1]. In particular, tin-doped indium oxide (ITO), has been widely used in solar cells, heat mirrors, and many opto-electronic devices including liquid crystal (LCD) and organic light emitting (OLED) displays [2]. For many of these applications, films must exhibit a low resistivity (~10-4 Ω cm) while maintaining transparency (>85 %) across the visible spectrum. High quality ITO films can meet these requirements due to the large band gap energy (Eg = ~3.7 eV) with large carrier concentrations (~1021 cm-3). These high quality ITO films are typically deposited on glass substrates using sputtering or thermal evaporation at elevated substrate temperatures (>300 ºC) often followed by furnace annealing. However, as interest has grown recently in developing flat panel displays on low temperature flexible plastic substrates, it is desired to deposit high quality ITO on these substrates instead of glass. ITO films deposited at low temperatures (70 mJ/cm2). For high number of shots (30 ~ 100 shots), rapid increase starts at 70 mJ/cm2 while only slow increase is found at even higher fluences at a small number of shots (1 ~ 10 shots). Although electrical properties of ITO films were improved by laser annealing, optical transmittance was found to remain almost identical after laser annealing as shown in the plot of optical transmittance as a function of wavelength in figure 2 (b). Figure 3 shows microscopic images of 2 µm line etching test using undiluted HCl (38 %) at room temperature. As clearly shown, 2 µm lines can be very well defined on the laser-annealed ITO while as-deposited films exhibited only poorly defined lines due to non-uniform overetch, a common problem for low-temperature ITO etc
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