Low Damage Sputter Deposition of ITO Films on Organic Light Emitting Films

Transaction of the Materials Research Society of Japan 34[2] 321-324 (2009)

Hao Leia,bKeisuke Ichikawaa,b, Yoichi Hoshia,b*, Meihan Wanga,c, Yutaka Sawadaa,c, Takayuki Uchidaa,d a. Center for Hyper Media Research, Tokyo Polytechnic University, 1583 Iiyama, Atsugi, Kanagawa 243-0297, Japan b. Department of System Electronics and Information Technology, Graduate School of Engineering, Tokyo Polytechnic University, 1583 Iiyama, Atsugi, Kanagawa 243-0297, Japan c. Department of Nanochemistry, Faculty of Engineering, Tokyo Polytechnic University, 1583 Iiyama, Atsugi, Kanagawa 243-0297, Japan d. Department of Image Engineering, Graduate School of Engineering, Tokyo Polytechnic University, 1583 Iiyama, Atsugi, Kanagawa 243-0297, Japan Fax: +81-46-242-9566, E-mail: [email protected]

Damage to an aluminum (III) bis(2-methyl-8-quninolinato)-4-phenylphenolate (BAlq) layer was investigated during indium-tin oxide (ITO) deposition using a facing target sputtering (FTS) system in which the bombardment of negative oxygen ions and -electrons onto the substrate can be completely suppressed. The photoluminescence (PL) intensity of BAlq was used to evaluate the damage on the organic layer after the deposition of an ITO thin film in Ar and Kr gas at different gas pressures and input power levels. The results suggest that the bombardment of reflected neutral atoms is not the mainreason for the damage. The remnant damage from the bombardment of sputtered atoms can be reduced by sputtering at a higher gas pressure. Finally, the bombardment of the organic film by high-energy particles such as negative oxygen ions, - electrons, and sputter-emitted atoms was completely suppressed using an FTS system to attain low-damage and high-rate sputter ITO deposition. Key words: ITO thin film, facing target sputtering, low-damage

1. INTRODUCTION ITO films on organic light emitting materials to attain For fabricating transparent organic light-emitting low-damage and high-rate sputter deposition. devices (TOLEDs) and top-emitting OLEDs (TEOLEDs), deposition of a transparent and conductive 2. EXPERIMENTAL film as a cathode on an organic emission layer is BAlq was chosen as the light emitting organic layer required [1]. Indium-tin oxide (ITO) thin film is a since it is more stable than Alq3 in atmosphere. The promising candidate to be deposited as the electrode BAlq film (thickness: ~40 nm) was deposited at room because of its low resistivity and high transmittance. temperature on a clean glass substrate by thermal Usually, deposition of ITO film on an organic layer is evaporation, and then the ITO film (thickness: ~150 nm) performed using a traditional magnetron sputtering was deposited on the BAlq-coated substrate using a method; however, large radiation damage to the organic facing target sputtering (FTS) system (see Fig. 1). layer occurs from the bombardment of high energetic To evaluate the damages produced in the BAlq films particles including sputtered atoms, reflected neutral Ar by the sputter-deposition of ITO, photoluminescence atoms, negative oxygen ions and -electrons [2,3]. To (PL) intensity of the BAlq film was measured before and minimize this radiation damage, some researchers have after ITO deposition. PL measurement was carried out deposited a protective buffer layer before sputtering ITO, using a procedure described in a previous study [5]. and some semitransparent cathode materials were also To confirm the ion and electron bombardment effects used [4]. However these procedures make the design of on the organic layer, direct bombardment of ions and the OLED stack more complex. Developing a process electrons on the layer was performed using radio for low-damage-sputter deposition of ITO on top of frequency (RF) bias in the FTS [5] and tungsten filament organic layers is significant for simplifying the device as an electron emission source [6], respectively. structure andimproving performance. Our previous study showed that bombardment of 3. RESULTS AND DISCUSSION high-energy particles, such as ions and electrons, to the Figure 2 shows the PL spectra of the BAlq layer organic layer leads to serious degradation of the organic before and after electron or ion bombardment for 10min. layer [5,6]. Suppression of these high-energy particles is It is clear that a significant reduction in PL intensity of necessary to reduce this damage. the BAlq film occurred after bombardment. These We propose a new sputtering process for deposition of results indicate that bombardment by high-energy ions

321 322 Low Damage Sputter Deposition of ITO Films on Organic Light Emitting Films

Fig. 1. Schematic configuration of facing target sputtering Fig. 2. PL spectra of BAlq before and after ion and electron system with evaporation source for organic film bombardment for 10 min ( EBElectron Bombardment, voltage: 200 V, amount of electrons: 1.6 1017, substrate current density: 43.0 μA/cm2; IB: Ion Bombardment, voltage: 100 V)

and electrons causes considerable damage to the organic layer. FTS has been proven to be a good method for suppressing damage from negative oxygen ions and -electrons [5,7,8]. The set-up of a semi-sector-shaped metal shield on targets can also completely prevent damage from -electrons and enhance the photoluminescence properties of the organic layer [6]. However, the PL intensity still decreased markedly due to the deposition of ITO films when sputtering was performed at 50 W in Ar with a gas pressure below 2 mTorr (see Fig. 3). This reduction may be caused by the bombardment of high-energy reflected gas atoms and/or high-energy sputter-emitted atoms from the target. To clarify which type of particles is the main source for the reduction in PL intensity, Kr gas was selected as the sputtering gas during deposition of the ITO film. Owing to its larger atomic mass and radius, changing the sputtering gas from Ar to Kr gas is thought to reduce the amount of reflected sputtering gas atoms at the target surface, which is thought to be effective in reducing the bombardment of high-energy particles [9]. Comparison of the damage produced by the sputter- deposition in Ar and Kr gas is shown in Fig. 3, only a small improvement was observed using the Kr gas. There was still a significant reduction in PL intensity when sputter-deposition was performed at a lower gas pressure (2 mTorr). These results suggest that high-energy reflected sputtering gas atoms has little effect on the damage of organic film, that is high-energy sputter-emitted atoms from the target are the main Fig. 3. PL spectra of BAlq and BAlq/ITO deposited at 50 W in source of damage in an FTS system. Ar and Kr gas (gas pressure: 2 mTorr)

H. Lei et al. Transaction of the Materials Research Society of Japan 34[2] 321-324 (2009) 323

Fig. 4. PL spectra of BAlq and BAlq/ITO deposited at 50 W in Fig. 5. PL spectra of BAlq and BAlq/ITO deposited at 50 and Ar and Kr gas (gas pressure: 10 mTorr) 200 W in Kr gas (gas pressure: 10 mTorr)

Generally, the kinetic energy of incident particles to 4. CONCLUSION the substrate reduces with an increase in sputtering gas Low-damage and high-rate sputter deposition of ITO pressures due to gas scattering. The PL spectra of BAlq films on an organic layer can be attained using an FTS before and after ITO deposition in Ar and Kr under a gas system, where bombardment of the organic film by pressure of 10 mTorr are shown in Fig. 4. It was found high-energy particles, such as negative oxygen ions, - that the reduction in PL intensity can be markedly electrons, and sputter-emitted atoms, are completely suppressed, which indicates that the damage caused by suppressed. high-energy sputter-emitted atoms was reduced at a higher sputtering gas pressure. Although increasing the sputtering input power results in high kinetic energy of incident particles, it is REFERENCE: necessary to improve the deposition rate and decrease [1] G. Gu, V. Bulovic, P. E. Burrows, S. R. Forrest and the sputtering time synchronously [7]. Figure 5 shows M. E. Thompson, Appl. Phys. Lett., 68, No. 19, the PL spectra of BAlq after ITO deposition below 10 2606-2608 (1996). mTorr Kr with an input power of 50 and 200 W. It [2] S. M. Rossnagel, J. Vac. Sci. Technol. A, 7, No. 3, should be noted that the PL intensity decreased only a 1025-1029 (1989). little even when sputtering was performed using four [3] K. Tominaga, M. Chong and Y. Shintani, J. Vac. Sci. times the power. The deposition rate, however, increased Technol. A, 12, No. 4, 1435-1438 (1994). from about 6.5 nm/min (50 W) to 20 nm/min (200 W). [4] B. J. Chen, X. W. Sun, and S. C. Tan, Optics Exp., The higher input power does not play an important role 13, No. 3, 937-941 (2005). in the damage compared with sputtering gas pressure. [5] Y. Onai, T. Uchida, Y. Kasahara, K. Ichikawa and Y. Therefore, high-rate sputter deposition can be attained in Hoshi, Thin Solid Films, 516, No. 17, 5911-5915 (2008). an FTS system. [6] H. LeiK. Ichikawa, M. H. Wang, Y. Hoshi, T. One interesting phenomenon observed in our current Uchida and Y. Sawada, IEICE Trans. on Electronics, study is that the PL peak shifts to a larger or smaller E-91-C, No. 10, 1658-1662 (2008). wavelength after ITO deposition (see Figs. 3-5). This [7] Y. Hoshi, H. Kato and K. Funatu, Thin Solid Films, shift is thought to originate from the so-called 445, No. 2, 245-250 (2003). microcavity effect [10], that is BAlq and ITO films work [8] O. Kamiya , Y. Onai, H. Kato and Y. Hoshi, Journal as microcavities. The wavelength of the light confined in of Materials Science: Materials in Electronics, 18, the microcavity depends on the thickness of the ITO Suppl. 1, S359-S362 (2007). films, so that the peak shift in the PL spectra occurs due [9] Y. Hoshi and H. Shimizu, IEICE Trans. on to the deposition of ITO film. Electronics (Inst Electron Inf Commun Eng), E87-C, No. 2, 212-217 (2004). 324 Low Damage Sputter Deposition of ITO Films on Organic Light Emitting Films

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(Received December 9, 2008; Accepted February 26, 2009)