Ag Ti Alloy Used in ITO Metal ITO Transparency Conductive Thin Film with Good Durability against Moisture

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1 Materials Transactions, Vol. 46, No. 11 (2005) pp to 25 #2005 The Japan Institute of Metals EXPRESS REGULAR ARTICLE Ag Ti Alloy Used in ITO Metal ITO Transparency Conductive Thin Film with Good Durability against Moisture Shi-Wei Chen 1; * 1, Chun-Hao Koo 1; * 2, Hsin-Erh Huang 2 and Chia-Hua Chen 2 1 Department of Materials Science and Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei, Taiwan 106, R. O. China 2 Metallurgy Division, Materials & Electro-Optics Research Division, Ghung-Shan Institute of Science & Technology, Lungtan, P. O. Box 90008, Lung-Tan, Tao-Yuan, Taiwan, R. O. China This study investigates the characteristics of IMI (ITO Metal ITO) transparent conductive thin films, with an Ag Ti alloy intermediate layer. Multi-layers were deposited by sputtering. ITO AgTi ITO films have better transmittance than ITO Ag ITO films in the visible wavelength range. The maximum transparency is 94% after being annealed at 573 K in a vacuum. Although the resistivity of ITO AgTi ITO films is slightly higher than that of ITO Ag ITO films, the former apparently exhibit greater thermal stability and durability. After exposure to the environment at 323 K with a relative humidity of 90% for 144 h, corrosive spots appear on ITO Ag ITO films but not on ITO AgTi ITO films. (Received August 18, 2005; Accepted September 26, 2005; Published November 15, 2005) Keywords: indium tin oxide Metal indium tin oxide transparent conductive thin film, silver titanium alloy, resistivity, transparency, durability 1. Introduction Transparent conductive oxide (TCO) thin films, e.g. ITO, with good conductivity and transmittance in the wavelength range of nm have been widely used as the transparent conductor for opto-electronic devices. 1 3) Presently, along with the progress of display technology, TCOs for high performance flat panel displays (FPD) require even lower resistivity for high response rate, lower power consumption and driving voltage. ITO with higher resistivity seems not good enough to meet the requirement. In recent years, the demand and application of TCOs with excellent performance increase continuously. There are many researches in advancing the properties of TCOs films and improved materials. 4 6) But most of these researches without ITO can t surpass ITO in conductivity. The simplest and most effective way is to use ITO Metal ITO (IMI) multilayer structures. 7 10) IMI structures have quit low sheet resistance and relatively lower thickness than single-layer TCO films, e.g. ITO, although the transmittance is slight lower than ITO. The properties of IMI films are strongly dependent on the intermediate layer. 2,8) Silver is the best choice as the intermediate layer of IMI films, because it owns both greatest transmittance and conductivity than other metals when the thickness of films gets very thin. 11) Pure silver films are unstable and easy to agglomerate and corrode at high temperature, the conductivity and transmittance will be reduced by the increase of surface roughness and corrosion or oxidation. 9,10,12 14) IMI films, with the Ag intermediate layer, will produce spots under environment with high temperature and moisture for a long time. The generation of spots is related with the corrosion of Ag. It was reported 10) that if silver layers become stable by alloying with doping precious metal, e.g. Ag Pd alloy, the corrosion and * 1 Graduate Student, National Taiwan University * 2 Corresponding author, chkoo@ccms.ntu.edu.tw oxidation will be restrained. However, palladium is a rare metal and very expensive. This investigation develops Ag Ti alloy to replace pure Ag as the intermediate layer of IMI structures, by which the resistivity, transmittance and crystalline after being annealed in air and vacuum are measured. It is also to test the durability in order to realize the difference in stability between pure silver and silver titanium alloy as the intermediate layer of IMI films. 2. Experimental IMI multilayer films were deposited on glass substrates (coring 7059) by continuous magnetron sputtering without vacuum breaking. The thickness of the films was nm (ITO Ag or Ag alloy ITO). ITO films were deposited from a sintered ITO target with high density and a RF power source. Ag or Ag Ti alloy was applied as metal targets. A DC power source was supplied. The targets of Ag and Ag Ti alloy were produced in a vacuum arc furnace. The chamber, which was equipped with a cyro pump, had a base pressure of 6: Pa. Deposition was conducted at a pressure of 6: Pa in an atmosphere of extra pure Ar. Films after deposition were annealed in a vacuum (6: Pa) and in air atmosphere for one hour at a temperature range from 573 to 773 K. AFM (atomic force microscopy) was used to measure the thickness of the films and estimate the sputtering rate. Low angle XRD was used to explore the crystallization, and the thickness of films used for analysis was nm (ITO Ag or Ag alloy ITO). The four point probe method was used to measure the resistivity. UV visible spectroscopy was used to analyze the transmittance. The durability test was conducted at 323 K in a testing box with a relative humidity of 90%. After the samples had been exposed for 144 h, optical microscopy was employed to observe the surface and the resistivity and transmittance were measured.

2 Ag Ti Alloy Used in ITO Metal ITO Transparency Conductive Thin Film with Good Durability against Moisture 2537 Table 1 Resistivity of the Ag, Ag Ti and IMI films as deposition at 298 K. Ag film Ag Ti film ITO Ag ITO film ITO AgTi ITO film 3. Result and Discussion Resistivity, =cm 4: : : : Resistivity As deposition Table 1 presents the resistivity of the Ag film, Ag Ti film and IMI films with different intermediate layer deposited at room temperature. The resistivity of the Ag Ti film exceeds that of the Ag film, according to the theory of the free electron model, 15) that is because of the increase of the probability that carriers are scattered by impurities. Table 1 also shows that the resistivity of the ITO AgTi ITO film exceeds that of the ITO Ag ITO film. The conductive properties of IMI films are dominated by the intermediate layer, 2,8) which indicates that the resistivity of AgTi alloy exceeding that of pure Ag causes the resistivity of ITO AgTi ITO films to exceed that of ITO Ag ITO films Heat treatment in air atmosphere Figure 1 presents the resistivity of IMI films after being annealed in air atmosphere or in a vacuum. The resistivity of ITO Ag ITO and ITO AgTi ITO films increase slightly after being annealed at K in air atmosphere. It seems possible for the following referred reasons. (1) identifying with the gas sensor, the surface resistance increases along with the adsorption of oxygen. 16) (2) oxygen in air diffuses into the ITO layer at the top of the multi-layer, the resistivity of the ITO film increases as the number of conductive carriers is reduced by the decrease of the number of oxygen vacancies ) (3) oxygen atoms diffuse through the ITO layer and oxidize the metal layer inside. 10) When the temperature exceeds 673 K, the resistivity of ITO Ag ITO and ITO AgTi ITO films raise sharply. Jung et al. 10) presented similar results too. It is mainly and probably because the inter-diffusion of atoms becomes severer. 20) Silver atoms diffuse into ITO layers or oxygen atoms diffuse through ITO and react with intermediate layers, causing the oxidation of the metal layers, so the conductivity of both the metal layer and the IMI films decline. The resistivity of the ITO AgTi ITO film is lower than that of the ITO Ag ITO film after being annealed at 773 K in air atmosphere, it apparently reveals that Ag Ti alloy is more resistant than pure silver to oxidation Heat treatment in a vacuum Figure 1 shows that IMI films annealed in a vacuum can reduce the resistivity below that obtained by annealing in air atmosphere. It is suggested that, during annealing in a vacuum, more rare oxygen atoms are adsorbed onto the surface of the films or diffuse into ITO, a fact that will not increase the resistivity of ITO as well as the IMI films. Moreover, the Ag or Ag Ti layer is not oxidized, by which the resistivity of IMI films will not decline. Heat treatment reduces the concentration of defects, so the resistivity of IMI films decreases according to the decrease of the number of carriers scattered by the defects. 3.2 Transmittance Heat treatment in air atmosphere Figure 2 shows the result of X-ray diffraction of ITO AgTi ITO films as deposition and annealed at K in air atmosphere. ITO film is amorphous as depositing at room temperature, but changes to crystalline after being annealed in air atmosphere at above 573 K. Meanwhile, defects decrease and the transmittance of ITO will increase according to the decrease of light scattering by defects, as proposed by Kikuchi et al ) Resistivity, ρ/ Ω cm ITO - AgTi - ITO in air 250 ITO - AgTi - ITO in a vacuum ITO - Ag - ITO in air ITO - Ag - ITO in a vacuum Temperature, T/K Fig. 1 Resistivity of IMI films after being annealed. ITO AgTi ITO films annealed in air atmosphere ( ). ITO AgTi ITO films annealed in a vacuum ( ). ITO Ag ITO films annealed in air atmosphere ( ). ITO Ag ITO films annealed in a vacuum ( ) Intensity ITO (222) ITO (0) Ag (111) Ag (200) two theta, 2θ 300 C 200 C 25 C Fig. 2 XRD spectra of ITO AgTi ITO films annealed at a temperature range from 298 to 573 K in air atmosphere.

3 2538 S.-W. Chen, C.-H. Koo, H.-E. Huang and C.-H. Chen :298K :573K :673K :723K (e):773k (e) :298K :573K :673K :773K Fig. 3 Transmittance curves of the ITO Ag ITO films annealed in air atmosphere. Fig. 5 Transmittance curves of the ITO Ag ITO films annealed at a temperature range from 298 to 773 K in a vacuum (e) :298K :573K :673K :723K (e):773k Fig. 4 Transmittance curves of the ITO AgTi ITO films annealed in air atmosphere :298K :573K :673K :773K Fig. 6 Transmittance curves of the ITO AgTi ITO films annealed at 298 to 773 K in a vacuum. Figure 3 plots transmittance curves of ITO Ag ITO films annealed in air atmosphere at various temperatures. The transmittance declines when the film is annealed in air atmosphere at 673 K. Annealing at over 673 K in air atmosphere might cause severe inter-diffusion of atoms between silver and ITO layers. 10,20) Therefore, Ag atoms diffuse into ITO and reduce the transmittance of the ITO films as well as the IMI films. Under the same conditions, the transmittance of ITO AgTi ITO films does not decline, as plotted in Fig. 4, probably because Ag Ti alloy is more stable than pure Ag, and inter-diffusion could not occur easily between Ag Ti alloy and ITO. Furthermore, as shown in Figs. 3 and 4, when ITO Ag ITO films are annealed in air atmosphere at above 723 K (Fig. 3) and ITO AgTi ITO films at above 773 K (Fig. 4), transmittance curves change and have the similar shape as ITO single layer with the same thickness ( nm). It is suggested that because ITO and silver (or silver alloy) interdiffused fully, films might change from sandwich structures to single layers of the mixture of Ag and ITO, and the optical properties for single layer are presented. Meanwhile, the temperature at which the transmittance curves of ITO AgTi ITO films changed is higher than that of ITO Ag ITO films also because of the better stability of Ag Ti alloy Heat treatment in a vacuum Figure 5 pots the transmittance curves of ITO Ag ITO films annealed in a vacuum. The curves shift towards the direction of shorter wavelength. The maximum transparency is approximately 82% after being annealed at 573 K, and the

4 Ag Ti Alloy Used in ITO Metal ITO Transparency Conductive Thin Film with Good Durability against Moisture 2539 µm 50 µm 50 µm Fig. 7 Surface observation of IMI films after exposure for 144 h at 323 K with a relative humidity of 90% by OM. image of the ITO Ag ITO film. image of the ITO AgTi ITO film. transmittance declines during annealing at over 673. Kim et al. 13) proposed that silver films agglomerated when they were heat-treated at over 673 K in a vacuum. It is suggested that the agglomeration of silver layers, as mentioned above, also occurs herein, scattering light, which thereby reduces the transmittance. Figure 6 plots the transmittance curves of ITO AgTi ITO films annealed in a vacuum at various temperatures. The maximum transparency of ITO AgTi ITO films is approximately 94% after being annealed at 573 K. This excellent result has seldom been obtained for IMI systems with the same structure. ITO AgTi ITO films keep high transmittance after being annealed at a relatively higher temperature, probably because Ag Ti layers keep smooth surface by which rare light is scattered. Silver layers are stable and keep smooth surface at a relatively high temperature, because the addition of Ti into Ag reduces the diffusivity and thereby retards the agglomeration. The transmittance curves of ITO Ag ITO films shift towards the direction of shorter wavelength after being annealed seems because of the decrease of the refractive indices of ITO due to the relaxation of compressive stress as proposed by Jung. 10) The stress relaxation of ITO associates with the intermediate layer (Ag or Ag alloy). Jeong et al. 21) proposed that Ag alloy (Ag Pd Cu) films started to relax the compressive stress at a relatively higher temperature than pure Ag films. It is suggested that the addition of Ti into Ag can retard the compressive-stress relaxation of silver films, as well as ITO layers. Thus, the refractive indices of ITO does not decrease and the transmittance curves of ITO AgTi ITO films do not shift after being annealed. 3.3 Durability Deposited films exposed for 144 h at 323 K with 90% relative humidity were tested for durability. Figures 7 reveals that numerous spots form on the surfaces of the ITO Ag ITO films. It was reported that the generation of the defects is related with the corrosion of Ag. 10) As presented in Fig. 7, observations of the surface reveal that no corrosive spot appears on the ITO AgTi ITO

5 25 S.-W. Chen, C.-H. Koo, H.-E. Huang and C.-H. Chen Table 2 film after testing. A fact indicates that ITO AgTi ITO films are more durable than ITO Ag ITO films, and it seems because Ag Ti alloy is more resistant than pure silver to corrosion. Wei et al. 22) proposed that the addition of Ti into Ag can reduce the activity, increasing the chemical stability and resistance to corrosion. Table 2 presents the resistivity of IMI films after testing. The resistivity of ITO Ag ITO films increases and that of ITO AgTi ITO films is almost unchanged after testing. It is probably because Ag Ti alloy is more resistant than pure Ag to corrosion as mentioned above, the corrosion of Ag films increases the resistivity of both Ag films and ITO Ag ITO films. Figure 8 reveals the transmittance of IMI films after testing. ITO AgTi ITO films keeps high transmittance at a wavelength of 550 nm after testing, but the transmittance of ITO Ag ITO films declines apparently at the same wavelength due to the existence of sopts on the surface, scattering light. However, no spot appears on the surface of ITO AgTi ITO films after testing. 4. Conclusion Resistivity of IMI films before and after durability testing. before durability test (resistivity, cm) after durability test (resistivity, cm) ITO Ag ITO film 3: ITO AgTi ITO film 4: The transparency of the ITO AgTi ITO film at a wavelength of 550 nm raises to 94% after being annealed at 573 K in a vacuum, and the transparency exceeds that of the ITO a b c a: ITO-AgTi-ITO after testing b: ITO-AgTi-ITO before testing c: ITO- Ag -ITO after testing d: ITO- Ag -ITO before testing Fig. 8 Transmittance curves of ITO Ag ITO films and ITO AgTi ITO films after durability testing. d Ag ITO film with the best transparency of only 82%. This excellent result of transmittance has seldom been obtained for IMI systems with the same structure. The results for resistivity and transparency following annealing in air atmosphere or exposing to moist environment, and observations of the surface of films after durability testing, all indicate that using Ag Ti alloy as the intermediate layer increases the stability and durability of IMI films. The IMI film with Ag Ti alloy as the intermediate layer is a good candidate for transparent conductive electrodes. Acknowledgement The authors would like to thank Metallurgy Division, Materials & Electro-Optics Research Division, Ghung-Shan Institute of Science & Technology, and also show their appreciation to professor Kuo and Dr. Sun, Department of Materials Science and Engineering, National Taiwan University for support of equipments. REFERENCES 1) M. Sawada, M. Higuchi, S. Kondo and H. Saka: Jpn. J. Appl. Phys. (2001) ) J. Y. Kim, D. Sohn and E. R. Kim: Appl. Phys. A 72 (2001) ) S. F. Hsu, C. C. Lee, S. W. Hwang, H. H. Chen, C. H. Chen and A. T. Hu: Thin Solid Films 478 (2004) ) C. Agashe, O. Kluth, J. Hupkes, U. Zastrow and B. Rech: J. Appl. Phys. 95 (2004) ) C. Warmsingh, Y. Yoshida, D. W. Readey, C. W. Teplin, J. D. Perkins, P. A. Parilla, L. M. Gedvilas, B. M. Keyes and D. S. Ginley: J. Appl. Phys. 95 (2004) ) A. Wang, J. Dai, J. Cheng, M. P. Chudzik, T. J. Marks, R. P. H. Chang and C. R. Kannewurj: Appl. Phys. Lett. 73 (1998) ) A. KlÖppel, B. Meyer and J. Trube: Thin Solid Films 392 (2001) ) M. Bender, W. Seelig, C. Daube, H. Frankenberger, B. Ocker and J. Stollenwerk: Thin Solid Films 326 (1998) ) K. H. Choi, J. Y. Kim, Y. S. Lee and H. J. Kim: Thin Solid Films 341 (1999) ) Y. S. Jung, Y. W. Choi, H. C. Lee and D. W. Lee: Thin Solid Films 4 (2003) ) Y. Guan, M. A. Matin and T. M. Stephen: Electron. Lett. 39 (2002) ) H. C. Kim and T. C. Alford: Appl. Phys. Lett. 81 (2002) ) H. C. Kim and T. L. Alford: J. Appl. Phys. 94 (2003) ) T. Suzuki, Y. Abe, M. Kawamura, K. Sasaki, T. Shouzu and K. Kawamata: Vacuum 66 (2002) ) M. A. Omar: Elementary Solid State Physics, (Addison Wesley, 1993) pp ) J. F. McAleer, P. T. Mosely, J. O. Norris and D. E. Williams: J. Chem. Soc. Faraday Trans. 83 (1987) ) M. T. Bhatti, A. M. Rana and A. F. Khan: Mater. Chem. Phys. 84 (2004) ) Y. Hu, X. Diao, C. Wang, W. Hao and T. Wang: Vacuum 75 (2004) ) N. Kikuchi, E. Kusano, H. Nanto, A. Kinbara and H. Hosono: Vacuum 59 (2000) ) A. KlÖppel, W. Kriegseis, B. K. Meyer, A. Scharmann, C. Daube, J. Stollenwerk and J. Trube: Thin Solid Films 365 (2000) ) C. O. Feong, N. S. Roh, S. G. Kim et al.: J. Electron. Mater. 31 (2002) ) P. Wei, L. Rongti, C. Jian, S. Ruifeng and L. Jie: Mater. Sci. Eng. A 287 (2000)