SiOx Barrier Films for Flexible Displays

Transcription

1 SiOx Barrier Films for Flexible Displays T. Komori*, H. Kobayashi, and H. Uyama Technical Research Institute, TOPPAN PRINTING CO., LTD , Takanodai-Mimami, Sugito-machi, Kitakatushika-gun, Saitama-ken Japan Abstract High barrier SiOx film was obtained by a roll-to-roll PECVD system using hexamethyldisiloxane (HMDSO) and oxygen. We found that SiOx film deposited by PECVD with HMDSO contains little carbon, and the refractive index and density were near 1.46 and 2.2g/cm 3 respectively, comparable to those of thermal silicon oxide. The water vapor transmission rate (WVTR) of only 40nm thick SiOx on COP (100µm) reached the detection limit of MOCON and we confirmed the possibility of application of the SiOx film to OLED by an encapsulation test. Introduction The improvement of barrier properties of polymers against oxygen and/or water vapor permeation using transparent ceramic coatings has been widely investigated for food packaging, pharmaceutical, and electronics applications. AlOx and SiOx coating on polymer films using high speed roll-to-roll processes enabled large-scale production for food packaging. Recently high barrier coatings on flexible substrates are receiving much attention in the field of flat panel display (FPD) applications: liquid crystal display (LCD), electronic paper and organic light emitting diode [1, 2] (OLED). Especially OLED attracts much expectation of the application to a flexible display, since OLED has features that are possible to reduce weight and thickness compared to other flat panel displays. The substrate for OLED is required a high barrier against water vapor, because OLED has a negative electrode and light emitting layer which are very sensitive to moisture. It is supposed that the required barrier level for OLED is less than * Electronic mail: tsunenori.komori@toppan.co.jp 10-5 g/m 2 /day, though the barrier level for packaging [3] is about 10-1 g/m 2 /day. In order to achieve this high barrier property, various techniques have been examined, such as organic/inorganic multilayer [4], SiON/CN multilayer [5] and SiNx layer [6]. We directed our attention to a roll-to-roll PECVD system in order to obtain high barrier SiOx films using hexamethyldisiloxane (HMDSO) as a precursor and oxygen. It is well known that PECVD is one of the techniques that obtain layers with high transparency and high barrier. HMDSO has high chemical stability and relatively higher vapor pressure than other organosilicon precursors, therefore it is used as an alternative to silanes (e.g. SiH4). In this report, we optimized deposition conditions from the view of SiOx layer and the possibility of application to OLED was considered. Experimental Procedures The PECVD coating was carried out in a pilot-scale roll-to-roll deposition system equipped with one electrode. In order to obtain a thicker SiOx film and avoid scratching SiOx surface at the time of

2 rewinding, these experiments carried out without rewinding, so films were fixed on main roll when SiOx was deposited. The outline of the apparatus is shown in Fig.1. When roll-to-roll deposition was performed, the barrier against water vapor achieved was about 0.05 g/m 2 /day using 125µm thick PET film as the substrate. Fig. 1 Schematic illustration of roll-to-roll PECVD deposition system. The winding roll and the rewinding roll were not used these experiments. The deposition room and the winding room were evacuated separately, and plasma was generated only in the deposition room. The main roll was kept at 50 degrees C, the substrate was fixed on the main roll, and the rise of temperature was not observed during the deposition. The plasma was generated by radio frequency (RF MHz) and the electrode also served as oxygen introduction. HMDSO was fed from the inlet in the vicinity of the electrode. The main roll rotated at a constant speed, the substrate passed throughthe deposition room and SiOx was deposited on the substrate. Results and Discussion The chemical bonding states of the deposited films were characterized by Fourier transform infrared spectroscopy (FT-IR; PerkinElmer Spectrum One). The FTIR spectra are represented in Fig. 2, which are obtained from SiOx films deposited on Si wafers with varying HMDSO/O2 flow ratios: (a), (b) and (c) indicate HMDSO/O2 flow ratio of 0.1, 0.2 and 0.4, respectively. The FTIR spectra [7, 8] exhibit three characteristic IR absorption bands of Si-O-Si bonds: the rocking mode around 450 cm -1, the bending mode around 800 cm -1 and asymmetrical stretching mode around cm -1. In the case of using HMDSO as the precursor, there are some peaks that are derived from incorporated carbon atom from HMDSO precursor such as CH3 asymmetric stretching at 2985cm -1 and CH3 symmetric bending in Si-(CH3)n at 1270cm -1. However, those peaks become smaller with reduction in flow ratio, and there are no peaks derived from carbon groups for flow ratios less than 0.2. It is noted that carbon content is less than 3 at. % with HMDSO/O2 flow ratio of 0.1 from XPS analysis. From these results, it can be said that almost no carbon is contained in SiOx film with HMDSO/O2 flow ratio of less than 0.2. Fig.2 FTIR spectra obtained from SiOx films deposited with varying HMDSO/O 2 flow ratio, (a), (b) and (c) indicate HMDSO/O 2 flow ratio of 0.1, 0.2 and 0.4

3 Fig.3 Refractive index measurement of SiOx film as a function of HMDSO/O 2 flow ratio and RF power 0.5 kw (), 1.0kW (). Fig.4 Density measurement of SiOx film as a function of HMDSO/O 2 flow ratio and RF power 0.5 kw (), 1.0kW (). Figure 3 shows the refractive index [9] of SiOx films deposited on Si wafers as a function of HMDSO/O2 flow ratio and RF power. Refractive index was measured by elipsometry with the light of 632.8nm. Refractive index, n rises with increased flow ratio, and when the high electrical power was applied, n tends to be higher, which suggests that SiOx films become more Si-rich. Stoichiometric thermal silicon oxides have n ~1.46, whereas Si-rich films have n > 1.46 and O-rich films or lower density films including Si-OH group and un-decomposed CH group have n < Refractive index of SiOx films deposited by PECVD with flow ratio of 0.1 takes the value of ~1.46. The film density was determined by X-ray reflectivity (XRR) using advanced thin film X-ray system (Rigaku ATX-G). Fig. 4 shows that SiOx films deposited on Si wafers under the sameconditions as in Fig. 3 have a density of around 2.2 g/cm 3.Since the density of SiOx deposited by thermal silicon oxide is 2.2g/cm 3, the densities of SiOx deposited by PECVD with HMDSO are comparable to those of thermal silicon oxide. When power is 0.5kW and flow ratio is 0.1, density reaches 2.2g/cm 3 and decreases with a flow ratio increase. In the case of 1.0kW and the flow ratio between 0.1 and 0.3, the density remains near 2.2g/cm 3. Fig.5 WVTR as a function of SiOx thickness using various substrates: PET175m (), COP 100m (), PES 200m (). We took the above-mentioned factors into considerationstructure, composition and density of SiOx films. We measured the water vapor transmission rate (WVTR) as a function of SiOx thickness using various substrates: PET 175µm ( ), polyether sulphone (PES) 200µm ( ), cyclic-olefin polymer (COP) 100µm ( ), and the results are indicated in Fig. 5. The WVTR measurements were taken on a MOCON PERMATRAN 3/33 at 40

4 Fig.6 Encapsulation test comparing dark spot areas using barrier films. degrees C, 90% relative humidity. From these results, WVTR decreases rapidly with increasing SiOx Film thickness, and became below the detection limit of MOCON (0.01g/m 2 /day) when the thickness of SiOx was 40nm. It is thought that the barrier property difference between substrates is caused by the roughness and the barrier of the base substrates. Fig. 7 Area ratio of Dark Spot after OLED encapsulation using barrier films: sample A(), B (), and glass (). In order to investigate the possibility of application to OLED, We fabricated a polymer type OLED on a glass substrate which was encapsulated with barrier film deposited by PECVD in a water and oxygen free environment. The thin calcium layer which is the negative electrode of an OLED is very sensitive to water, and the resulting calcium corrosion forms a dark spot (DS) which doesn t emit light. To estimate the amount of water vapor permeation, the area ratio of DS of the OLED samples aged at 60 degrees C, 90% relative humidity was measured. Fig. 6 indicates the results of this DS test, where the difference between Samples A and B is the difference of the substrate used in the barrier film.fig. 7 represents the area ratio of DS of the OLED samples A ( ) and B ( ). As a reference, a sample encapsulated by glass ( ) was also examined at the same time. Sample A kept a low area ratio of DS for 500h, comparable to that of glass encapsulation. On other hand sample B, DS growth was observed and the barrier film delaminated from OLED. These results indicate that barrier films for OLED must take into consideration not only barrier property but also the selection and pretreatment of the substrates. Finally, we fabricated a flexible OLED using the barrier film instead of a glass substrate and its picture is shown in Fig.8.

5 Fukuda, Teruichi Watanabe, Hideo Ochi, Tsuyoshi Sakamoto, Takako Miyake, Masami Tsuchida, Isamu Ohshita, Teruo Tohma, Journal of Luminescence (2000) 56. [7] Min Tae Kim, Thin Solid Films 311(1997) 157 [8] F. Benitez, E. Martinez, and J. Esteve, Thin Solid Films 377 (2000) 109 [9] R. Rashid, A. J. Flewitt, and J. Robertson, J. Vac. Sci. Technol. A, 21 (2003) 726 Fig. 8 Flexible OLED using barrier film instead of glass substrate. Conclusion In this report we investigated high barrier SiOx film obtained by a roll-to-roll PECVD deposition system. We found out that SiOx film deposited by PECVD with HMDSO contains little carbon, and the refractive index and density of obtained SiOx film were near 1.46 and 2.2g/cm 3, respectively. The resulting WVTR of the barrier film reached the detection limit of MOCON with only 40nm thick SiOx and were confirmed comparable to glass by an encapsulation test of an OLED display. References [1] Ayako Yoshida, Akira Sugimoto, Toshiyuki Miyadera, Satoshi Miyaguchi, J. Photopolym. Sci. Technol. 14 (2001) 327. [2] Anna B. Chwang, Mark A. Rothman, Sokhanno Y. Mao, Richard H. Hewitt, Michael S. Weaver, Jeff A. Silvernail, Kamala Rajan, Michael Hack, and Julie J. Brown, SID 03 Digest 34 (2003) 868. [3] e.g. Toppan GX film. ( [4] M. S. Weaver, L. A. Michalski, K. Rajan, M. A. Rothman, J. A. Silvernail, P. E. Burrows, G. L. Graff, M. E. Gross, P. M. Martin, M. Hall, E. Mast, C. Bonham, W. Bennett, and M. Zumhoff, Appl. Phys. Lett., 81 (2002) [5] Kunio Akedo, Atsushi Miura, Hisayoshi Fujikawa, and Yasunori Taga, SID 03 Digest 34 (2003) 559. [6] Hirofumi Kubota, Satoshi Miyaguchi, Shinichi Ishizuka, Takeo Wakimoto, Jun Funaki, Yoshinori

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