Thin film passivation of organic light emitting diodes by inductively coupled plasma chemical vapor deposition

Transcription

1 Thin Solid Films 515 (2007) Thin film passivation of organic light emitting diodes by inductively coupled plasma chemical vapor deposition Han-Ki Kim a,, Sang-Woo Kim a, Do-Geun Kim b, Jae-Wook Kang c, Myung Soo Kim d, Woon Jo Cho e a Department of Information and Nano Materials Engineering, Kumoh National Institute of Technology (KIT), 1 Yangho-dong, Gumi, Gyeongbuk, , South Korea b Surface Technology Research Center, Korea Institute of Machinery and Materials, 66 Sangnam-dong, Changwon-si, Gyeongnam, , South Korea c Organic Light Emitting Diodes (OLED) Center, Seoul National University, Silim-dong, Seoul , South Korea d Core Technology Laboratory, Samsung SDI, Co., LTD., 575 Shin-dong, Youngtong-Gu, Suwon, Gyeonggi-Do, , South Korea e Nano Device Research Center, Korea Institute of Science and Technology, 39-1, Haweolgok-Dong, Seongbuk-Gu, Seoul, , South Korea Received 18 January 2006; received in revised form 9 October 2006; accepted 10 November 2006 Available online 28 December 2006 Abstract The characteristics of an SiN x passivation layer grown by a specially designed inductively coupled plasma chemical vapor deposition (ICP- CVD) system with straight antennas for the top-emitting organic light emitting diodes (TOLEDs) are investigated. Using a high-density plasma on the order of electrons/cm 3 formed by nine straight antennas connected in parallel, a high-density SiN x passivation layer was deposited on a transparent Mg Ag cathode at a substrate temperature of 40 C. Even at a low substrate temperature, single SiN x passivation layer prepared by ICP-CVD showed a low water vapor transmission rate of g/m 2 /day and a transparency of 85% respectively. In addition, current voltage luminescence results of the TOLED passivated by the SiN x layer indicated that the electrical and optical properties of the TOLED were not affected by the high-density plasma during the SiN x deposition process Elsevier B.V. All rights reserved. Keywords: ICP-CVD; Straight antenna; TOLED; SiN x 1. Introduction Top-emitting organic light-emitting diodes (TOLEDs) are of great interest for their potential applications in high-resolution active matrix OLEDs (AMOLEDs) and flexible displays, because their geometrical advantages include a high pixel resolution and integration on the Si substrate [1 5]. However, the long-term stability of TOLEDs is still limited due to the instability of the luminescent organic materials, interfacial reactions between electrode and organic layer, and chemical reactions with oxygen and moisture in the air [6 8]. Organic layers are particularly sensitive to moisture and oxygen when they are exposed to ambient air. To prevent the effects of moisture and oxygen, a metal lid or transparent glass lid attached by a UV-cured epoxy resin is widely used in the Corresponding author. Tel.: ; fax: address: hkkim@kumoh.ac.kr (H.-K. Kim). encapsulation process of OLEDs. However, the lid type encapsulation process is not applicable in the case of a flexible substrate and one of the troublesome processes in OLEDs fabrication. To achieve further advances in the production of TOLEDs and flexible displays, it will be necessary to develop high quality thin film passivation with a low water vapor transmission rate (WVTR), excellent reliability, long-term stability, and a high degree of transparency. In particular, a low temperature thin film deposition process would be desirable because high process temperatures are incompatible with the OLEDs fabrication process. SiN x, SiO x, SiO x N y, AlO x, and Al 2 O 3 :N films are currently employed as inorganic passivation layers for OLEDs [9 14]. Huang et al. reported that the SiN x films, grown by plasma enhanced chemical vapor deposition (PECVD), showed good moisture resistance even at a low substrate temperature. In addition, Lifka et al. proposed a multiplayer stack of SiN x /SiO x /SiN x /SiO x /SiN x (NONON) deposited by PECVD which showed a very low water /$ - see front matter 2006 Elsevier B.V. All rights reserved. doi: /j.tsf

2 H.-K. Kim et al. / Thin Solid Films 515 (2007) permeability of 10 6 g/m 2 /day at 85 C [15]. Yun et al. recently reported that an Al 2 O 3 :N layer grown by plasma enhanced atomic layer deposition can serve as a very good passivation layer for OLEDs [14]. Although SiN x layers are well recognized as a passivation layer in OLEDs and solar cells, the characteristics of SiN x grown using an inductively coupled plasma chemical vapor deposition (ICP-CVD) system at low substrate temperatures for the passivation of OLED is still lacking [16,17]. In this work, we reported on an investigation of the characteristics of an SiN x passivation layer grown using an ICP-CVD system with straight antennas and a substrate cooling system. The ICP-CVD system has several advantages including a high deposition rate, square type nozzle and a cooling line, a metal mask cooling system but also minimizes damage to OLEDs by the plasma. Even at a low substrate temperature of 40 C, a 100 nm-thick SiN x film showed a low WVTR of g/m 2 /day and a high transparency of 85%. In addition, a TOLED with an ICP-CVD grown SiN x passivation layer showed a low leakage current density and luminance that was identical to that for nonpassivated TOLEDs. 2. Experimental details For the deposition of high-density SiN x passivation layer at a low substrate temperature, a specially designed ICP-CVD system with straight antennas was employed. A schematic of the ICP- CVD system and straight antennas is shown in Fig. 1. Highdensity plasma is generated by the straight antennas located on the outer portion of the process chamber adjacent to the dielectric upper wall. The plasma density is controlled by radio frequency (rf) power of MHz supplied to the ICP antenna, while the ion energy is controlled by an rf power of MHz supplied to the susceptor. Plasma density and uniformity generated in the ICP- CVD were measured using a Langmuir probe penetrating the sidewall of the chamber as shown in the inset of Fig. 2. To maintain a low substrate and a metal mask temperature below 40 C, a mechanical chucking system equipped with an He (15 sccm) cooling groove was employed. The distance between the substrate and the top gas nozzle was maintained constant at 200 mm to minimize plasma damage effect during the SiN x deposition. Using the ICP-CVD system, an SiN x passivation layer was deposited on the bare glass and the test cell (Transparent cathode/organic layers/ Indium Tin oxide (ITO) anode) at substrate temperature of 40 C. α-napthylphenlylbiphenyl (NPB) and tris-(8-hydroxyquinoline) aluminum (Alq 3 ) were used as the HTL and ETL (EL) layers, respectively. A 5 Å-thick LiF layer was then thermally evaporated on the Alq 3 layer. After the LiF deposition, transparent Mg Ag cathode layers were deposited on the LiF/Alq 3 layer. A mixture of SiH 4, N 2, and Ar was used for the deposition of the SiN x passivation layer. Ellipsometry (SOPRA, SE-5 FPD) and profilometer (α-step, KLA-Tencor coporation) were used to measure the refractive index and thickness of SiN x films. Optical transmittance through the SiN x films was measured in the wavelength range from 350 to 800 nm. The WVTR of the SiN x films grown on polycarbonate (PC) substrates (10 cm 10 cm) were measured at 38±2 C, 100% R. H. using a Permatran-W (MOCON, Inc.) for 72 h. To investigate the effect of plasma damage on the TOLED during the SiN x deposition process, a 100 nm-thick SiN x passivation layer was deposited on the test sample with a structure of Mg Ag cathode/lif/alq 3 /NPB/ITO anode/glass. After deposition of the SiN x passivation layer, the current voltage Luminescence (I V L) characteristics were measured by using a Photo Research PR-650 spectrophotometer driven by a programmable direct current source. 3. Results and discussion Fig. 2 shows the electron density of the plasma generated by nine straight antennas as a function of ICP power. The electron density of the plasma was measured using a Langmuir probe installed at a distance of 5 cm above the susceptor as shown in the inset of Fig. 2. It is shown that an increase in ICP power leads to an increase in electron density. At 2000 W of ICP power, the electron density was cm 3 indicating the formation of a high-density plasma by the straight antennas. This large area plasma source, consisting of nine Cu lines connected in parallel, is similar to the large area plasma source reported by Lieberman [18]. They reported that the plasma density generated by the linear type source is comparable to that generated by other high density inductive sources. In general, the growth rate and density of SiN x films in ICP-CVD are mainly affected by the electron density and the ion energy flux in the plasma. Therefore, it would be expected that high-density Fig. 1. Schematic diagram of an inductively coupled plasma chemical vapor deposition (ICP-CVD) system and the structure of nine-straight antennas.

3 4760 H.-K. Kim et al. / Thin Solid Films 515 (2007) Fig. 2. Electron density of the plasma generated in ICP-CVD as a function of ICP power with inset showing the Langmuir probe installed above the susceptor to measure plasma density. plasma generated by straight antennas would increase the rate of deposition of the SiN x passivation layer. Fig. 3 shows the deposition rate of the SiN x passivation layer as a function of ICP power and SiH 4 flow ratio respectively. The deposition rate was measured as a function of ICP power in the range of W with a constant bias power of 100 W, a Fig. 4. Transmittance of the SiN x films prepared by ICP-CVD at 40 C as a function of SiH 4 flow ratios at a constant ICP power, bias power, working pressure, and N 2 /Ar ratio. reactive gas flow ratio of SiH 4 (15 sccm)/n 2 (110 sccm)/ar (30 sccm), and a working pressure of 5 mtorr (Fig. 3(a)). It is noteworthy that the deposition rate increased monotonically with increasing ICP power. The increase in the density of reactive species by an increase ICP power is believed to be Fig. 3. Deposition rate and refractive index of SiN x films prepared by ICP-CVD at 40 C as a function of (a) ICP power and (b) SiH 4 flow ratio, respectively. Fig. 5. (a) I V and (b) L V characteristics of TOLEDs with an SiN x thin passivation layer and only an Mg Ag cathode (reference) with the inset of EL images of a TOLED passivated by SiN x layer as the TOLED aged (after 0, 100, 200, and 300 h).

4 H.-K. Kim et al. / Thin Solid Films 515 (2007) responsible for the monotonic increase in deposition rates. In addition the refractive index of the SiN x films increases with increasing ICP power, suggesting that the SiN x film becomes silicon rich. This behavior is similar to that observed for SiN x films prepared from SiH 4 and NH 3 in the PECVD. Huang et al., in an investigation of an SiN x layer applied in OLED packaging reported that both the deposition rate and the Si/N ratio increased with increasing rf power because the dissociation of reactant gases increased with the rf power [13]. Fig. 3(b) shows the deposition rate for an SiN x film as a function of SiH 4 content in SiH 4 /N 2 /Ar mixture gas at a constant N 2 (110 sccm), and Ar (30 sccm) flow rate. It can be seen that the deposition rate of the SiN x film increases almost linearly with increasing SiH 4 content at a constant substrate temperature of 40 C [19]. However, an increase in N 2 content resulted in a fairly small increase in deposition rate. These results indicate that the content of SiH 4 is critical factor in controlling the deposition rate of the SiN x layer because the flow rate of SiH 4 gas in ICP-CVD is very low. However, the refractive index was maintained at a constant level regardless of the SiH 4 flow ratio during the SiN x deposition. The optical transmittance of the SiN x passivation layer in the visible range shown in Fig. 4 as a function of SiH 4 flow ratio at constant ICP power (1000 W), bias power (100 W), and N 2 (110 sccm)/ar (30 sccm) flow ratio. It is clearly shown that the transmittance of the SiN x passivation layer is very high regardless of the SiH 4 flow ratio. The highest optical transmittance of the SiN x film is 85% at 550 nm. To apply the SiN x thin passivation layer to TOLEDs, it is very important to deposit passivation layer with a high transmittance because most of the light is extracted through the passivation layer. Therefore, the SiN x passivation layer grown by the linear antenna type ICP- CVD with a high transmittance of 85% could be employed as the top thin film passivation layer in TOLEDs. The WVTR of 100 nm-thick SiN x passivation layer deposited on a PC substrate was measured. Compared to the WVTR ( g/m 2 /day) of the SiN x film prepared at 1000 W ICP power without bias power (0 W), SiN x film with bias power above 100 W had a much lower WVTR of g/m 2 /day due to the improved film density. Therefore, it is necessary to apply a bias voltage to the substrate to prepare a high-density SiN x passivation layer at a low temperature. However, to prevent the plasma damage by bombardment of ions in plasma, bias power should be maintained at low rf power range. To investigate damage resulting from plasma exposure on the electrical and optical properties of TOLEDs, a 100 nm-thick SiN x passivation layer was deposited over a thin Mg Ag cathode layer of a test sample at 5 mtorr with an ICP power of 1000 W and a bias power of 100 W. Fig. 5(a) shows the I V characteristics of the two types TOLEDs, one with an SiN x passivation layer prepared by ICP-CVD and the other with only an Mg Ag cathode (for reference). The I V curve of the TOLEDs with an SiN x passivation layer prepared by ICP-CVD shows a similar forward bias current density to that of TOLEDs with only Mg Ag cathode (for reference). In addition, TOLEDs with an SiN x passivation layer show a low leakage current density at a reverse bias, which is comparable to reference sample. Fig. 5(b) shows L V curves of a TOLED with SiN x passivation layer and a reference sample (non-passivated TOLED). As expected from the I V curve, the TOLED with an SiN x thin passivation layer shows an identical turn-on voltage and luminance to the reference sample. The slight discrepancy is within the error range of reproducibility of TOLEDs. This I V L curve indicates that the electrical and optical characteristics of TOLED are not critically affected by plasma exposure during the SiN x deposition. The electroluminescence (EL) images of TOLED with SiN x thin passivation layer are shown in inset of Fig. 5(b). The initial formation of very small dark spot and side shrinkage in the 0 h sample indicated by arrows is believed to be caused by unintentional exposure of the test sample to particulates, oxygen, and water vapor during its loading into the ICP-CVD chamber. However no new dark spots appeared as the TOLED aged but the initial dark spot increased in size. This indicates that the thin SiN x passivation layer prepared by the linear antenna type ICP-CVD is a promising thin film passivation layer for high-quality TOLEDs and flexible displays. 4. Conclusions In summary, a straight antenna type ICP-CVD generating high-density plasma ( cm 3 ) was developed for the deposition of a thin film passivation layer in OLEDs. Even at a low substrate temperature of 40 C the 100 nm thick SiN x passivation layer showed superior barrier and optical properties. Thus, SiN x films prepared by linear antenna type ICP-CVD appears to be a promising thin film passivation layer for high quality TOLEDs and flexible displays. Due to high-density plasma that is formed far away from the substrate region and the intentional susceptor bias power of the ICP-CVD system, highdensity SiN x passivation layers for TOLEDs could be prepared at low substrate temperatures. Further studies are underway regarding detailed lifetime tests and correlations between the characteristics of SiN x films and barrier properties. Acknowledgment This work was supported by Korea Research Foundation Grant funded by Korea Government (MOEHRD: Basic Research Promotion Fund) ( KRF D00243). References [1] G. Gu, V. Bulovic, P.E. Burrows, S.R. Forrest, M.E. Thompson, Appl. Phys. Lett. 68 (1996) [2] G. Parthasarathy, P.E. Burrows, V. Khalfin, V.G. Koziov, S.R. Forrest, Appl. Phys. Lett. 72 (1998) [3] G. Parthasarathy, C. Adachi, P.E. Burrows, S.R. Forrest, Appl. Phys. Lett. 76 (2000) [4] S. Han, X. Feng, Z.H. Lu, D. Johnson, R. Wood, Appl. Phys. Lett. 82 (2003) [5] H.-K. Kim, K.-S. Lee, M.-J. Keum, K.-H. Kim, Electrochem. Solid-State Lett. 8 (2005) H103. [6] P.E. Burrows, V. Bulovic, S.R. Forrest, L.S. Sapochak, D.M. McCarty, M.E. Thompson, Appl. Phys. Lett. 65 (1994) [7] H. Aziz, Z. Popovic, S. Xie, A.-M. Hor, N.-X. Hu, C. Tripp, G. Xu, Appl. Phys. Lett. 72 (1998) 756.

5 4762 H.-K. Kim et al. / Thin Solid Films 515 (2007) [8] L. Ke, S.-J. Chua, K. Zhang, N. Yakovlev, Appl. Phys. Lett. 80 (2002) [9] A.B. Chwang, M.A. Rothman, S.Y. Mao, R.H. Hewitt, X. Chu, L. Moro, T. Trajewski, N. Rutherford, Appl. Phys. Lett. 83 (2003) 413. [10] M. Schaepkens, T.W. Kim, A.G. Erlat, M. Yan, K.W. Flanagan, C.M. Heller, P.A. McConnelee, J. Vac. Sci. Technol., A 22 (2004) [11] H. Kubota, S. Miyaguchi, S. Ishizuka, T. Wakimoto, Y. Fukuda, T. Watanabe, H. Ochi, T. Sakamoto, T. Miyake, M. Tsuchida, I. Ohshita, T. Tohma, J. Lumin. 87 (2000) 56. [12] S.-H.K. Park, J. Oh, C.-S. Hwang, J.-I. Lee, Y.S. Yang, H.Y. Chu, Electrochem. Solid-State Lett. 8 (2005) H21. [13] W. Huang, X. Wang, M. Sheng, L. Xu, F. Stubhan, L. Luo, T. Feng, X. Wang, F. Zhang, S. Zou, Mater. Sci. Eng., B, Solid-state Mater. Adv. Technol. 98 (2003) 248. [14] S.J. Yun, Y.-W. Ko, J.W. Lim, Appl. Phys. Lett. 85 (2004) [15] H. Lifka, H.A. van Esch, J.J.W.M. Rosink, SID Symposium Digest, vol. 35, 2004, p [16] A.G. Aberle, Sol. Energy Mater. Sol. Cells 65 (2001) 239. [17] H. Mackel, R. Ludemann, J. Appl. Phys. 92 (2002) [18] Y. Wu, M.A. Lieberman, Appl. Phys. Lett. 72 (1998) 777. [19] L.S. Zambom, R.D. Mansano, Vacuum 71 (2003) 439.