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Thin film passivation of organic light emitting diodes by inductively coupled plasma chemical vapor deposition
Thin Solid Films 515 (2007) 4758 – 4762 www.elsevier.com/locate/tsf
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
b
Department of Information and Nano Materials Engineering, Kumoh National Institute of Technology (KIT), 1 Yangho-dong, Gumi, Gyeongbuk, 730-701, South Korea Surface Technology Research Center, Korea Institute of Machinery and Materials, 66 Sangnam-dong, Changwon-si, Gyeongnam, 641-831, South Korea c Organic Light Emitting Diodes (OLED) Center, Seoul National University, Silim-dong, Seoul 151-741, South Korea d Core Technology Laboratory, Samsung SDI, Co., LTD., 575 Shin-dong, Youngtong-Gu, Suwon, Gyeonggi-Do, 442-391, South Korea e Nano Device Research Center, Korea Institute of Science and Technology, 39-1, Haweolgok-Dong, Seongbuk-Gu, Seoul, 136-791, 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 SiNx passivation layer grown by a specially designed inductively coupled plasma chemical vapor deposition (ICPCVD) system with straight antennas for the top-emitting organic light emitting diodes (TOLEDs) are investigated. Using a high-density plasma on the order of ∼ 1011 electrons/cm3 formed by nine straight antennas connected in parallel, a high-density SiNx passivation layer was deposited on a transparent Mg–Ag cathode at a substrate temperature of 40 °C. Even at a low substrate temperature, single SiNx passivation layer prepared by ICP-CVD showed a low water vapor transmission rate of 5 × 10− 2 g/m2/day and a transparency of ∼ 85% respectively. In addition, current– voltage–luminescence results of the TOLED passivated by the SiNx layer indicated that the electrical and optical properties of the TOLED were not affected by the high-density plasma during the SiNx deposition process. © 2006 Elsevier B.V. All rights reserved. Keywords: ICP-CVD; Straight antenna; TOLED; SiNx
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.: +82 54 478 7746; fax: +82 54 478 7769. E-mail address: [email protected] (H.-K. Kim). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.11.030
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. SiNx, SiOx, SiOxNy, AlOx, and Al2O3:N films are currently employed as inorganic passivation layers for OLEDs [9–14]. Huang et al. reported that the SiNx 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 SiNx/SiOx/SiNx/SiOx/SiNx (NONON) deposited by PECVD which showed a very low water
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permeability of 10− 6 g/m2/day at 85 °C [15]. Yun et al. recently reported that an Al2O3:N layer grown by plasma enhanced atomic layer deposition can serve as a very good passivation layer for OLEDs [14]. Although SiNx layers are well recognized as a passivation layer in OLEDs and solar cells, the characteristics of SiNx 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 SiNx 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 SiNx film showed a low WVTR of 5 × 10− 2 g/m2/day and a high transparency of ∼85%. In addition, a TOLED with an ICP-CVD grown SiNx 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 SiNx passivation layer at a low substrate temperature, a specially designed ICP-CVD system with straight antennas was employed. A schematic of the ICPCVD 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 13.56 MHz supplied to the ICP antenna, while the ion energy is controlled by an rf power of 13.56 MHz supplied to the susceptor. Plasma density and uniformity generated in the ICPCVD 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 SiNx deposition. Using the ICP-CVD system, an SiNx 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 (Alq3) were used as the HTL and ETL (EL) layers, respectively. A 5 Å-thick LiF layer was then thermally evaporated on the Alq3 layer. After the LiF deposition, transparent Mg–Ag cathode layers were deposited on the LiF/Alq3 layer. A mixture of SiH4, N2, and Ar was used for the deposition of the SiNx passivation layer. Ellipsometry (SOPRA, SE-5 FPD) and profilometer (α-step, KLA-Tencor coporation) were used to measure the refractive index and thickness of SiNx films. Optical transmittance through the SiNx films was measured in the wavelength range from 350 to 800 nm. The WVTR of the SiNx films grown on polycarbonate (PC) substrates (10 cm× 10 cm) were measured at 38 ± 2 °C, 100% R. H. using a Permatran-W®
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(MOCON, Inc.) for 72 h. To investigate the effect of plasma damage on the TOLED during the SiNx deposition process, a 100 nm-thick SiNx passivation layer was deposited on the test sample with a structure of Mg–Ag cathode/LiF/Alq3/NPB/ITO anode/Glass. After deposition of the SiNx 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 1.5 × 1011 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 SiNx 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.
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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 SiNx passivation layer. Fig. 3 shows the deposition rate of the SiNx passivation layer as a function of ICP power and SiH4 flow ratio respectively. The deposition rate was measured as a function of ICP power in the range of 500–2000 W with a constant bias power of 100 W, a
Fig. 3. Deposition rate and refractive index of SiNx films prepared by ICP-CVD at 40 °C as a function of (a) ICP power and (b) SiH4 flow ratio, respectively.
Fig. 4. Transmittance of the SiNx films prepared by ICP-CVD at 40 °C as a function of SiH4 flow ratios at a constant ICP power, bias power, working pressure, and N2/Ar ratio.
reactive gas flow ratio of SiH4 (15 sccm)/N2(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. 5. (a) I–V and (b) L–V characteristics of TOLEDs with an SiNx thin passivation layer and only an Mg–Ag cathode (reference) with the inset of EL images of a TOLED passivated by SiNx layer as the TOLED aged (after 0, 100, 200, and 300 h).
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responsible for the monotonic increase in deposition rates. In addition the refractive index of the SiNx films increases with increasing ICP power, suggesting that the SiNx film becomes silicon rich. This behavior is similar to that observed for SiNx films prepared from SiH4 and NH3 in the PECVD. Huang et al., in an investigation of an SiNx 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 SiNx film as a function of SiH4 content in SiH4/N2/Ar mixture gas at a constant N2 (110 sccm), and Ar (30 sccm) flow rate. It can be seen that the deposition rate of the SiNx film increases almost linearly with increasing SiH4 content at a constant substrate temperature of 40 °C [19]. However, an increase in N2 content resulted in a fairly small increase in deposition rate. These results indicate that the content of SiH4 is critical factor in controlling the deposition rate of the SiNx layer because the flow rate of SiH4 gas in ICP-CVD is very low. However, the refractive index was maintained at a constant level regardless of the SiH4 flow ratio during the SiNx deposition. The optical transmittance of the SiNx passivation layer in the visible range shown in Fig. 4 as a function of SiH4 flow ratio at constant ICP power (1000 W), bias power (100 W), and N2 (110 sccm)/Ar (30 sccm) flow ratio. It is clearly shown that the transmittance of the SiNx passivation layer is very high regardless of the SiH4 flow ratio. The highest optical transmittance of the SiNx film is 85% at 550 nm. To apply the SiNx 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 SiNx passivation layer grown by the linear antenna type ICPCVD 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 (3 × 10− 1 g/m2/day) of the SiNx film prepared at 1000 W ICP power without bias power (0 W), SiNx film with bias power above 100 W had a much lower WVTR of 5 × 10− 2 g/m2/day due to the improved film density. Therefore, it is necessary to apply a bias voltage to the substrate to prepare a high-density SiNx 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 SiNx 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 SiNx 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 SiNx 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 SiNx 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 SiNx
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passivation layer and a reference sample (non-passivated TOLED). As expected from the I–V curve, the TOLED with an SiNx 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 SiNx deposition. The electroluminescence (EL) images of TOLED with SiNx 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 SiNx 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 (1.5 × 1011 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 SiNx passivation layer showed superior barrier and optical properties. Thus, SiNx 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 SiNx 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 SiNx films and barrier properties. Acknowledgment This work was supported by Korea Research Foundation Grant funded by Korea Government (MOEHRD: Basic Research Promotion Fund) ( KRF-2006-331-D00243). References [1] G. Gu, V. Bulovic, P.E. Burrows, S.R. Forrest, M.E. Thompson, Appl. Phys. Lett. 68 (1996) 2606. [2] G. Parthasarathy, P.E. Burrows, V. Khalfin, V.G. Koziov, S.R. Forrest, Appl. Phys. Lett. 72 (1998) 2138. [3] G. Parthasarathy, C. Adachi, P.E. Burrows, S.R. Forrest, Appl. Phys. Lett. 76 (2000) 2128. [4] S. Han, X. Feng, Z.H. Lu, D. Johnson, R. Wood, Appl. Phys. Lett. 82 (2003) 2175. [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) 2922. [7] H. Aziz, Z. Popovic, S. Xie, A.-M. Hor, N.-X. Hu, C. Tripp, G. Xu, Appl. Phys. Lett. 72 (1998) 756.
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[8] L. Ke, S.-J. Chua, K. Zhang, N. Yakovlev, Appl. Phys. Lett. 80 (2002) 2195. [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) 1716. [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) 4896. [15] H. Lifka, H.A. van Esch, J.J.W.M. Rosink, SID Symposium Digest, vol. 35, 2004, p. 1387. [16] A.G. Aberle, Sol. Energy Mater. Sol. Cells 65 (2001) 239. [17] H. Mackel, R. Ludemann, J. Appl. Phys. 92 (2002) 2602. [18] Y. Wu, M.A. Lieberman, Appl. Phys. Lett. 72 (1998) 777. [19] L.S. Zambom, R.D. Mansano, Vacuum 71 (2003) 439.
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
b
Department of Information and Nano Materials Engineering, Kumoh National Institute of Technology (KIT), 1 Yangho-dong, Gumi, Gyeongbuk, 730-701, South Korea Surface Technology Research Center, Korea Institute of Machinery and Materials, 66 Sangnam-dong, Changwon-si, Gyeongnam, 641-831, South Korea c Organic Light Emitting Diodes (OLED) Center, Seoul National University, Silim-dong, Seoul 151-741, South Korea d Core Technology Laboratory, Samsung SDI, Co., LTD., 575 Shin-dong, Youngtong-Gu, Suwon, Gyeonggi-Do, 442-391, South Korea e Nano Device Research Center, Korea Institute of Science and Technology, 39-1, Haweolgok-Dong, Seongbuk-Gu, Seoul, 136-791, 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 SiNx passivation layer grown by a specially designed inductively coupled plasma chemical vapor deposition (ICPCVD) system with straight antennas for the top-emitting organic light emitting diodes (TOLEDs) are investigated. Using a high-density plasma on the order of ∼ 1011 electrons/cm3 formed by nine straight antennas connected in parallel, a high-density SiNx passivation layer was deposited on a transparent Mg–Ag cathode at a substrate temperature of 40 °C. Even at a low substrate temperature, single SiNx passivation layer prepared by ICP-CVD showed a low water vapor transmission rate of 5 × 10− 2 g/m2/day and a transparency of ∼ 85% respectively. In addition, current– voltage–luminescence results of the TOLED passivated by the SiNx layer indicated that the electrical and optical properties of the TOLED were not affected by the high-density plasma during the SiNx deposition process. © 2006 Elsevier B.V. All rights reserved. Keywords: ICP-CVD; Straight antenna; TOLED; SiNx
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.: +82 54 478 7746; fax: +82 54 478 7769. E-mail address: [email protected] (H.-K. Kim). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.11.030
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. SiNx, SiOx, SiOxNy, AlOx, and Al2O3:N films are currently employed as inorganic passivation layers for OLEDs [9–14]. Huang et al. reported that the SiNx 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 SiNx/SiOx/SiNx/SiOx/SiNx (NONON) deposited by PECVD which showed a very low water
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permeability of 10− 6 g/m2/day at 85 °C [15]. Yun et al. recently reported that an Al2O3:N layer grown by plasma enhanced atomic layer deposition can serve as a very good passivation layer for OLEDs [14]. Although SiNx layers are well recognized as a passivation layer in OLEDs and solar cells, the characteristics of SiNx 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 SiNx 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 SiNx film showed a low WVTR of 5 × 10− 2 g/m2/day and a high transparency of ∼85%. In addition, a TOLED with an ICP-CVD grown SiNx 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 SiNx passivation layer at a low substrate temperature, a specially designed ICP-CVD system with straight antennas was employed. A schematic of the ICPCVD 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 13.56 MHz supplied to the ICP antenna, while the ion energy is controlled by an rf power of 13.56 MHz supplied to the susceptor. Plasma density and uniformity generated in the ICPCVD 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 SiNx deposition. Using the ICP-CVD system, an SiNx 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 (Alq3) were used as the HTL and ETL (EL) layers, respectively. A 5 Å-thick LiF layer was then thermally evaporated on the Alq3 layer. After the LiF deposition, transparent Mg–Ag cathode layers were deposited on the LiF/Alq3 layer. A mixture of SiH4, N2, and Ar was used for the deposition of the SiNx passivation layer. Ellipsometry (SOPRA, SE-5 FPD) and profilometer (α-step, KLA-Tencor coporation) were used to measure the refractive index and thickness of SiNx films. Optical transmittance through the SiNx films was measured in the wavelength range from 350 to 800 nm. The WVTR of the SiNx films grown on polycarbonate (PC) substrates (10 cm× 10 cm) were measured at 38 ± 2 °C, 100% R. H. using a Permatran-W®
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(MOCON, Inc.) for 72 h. To investigate the effect of plasma damage on the TOLED during the SiNx deposition process, a 100 nm-thick SiNx passivation layer was deposited on the test sample with a structure of Mg–Ag cathode/LiF/Alq3/NPB/ITO anode/Glass. After deposition of the SiNx 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 1.5 × 1011 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 SiNx 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.
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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 SiNx passivation layer. Fig. 3 shows the deposition rate of the SiNx passivation layer as a function of ICP power and SiH4 flow ratio respectively. The deposition rate was measured as a function of ICP power in the range of 500–2000 W with a constant bias power of 100 W, a
Fig. 3. Deposition rate and refractive index of SiNx films prepared by ICP-CVD at 40 °C as a function of (a) ICP power and (b) SiH4 flow ratio, respectively.
Fig. 4. Transmittance of the SiNx films prepared by ICP-CVD at 40 °C as a function of SiH4 flow ratios at a constant ICP power, bias power, working pressure, and N2/Ar ratio.
reactive gas flow ratio of SiH4 (15 sccm)/N2(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. 5. (a) I–V and (b) L–V characteristics of TOLEDs with an SiNx thin passivation layer and only an Mg–Ag cathode (reference) with the inset of EL images of a TOLED passivated by SiNx layer as the TOLED aged (after 0, 100, 200, and 300 h).
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responsible for the monotonic increase in deposition rates. In addition the refractive index of the SiNx films increases with increasing ICP power, suggesting that the SiNx film becomes silicon rich. This behavior is similar to that observed for SiNx films prepared from SiH4 and NH3 in the PECVD. Huang et al., in an investigation of an SiNx 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 SiNx film as a function of SiH4 content in SiH4/N2/Ar mixture gas at a constant N2 (110 sccm), and Ar (30 sccm) flow rate. It can be seen that the deposition rate of the SiNx film increases almost linearly with increasing SiH4 content at a constant substrate temperature of 40 °C [19]. However, an increase in N2 content resulted in a fairly small increase in deposition rate. These results indicate that the content of SiH4 is critical factor in controlling the deposition rate of the SiNx layer because the flow rate of SiH4 gas in ICP-CVD is very low. However, the refractive index was maintained at a constant level regardless of the SiH4 flow ratio during the SiNx deposition. The optical transmittance of the SiNx passivation layer in the visible range shown in Fig. 4 as a function of SiH4 flow ratio at constant ICP power (1000 W), bias power (100 W), and N2 (110 sccm)/Ar (30 sccm) flow ratio. It is clearly shown that the transmittance of the SiNx passivation layer is very high regardless of the SiH4 flow ratio. The highest optical transmittance of the SiNx film is 85% at 550 nm. To apply the SiNx 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 SiNx passivation layer grown by the linear antenna type ICPCVD 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 (3 × 10− 1 g/m2/day) of the SiNx film prepared at 1000 W ICP power without bias power (0 W), SiNx film with bias power above 100 W had a much lower WVTR of 5 × 10− 2 g/m2/day due to the improved film density. Therefore, it is necessary to apply a bias voltage to the substrate to prepare a high-density SiNx 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 SiNx 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 SiNx 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 SiNx 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 SiNx 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 SiNx
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passivation layer and a reference sample (non-passivated TOLED). As expected from the I–V curve, the TOLED with an SiNx 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 SiNx deposition. The electroluminescence (EL) images of TOLED with SiNx 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 SiNx 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 (1.5 × 1011 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 SiNx passivation layer showed superior barrier and optical properties. Thus, SiNx 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 SiNx 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 SiNx films and barrier properties. Acknowledgment This work was supported by Korea Research Foundation Grant funded by Korea Government (MOEHRD: Basic Research Promotion Fund) ( KRF-2006-331-D00243). References [1] G. Gu, V. Bulovic, P.E. Burrows, S.R. Forrest, M.E. Thompson, Appl. Phys. Lett. 68 (1996) 2606. [2] G. Parthasarathy, P.E. Burrows, V. Khalfin, V.G. Koziov, S.R. Forrest, Appl. Phys. Lett. 72 (1998) 2138. [3] G. Parthasarathy, C. Adachi, P.E. Burrows, S.R. Forrest, Appl. Phys. Lett. 76 (2000) 2128. [4] S. Han, X. Feng, Z.H. Lu, D. Johnson, R. Wood, Appl. Phys. Lett. 82 (2003) 2175. [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) 2922. [7] H. Aziz, Z. Popovic, S. Xie, A.-M. Hor, N.-X. Hu, C. Tripp, G. Xu, Appl. Phys. Lett. 72 (1998) 756.
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[8] L. Ke, S.-J. Chua, K. Zhang, N. Yakovlev, Appl. Phys. Lett. 80 (2002) 2195. [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) 1716. [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) 4896. [15] H. Lifka, H.A. van Esch, J.J.W.M. Rosink, SID Symposium Digest, vol. 35, 2004, p. 1387. [16] A.G. Aberle, Sol. Energy Mater. Sol. Cells 65 (2001) 239. [17] H. Mackel, R. Ludemann, J. Appl. Phys. 92 (2002) 2602. [18] Y. Wu, M.A. Lieberman, Appl. Phys. Lett. 72 (1998) 777. [19] L.S. Zambom, R.D. Mansano, Vacuum 71 (2003) 439.