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Organosilicon function of gas barrier films purely deposited by inductively coupled plasma chemical vapor deposition system
Journal of Alloys and Compounds 542 (2012) 11–16
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Organosilicon function of gas barrier films purely deposited by inductively coupled plasma chemical vapor deposition system Tung-Ying Lin a,b, Ching-Ting Lee a,c,⇑ a
Institute of Nanotechnology and Microsystems Engineering, Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan, ROC Laser Application Technology Center, Industrial Technology Research Institute, Tainan 734, Taiwan, ROC c Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, Tainan 701, Taiwan, ROC b
a r t i c l e
i n f o
Article history: Received 20 April 2012 Received in revised form 30 June 2012 Accepted 10 July 2012 Available online 20 July 2012 Keywords: OLED thin film encapsulation Polymer-like organosilicon Inductively coupled plasma chemical vapor deposition
a b s t r a c t The novel design of inductively coupled plasma (ICP) source with the parallel electrodes embedded in quartz tubes was developed in this study. The advantages of the inductively coupled plasma chemical vapor deposition (ICPCVD) system were less ion-bombardment effect during the OLED encapsulation process and low cost to manufacture. The encapsulation structure of organosilicon/SiOx thin films deposited on flexible plastic substrates was purely deposited by the single chamber ICPCVD system under various hexamethyldisiloxane (HMDSO) and Ar flow ratios. To investigate the organosilicon film function, the ratio of HMDSO ambient was varied during deposition process and the associated bonding configurations were measured. With an adequate power of 400 W and HMDSO atmosphere of 60%, the polymer-like organosilicon films were obtained due to the long chain structures. Finally, the water vapor transmission rate (WVTR) of one dyad barrier decreases to the 0.021 g/m2/day due to the stress release of SiOx films caused by the polymer-like films. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Organic light-emitting diodes (OLEDs) are particularly attractive for displays since they exhibit the inherent advantages of high brightness, wide viewing angle, fast response time, low power consumption and suited to being fabricated on flexible substrates. However, flexible OLEDs require permeation barriers to prevent their degradation caused by the absorption of moisture and oxygen from the environment. Hence, transparent materials such as silicon oxide (SiOx) and silicon nitride (SiNx) were widely used as gas barrier films [1,2]. However, these inorganic oxide films suffer from the crack or peel as a result of the difference in thermal expansion coefficient between the substrate and the coated films [3]. To circumvent this problem, a multilayer barrier structure consisting of soft organic and hard inorganic films was commonly used, which benefiting in improving the water vapor barrier performance [4,5]. However, these gas barrier films were prepared in various vacuum equipments, which resulted in significant inconvenience for multilayer coating deposition. Based on the above consideration, it is required to develop a gas barrier film with high reliability and low cost deposited in a single-chamber process. In this study, single chamber thin-film encapsulation technology ⇑ Corresponding author at: Institute of Microelectronics, Department of Electrical Engineering, Department of Material Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan, ROC, Tel.: +886 6 2379582; fax: +886 6 2362303. E-mail address: [email protected] (C.-T. Lee). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.07.053
was developed using the liquid precursor hexamethyldisiloxane (HMDSO) and the novel inductively coupled plasma chemical vapor deposition (ICPCVD) system. This work focused on the investigation of the soft organic films which affected the barrier structures. Details of the deposition parameters, chemical bonding, and water vapor transmission rate of the organic and SiOx thin films were investigated and discussed in this work.
2. Experimental The structures of the ICPCVD system equipped with parallel coils embedded in quartz tubes, as shown in Fig. 1a and b, was used to deposit the organosilicon film and SiOx film on 220 lm thick polyethersulfone (PES, 5 5 cm2) substrates and 525 lm thick Si substrates, respectively. Compared to the traditional ICP coil design, the advantages of parallel coils design was to prevent the standing wave effect, low cost, and maintaining the plasma uniformity when the system was scaled up. Moreover, inductively coupled plasma has less ion-bombardment and still keeping high plasma density compared with the capacitively coupled plasma (CCP) system. Therefore, it is more suitable for using in the OLED thin film encapsulation. The ICP was excited by 13.56 MHz RF power. The plasma density was measured with Ar that was used a Langmuir probe at 10 cm above the ICP coil. Fig. 2 shows the plasma density as a function of the RF power at working pressure of 5, 30 and 40 mTorr, respectively. The plasma density increased with RF power from 400 to 1400 W. At 40 mTorr, it could reach more than 1011 /cm3 with the increasing of RF power over 1000 W. At working pressure of 5 mTorr, the plasma density reached around 1010 /cm3 due to the less process gas. The measurement of the plasma density is the guideline of process setting. Organosilicon films were deposited at a higher working pressure due to the investigation of correlation between various fraction of HMDSO and RF power. The dense SiOx films were deposited at a low working pressure [6].
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Fig. 1. (a) Quartz tube embedded type of ICP. (b) Vaporizer and gas system.
Fig. 3. The structure of one dyad encapsulation.
Fig. 2. Plasma density as a function of RF power at various working pressures.
copy was measured by plasus Emicon MC (range from 200 to 1000 nm). The optical transmittance of barrier films was measured using an UV/Vis 4001 spectrophotometer. The water vapor permeation of these samples was measured using a water vapor transmission rate measurement system (MOCON Inc., PERMATRANW 3/61) at a temperature of 40 °C with a relative humidity of 100%.
3. Experimental results and discussion
Table 1 Process setting. Setting
Organosilicon film 20% 40%
SiOx film
Ar (sccm) HMDSO (sccm) O2 (sccm) Pressure (mTorr) RF power (W)
32 24 16 8 16 24 N/A N/A N/A 40 40 40 400, 600, and 800 for each HMDSO
60%
80% 8 32 N/A 40 fraction
50 5 60 5 1000
Table 1 shows the deposition conditions of organosilicon and SiOx thin films. Pure HMDSO (99.99%) were used as the precursor and the deposition RF power was fixed at 400, 600 and 800 W, respectively. The vaporizer system of HMDSO consisted of a mass flow controller and a metal tank heated to 50 °C as shown in Fig. 1b. To control the (CH)x contents in the organosilicon films, the HMDSO/(HMDSO+Ar) flow ratio was respectively kept at 20%, 40%, 60%, 80% and the total flow rate was kept at 40 sccm. The working pressure of the ICPCVD system was kept at 40 mTorr. For the deposition of inorganic films, the oxygen gas was added to form the SiOx structure. In the deposition process, the associated RF power, the working pressure, and oxygen flow rate were 1000 W, 5 mTorr, and 60 sccm, respectively. The one dyad encapsulation structure of organosilicon (SiOxCyHz)/inorganic (SiOx)/flexible substrate was shown in Fig. 3. The chemical bonding states were examined using a Fourier transform infrared spectrometer (JASCO-4100). The surface morphologies were studied by field emission scanning electron microscopy (JEOL JSM-7001F). Optical emission spectros-
In the previous studies [7,8], different kinds of plasma, such as Ar, He, N2, could be used to treat polymer substrates. After plasma treatment, the contaminants, oligomers and amorphous layers existing on the polymer substrates would be removed [9]. Furthermore, the polymer substrates became more hydrophilic for improving their wettability and their adhesion properties. Therefore, after cleaning using the deionized water, the substrates were pre-cleaned using the pure Ar plasma and the treatment time was 1 min. Fig. 4a–c shows the SEM results of PES substrates cleaned under RF power of 100, 400, and 800 W, respectively. It could be found that the smooth surface of substrate was obtained after clean with RF power at 100 W, as shown in Fig. 4a. The associated contact angle of the PES substrate was decreased from 58° to 12°. With an increase of RF powers up to 400–800 W, the PES substrates were damaged due to an increase of sheath voltage. The sheath voltage was measured by the Langmuir probe under the Ar plasma condition. The sheath voltage of the plasma power operated at 100, 400, 600, and 800 W was 13.5, 20.8, 23.0, and 27.7 V respectively. These values were lower compared to the traditional capacitively coupled plasma system. The sheath voltage was attributed to the increase of capacitive coupling in our ICP system. In general, the higher sheath voltage would cause more ion bombardment on
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13
methyl groups. With an increase of HMDSO/(HMDSO+Ar) flow ratio, the C–H and Ha emission decreased. This phenomenon was attributed to the more monomer resided in the plasma system which could mitigate the dissociation ability at the same RF power. Furthermore, the Si–H emission was resulted from the dangling bond formed by the break of Si–C bond or the Si–O–Si bond which attracts the H ions [14]. On the other hand, C2 emission was only observed obviously in the flow ratio of 20–40%. This phenomenon could be attributed to the hardness of the total dissociation of methyl groups. In the flow ratio of 20%, the highest emission of all spices were exhibited whatever RF power. The ions and electrons dissociated from Ar reacted strongly with the HMDSO. From the OES, the trend of process can be observed. The actual bonding structure need to be measured by a Fourier transform infrared spectrometer. 3.2. Fourier transform infrared spectrometer analysis
Fig. 4. SEM micrographs of PES after Ar plasma treatment of (a) 100 W, (b) 400 W, and (c) 800 W.
the polymer substrate. Therefore, the 100 W RF power pre-cleaned procedure was applied in the following experiments. 3.1. Optical emission spectroscopy analysis Fig. 5 shows the intensity of Ha, CH, SiH, and C2 emitted from the dissociation of methyl groups during the HMDSO ionized process measured by the CCD sensor of the optical emission spectroscopy (OES). The associated transition level and wavelength were listed in Table 2 [10–12]. In general, the bonds of Si–C and C–H in HMDSO monomer are easily dissociated in high plasma density environment due to the bonding energy of Si–O (8.3 eV) was higher than that of Si–C (4.6 eV) and C–H (3.5 eV). Therefore, the C–H emission and Ha emission would come from the dissociation of
The Fourier transform infrared spectrometer (FTIR) spectra of the plasma-polymerized coatings from 900 to 1350 cm 1 shown in Fig. 6 were deconvolved into six components including linear, network, cage and methyl related groups at 60% flow ratio and RF power at 400 W. The related vibration modes of FTIR absorption spectroscopy were listed in Table 3 [10–13,15]. Usually, the Si–O absorption peak could be classified into three structures. The peak at 1023 cm 1 represents the long-chain structure of the siloxane, called linear. The peak at 1132 cm 1 represents 3D Si–OC structure, called cage, and the 1072 cm 1 called network [16]. The above mentioned three type structures were shown in Fig. 7. The peak at 1263 cm 1 represents symmetrical deformation of the Si– (CH3)x bonds. Fig. 8a shows the cage/linear ratios at various flow ratios and various powers. At 400 W, the cage/linear structure ratio was decreased with the increase of HMDSO ratio. It could be found that the cage/linear structure ratio at low power (400 W) was less than that at higher power. This phenomenon could be attributed to that lower dissociation rate of Si–O bonds. The formation of the cage structure was restrained. At 800 W, the variations of cage/linear structure ratio were unapparent. This behavior was attributed to that an increasing rate of HMDSO could make up cage/linear structure ratio owing to the decrease of Ar. At 600 W, the cage/linear structure ratio increased firstly and then decreased with an increase of HMDSO ratio caused by the limitation of dissociation. The lowest (cage/linear) ratio reached to 0.062 at the 400 W and flow ratio of 60%. The linear structure contains siloxane chains which performed the polymer-like characteristics [17]. The cage structure results in the pores of the organosilicon films [18]. Therefore, the less cage/linear structure ratio contributes the organosilicon films to polymerization. From the absorption intensity of Si– (CH3)x bonds, the polymerization could also be observed. The polymerization ratio (A(Si–(CH3)x)/A(linear+network+cage)) is defined by the ratio of the intensity of the peak at 1260 cm 1 (Si–(CH3)x bonds) to the intensity of the peak 1000–1100 cm 1 (Si–O–Si bonds) [19]. In this work, the range of inorganic groups enlarged from 1000–1200 cm 1 including linear, network, and cage. Fig. 8b shows the ratio of polymerization as a function of the amount of HMDSO in the gas mixture which meant the organic trend of organosilicon films. In the power of 400 W, the polymerization ratio was increased with the increase of HMDSO ratio. The polymerization ratio in the power of 400 W was larger than that in higher power. This result consisted with the trend of CH emission as shown in Fig. 5b. The OES could detect more CH radical at a high power and a low fraction of HMDSO, but it resulted in less methyl group in the organosilicon film. In this work, cage/linear structure ratio and polymerization ratio were the degree of the polymerization. The major factor for the polymerization was the
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Fig. 5. Variations of the line intensity of (a) Ha, (b) CH, (c) SiH, and (d) C2 as a function of the amount of HMDSO in the gas mixture in organosilicon process.
Table 2 Transitions detected by OES. Species
Transition
Wavelength (nm)
Ha C2 CH SiH Ar
H(n = 3)–H(n = 2) A3Pg–X3Pu A2D–X2P A2D–X2P 4s–4p
656 516 431 414.2 750
Table 3 Vibration modes by FTIR absorption spectroscopy. Species
Vibrating mode
Wave number (cm
Si–O–Si Si–O–Si Si–O–Si Si–(CH3)x
asymmetric stretching [linear] asymmetric stretching [network-like] asymmetric stretching [cage-like] symmetrical deformation
1023 1072 1132 1263
1
)
RF power. When the RF power decreased, the monomer structure was better preserved. Therefore, the higher degree polymer-like organosilicon film could be obtained as the buffer layer. The function of the buffer layer could release the stress and prevent the crack from SiOx film. 3.3. Water vapor transmission rate and optical transmittance analyses
Fig. 6. Deconvoluted absorption peak obtained by FTIR at 400 W in organosilicon process.
Fig. 9 shows the water vapor transmission rate (WVTR) of the one dyad encapsulation structure fabricated under various conditions. The WVTR values show better result in the RF power of 400 W due to the less cage/linear ratio. The polymerization ratio also exhibited the correlation between the contained methyl group and WVTR. In the power of 600–800 W, the associated WVTR increased due to the high dissociation which was attributed to the Ar. High cage/linear ratio and low polymerization ratio were the reasons. Using the cage/linear ratio at 0.062 (60% HMDSO and 400 W) as buffer layer, the lowest WVTR of the PES substrates was significantly decreased from 50 to 0.021 g/m2/day. Fig. 10 shows the optical transmittance spectra of the one dyad encapsulation structure fabricated at 60% HMDSO and 400 W RF power. It could be seen that the films exhibited a high optical transmittance,
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Fig. 7. The structure of HMDSO molecule, linear, cage, and network.
HMDSO [%] Fig. 9. WVTR (at 40 °C, 100% RH) of one dyad encapsulation structure as a function of the amount of HMDSO in the gas mixture.
which was higher than 90% in the wavelength range from 400 to 700 nm.
4. Conclusions
Fig. 8. (a) Ratio of A(cage)/A(linear) IR density as a function of the amount of HMDSO in the gas mixture; (b) Ratio of A(Si–(CH3)x)/A(linear + network + cage) IR density as a function of the amount of HMDSO in the gas mixture.
In this work, the designed ICP source with the parallel electrodes embedded in quartz tubes was used for flexible substrate encapsulation in a single-chamber process. The process area of the ICP apparatus can be easily enlarged for larger substrate. Organic and inorganic processes are fabricated sequentially. It is convenient for multi barrier encapsulation up to several pairs. In the OES analysis, the trend of dissociation could be observed. In the FTIR analysis, the cage structure is the main effect of the organosilicon film. Polymerization ratio also indicated the organic degree of the organosilicon films. For lower cage/linear ratio and higher polymerization ratio, the better WVTR could be obtained. Low RF
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Center of Industrial Technology Research Institute, Advanced Optoelectronic Technology Center and Research Center Energy Technology and Strategy of the National Cheng Kung University. References
Fig. 10. The optical transmittance spectra of the one dyad encapsulation structure fabricated at 60% HMDSO and 400 W RF power.
power is suitable for the deposition of the organosilicon film. The best result was 0.021 g/m2/day at 400 W and 60% HMDSO. In addition, the high optical transmittance within the visible spectrum is quite suitable for the encapsulation of OLED. The thickness of one dyad encapsulation structure was only 68 nm which was measured by a cross section micrograph. The ICPCVD system can be expected as the great potential for the OLED encapsulation. Acknowledgements The authors gratefully acknowledge the support from the National Science Council of Taiwan, Laser Application Technology
[1] T.N. Chen, D.S. Wuu, C.C. Wu, C.C. Chiang, Y.P. Chen, R.H. Horng, Plasma Process. Polym. 4 (2007) 180–185. [2] J. Shim, H.G. Yoon, S.H. Na, I. Kim, S. Kwak, Surf. Coat. Technol. 202 (2008) 2844–2849. [3] G.L. Graff, R.E. Williford, P.E. Burrows, J. Appl. Phys. 96 (2004) 1840–1849. [4] J.S. Lewis, M.S. Weaver, IEEE J. Sel. Top. Quantum Electron. 10 (2004) 45–57. [5] J.S. Park, H. Chae, H.K. Chung, S.I. Lee, Semicond. Sci. Technol. 26 (2011) 034001. [6] L. Zajícková, V. Buršíková, D. Franta, A. Bousquet, A. Granier, A. Goullet, J. Buršík, Plasma Process. Polym. 4 (2007) S287–S293. [7] R. Morent, N. De Geyter, C. Leys, L. Gengembre, E. Payen, Surf. Coat. Technol. 201 (2007) 7847–7854. [8] D. Hegemann, H. Brunner, C. Oehr, Nucl. Instrum. Methods B 208 (2003) 281– 286. [9] I.A. Rusu, G. Borcia, S.O. Sayed, J.L. Sullivan. 28th ICPIG, (2007) 704–707. [10] M. Goujon, T. Belmonte, G. Henrion, Surf. Coat. Technol. 188–189 (2004) 756– 761. [11] C. Vautrin-Ul, C. Boisse-Laporte, N. Benissad, A. Chausse, P. Leprince, R. Messina, Prog. Org. Coat. 38 (2000) 9–15. [12] R. Lamendola, R. d’Agostino, F. Fracassi, Plasmas Polym. 2 (1997) 147–164. [13] A. Granier, M. Vervloet, K. Aumaille, C. Vallée, Plasma Sources Sci. Technol. 12 (2003) 89–96. [14] Y. Wang, J. Zhang, X. Shen, Mater. Chem. Phys. 96 (2006) 498–505. [15] N. Klymko, Future Fab Intl. 17 (2004). [16] S. Yasuhara, T. Sasaki, T. Shimayama, K. Tajima, H. Yano, S. Kadomura, M. Yoshimaru, N. Matsunaga, S. Samukawa, J. Phys. D: Appl. Phys. 43 (2010) 065203. [17] P. Raynaud, B. Despax, Y. Segui, H. Caquineau, Plasma Process. Polym. 2 (2005) 45–52. [18] C.Y. Kim, S.H. Kim, H.S. Kim, R. Navamathavan, C.K. Choi, J. Korean Phys. Soc. 50 (2007) 1119–1124. [19] S. Roualdes, A. Van der Lee, R. Berjoan, J. Sanchez, J. Durand, AIChE J. 45 (1999) 1566–1575.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Organosilicon function of gas barrier films purely deposited by inductively coupled plasma chemical vapor deposition system Tung-Ying Lin a,b, Ching-Ting Lee a,c,⇑ a
Institute of Nanotechnology and Microsystems Engineering, Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan, ROC Laser Application Technology Center, Industrial Technology Research Institute, Tainan 734, Taiwan, ROC c Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, Tainan 701, Taiwan, ROC b
a r t i c l e
i n f o
Article history: Received 20 April 2012 Received in revised form 30 June 2012 Accepted 10 July 2012 Available online 20 July 2012 Keywords: OLED thin film encapsulation Polymer-like organosilicon Inductively coupled plasma chemical vapor deposition
a b s t r a c t The novel design of inductively coupled plasma (ICP) source with the parallel electrodes embedded in quartz tubes was developed in this study. The advantages of the inductively coupled plasma chemical vapor deposition (ICPCVD) system were less ion-bombardment effect during the OLED encapsulation process and low cost to manufacture. The encapsulation structure of organosilicon/SiOx thin films deposited on flexible plastic substrates was purely deposited by the single chamber ICPCVD system under various hexamethyldisiloxane (HMDSO) and Ar flow ratios. To investigate the organosilicon film function, the ratio of HMDSO ambient was varied during deposition process and the associated bonding configurations were measured. With an adequate power of 400 W and HMDSO atmosphere of 60%, the polymer-like organosilicon films were obtained due to the long chain structures. Finally, the water vapor transmission rate (WVTR) of one dyad barrier decreases to the 0.021 g/m2/day due to the stress release of SiOx films caused by the polymer-like films. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Organic light-emitting diodes (OLEDs) are particularly attractive for displays since they exhibit the inherent advantages of high brightness, wide viewing angle, fast response time, low power consumption and suited to being fabricated on flexible substrates. However, flexible OLEDs require permeation barriers to prevent their degradation caused by the absorption of moisture and oxygen from the environment. Hence, transparent materials such as silicon oxide (SiOx) and silicon nitride (SiNx) were widely used as gas barrier films [1,2]. However, these inorganic oxide films suffer from the crack or peel as a result of the difference in thermal expansion coefficient between the substrate and the coated films [3]. To circumvent this problem, a multilayer barrier structure consisting of soft organic and hard inorganic films was commonly used, which benefiting in improving the water vapor barrier performance [4,5]. However, these gas barrier films were prepared in various vacuum equipments, which resulted in significant inconvenience for multilayer coating deposition. Based on the above consideration, it is required to develop a gas barrier film with high reliability and low cost deposited in a single-chamber process. In this study, single chamber thin-film encapsulation technology ⇑ Corresponding author at: Institute of Microelectronics, Department of Electrical Engineering, Department of Material Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan, ROC, Tel.: +886 6 2379582; fax: +886 6 2362303. E-mail address: [email protected] (C.-T. Lee). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.07.053
was developed using the liquid precursor hexamethyldisiloxane (HMDSO) and the novel inductively coupled plasma chemical vapor deposition (ICPCVD) system. This work focused on the investigation of the soft organic films which affected the barrier structures. Details of the deposition parameters, chemical bonding, and water vapor transmission rate of the organic and SiOx thin films were investigated and discussed in this work.
2. Experimental The structures of the ICPCVD system equipped with parallel coils embedded in quartz tubes, as shown in Fig. 1a and b, was used to deposit the organosilicon film and SiOx film on 220 lm thick polyethersulfone (PES, 5 5 cm2) substrates and 525 lm thick Si substrates, respectively. Compared to the traditional ICP coil design, the advantages of parallel coils design was to prevent the standing wave effect, low cost, and maintaining the plasma uniformity when the system was scaled up. Moreover, inductively coupled plasma has less ion-bombardment and still keeping high plasma density compared with the capacitively coupled plasma (CCP) system. Therefore, it is more suitable for using in the OLED thin film encapsulation. The ICP was excited by 13.56 MHz RF power. The plasma density was measured with Ar that was used a Langmuir probe at 10 cm above the ICP coil. Fig. 2 shows the plasma density as a function of the RF power at working pressure of 5, 30 and 40 mTorr, respectively. The plasma density increased with RF power from 400 to 1400 W. At 40 mTorr, it could reach more than 1011 /cm3 with the increasing of RF power over 1000 W. At working pressure of 5 mTorr, the plasma density reached around 1010 /cm3 due to the less process gas. The measurement of the plasma density is the guideline of process setting. Organosilicon films were deposited at a higher working pressure due to the investigation of correlation between various fraction of HMDSO and RF power. The dense SiOx films were deposited at a low working pressure [6].
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Fig. 1. (a) Quartz tube embedded type of ICP. (b) Vaporizer and gas system.
Fig. 3. The structure of one dyad encapsulation.
Fig. 2. Plasma density as a function of RF power at various working pressures.
copy was measured by plasus Emicon MC (range from 200 to 1000 nm). The optical transmittance of barrier films was measured using an UV/Vis 4001 spectrophotometer. The water vapor permeation of these samples was measured using a water vapor transmission rate measurement system (MOCON Inc., PERMATRANW 3/61) at a temperature of 40 °C with a relative humidity of 100%.
3. Experimental results and discussion
Table 1 Process setting. Setting
Organosilicon film 20% 40%
SiOx film
Ar (sccm) HMDSO (sccm) O2 (sccm) Pressure (mTorr) RF power (W)
32 24 16 8 16 24 N/A N/A N/A 40 40 40 400, 600, and 800 for each HMDSO
60%
80% 8 32 N/A 40 fraction
50 5 60 5 1000
Table 1 shows the deposition conditions of organosilicon and SiOx thin films. Pure HMDSO (99.99%) were used as the precursor and the deposition RF power was fixed at 400, 600 and 800 W, respectively. The vaporizer system of HMDSO consisted of a mass flow controller and a metal tank heated to 50 °C as shown in Fig. 1b. To control the (CH)x contents in the organosilicon films, the HMDSO/(HMDSO+Ar) flow ratio was respectively kept at 20%, 40%, 60%, 80% and the total flow rate was kept at 40 sccm. The working pressure of the ICPCVD system was kept at 40 mTorr. For the deposition of inorganic films, the oxygen gas was added to form the SiOx structure. In the deposition process, the associated RF power, the working pressure, and oxygen flow rate were 1000 W, 5 mTorr, and 60 sccm, respectively. The one dyad encapsulation structure of organosilicon (SiOxCyHz)/inorganic (SiOx)/flexible substrate was shown in Fig. 3. The chemical bonding states were examined using a Fourier transform infrared spectrometer (JASCO-4100). The surface morphologies were studied by field emission scanning electron microscopy (JEOL JSM-7001F). Optical emission spectros-
In the previous studies [7,8], different kinds of plasma, such as Ar, He, N2, could be used to treat polymer substrates. After plasma treatment, the contaminants, oligomers and amorphous layers existing on the polymer substrates would be removed [9]. Furthermore, the polymer substrates became more hydrophilic for improving their wettability and their adhesion properties. Therefore, after cleaning using the deionized water, the substrates were pre-cleaned using the pure Ar plasma and the treatment time was 1 min. Fig. 4a–c shows the SEM results of PES substrates cleaned under RF power of 100, 400, and 800 W, respectively. It could be found that the smooth surface of substrate was obtained after clean with RF power at 100 W, as shown in Fig. 4a. The associated contact angle of the PES substrate was decreased from 58° to 12°. With an increase of RF powers up to 400–800 W, the PES substrates were damaged due to an increase of sheath voltage. The sheath voltage was measured by the Langmuir probe under the Ar plasma condition. The sheath voltage of the plasma power operated at 100, 400, 600, and 800 W was 13.5, 20.8, 23.0, and 27.7 V respectively. These values were lower compared to the traditional capacitively coupled plasma system. The sheath voltage was attributed to the increase of capacitive coupling in our ICP system. In general, the higher sheath voltage would cause more ion bombardment on
T.-Y. Lin, C.-T. Lee / Journal of Alloys and Compounds 542 (2012) 11–16
13
methyl groups. With an increase of HMDSO/(HMDSO+Ar) flow ratio, the C–H and Ha emission decreased. This phenomenon was attributed to the more monomer resided in the plasma system which could mitigate the dissociation ability at the same RF power. Furthermore, the Si–H emission was resulted from the dangling bond formed by the break of Si–C bond or the Si–O–Si bond which attracts the H ions [14]. On the other hand, C2 emission was only observed obviously in the flow ratio of 20–40%. This phenomenon could be attributed to the hardness of the total dissociation of methyl groups. In the flow ratio of 20%, the highest emission of all spices were exhibited whatever RF power. The ions and electrons dissociated from Ar reacted strongly with the HMDSO. From the OES, the trend of process can be observed. The actual bonding structure need to be measured by a Fourier transform infrared spectrometer. 3.2. Fourier transform infrared spectrometer analysis
Fig. 4. SEM micrographs of PES after Ar plasma treatment of (a) 100 W, (b) 400 W, and (c) 800 W.
the polymer substrate. Therefore, the 100 W RF power pre-cleaned procedure was applied in the following experiments. 3.1. Optical emission spectroscopy analysis Fig. 5 shows the intensity of Ha, CH, SiH, and C2 emitted from the dissociation of methyl groups during the HMDSO ionized process measured by the CCD sensor of the optical emission spectroscopy (OES). The associated transition level and wavelength were listed in Table 2 [10–12]. In general, the bonds of Si–C and C–H in HMDSO monomer are easily dissociated in high plasma density environment due to the bonding energy of Si–O (8.3 eV) was higher than that of Si–C (4.6 eV) and C–H (3.5 eV). Therefore, the C–H emission and Ha emission would come from the dissociation of
The Fourier transform infrared spectrometer (FTIR) spectra of the plasma-polymerized coatings from 900 to 1350 cm 1 shown in Fig. 6 were deconvolved into six components including linear, network, cage and methyl related groups at 60% flow ratio and RF power at 400 W. The related vibration modes of FTIR absorption spectroscopy were listed in Table 3 [10–13,15]. Usually, the Si–O absorption peak could be classified into three structures. The peak at 1023 cm 1 represents the long-chain structure of the siloxane, called linear. The peak at 1132 cm 1 represents 3D Si–OC structure, called cage, and the 1072 cm 1 called network [16]. The above mentioned three type structures were shown in Fig. 7. The peak at 1263 cm 1 represents symmetrical deformation of the Si– (CH3)x bonds. Fig. 8a shows the cage/linear ratios at various flow ratios and various powers. At 400 W, the cage/linear structure ratio was decreased with the increase of HMDSO ratio. It could be found that the cage/linear structure ratio at low power (400 W) was less than that at higher power. This phenomenon could be attributed to that lower dissociation rate of Si–O bonds. The formation of the cage structure was restrained. At 800 W, the variations of cage/linear structure ratio were unapparent. This behavior was attributed to that an increasing rate of HMDSO could make up cage/linear structure ratio owing to the decrease of Ar. At 600 W, the cage/linear structure ratio increased firstly and then decreased with an increase of HMDSO ratio caused by the limitation of dissociation. The lowest (cage/linear) ratio reached to 0.062 at the 400 W and flow ratio of 60%. The linear structure contains siloxane chains which performed the polymer-like characteristics [17]. The cage structure results in the pores of the organosilicon films [18]. Therefore, the less cage/linear structure ratio contributes the organosilicon films to polymerization. From the absorption intensity of Si– (CH3)x bonds, the polymerization could also be observed. The polymerization ratio (A(Si–(CH3)x)/A(linear+network+cage)) is defined by the ratio of the intensity of the peak at 1260 cm 1 (Si–(CH3)x bonds) to the intensity of the peak 1000–1100 cm 1 (Si–O–Si bonds) [19]. In this work, the range of inorganic groups enlarged from 1000–1200 cm 1 including linear, network, and cage. Fig. 8b shows the ratio of polymerization as a function of the amount of HMDSO in the gas mixture which meant the organic trend of organosilicon films. In the power of 400 W, the polymerization ratio was increased with the increase of HMDSO ratio. The polymerization ratio in the power of 400 W was larger than that in higher power. This result consisted with the trend of CH emission as shown in Fig. 5b. The OES could detect more CH radical at a high power and a low fraction of HMDSO, but it resulted in less methyl group in the organosilicon film. In this work, cage/linear structure ratio and polymerization ratio were the degree of the polymerization. The major factor for the polymerization was the
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Fig. 5. Variations of the line intensity of (a) Ha, (b) CH, (c) SiH, and (d) C2 as a function of the amount of HMDSO in the gas mixture in organosilicon process.
Table 2 Transitions detected by OES. Species
Transition
Wavelength (nm)
Ha C2 CH SiH Ar
H(n = 3)–H(n = 2) A3Pg–X3Pu A2D–X2P A2D–X2P 4s–4p
656 516 431 414.2 750
Table 3 Vibration modes by FTIR absorption spectroscopy. Species
Vibrating mode
Wave number (cm
Si–O–Si Si–O–Si Si–O–Si Si–(CH3)x
asymmetric stretching [linear] asymmetric stretching [network-like] asymmetric stretching [cage-like] symmetrical deformation
1023 1072 1132 1263
1
)
RF power. When the RF power decreased, the monomer structure was better preserved. Therefore, the higher degree polymer-like organosilicon film could be obtained as the buffer layer. The function of the buffer layer could release the stress and prevent the crack from SiOx film. 3.3. Water vapor transmission rate and optical transmittance analyses
Fig. 6. Deconvoluted absorption peak obtained by FTIR at 400 W in organosilicon process.
Fig. 9 shows the water vapor transmission rate (WVTR) of the one dyad encapsulation structure fabricated under various conditions. The WVTR values show better result in the RF power of 400 W due to the less cage/linear ratio. The polymerization ratio also exhibited the correlation between the contained methyl group and WVTR. In the power of 600–800 W, the associated WVTR increased due to the high dissociation which was attributed to the Ar. High cage/linear ratio and low polymerization ratio were the reasons. Using the cage/linear ratio at 0.062 (60% HMDSO and 400 W) as buffer layer, the lowest WVTR of the PES substrates was significantly decreased from 50 to 0.021 g/m2/day. Fig. 10 shows the optical transmittance spectra of the one dyad encapsulation structure fabricated at 60% HMDSO and 400 W RF power. It could be seen that the films exhibited a high optical transmittance,
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Fig. 7. The structure of HMDSO molecule, linear, cage, and network.
HMDSO [%] Fig. 9. WVTR (at 40 °C, 100% RH) of one dyad encapsulation structure as a function of the amount of HMDSO in the gas mixture.
which was higher than 90% in the wavelength range from 400 to 700 nm.
4. Conclusions
Fig. 8. (a) Ratio of A(cage)/A(linear) IR density as a function of the amount of HMDSO in the gas mixture; (b) Ratio of A(Si–(CH3)x)/A(linear + network + cage) IR density as a function of the amount of HMDSO in the gas mixture.
In this work, the designed ICP source with the parallel electrodes embedded in quartz tubes was used for flexible substrate encapsulation in a single-chamber process. The process area of the ICP apparatus can be easily enlarged for larger substrate. Organic and inorganic processes are fabricated sequentially. It is convenient for multi barrier encapsulation up to several pairs. In the OES analysis, the trend of dissociation could be observed. In the FTIR analysis, the cage structure is the main effect of the organosilicon film. Polymerization ratio also indicated the organic degree of the organosilicon films. For lower cage/linear ratio and higher polymerization ratio, the better WVTR could be obtained. Low RF
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Center of Industrial Technology Research Institute, Advanced Optoelectronic Technology Center and Research Center Energy Technology and Strategy of the National Cheng Kung University. References
Fig. 10. The optical transmittance spectra of the one dyad encapsulation structure fabricated at 60% HMDSO and 400 W RF power.
power is suitable for the deposition of the organosilicon film. The best result was 0.021 g/m2/day at 400 W and 60% HMDSO. In addition, the high optical transmittance within the visible spectrum is quite suitable for the encapsulation of OLED. The thickness of one dyad encapsulation structure was only 68 nm which was measured by a cross section micrograph. The ICPCVD system can be expected as the great potential for the OLED encapsulation. Acknowledgements The authors gratefully acknowledge the support from the National Science Council of Taiwan, Laser Application Technology
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