- Email: [email protected]
Plastic substrate with gas barrier layer and transparent conductive oxide thin film for flexible displays
Thin Solid Films 518 (2010) 3089–3092
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Plastic substrate with gas barrier layer and transparent conductive oxide thin film for flexible displays Toru Hanada ⁎, Tuyoto Negishi, Isao Shiroishi, Takashi Shiro Electronics Material Research Laboratories, Teijin Ltd., 4-3-2 Asahigaoka, Hino, Tokyo 191, Japan
a r t i c l e
i n f o
Available online 13 October 2009 Keywords: Plastic substrate Flexible displays Transparent conductive oxide Gas barrier Sputtering
a b s t r a c t A novel plastic substrate for flexible displays was developed. The substrate consisted of a polycarbonate (PC) base film coated with a gas barrier layer and a transparent conductive thin film. PC with ultra-low intrinsic birefringence and high temperature dimensional stability was developed for the base film. The retardation of the PC base film was less than 1 nm at a wavelength of 550 nm (film thickness, 120 µm). Even at 180 °C, the elastic modulus was 2 GPa, and thermal shrinkage was less than 0.01%. The surface roughness of the PC base film was less than 0.5 nm. A silicon oxide (SiOx) gas barrier layer was deposited on the PC base film by a roll-to-roll DC magnetron reactive sputtering method. The water vapor transmission rate of the SiOx film was less than 0.05 g/ m2/day at 40 °C and 100% relative humidity (RH), and the permeation of oxygen was less than 0.5 cc/m2 day atm at 40 °C and 90% RH. As the transparent conductive thin film, amorphous indium zinc oxide was deposited on the SiOx by sputtering. The transmittance was 87% and the resistivity was 3.5 × 10− 4 ohm cm. © 2009 Elsevier B.V. All rights reserved.
1. Introduction There has been growing interest in the use of plastic substrates in the fabrication of future electronic devices such as flexible displays [1–3], photovoltaics, batteries, and sensors. These flexible devices have a high potential to realize thin, lightweight, robust, and bendable electronic products. However, conventional plastic substrates made from commercial polymers are not sufficient to satisfy the demands of display designers; further improvements in birefringence, heat resistance, and gas barrier performance are especially demanded. In the manufacture of liquid crystal displays (LCDs) by using plastic substrates, the retardation value, which is the product of the substrates' birefringence and thickness, should be less than 10 nm to avoid interference colors. In addition, they should be capable of withstanding temperatures of at least 150 °C, and high gas barrier performance against water vapour (≤1 g/m2/day at 40 °C and 90% RH) and oxygen (≤1 cc/m2/day at 40 °C and 90% RH) is demanded to ensure endurance and reliability of the final products. In this study, a high-temperature PC substrate with a gas barrier layer and a transparent conductive oxide thin film was developed to improve the properties that are especially important for flat panel display devices. A cross-sectional view of the substrate is illustrated in Fig. 1. The substrate was composed of a PC base film (120 µm thick), a silicon oxide gas barrier layer (30 nm thick), a hard coating (2 µm thick), and
⁎ Corresponding author. Tel.: + 81 42 586 8164; fax: + 81 42 587 5510. E-mail address: [email protected] (T. Hanada). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.09.166
an indium zinc oxide transparent conductive layer (130 nm thick). The main properties of the substrate are shown in Table 1. Remarkable improvements were achieved in some properties, for instance: the glass transition temperature (Tg) of the base film was 215 °C, the retardation was 1 nm, the water vapor permeation rate was 0.05 g/ m2/day, thermal shrinkage was 0.01% after heating at 180 °C for 2 h, and Young's modulus was 2.8 GPa.
2. Experimental details The high-temperature PC base film was produced by the solvent casting method from dichloromethane solution. As a gas barrier layer, silicon oxide (SiOx) was deposited on the base film by DC magnetron reactive sputtering using a silicon target. The sputtering chamber was evacuated to 2 × 10− 4 Pa. The substrate temperature was set at 20 °C. Sputtering was carried out under Ar ± O2 mixed gas. The total gas pressure was 0.1 Pa at a power density of 4 W/cm2. A hard coating composed of siloxane-containing material was applied on the SiOx surface of the substrate, and an acrylic-type hard coating was applied on the surface opposite the SiOx layer by using a conventional gravure coating method. A transparent conductive film was deposited on the acrylic-type hard coating by DC magnetron sputtering using either In2O3–ZnO (Zn: 10 wt.%; IZO) or In2O3–SnO2 (Sn: 5 wt.%; ITO). The sputtering chamber was evacuated to 9 × 10− 5 Pa. The substrate temperature was set at 20 °C. Sputtering was carried out under Ar or Ar ± O2 mixed gas. The total gas pressure was 0.5 Pa at a power density of 4 W/cm2. The thicknesses of the IZO and ITO films were adjusted to be about 130 nm by controlling the deposition time.
3090
T. Hanada et al. / Thin Solid Films 518 (2010) 3089–3092
Fig. 1. Cross-sectional view of the plastic substrate developed for flexible displays.
The glass transition temperature Tg and Young's modulus of the PC base film were measured by differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA), respectively. The total light transmittance of the substrate was measured by a colorimeter (COH 400, Nippon Denshoku Industries), and the retardation was estimated by spectroscopic ellipsometry. The film thicknesses of the SiOx and transparent conductive oxides were measured using a surface profiler (Dektak3, Sloan Tech). To evaluate the gas-barrier properties of the substrate, the water vapor permeability at 40 °C and 100% RH and the oxygen permeability at 40 °C and 90% RH were measured with permeation instruments (OX-TRAN and PERMATRAN, mocon®). The surface roughness of the substrate was analyzed by using tapping mode atomic force microscopy (AFM). The crystallization temperature of the transparent conductive oxides was estimated by DSC measurement of isolated thin films flaked from the substrate. 3. Results and discussion 3.1. Ultra-low birefringence Large birefringence of the PC base film is undesirable when the plastic substrates are used for LCDs. As the birefringence becomes larger, the display panel tends to have strong interference colors. The birefringence of a polymer film is expressed as Δn0 = Δn0f, where Δn0 is the intrinsic birefringence of the polymer, and f is the orientation function. According to this equation, Δn0 and f must be reduced to decrease the birefringence. The base films of conventional substrates are typically made from bis-A type polycarbonate (bis-A PC). This polymer is obtained by reacting 2,2-bis (4-hydroxyphenyl) propane and phosgene in the presence of pyridine. In this polymer, the segment formed from 2,2-bis (4-hydroxyphenyl) propane increases the intrinsic birefringence because the polarizability of the segment parallel to the axis of the polymer chain is larger. In order to reduce the intrinsic birefringence, we designed a new polycarbonate, called “high-temperature PC”, for the substrate. A solvent casting process is suitable for reducing the function f. This process yields a highly
Fig. 2. Retardation of high-temperature PC base film (solid line) and conventional bis-A PC base film (broken line) versus wavelength (thickness 0.12 mm).
amorphous, optically clear, and colorless film. The retardation (Re = Δn · d, where d is the substrate thickness) of the hightemperature PC base film was less than 1 nm, which is one order of magnitude smaller than that of bis-A PC (Fig. 2). This property is ideal for LCDs. The high-temperature PC base film had not only ultra-low birefringence but also high transmittance of visible light, at more than 90%. 3.2. Improved heat resistance The most promising way to improve the thermal stability of polymers is to introduce highly stable structural units into the polymer systems, such as aromatic and/or heterocyclic rings. Our new high-temperature PC was designed not only to reduce the birefringence but also to improve thermal stability. Fig. 3 shows the dynamic shear modulus E′ and the mechanical damping factor tan δ of the high-temperature PC and conventional bis-A PC versus temperature. The high-temperature PC displayed favorable thermal stability and mechanical properties over a temperature range from 30 to 200 °C. In general, when a plastic substrate is exposed to temperatures close to the Tg of the polymer base film, a relaxation process in the polymer makes the substrate shrink, which can lead to structural distortion, cracks, and film peeling during the process. However, the thermal shrinkage of the high-temperature PC base film was less than 0.01%
Table 1 Properties of the plastic substrate coated with indium zinc oxide (IZO) transparent conductive layer. Properties
Characteristics (remarks)
Thickness Glass transition temperature Surface resistance Light transmittance Retardation O2 permeation rate H2O permeation rate Thermal shrinkage Young's modulus Tensile Elongation
125 µm 215 °C (basefilm) 30 ohm/sq. 87% (91% w/o IZO) 1 nm 0.5 cc/m2/day at 40 °C/90%RH 0.05 g/m2/day at 40 °C/100%RH 0.01% at 180 °C 2 h 2.8 GPa 13%
Fig. 3. The shear modulus E′ and damping factor tan δ versus temperature of hightemperature PC (solid line) and conventional bis-A PC (broken line). The oscillating frequency in the thermo-mechanical analysis was 1 Hz over the entire temperature range from 30 to 250 °C.
T. Hanada et al. / Thin Solid Films 518 (2010) 3089–3092 Table 2 Water vapor permeation rate (WVPR) for 40 nm silicon oxide (SiOx) layers deposited on high-temperature PC base film and biaxially oriented polyester (PET) base film.
Table 4 Comparison of properties of IZO and ITO deposited on polycarbonate. Target material
WVTR (g/m2/day)
Sample Material
Thickness
With SiOx
Bare
High-temperature PC PET
0.12 mm 0.125 mm
0.05 0.5
35 5
3091
Resistivity (Ω cm) Total transmittance (%) Crystallization temperature (°C)
IZO
ITO
(ZnO: 10 wt.%)
(SnO2: 5 wt.%)
3.5 × 10− 4 87 404
4.7 × 10− 4 87 130
3.3. Gas barrier improvement Table 3 Oxygen permeation rate (O2PR) for 40 nm silicon oxide (SiOx) layers deposited on hightemperature PC base film and biaxially oriented polyester (PET) base film. O2TR(cc/m2/day/atm)
Base film Material
Thickness
With SiOx
Bare
High-temperature PC PET
0.12 mm 0.125 mm
0.8 0.7
500 17
after heating at 180 °C for 2 h. Furthermore, once the substrate was heated under these conditions, the thermal shrinkage of the substrate was negligible below 180 °C.
Fig. 4. The surface roughness of the high-temperature PC base-film (top) compared with the plane surface of a biaxially oriented polyester (PET) base film (bottom). The surface roughness was measured by tapping mode AFM.
The permeability of polymer films is not negligible, in contrast to glass substrates of flat panel displays, which are non-permeable. The poor gas barrier properties of the films lead to problems such as deterioration of the liquid crystal material due to permeation of oxygen, and an increase in power consumption of the display device due to permeation of moisture. A solution to these problems is to apply diffusion barriers to the films. Optically transparent, dense inorganic layers are most appropriate for this function. In this study, a silicon oxide (SiOx) layer was deposited on the plastic films. The water vapor and oxygen permeation through 40 nm-thick silicon oxide layers deposited on a 0.12 mm-thick high-temperature PC base film and a 0.125 mmthick PET base film (Teijin DuPont Films, Tetoron® biaxially oriented polyester film “HSL”) were measured (Tables 2 and 3). The water vapor permeation rate (WVPR) for the high-temperature PC sample at 40 °C
Fig. 5. The root-mean-square surface roughness (Rrms) of the IZO film deposited on the substrate, compared with the surface before the IZO deposition. The surface roughness of a 20 µm × 20 µm square was measured by tapping mode AFM.
3092
T. Hanada et al. / Thin Solid Films 518 (2010) 3089–3092
and 100% RH was 0.05 g/m2/day, and the oxygen permeation rate (O2PR) at 40 °C and 90% RH was 0.5 cc/m2/day/atm. By depositing the SiOx layer on the high-temperature PC base film, the WVPR was reduced by three orders of magnitude and the O2PR was reduced by four orders of magnitude. The O2PR for the PET sample was equivalent to the value for the high-temperature PC sample, and the WVPR was ten times larger than that for the high-temperature PC sample. Although the reduction of permeation by coating the SiOx layer on the PET base film was only about one order of magnitude, the permeation rates for the PET base film are smaller than the permeation rates for the high-temperature PC base film. Generally, the reduction in permeation due to a thin layer is limited by transport through coating defects such as pinholes, grain boundaries, or micro cracks [4-6]. The coating defects in vacuum-deposited thin layers result from geometric shadowing during growth of the layer due to the surface roughness of the base film surface. Fig. 4 shows a surface roughness comparison between the high-temperature PC and PET base films. The root-mean-square (RMS) surface roughness, Rrms, of the hightemperature PC film was less than 0.5 nm, as measured by tapping mode AFM. On the other hand, Rrms of the PET base film was over 50 times larger. As a result, we concluded that the extremely smooth surface of the high-temperature PC base film plays an important role in improving the gas barrier properties when the silicon oxide is deposited on it. 3.4. Transparent conductive oxide thin film suitable for plastic substrate Indium tin oxide (ITO) is the most widely used transparent conductor in the display industry. There are few other materials that have such an optimal combination of transparency and conductivity. Crystallization of ITO thin films is one of the most important factors to achieve low resistance and high transparency, and high-temperature heat treatment is essential to increase the crystallinity. However, plastic substrates have low heat tolerance, and the substrate temperature during ITO deposition is thus restricted. Crystallization by post annealing is an effective procedure for improving the ITO performance for plastic substrates. However, as the crystallinity increases, the compressive stress in the ITO grows and causes substrate bending.
Amorphous zinc-doped indium oxide is one promising material for avoiding this bending problem. Table 4 shows a comparison of the properties of IZO and ITO deposited on polycarbonate by DC magnetron sputtering. The IZO film crystallized at 404 °C, whereas the ITO crystallized at 130 °C. The RMS surface roughness of the IZO was less than 2 nm (Fig. 5), and no grains were observed. Furthermore, the resistivity of the IZO as deposited was 3.5 × 10− 4 Ω cm, which is superior to that of ITO (4.7× 10− 4 Ω cm). The total transmittances of both the IZO and ITO were almost the same at 87%.
4. Summary A new plastic substrate suitable for flexible displays was developed. High-temperature polycarbonate obtained by a solvent casting process was used as a base film of the substrate. The base film produced had high optical transmittance, low retardation (1 nm), good elastic and dimensional stability, resistance to high heat treatment, and an extremely smooth surface (Rrms = 0.5 nm). Excellent gas barrier performance against moisture (WVPR = 0.05 g/m2/day) and oxygen (O2PR = 0.8 cc/m2/day) was achieved by coating a 40 nm-thick silicon oxide layer on the smooth surface of the 0.12 mm-thick hightemperature PC base film. An amorphous IZO thin film is suitable for depositing on the plastic substrate from the viewpoint of transparency, resistivity, and especially the avoidance of bending stress. This new plastic substrate shows promise in overcoming the obstacles in producing many kinds of thin, lightweight, robust, and flexible displays, such as LCDs, OLEDs, and electrophoresis displays.
References [1] [2] [3] [4] [5] [6]
F. Matumoto, T. Nagata, T. Miyabori, H. Tanaka, S. Tsushima, Proc. SID'93 965 (1993). X.-Y. Huang, A. Khan, N. Miller, B. Wall, J.W. Doane, Proc. IDW'01 465 (2001). R. Baeuerle, J. Baumbach, E. Lueder, J. Siegordner, Proc. SID'99 14 (1999). H. Chatham, Surf. Coat. Technol. 78 (1996) 1. W. Prins, J.J. Hermans, J. Phys. Chem. 63 (1959) 716. G. Rossi, M. Nulman, J. Appl. Physi. 74 (1993) 5471.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Plastic substrate with gas barrier layer and transparent conductive oxide thin film for flexible displays Toru Hanada ⁎, Tuyoto Negishi, Isao Shiroishi, Takashi Shiro Electronics Material Research Laboratories, Teijin Ltd., 4-3-2 Asahigaoka, Hino, Tokyo 191, Japan
a r t i c l e
i n f o
Available online 13 October 2009 Keywords: Plastic substrate Flexible displays Transparent conductive oxide Gas barrier Sputtering
a b s t r a c t A novel plastic substrate for flexible displays was developed. The substrate consisted of a polycarbonate (PC) base film coated with a gas barrier layer and a transparent conductive thin film. PC with ultra-low intrinsic birefringence and high temperature dimensional stability was developed for the base film. The retardation of the PC base film was less than 1 nm at a wavelength of 550 nm (film thickness, 120 µm). Even at 180 °C, the elastic modulus was 2 GPa, and thermal shrinkage was less than 0.01%. The surface roughness of the PC base film was less than 0.5 nm. A silicon oxide (SiOx) gas barrier layer was deposited on the PC base film by a roll-to-roll DC magnetron reactive sputtering method. The water vapor transmission rate of the SiOx film was less than 0.05 g/ m2/day at 40 °C and 100% relative humidity (RH), and the permeation of oxygen was less than 0.5 cc/m2 day atm at 40 °C and 90% RH. As the transparent conductive thin film, amorphous indium zinc oxide was deposited on the SiOx by sputtering. The transmittance was 87% and the resistivity was 3.5 × 10− 4 ohm cm. © 2009 Elsevier B.V. All rights reserved.
1. Introduction There has been growing interest in the use of plastic substrates in the fabrication of future electronic devices such as flexible displays [1–3], photovoltaics, batteries, and sensors. These flexible devices have a high potential to realize thin, lightweight, robust, and bendable electronic products. However, conventional plastic substrates made from commercial polymers are not sufficient to satisfy the demands of display designers; further improvements in birefringence, heat resistance, and gas barrier performance are especially demanded. In the manufacture of liquid crystal displays (LCDs) by using plastic substrates, the retardation value, which is the product of the substrates' birefringence and thickness, should be less than 10 nm to avoid interference colors. In addition, they should be capable of withstanding temperatures of at least 150 °C, and high gas barrier performance against water vapour (≤1 g/m2/day at 40 °C and 90% RH) and oxygen (≤1 cc/m2/day at 40 °C and 90% RH) is demanded to ensure endurance and reliability of the final products. In this study, a high-temperature PC substrate with a gas barrier layer and a transparent conductive oxide thin film was developed to improve the properties that are especially important for flat panel display devices. A cross-sectional view of the substrate is illustrated in Fig. 1. The substrate was composed of a PC base film (120 µm thick), a silicon oxide gas barrier layer (30 nm thick), a hard coating (2 µm thick), and
⁎ Corresponding author. Tel.: + 81 42 586 8164; fax: + 81 42 587 5510. E-mail address: [email protected] (T. Hanada). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.09.166
an indium zinc oxide transparent conductive layer (130 nm thick). The main properties of the substrate are shown in Table 1. Remarkable improvements were achieved in some properties, for instance: the glass transition temperature (Tg) of the base film was 215 °C, the retardation was 1 nm, the water vapor permeation rate was 0.05 g/ m2/day, thermal shrinkage was 0.01% after heating at 180 °C for 2 h, and Young's modulus was 2.8 GPa.
2. Experimental details The high-temperature PC base film was produced by the solvent casting method from dichloromethane solution. As a gas barrier layer, silicon oxide (SiOx) was deposited on the base film by DC magnetron reactive sputtering using a silicon target. The sputtering chamber was evacuated to 2 × 10− 4 Pa. The substrate temperature was set at 20 °C. Sputtering was carried out under Ar ± O2 mixed gas. The total gas pressure was 0.1 Pa at a power density of 4 W/cm2. A hard coating composed of siloxane-containing material was applied on the SiOx surface of the substrate, and an acrylic-type hard coating was applied on the surface opposite the SiOx layer by using a conventional gravure coating method. A transparent conductive film was deposited on the acrylic-type hard coating by DC magnetron sputtering using either In2O3–ZnO (Zn: 10 wt.%; IZO) or In2O3–SnO2 (Sn: 5 wt.%; ITO). The sputtering chamber was evacuated to 9 × 10− 5 Pa. The substrate temperature was set at 20 °C. Sputtering was carried out under Ar or Ar ± O2 mixed gas. The total gas pressure was 0.5 Pa at a power density of 4 W/cm2. The thicknesses of the IZO and ITO films were adjusted to be about 130 nm by controlling the deposition time.
3090
T. Hanada et al. / Thin Solid Films 518 (2010) 3089–3092
Fig. 1. Cross-sectional view of the plastic substrate developed for flexible displays.
The glass transition temperature Tg and Young's modulus of the PC base film were measured by differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA), respectively. The total light transmittance of the substrate was measured by a colorimeter (COH 400, Nippon Denshoku Industries), and the retardation was estimated by spectroscopic ellipsometry. The film thicknesses of the SiOx and transparent conductive oxides were measured using a surface profiler (Dektak3, Sloan Tech). To evaluate the gas-barrier properties of the substrate, the water vapor permeability at 40 °C and 100% RH and the oxygen permeability at 40 °C and 90% RH were measured with permeation instruments (OX-TRAN and PERMATRAN, mocon®). The surface roughness of the substrate was analyzed by using tapping mode atomic force microscopy (AFM). The crystallization temperature of the transparent conductive oxides was estimated by DSC measurement of isolated thin films flaked from the substrate. 3. Results and discussion 3.1. Ultra-low birefringence Large birefringence of the PC base film is undesirable when the plastic substrates are used for LCDs. As the birefringence becomes larger, the display panel tends to have strong interference colors. The birefringence of a polymer film is expressed as Δn0 = Δn0f, where Δn0 is the intrinsic birefringence of the polymer, and f is the orientation function. According to this equation, Δn0 and f must be reduced to decrease the birefringence. The base films of conventional substrates are typically made from bis-A type polycarbonate (bis-A PC). This polymer is obtained by reacting 2,2-bis (4-hydroxyphenyl) propane and phosgene in the presence of pyridine. In this polymer, the segment formed from 2,2-bis (4-hydroxyphenyl) propane increases the intrinsic birefringence because the polarizability of the segment parallel to the axis of the polymer chain is larger. In order to reduce the intrinsic birefringence, we designed a new polycarbonate, called “high-temperature PC”, for the substrate. A solvent casting process is suitable for reducing the function f. This process yields a highly
Fig. 2. Retardation of high-temperature PC base film (solid line) and conventional bis-A PC base film (broken line) versus wavelength (thickness 0.12 mm).
amorphous, optically clear, and colorless film. The retardation (Re = Δn · d, where d is the substrate thickness) of the hightemperature PC base film was less than 1 nm, which is one order of magnitude smaller than that of bis-A PC (Fig. 2). This property is ideal for LCDs. The high-temperature PC base film had not only ultra-low birefringence but also high transmittance of visible light, at more than 90%. 3.2. Improved heat resistance The most promising way to improve the thermal stability of polymers is to introduce highly stable structural units into the polymer systems, such as aromatic and/or heterocyclic rings. Our new high-temperature PC was designed not only to reduce the birefringence but also to improve thermal stability. Fig. 3 shows the dynamic shear modulus E′ and the mechanical damping factor tan δ of the high-temperature PC and conventional bis-A PC versus temperature. The high-temperature PC displayed favorable thermal stability and mechanical properties over a temperature range from 30 to 200 °C. In general, when a plastic substrate is exposed to temperatures close to the Tg of the polymer base film, a relaxation process in the polymer makes the substrate shrink, which can lead to structural distortion, cracks, and film peeling during the process. However, the thermal shrinkage of the high-temperature PC base film was less than 0.01%
Table 1 Properties of the plastic substrate coated with indium zinc oxide (IZO) transparent conductive layer. Properties
Characteristics (remarks)
Thickness Glass transition temperature Surface resistance Light transmittance Retardation O2 permeation rate H2O permeation rate Thermal shrinkage Young's modulus Tensile Elongation
125 µm 215 °C (basefilm) 30 ohm/sq. 87% (91% w/o IZO) 1 nm 0.5 cc/m2/day at 40 °C/90%RH 0.05 g/m2/day at 40 °C/100%RH 0.01% at 180 °C 2 h 2.8 GPa 13%
Fig. 3. The shear modulus E′ and damping factor tan δ versus temperature of hightemperature PC (solid line) and conventional bis-A PC (broken line). The oscillating frequency in the thermo-mechanical analysis was 1 Hz over the entire temperature range from 30 to 250 °C.
T. Hanada et al. / Thin Solid Films 518 (2010) 3089–3092 Table 2 Water vapor permeation rate (WVPR) for 40 nm silicon oxide (SiOx) layers deposited on high-temperature PC base film and biaxially oriented polyester (PET) base film.
Table 4 Comparison of properties of IZO and ITO deposited on polycarbonate. Target material
WVTR (g/m2/day)
Sample Material
Thickness
With SiOx
Bare
High-temperature PC PET
0.12 mm 0.125 mm
0.05 0.5
35 5
3091
Resistivity (Ω cm) Total transmittance (%) Crystallization temperature (°C)
IZO
ITO
(ZnO: 10 wt.%)
(SnO2: 5 wt.%)
3.5 × 10− 4 87 404
4.7 × 10− 4 87 130
3.3. Gas barrier improvement Table 3 Oxygen permeation rate (O2PR) for 40 nm silicon oxide (SiOx) layers deposited on hightemperature PC base film and biaxially oriented polyester (PET) base film. O2TR(cc/m2/day/atm)
Base film Material
Thickness
With SiOx
Bare
High-temperature PC PET
0.12 mm 0.125 mm
0.8 0.7
500 17
after heating at 180 °C for 2 h. Furthermore, once the substrate was heated under these conditions, the thermal shrinkage of the substrate was negligible below 180 °C.
Fig. 4. The surface roughness of the high-temperature PC base-film (top) compared with the plane surface of a biaxially oriented polyester (PET) base film (bottom). The surface roughness was measured by tapping mode AFM.
The permeability of polymer films is not negligible, in contrast to glass substrates of flat panel displays, which are non-permeable. The poor gas barrier properties of the films lead to problems such as deterioration of the liquid crystal material due to permeation of oxygen, and an increase in power consumption of the display device due to permeation of moisture. A solution to these problems is to apply diffusion barriers to the films. Optically transparent, dense inorganic layers are most appropriate for this function. In this study, a silicon oxide (SiOx) layer was deposited on the plastic films. The water vapor and oxygen permeation through 40 nm-thick silicon oxide layers deposited on a 0.12 mm-thick high-temperature PC base film and a 0.125 mmthick PET base film (Teijin DuPont Films, Tetoron® biaxially oriented polyester film “HSL”) were measured (Tables 2 and 3). The water vapor permeation rate (WVPR) for the high-temperature PC sample at 40 °C
Fig. 5. The root-mean-square surface roughness (Rrms) of the IZO film deposited on the substrate, compared with the surface before the IZO deposition. The surface roughness of a 20 µm × 20 µm square was measured by tapping mode AFM.
3092
T. Hanada et al. / Thin Solid Films 518 (2010) 3089–3092
and 100% RH was 0.05 g/m2/day, and the oxygen permeation rate (O2PR) at 40 °C and 90% RH was 0.5 cc/m2/day/atm. By depositing the SiOx layer on the high-temperature PC base film, the WVPR was reduced by three orders of magnitude and the O2PR was reduced by four orders of magnitude. The O2PR for the PET sample was equivalent to the value for the high-temperature PC sample, and the WVPR was ten times larger than that for the high-temperature PC sample. Although the reduction of permeation by coating the SiOx layer on the PET base film was only about one order of magnitude, the permeation rates for the PET base film are smaller than the permeation rates for the high-temperature PC base film. Generally, the reduction in permeation due to a thin layer is limited by transport through coating defects such as pinholes, grain boundaries, or micro cracks [4-6]. The coating defects in vacuum-deposited thin layers result from geometric shadowing during growth of the layer due to the surface roughness of the base film surface. Fig. 4 shows a surface roughness comparison between the high-temperature PC and PET base films. The root-mean-square (RMS) surface roughness, Rrms, of the hightemperature PC film was less than 0.5 nm, as measured by tapping mode AFM. On the other hand, Rrms of the PET base film was over 50 times larger. As a result, we concluded that the extremely smooth surface of the high-temperature PC base film plays an important role in improving the gas barrier properties when the silicon oxide is deposited on it. 3.4. Transparent conductive oxide thin film suitable for plastic substrate Indium tin oxide (ITO) is the most widely used transparent conductor in the display industry. There are few other materials that have such an optimal combination of transparency and conductivity. Crystallization of ITO thin films is one of the most important factors to achieve low resistance and high transparency, and high-temperature heat treatment is essential to increase the crystallinity. However, plastic substrates have low heat tolerance, and the substrate temperature during ITO deposition is thus restricted. Crystallization by post annealing is an effective procedure for improving the ITO performance for plastic substrates. However, as the crystallinity increases, the compressive stress in the ITO grows and causes substrate bending.
Amorphous zinc-doped indium oxide is one promising material for avoiding this bending problem. Table 4 shows a comparison of the properties of IZO and ITO deposited on polycarbonate by DC magnetron sputtering. The IZO film crystallized at 404 °C, whereas the ITO crystallized at 130 °C. The RMS surface roughness of the IZO was less than 2 nm (Fig. 5), and no grains were observed. Furthermore, the resistivity of the IZO as deposited was 3.5 × 10− 4 Ω cm, which is superior to that of ITO (4.7× 10− 4 Ω cm). The total transmittances of both the IZO and ITO were almost the same at 87%.
4. Summary A new plastic substrate suitable for flexible displays was developed. High-temperature polycarbonate obtained by a solvent casting process was used as a base film of the substrate. The base film produced had high optical transmittance, low retardation (1 nm), good elastic and dimensional stability, resistance to high heat treatment, and an extremely smooth surface (Rrms = 0.5 nm). Excellent gas barrier performance against moisture (WVPR = 0.05 g/m2/day) and oxygen (O2PR = 0.8 cc/m2/day) was achieved by coating a 40 nm-thick silicon oxide layer on the smooth surface of the 0.12 mm-thick hightemperature PC base film. An amorphous IZO thin film is suitable for depositing on the plastic substrate from the viewpoint of transparency, resistivity, and especially the avoidance of bending stress. This new plastic substrate shows promise in overcoming the obstacles in producing many kinds of thin, lightweight, robust, and flexible displays, such as LCDs, OLEDs, and electrophoresis displays.
References [1] [2] [3] [4] [5] [6]
F. Matumoto, T. Nagata, T. Miyabori, H. Tanaka, S. Tsushima, Proc. SID'93 965 (1993). X.-Y. Huang, A. Khan, N. Miller, B. Wall, J.W. Doane, Proc. IDW'01 465 (2001). R. Baeuerle, J. Baumbach, E. Lueder, J. Siegordner, Proc. SID'99 14 (1999). H. Chatham, Surf. Coat. Technol. 78 (1996) 1. W. Prins, J.J. Hermans, J. Phys. Chem. 63 (1959) 716. G. Rossi, M. Nulman, J. Appl. Physi. 74 (1993) 5471.