Heat-Resistant Properties of a SiO 2 -Coated PET Film Prepared by Irradiating a Polysilazane-Coated Film with Excimer Light

Flexible electronics have been recently paid much attention. A flexible substrate (Organic resin film) is indispensable component for flexible devices. Though PET film is low-cost organic film, low heat-resistance of PET film limits its application as a flexible device substrate. We have developed heat-resistant PET which does not deteriorate even at 190˚C heat treatment for one hour. An excimer light was irradiated onto a polysilazane (PHPS: perhydropolysilane)-coated film to form a dense silicon-dioxide (SiO 2 ) layer on a PET film, and the heat-resistance property of the formed film was examined. Changes of surface state and cross-sectional structure of the formed film due to heat treatment were investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM). Compared to normal PET, which is deteriorated and whitened by heat treatment of about 110˚C 120˚C, the SiO 2 -coated PET film maintains transparency and does not deteriorate after heat treatment at 180˚C - 190˚C for one hour. This high heat resistance is due to a dense SiO 2 film formed on the surface that prevents surface precipitation and crystallization of low-molecular-weight oligomers (which are the cause of thermal degradation of PET). It is expected that enhancing the heat resistance of PET—which has high versatility and low cost—to about 180˚C to 190˚C will allow SiO 2 -film-coated PET to be developed as a film substrate for flexible devices.

T. Ohishi [6]. A flexible substrate is an indispensable component for producing a flexible device, and an organic polymer film is usually used for that substrate. When various devices are fabricated on an organic polymer film, heat treatment-for processes like forming films on elemental devices and curing the film-is required. For that reason, the organic polymer film must have sufficient heat resistance. Accordingly, organic films with relatively good heat-resistance properties, such as polyimide (PI), polyethylene naphthalate (PEN), and polyether sulfone (PES), are used [7]- [11]. Known as "functional organic polymer films", these organic films have the drawback that their cost is high. And the cost of the film is reflected in the final product. Although it may be possible to use PET as a general-purpose, low-cost organic film, the low heat resistance of PET limits its application as a flexible device substrate. For that reason, practical flexible devices using a PET film are few and far between.
We have developed a technology for forming a dense silica film at low temperature by irradiating a polysilazane-coated film with ultraviolet or excimer light. By utilizing the technology, we have succeeded in improving the gas-barrier properties of the organic polymer film [12] [13] [14]. It is expected that forming a dense silica film in this way will prevent surface deposition of low-molecular weight oligomers, which cause thermal deterioration of the PET film, and thereby improve the heat resistance of PET. PET films are known to contain cyclic oligomers with low molecular weight as natural impurities that are by-products generated during polymerization. Cyclic oligomers are precipitated on the PET surface after heat treatment, resulting in degradation of the PET such as reducing of transparency and flexibility [15] [16]. In the present study, the silica thin film was formed by irradiating an excimer light onto a polysilazane (PHPS: perhydropolysilane)-coated film formed on PET, and the heat-resistance characteristics of the formed PHPS-coated PET film were investigated and compared with those of a normal PET film. According to the results of the investigation, thermal degradation of normal PET starts around 110˚C -120˚C, whereas the PET developed in this study has higher heat resistance-with almost no deterioration-even after heat treatment at 180˚C -190˚C.  The prepared thin film was heat treated at temperatures of 90˚C, 120˚C, 150˚C, 180˚C, 190˚C, and 210˚C for one hour in a heat-treatment furnace. Change in transmittance of the thin film was measured by a visible-UV spectrophotometer (Shimadzu UV 2450). The normal PET film was similarly heat treated, and the two films (silica-coated PET and normal PET) were compared.

Observation of Surface and Cross Section of Thin Film
The surface state of the excimer-light-treated film was observed by optical microscope (Kyowa Riken ME-LUX2), laser microscope (Olympus LEXT OLS4000), scanning electron microscope (Nihon Electronics JSM-7610F), and atomic-force microscope (Hitachi AFM-5100N). The cross section of the thin film (as a focused ion beam (FIB)-processed sample) was observed by transmission electron microscope (Hitachi H-9500).

Formation and Cross-Sectional Structure of Dense SiO2 Film Formed on PET by Excimer-Light Irradiation of PHPS Film
As an inorganic polymer with Si-N bonds, PHPS is soluble in a solvent.  In order to investigate heat resistance of the PET, change of surface condition and transmittance by varying heat-treatment temperature was examined by optical microscopy and absorption spectroscopy.

Change of Surface Condition and Transmittance by Varying Heat-Treatment Temperature
Optical micrographs of the change in the surface state due to the heat treatment (at each previously stated temperature) of the PET-only film and the PET coated with a SiO 2 film derived from PHPS are shown in Figure 2. As for the PET-only film, it was heat treated at 90˚C to 210˚C for one hour. No significant change in the surface state was observed up to 120˚C; however, the surface started becoming cloudy at 130˚C, and it became cloudy, and then it became significantly hazy at 180˚C or higher. In contrast, as for the thin-film-coated PET film, its transparency was maintained from 150˚C to 190˚C, and it only started to become hazy at 210˚C. However, the amount of haze due to heat treatment at 210˚C was significantly reduced compared to that in the case of the PET-only film.
Changes in the transmittance spectra for the films heat treated at each temperature are shown in Figure 3. As shown in Figure 3  greatly reduced after heating at 150˚C, and after heating at 180˚C to 210˚C, it was about 30%. This change in transmittance corresponds to the degree of surface hazing at each heat-treatment temperature.
On the contrary, as shown in Figure 3(b), the PET coated with a PHPS-derived   Figure 4 and Figure 5. The surface of the PET only is very smooth and uniform (Figure 4(a) "Before"); however, after it was heat treated at 150˚C, particles were deposited on the surface. As the processing temperature increased (to 180˚C and 210˚C), the size of the precipitated particles increased while their number increased. In the SEM micrographs shown in Figure 5(a), particles with size of about 1 μm precipitated at 150˚C become larger (about 2 to 4 μm) at 180˚C, and particles with size of several microns or more are observed at 210˚C.

Microstructural Observations of Surface Condition
It is considered that light is irregularly reflected from the surface on which particles were deposited, so the surface appears white and hazy. These precipitated particles are understood to be low-molecular-weight cyclic oligomers of PET produced during synthesis of PET [15] [16]. It is considered that as the temperature rises, the oligomers present inside the PET move to the surface and crystallize there.
On the contrary, such precipitation of particles is not observed in the case of the PHPS-derived-SiO 2 -film-coated PET ( Figure 5(b)). Even for the heat treatment at 180˚C, the surface was smooth and did not change compared to that before the heat treatment. As for the heat treatment at 210˚C, a white-foggy-lined surface state is observed. This surface observation by SEM appears to correspond to the linear cracks observed in the OM image in Figure 4(b) 210˚C. However, unevenness on the outermost surface was not observed; that is, the surface was in a relatively smooth state. The OM images obtained by transmitted light can obtain information about the interior of the film at best; however, the SEM can obtain information about the outermost surface. In view of that fact, it is supposed that the linear precipitates observed in the heat-treated film at 210˚C are deposited at the interface between the PET and SiO 2 film. To confirm that supposition, the surface roughness was observed by laser microscope. Laser-microscope  On the contrary, as for the PHPS-derived SiO 2 -film-coated PET (Figure 6(b)), no surface unevenness is observed up to 180˚C, and very few precipitates are observed at 190˚C. Although the number of precipitates increases at 210˚C, the surface irregularities are very few compared to those on the PET-only surface. This result indicates that 1) the heat resistance of the PHPS-derived-SiO 2 -film-coated PET is very high compared to that of PET only and 2) the surface stays relatively flat even after heat treatment at 210˚C. It is also understood from the above results that the cracks observed by OM are due to precipitation of linear precipitates near the interface between the PET and SiO 2 .
To investigate finer areas of the surface morphology by heat treatment of the SiO 2 -film-coated PET, the film was observed by AFM. The AFM images are shown in Figure 7. The SiO 2 -thin-film-coated PET is smoother than the PET-only film (PHPS/PET: Ra = 0.276 nm versus PET: Ra = 0.669 nm). This result is explained by the fact that the unevenness of the PET surface was smoothed by the application of PHPS. When this SiO 2 -coated PET film was heat treated at 150˚C, particles on the surface grew slightly, resulting in Ra of 0.437 nm. After that, the films were heat treated at 180˚C (Ra = 0.356 nm), 190˚C (Ra = 0.311 nm), and 210˚C (Ra = 0.443 nm); however, Ra of the film surfaces showed the value less than 0.5 nm, indicating smoothness of surface even after heat-treatment.

Cross-Sectional Observation after Heat Treatment (210˚C) of PHPS-Derived SiO2-Film-Coated PET
A cross-sectional TEM image of the SiO 2 -film-coated PET film heat treated at 210˚C for one hour is shown in Figure 8. Undulations in the PET film after the heat treatment are observed (×25,000 image). However, even after the heat treatment, thickness of the SiO 2 film on the PET was 55 to 60 nm, which is not significantly different from that before the heat treatment. A significant change from the state before the heat treatment is that a layer (with thickness of 20 to 30 nm) with different contrast is observed at the interface between the PET and the SiO 2 film (×300,000 image). This layer is supposed to be precipitates of low-molecular-weight oligomers originated from the PET. The precipitates are partially deposited at the surface in a manner that they pierce the SiO 2 film when

Discussion
Although the mechanism by which a PHPS film is converted to a SiO 2 film by light irradiation at low temperature has been reported elsewhere [12], it is restated simply as follows. The outline of the method is shown in Figure 9.  oligomers generated during synthesis of PET precipitate and crystallize on the PET surface by heat treatment, and the fine crystalline particles cause the surface to be whitened. In fact, in the case of PET, fine particles were deposited on the surface by heat treatment at 120˚C or more, and the number and size of the particles increased with increasing heat-treatment temperature. This deposition is considered to be surface precipitation of low-molecular-weight oligomers [15] [16]. And the precipitate significantly reduces transmittance and degrades transparency of the PET film.
As for the dense-SiO 2 -film-coated PET, precipitation of fine particles on the surface is not observed even after heat treatment at 180˚C -190˚C. After heat treatment at 210˚C, a layer of fine particles deposited at the interface between the PET and SiO 2 film was observed by cross-sectional TEM. Deposits of larger particles, partially penetrating the surface SiO 2 layer, were also observed. However, precipitation of large particles of the micron order, as in the case of the PET only film, was scarce, and that result suggests that the dense SiO 2 film on the PET surface suppresses deposition of low-molecular-weight oligomers on the surface.
Forming a dense SiO 2 film by this method not only greatly improves heat resistance of PET but also imparts good gas-barrier performance on the surface of general-purpose PET presently used in many fields. Moreover, the developed method is a low-temperature film-formation technology utilizing a simple coating that does not require a large-scale vacuum facility, so its cost is low. It is considered that as a high-heat-resistance and high-gas-barrier PET, the developed dense-SiO 2 -coated PET film will expand the application range of PET as a film for flexible devices.

Conclusion
Heat-resistance characteristics of a PET film coated with SiO 2 (formed by irra- 180˚C to 190˚C, in this manner, it will be possible to apply a PET film as a substrate for flexible devices. Next step, in order to enhance heat resistance of the film at higher temperature, we are planning to vary photo intensity of excimer light and irradiation time because photo intensity and irradiation time affect film quality of the SiO 2 film.