Surface Modification of Polyethylene Terephthalate (PET) by 172-nm Excimer Lamp

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1 Kasahara et al.: Surface Modification of Polyethylene Terephthalate (PET) (1/8) [Technical Paper] Surface Modification of Polyethylene Terephthalate (PET) by 172-nm Excimer Lamp Takashi Kasahara*, Shuichi Shoji*, and Jun Mizuno** *Major in Nano-Science and Nano-Engineering, Waseda University, Okubo, Shinjuku, Tokyo , Japan **Institute for Nanoscience and Nanotechnology, Waseda University, 513 Wasedatsurumakicho, Shinjuku, Tokyo , Japan (Received July 30, 2012; accepted October 11, 2012) Abstract We studied the effects of 172 nm Xe 2 * excimer lamp irradiation on polyethylene terephthalate (PET) surfaces. Two kinds of techniques were applied: vacuum ultraviolet (VUV) light irradiation and VUV irradiation in the presence of oxygen gas (VUV/O 3 ). The modified PET surfaces were investigated by using contact angle measurements which enabled the surface free energy to be calculated, X-ray photoelectron spectroscopy (XPS), nano-thermal analysis (nano-ta), and atomic force microscopy (AFM). The surface free energy increased significantly after the treatments. The results of XPS analysis showed that the elemental ratio of oxygen on the surface increased, whereas that of carbon decreased. From the deconvoluted C1s and O1s spectra, it was revealed that new oxidized functional groups such as alcoholic and carboxyl groups were generated. The nano-ta results showed that a low melting temperature (T m ) layer had formed on the VUV and VUV/O 3 treated PET surfaces. The results of AFM measurements showed there were no remarkable changes after the treatments compared with untreated PET. In summary, the VUV and VUV/O 3 treatments using a Xe 2 * excimer lamp not only change the surface functionalities but also reduce the T m of the PET surfaces without significantly affecting the surface morphologies. Keywords: Surface Modification, Vacuum Ultraviolet, PET, Surface Free Energy, XPS, Nano-TA 1. Introduction Polyethylene terephthalate (PET) films have been widely used in flexible substrates for organic light emitting diode (OLED) displays,[1] tactile sensors,[2] and roll to roll UV imprint lithography[3] because they have attractive properties, including a high melting temperature, low dielectric constant, and good mechanical strength. On the other hand, the low surface free energy and the chemical inertness of the PET often lead to poor adhesive bonding and poor adhesion of printing and coatings in practice. Surface modification techniques such as ion implantation,[4] laser ablation,[5, 6] plasma treatments,[7 12] ultraviolet-ozone (UV/O 3 ) cleaning,[13] and wet-processes[14] have been utilized to overcome this problem. Most of these processes can change the wettability and the chemical functional groups while increasing the surface roughness. It is essential for the surface modification of the polymers to affect the uppermost surface layer only and not alter the bulk properties. Recently, irradiation with UV excimer lamps for the photochemical modification has been attracted attention. Several polymers have been modified by using UV excimer lamps at different wavelengths, such as 126 nm using Ar 2 *,[15] 172 nm using Xe 2 *,[16 18] and 222 nm using KrCl*[19] in various gas environments. Additionally, vacuum ultraviolet treatments using Xe 2 * excimer lamps were utilized to improve the bond strength of the flip chip and three dimensional (3D) interconnections.[20 22] UV lamps can provide large area exposures and short reaction times at low temperature and only require simple and inexpensive apparatus. However, the surface modification effects depend on the lamp parameters such as the wavelength and the intensity as well as on the chamber pressure and atmosphere. In this study, PET films were modified by using a 172 nm Xe 2 * excimer lamp. Two kinds of treatment techniques were applied. The first was vacuum ultraviolet (VUV) light irradiation, and the other was VUV irradiation in the presence of oxygen gas (VUV/O 3 ).[23] The contact angles were measured to evaluate the wettability and to calculate 47

2 Transactions of The Japan Institute of Electronics Packaging Vol. 5, No. 1, 2012 the surface free energy. The surface chemical structures were investigated in detail by X-ray photoelectron spectroscopy (XPS). Nano-thermal analysis (nano-ta) was used to evaluate the local thermomechanical properties of the uppermost surface layer. The surface morphologies were analyzed by atomic force microscopy (AFM). 2. Experimental Procedure 2.1 Material Commercial, 50 μm thick PET film (Teijin DuPont Films Japan Ltd., G2) was used. The chemical structure of the PET is shown in Fig. 1. The film was cut into mm 2 square pieces for XPS and AFM and mm 2 square pieces for contact angle measurements and nano-ta. The surface of the PET was cleaned with isopropyl alcohol in an ultrasonic bath for 10 min before the experiments. 2.2 Surface treatments The VUV and VUV/O 3 treatments of the PET films were carried out using the Xe 2 * excimer lamp source (Ushio Inc., UER20-172). A schematic diagram of the experimental set-up is shown in Fig. 2. The central wavelength and the intensity at the lamp window were 172 nm and 10 mw/ cm 2, respectively. The distance between the lamp window and PET surfaces was fixed at 13 mm for both VUV and VUV/O 3 treatments. For the VUV treatment, the chamber was initially flushed with nitrogen gas and then evacuated to a base pressure of less than 20 mbar. The PET was directly exposed to 172 nm VUV light at room temperature. The chamber evacuation continued during the VUV irradiation. The duration of the VUV treatment was varied between 10 s and 60 s. The photon energy of VUV light is larger than that of conventional UV light (e.g., low pressure mercury lamps), and can break the various chemical bonds in organic molecules (e.g., C-C, C-H). For the VUV/ O 3 treatment, highly pure oxygen gas was introduced into the chamber to a pressure of 500 mbar after the initial chamber evacuation to 20 mbar, and the PET was irradiated by the VUV light in an oxygen atmosphere at room temperature for times that varied between 30 s and 300 s. The chamber was not evacuated, and the chamber pressure was kept at 500 mbar during the VUV/O 3 process. Because VUV irradiation was used instead of UV light, high-density ozone and excited oxygen atoms O( 1 D) were generated from O 2, and these could react with organic molecules on the polymer surface. 2.3 Contact angle measurements and surface free energy calculation The surface free energy of the PET was characterized by the contact angles. The contact angles on the PET surfaces were obtained using a contact angler (Kyowa Interface Science Co. Ltd., LCD-400S) and the sessile drop method. The surface free energy of the PET can be determined by using Young s equation, which can be written as follows[24]: γ s = γ sl + γ l cosθ, (1) where θ is the contact angle, γ s is the surface free energy of the solid, γ l is the surface tension of the liquid, and γ sl is the interfacial energy between the solid and liquid. According to Owens-Wendt theory,[25] γ s, γ l, and γ sl can be expressed as follows: γ s = γ p s + γ d s, (2) γ l = γ p l + γ d l, (3) γ sl = γ s + γ l - 2(γ p s γ p l ) 0.5-2(γ d s γ d l ) 0.5, (4) where γ p s and γ p l are the polar components, and γ d d s and γ l are the dispersive components of the solid and liquid. Equation (5) can be obtained from Eqs. (1) (4). γ l (1 + cosθ) = 2(γ s p γ l p ) (γ s d γ l d ) 0.5. (5) Fig. 1 Chemical structure of the PET. Fig. 2 Schematic diagram of the VUV and VUV/O 3 treatment system. When the values of γ l, γ p l, and γ d l of more than two test liquids are known, γ s, γ p s, and γ d s can be determined by the contact angles. In our case, four different liquids, i.e., water (H 2 O), glycerol (C 3 H 8 O 3 ), diiodomethane (CH 2 I 2 ), and formamide (CH 3 NO), were used. With each of the four liquids, five measurements were taken at the different locations. 2.4 XPS analysis The surface composition and chemical bonds of the PET were investigated using XPS (JEOL Ltd., JPS-9100TR). The X-ray source and the applied power were MgKα 48

3 Kasahara et al.: Surface Modification of Polyethylene Terephthalate (PET) (3/8) ( ev) and 100 W (10 kv and 10 ma), respectively. The photoelectron take-off angle was fixed at 90. The wide and high resolution scans were measured at pass energies of 50 ev and 10 ev, respectively. The surface elemental ratios were determined from the peak areas of the O1s and C1s spectra. The curve fitting was performed with a Gaussian/Lorentzian ratio of 70/30 using peak-fitting software (JEOL Ltd., SpecSurf) after a Shirley-type background subtraction. 2.5 Nano-TA The nano-ta is an AFM-based analysis technique used to determine the thermomechanical properties of materials.[26 28] In this method, a thermal probe placed in contact with the sample surface is heated. As the temperature rises, the deflection of the probe increases initially due to local thermal expansion of the substrate, and then decreases when the sample temperature reaches the softening temperature, which is a glass transition temperature (T g ) for amorphous polymers or a melting temperature (T m ) for semi-crystalline polymers. In this study, a nano- TA system (Anasys instruments Co., nano-ta) combined with AFM (Agilent Technologies Inc., 5500AFM) was used to evaluate the T m of the PET surfaces before and after treatments. The measurements of the local thermal analysis (LTA) were performed using a heating rate of 10 C/s at five different locations on each sample. 2.6 AFM The morphology of the PET was investigated by using AFM equipment (Shimadzu Co., SPM-9600) in dynamic mode. An area of 2 2 μm 2 was scanned in the air at room temperature. A root mean square surface roughness (R ms ) was obtained from the AFM images. 3. Results and Discussion 3.1 Contact angles and surface free energy Table 1 gives the results of the contact angle measurements before and after treatments. The treatment times of the VUV were 30 s and 60 s, while those of VUV/O 3 were 60 s and 300 s. The contact angles of water, glycerol, and Fig. 3 Calculated surface free energies of PET films (total, polar, and dispersive components) before and after VUV and VUV/O 3 treatment for different treatment times. Table 2 XPS elemental analysis of PET surfaces. Sample Treatment Chemical composition (%) time (s) O1s C1s Untreated VUV VUV VUV/O VUV/O formamide on the PET decreased drastically after both VUV and VUV/O 3 treatments. The calculated surface free energy of the PET is shown in Fig. 3. The surface free energy and its polar component of the untreated PET were mn/m and 8.11 mn/m, respectively, while the dispersive component was mn/m. After both VUV and VUV/O 3 treatments, a significant increase in the polar component was obtained, whereas the dispersive component showed no remarkable change. These results indicate that the VUV and VUV/O 3 treatments enhanced the hydrophilicity of the PET by the creation of additional polar components. 3.2 XPS The surface elemental ratios of the PET before and after treatments are listed in Table 2. The treatment times of the VUV were 30 s and 60 s, while those of VUV/O 3 were 60 s Sample Table 1 Changes in contact angles on PET surfaces. Treatment Contact angle ( ) time (s) Water Glycerol Diiodomethane Formamide Untreated VUV VUV VUV/O VUV/O

4 Transactions of The Japan Institute of Electronics Packaging Vol. 5, No. 1, 2012 and 300 s. After both VUV and VUV/O 3 treatments, the surface elemental ratio of the O1s increased, whereas that of the C1s decreased. The oxygen concentrations of the PET treated by VUV for 60 s and VUV/O 3 for 300 s increased from an initial value of 27.9% to 32.5% and 35.7%, respectively. These results show that the increased surface free energies were probably attributed to the incorporation of oxygen functional groups into the PET surface. In order to analyze the surface functional groups in more detail, the C1s and O1s spectra were deconvoluted. All spectra were referred to the C1s neutral carbon peak at ev. Figs. 4 (a) (e) show the C1s spectra of the untreated, 30 s and 60 s VUV treated, and 60 s and 300 s VUV/O 3 treated PET films. Based on its chemical structure, the untreated PET consists of three different carbon environments,[7 11, 18] because it has binding energies at ev corresponding to C-C bonding (C1), at ev corresponding to C-O bonding (ethers) (C2), and at ev corresponding to O=C-O bonding (esters) (C3). The broad peak at around 291 ev was a shake-up satellite due to the p p* transitions of the phenyl groups. After VUV and VUV/O 3 treatments, increases in C-O (ethers and alcoholic group) and O=C-O bonds (esters and carboxyl groups) were observed.[7, 18] However, the C-C bonding with bond energy of approximately 340 kj/mol decreased slightly, which was probably because of the chain scission induced by the photon energy of the Xe 2 * excimer lamp (697.5 kj/mol) and/or the oxidative decomposition by the excited oxygen atoms O( 1 D). These results indicated that the Xe 2 * excimer lamp has sufficient energy to break the C-C bond effectively, while the excited oxygen atoms O( 1 D) is expected to create oxygen functionalities of C-O Fig. 4 C1s XPS spectra of PET surfaces: (a) untreated; VUV treated for (b) 30 s and (c) 60 s; and VUV/O 3 treated for (d) 60 s and 300 s. Fig. 5 O1s XPS spectra of PET surfaces: (a) untreated; VUV treated for (b) 30 s and (c) 60 s; and VUV/O 3 treated for (d) 60 s and 300 s. 50

5 Kasahara et al.: Surface Modification of Polyethylene Terephthalate (PET) (5/8) and O=C-O bonds on the PET surfaces with the oxidative decomposition and volatilization.[21] The O1s spectra obtained from the untreated, VUV, and VUV/O 3 treated PET are shown in Figs. 5 (a)-(e), respectively. According to Ref.,[11, 18] the O1s spectrum of the untreated PET contains two peaks at ev and ev, which are assigned to O=C (esters) bonding (O1) and O-C (ethers) bonding (O2), respectively. The O1s spectra corresponding to VUV and VUV/O 3 treated PET showed a significant increase in O=C bonding of esters and carboxyl groups and O-C bonding of ethers and carboxyl groups.[10] Consequently, polar components such as alcoholic and carboxyl groups were formed on the PET surfaces by both VUV and VUV/O 3 treatments, indicating that the obtained XPS spectra are in agreement with the results of the surface free energy calculation shown in Fig Nano-TA The results of the nano-ta of the VUV and VUV/O 3 treated PET films with various treatment times are shown in Figs. 6 (a) and (b), respectively. In the case of the untreated PET, the increase in the probe temperature initially lead to an increase in the deflection because of the local thermal expansion of the PET surface, and then the probe penetrated into the material at approximately 240 C, which is taken to be T m. The T m of the VUV treated PET shifted downwards with increasing treatment times. When 60 s VUV treatment was carried out, the T m decreased to Fig. 6 Nano-TA measurements of (a) VUV and (b) VUV/O 3 treated PET with various treatment times. approximately 224 C. The formation of the low T m layer on the PET surfaces may be due to the change in the chemical structures induced by the photochemical modification of the VUV light.[20] These results also indicated that long treatment times were important for the modification of the thermomechanical properties of the PET in the case of the VUV treatments. For the VUV/O 3 treatments, a low T m layer had also formed on the PET surfaces, and a value of approximately 227 C was reached for treatment times longer than 60 s. Moreover, the deflections increased slowly compared with the untreated PET, and slow penetrations into the sample were observed. These changes in the thermomechanical properties were probably caused by the chain scission and additional components induced by the excited oxygen atoms O(1D), which were also observed in the results of the surface free energy calculation and XPS. From the nano-ta studies, we can conclude that the photochemical modification using a Xe 2 * excimer lamp changed the thermomechanical properties, indicating that the formation of a low T m layer can be controlled by the treatment time of VUV and with and without introduction of oxygen gas into the chamber. 3.4 AFM Figure 7 shows the AFM images and R ms roughness values of the untreated, VUV, and VUV/O 3 treated PET samples with various treatment times. The surface of the untreated PET was generally smooth, and its R ms was nm (Fig. 7 (a)). It can be clearly seen that after both VUV for 30 s and 60 s and VUV/O 3 for 60 s and 300 s, the morphologies of the PET has no remarkable change although sphere-like aggregates were formed on the surfaces. The R ms values of VUV treated PET for 30 s and 60 s were nm and nm, while those of VUV/O 3 treated PET for 60 s and 300 s were nm and nm. These results were probably due to the effect of the photon energy of the VUV light and/or the excited oxygen atoms O( 1 D) on chain scission. The changes in roughness were not significant in comparison with other treatments such as plasma methods, which indicating that polymer surface was etched by physical erosion by ion bombardments during plasma treatments,[8] while the excited oxygen atoms O( 1 D) and 172 nm photon energy modified PET surface at room temperature without ion bombardment. These results showed that the VUV and VUV/O 3 treatments using the Xe 2 * excimer lamp can modify the functional groups and thermomechanical properties of the PET surfaces without significantly changing the surface roughness. 51

6 Transactions of The Japan Institute of Electronics Packaging Vol. 5, No. 1, 2012 Fig. 7 AFM images and R ms roughness values of PET surfaces: (a) untreated; VUV treated for (b) 30 s and (c) 60 s; and VUV/O 3 treated for (d) 60 s and (e) 300 s. 4. Conclusion VUV and VUV/O 3 treatments of PET surfaces have been carried out with a 172 nm Xe 2 * excimer lamp. The wettability of the PET was dramatically improved due to a significant increase in the surface free energy. The results of the XPS analysis of the C1s and O1s spectra showed the formation of newly oxidized components on the PET surfaces, which agreed with the calculated PET surface free energy. The modified PET surfaces showed the formation of a low T m layer on the PET surfaces, as observed in the nano-ta results. After the surface treatments, the morphologies of the PET showed no remarkable changes. In conclusion, low T m layers and oxygen functionalities of C-O and O=C-O can be formed on PET surfaces without significantly affecting the surface profiles by VUV and VUV/O 3 treatments using a 172 nm Xe 2 * excimer lamp. Acknowledgements This work was partly supported by Japan Ministry of Education, Culture, Sports Science & Technology Grantin-Aid for Scientific Basic Research (S) No and by the Japan Society for the Promotion of Science (JSPS) through the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program), initiated by the Council for Science and Technology Policy (CSTP). The authors thank the Nanotechnology Support Project of Waseda University for their technical advice. The authors also thank Toyo Co. for the use of nano-ta equipment and technical advice. References [1] J. A. Jeong, H. S. Shin, K. H. Choi, and H. K. Kim, Flexible Al-doped ZnO films grown on PET substrates using linear facing target sputtering for flexible OLEDs, Journal of Physics D: Applied Physics, Vol. 43, p , [2] T. Kasahara, M. Mizushima, H. Shinohara, T. Obata, T. Futakuchi, S. Shoji, and J. Mizuno, Simple and low-cost fabrication of flexible capacitive tactile sensors, Japanese Journal of Applied Physics, Vol. 50, p , [3] P. Maury, N. Stroeks, M. Wijinen, R. Tacken, and R. V. Werf, Roll-to-roll UV imprint for bottom-up transistor fabrication, Journal of Photopolymer Science and Technology, Vol. 24, pp , [4] M. Drabik, K. Dworecki, R. Tanczyk, S. Wasik, and J. Zuk, Surface modification of PET membrane by ion implantation, Vacuum, Vol. 81, pp , [5] G. Wu, M. D. Paz, S. Chiussi, J. Serra, P. Gonzalez, Y. J. Wang, and B. Leon, Excimer laser chemical ammonia patterning on PET film, Journal of Materials Science, Materials in Medicine, Vol. 20, pp , [6] P. Laurens, S. Petit, and F. Arefi-Khonsari, Study of PET surfaces after laser or plasma treatment: surface modifications and adhesion properties towards Al deposition, Plasmas and Polymers, Vol. 8, pp , [7] H. Ardelean, S. Petit, P. Laurens, P. Marcus, and F. Arefi-Khonsari, Effects of different laser and plasma treatments on the interface and adherence between evaporated aluminium and polyethylene terephthalate films: X-ray photoemission, and adhesion studies, Applied Surface Science, Vol. 243, pp , 52

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8 Transactions of The Japan Institute of Electronics Packaging Vol. 5, No. 1, 2012 Takashi Kasahara was born in Saitama Jun Mizuno received his Ph.D. degree in Prefecture, Japan, in He received his applied physics from Tohoku University in BS and MS degree in the field of microsys He is currently an associate professor tems from Waseda University in 2010 and at Waseda University and works at the nano- 2012, respectively. He is presently Ph.D. stu- technology research center where is a dent at Waseda University. His current inter- research institute of nano-science and engi- ests are polymer microdevice technologies neering. His current interests are MEMS- such as OLED, flexible sensor, and surface modification. Shuichi Shoji received his BS, MS and Ph.D. degree in electronic engineering from Tohoku University in 1979, 1981 and 1984, respectively. He had been with Tohoku University as a research associate and associate professor from 1984 to In 1994 he moved to Waseda University as an associate professor and he is currently a professor of Department of Electronic and Photonic Systems, and Major in Nano-Science and Nano-Engineering, Waseda University. His current interests are micro-/nano-devices and systems for chemical/bio applications. 54 NEMS technology, bonding technology at a low temperature using plasma activation or excimer laser irradiation, printed electronics, and composite technology for UV or heat nanoimprint lithography combined with electrodeposition.