47 Kasahara et al.: Surface Modification of Polyethylene Terephthalate (PET) (1/8) 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 attrac- tive 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 implanta- tion,[4] laser ablation,[5, 6] plasma treatments,[7–12] ultra- violet-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 chemi- cal functional groups while increasing the surface rough- ness. It is essential for the surface modification of the poly- mers to affect the uppermost surface layer only and not alter the bulk properties. Recently, irradiation with UV excimer lamps for the pho- tochemical modification has been attracted attention. Sev- eral 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, vac- uum 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 inex- pensive apparatus. However, the surface modification effects depend on the lamp parameters such as the wave- length and the intensity as well as on the chamber pres- sure 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 pres- ence of oxygen gas (VUV/O 3 ).[23] The contact angles were measured to evaluate the wettability and to calculate [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, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan **Institute for Nanoscience and Nanotechnology, Waseda University, 513 Wasedatsurumakicho, Shinjuku, Tokyo 162-0041, 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 sur- face 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 analy- sis 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
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47
Kasahara et al.: Surface Modification of Polyethylene Terephthalate (PET) (1/8)
1. IntroductionPolyethylene 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 attrac-
tive 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 implanta-
Fig. 3 Calculated surface free energies of PET films (total, polar, and dispersive components) before and after VUV and VUV/O3 treatment for different treatment times.
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Transactions of The Japan Institute of Electronics Packaging Vol. 5, No. 1, 2012
and 300 s. After both VUV and VUV/O3 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/O3 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
284.6 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/O3 treated PET films. Based on its chemical struc-
ture, the untreated PET consists of three different carbon
environments,[7–11, 18] because it has binding energies at
284.6 eV corresponding to C-C bonding (C1), at 286.2 eV
corresponding to C-O bonding (ethers) (C2), and at 288.6
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/O3 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 Xe2* excimer lamp
(697.5 kJ/mol) and/or the oxidative decomposition by the
excited oxygen atoms O(1D). These results indicated that
the Xe2* excimer lamp has sufficient energy to break the
C-C bond effectively, while the excited oxygen atoms
O(1D) 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/O3 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/O3 treated for (d) 60 s and 300 s.
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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/O3 treated PET are shown in Figs. 5 (a)-(e), respec-
tively. According to Ref.,[11, 18] the O1s spectrum of the
untreated PET contains two peaks at 531.6 eV and 533.2
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/O3 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 car-
boxyl groups were formed on the PET surfaces by both
VUV and VUV/O3 treatments, indicating that the obtained
XPS spectra are in agreement with the results of the sur-
face free energy calculation shown in Fig. 3.
3.3 Nano-TAThe results of the nano-TA of the VUV and VUV/O3
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 ini-
tially 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 Tm. The Tm of the VUV treated PET
shifted downwards with increasing treatment times. When
60 s VUV treatment was carried out, the Tm decreased to
approximately 224°C. The formation of the low Tm layer on
the PET surfaces may be due to the change in the chemi-
cal 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/O3 treatments, a low Tm
layer had also formed on the PET surfaces, and a value of
approximately 227°C was reached for treatment times lon-
ger 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 ther-
momechanical 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 photo-
chemical modification using a Xe2* excimer lamp changed
the thermomechanical properties, indicating that the for-
mation of a low Tm layer can be controlled by the treatment
time of VUV and with and without introduction of oxygen
gas into the chamber.
3.4 AFMFigure 7 shows the AFM images and Rms roughness val-
ues of the untreated, VUV, and VUV/O3 treated PET sam-
ples with various treatment times. The surface of the
untreated PET was generally smooth, and its Rms was 1.894
nm (Fig. 7 (a)). It can be clearly seen that after both VUV
for 30 s and 60 s and VUV/O3 for 60 s and 300 s, the mor-
phologies of the PET has no remarkable change although
sphere-like aggregates were formed on the surfaces. The
Rms values of VUV treated PET for 30 s and 60 s were 2.193
nm and 1.884 nm, while those of VUV/O3 treated PET for
60 s and 300 s were 1.943 nm and 1.714 nm. These results
were probably due to the effect of the photon energy of the
VUV light and/or the excited oxygen atoms O(1D) on
chain scission. The changes in roughness were not signifi-
cant 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(1D) and 172 nm photon energy modified PET surface at
room temperature without ion bombardment. These
results showed that the VUV and VUV/O3 treatments
using the Xe2* excimer lamp can modify the functional
groups and thermomechanical properties of the PET sur-
faces without significantly changing the surface rough-
ness.Fig. 6 Nano-TA measurements of (a) VUV and (b) VUV/O3 treated PET with various treatment times.
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Transactions of The Japan Institute of Electronics Packaging Vol. 5, No. 1, 2012
4. ConclusionVUV and VUV/O3 treatments of PET surfaces have
been carried out with a 172 nm Xe2* 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 sur-
faces, which agreed with the calculated PET surface free
energy. The modified PET surfaces showed the formation
of a low Tm layer on the PET surfaces, as observed in the
nano-TA results. After the surface treatments, the mor-
phologies of the PET showed no remarkable changes. In
conclusion, low Tm 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/O3 treatments using a 172 nm Xe2* excimer lamp.
AcknowledgementsThis work was partly supported by Japan Ministry of
in-Aid for Scientific Basic Research (S) No. 23226010 and
by the Japan Society for the Promotion of Science (JSPS)
through the “Funding Program for World-Leading Innova-
tive 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.
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Takashi Kasahara was born in Saitama Prefecture, Japan, in 1987. He received his BS and MS degree in the field of microsys-tems from Waseda University in 2010 and 2012, respectively. He is presently Ph.D. stu-dent at Waseda University. His current inter-ests are polymer microdevice technologies
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 Uni-versity as a research associate and associate professor from 1984 to 1992. In 1994 he moved to Waseda University as an associate
professor and he is currently a professor of Department of Elec-tronic 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.
Jun Mizuno received his Ph.D. degree in applied physics from Tohoku University in 2000. He is currently an associate professor at Waseda University and works at the nano-technology research center where is a research institute of nano-science and engi-neering. His current interests are MEMS-
NEMS technology, bonding technology at a low temperature using plasma activation or excimer laser irradiation, printed elec-tronics, and composite technology for UV or heat nanoimprint lithography combined with electrodeposition.