8
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-

tion,[4] laser ablation,[5, 6] plasma treatments,[7–12] ultra-

violet-ozone (UV/O3) 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

Ar2*,[15] 172 nm using Xe2*,[16–18] and 222 nm using

KrCl*[19] in various gas environments. Additionally, vac-

uum ultraviolet treatments using Xe2* 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 Xe2* 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/O3).[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 LampTakashi 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 Xe2* 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/O3). 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 (Tm) layer had formed on the VUV

and VUV/O3 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/O3 treatments using a Xe2* excimer lamp

not only change the surface functionalities but also reduce the Tm 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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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 spec-

troscopy (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 Procedure2.1 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 10 × 10 mm2

square pieces for XPS and AFM and 20 × 20 mm2 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 treatmentsThe VUV and VUV/O3 treatments of the PET films were

carried out using the Xe2* excimer lamp source (Ushio

Inc., UER20-172). A schematic diagram of the experimen-

tal set-up is shown in Fig. 2. The central wavelength and

the intensity at the lamp window were 172 nm and 10 mW/

cm2, respectively. The distance between the lamp window

and PET surfaces was fixed at 13 mm for both VUV and

VUV/O3 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 tempera-

ture. 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 pres-

sure mercury lamps), and can break the various chemical

bonds in organic molecules (e.g., C-C, C-H). For the VUV/

O3 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 irradi-

ated 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 pres-

sure was kept at 500 mbar during the VUV/O3 process.

Because VUV irradiation was used instead of UV light,

high-density ozone and excited oxygen atoms O(1D) were

generated from O2, 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 sur-

faces were obtained using a contact angler (Kyowa Inter-

face Science Co. Ltd., LCD-400S) and the sessile drop

method. The surface free energy of the PET can be deter-

mined by using Young’s equation, which can be written as

follows[24]:

γs = γsl + γl cosθ, (1)

where θ is the contact angle, γs is the surface free energy

of the solid, γl is the surface tension of the liquid, and γsl is

the interfacial energy between the solid and liquid. Accord-

ing to Owens-Wendt theory,[25] γs, γl, and γsl can be

expressed as follows:

γs = γsp + γs

d, (2)

γl = γlp + γl

d, (3)

γsl = γs + γl - 2(γsp γl

p)0.5 - 2(γsd γl

d)0.5, (4)

where γsp and γl

p are the polar components, and γsd and γl

d

are the dispersive components of the solid and liquid.

Equation (5) can be obtained from Eqs. (1)–(4).

γl (1 + cosθ) = 2(γsp γl

p)0.5 + 2(γsd γl

d)0.5. (5)

When the values of γl, γlp, and γl

d of more than two test liq-

uids are known, γs, γsp, and γs

d can be determined by the

contact angles. In our case, four different liquids, i.e.,

water (H2O), glycerol (C3H8O3), diiodomethane (CH2I2),

and formamide (CH3NO), were used. With each of the

four liquids, five measurements were taken at the different

locations.

2.4 XPS analysisThe 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

Fig. 1 Chemical structure of the PET.

Fig. 2 Schematic diagram of the VUV and VUV/O3 treat-ment system.

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Kasahara et al.: Surface Modification of Polyethylene Terephthalate (PET) (3/8)

(1253.6 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 ele-

mental 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-fit-

ting software (JEOL Ltd., SpecSurf) after a Shirley-type

background subtraction.

2.5 Nano-TAThe nano-TA is an AFM-based analysis technique used

to determine the thermomechanical properties of materi-

als.[26–28] In this method, a thermal probe placed in con-

tact 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 soft-

ening temperature, which is a glass transition temperature

(Tg) for amorphous polymers or a melting temperature

(Tm) 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 Tm of the PET surfaces before and after

treatments. The measurements of the local thermal analy-

sis (LTA) were performed using a heating rate of 10°C/s

at five different locations on each sample.

2.6 AFMThe morphology of the PET was investigated by using

AFM equipment (Shimadzu Co., SPM-9600) in dynamic

mode. An area of 2 × 2 μm2 was scanned in the air at room

temperature. A root mean square surface roughness (Rms)

was obtained from the AFM images.

3. Results and Discussion3.1 Contact angles and surface free energy

Table 1 gives the results of the contact angle measure-

ments before and after treatments. The treatment times of

the VUV were 30 s and 60 s, while those of VUV/O3 were

60 s and 300 s. The contact angles of water, glycerol, and

formamide on the PET decreased drastically after both

VUV and VUV/O3 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

38.52 mN/m and 8.11 mN/m, respectively, while the dis-

persive component was 30.41 mN/m. After both VUV and

VUV/O3 treatments, a significant increase in the polar

component was obtained, whereas the dispersive compo-

nent showed no remarkable change. These results indi-

cate that the VUV and VUV/O3 treatments enhanced the

hydrophilicity of the PET by the creation of additional

polar components.

3.2 XPSThe 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/O3 were 60 s

Table 2 XPS elemental analysis of PET surfaces.

SampleTreatment

Chemical composition (%)

time (s) O1s C1s

Untreated 0 27.9 72.1VUV 30 31.6 68.4VUV 60 32.5 67.5VUV/O3 60 32.0 68.0VUV/O3 300 35.7 64.3

Table 1 Changes in contact angles on PET surfaces.

SampleTreatment Contact angle (°)

time (s) Water Glycerol Diiodomethane Formamide

Untreated 0 72.2 58.8 33.6 64.5VUV 30 33.8 31.3 23.9 8.3VUV 60 32.6 33.7 24.3 9.0VUV/O3 60 44.4 40.7 27.1 15.9VUV/O3 300 40.8 37.9 27.0 12.5

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

Education, Culture, Sports Science & Technology Grant-

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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Transactions of The Japan Institute of Electronics Packaging Vol. 5, No. 1, 2012

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.