Adhesive Properties of Ultraviolet Curable Pressure-Sensitive
Adhesive Tape for Semiconductor Processing (I)- Interpretation via Rheological Viewpointby Takehiro Ozawa *, Shinichi Ishiwata *2 and Yoshihisa Kano *For blends of acrylate copolymer [poly(2-ethyl hexyl acrylate-co-acrylic acid) ;P(2EHA-AA)] with ultraviolet (UV) curable oligomer [urethane acrylate oligomer ;UAO], pressure-sensitive adhesive (PSA) properties, such as peel adhesion, probe tack, andholding power were examined. The values of peel adhesion and probe tack of the P(2EHA-AA) /UAO blends were dramatically reduced by UV irradiation. However, all blends had a high holding
power even when cured by UV irradiation. The mechanism of reduced PSA properties was investigated via dynamic mechanical properties, DSC, and dynamic contact angle DCA. Peel adhesion
decreased monotonically with an increase of storage moduli E' and loss moduli E" for all non-UV
and UV cured blends. Since moduli values and glass transition temperatures Tg of these blends
after UV irradiation were higher than those of these blends before UV irradiation, we judged that
reduced peel adhesion and probe tack values were caused by the moduli increase and the Tg
increase due to UV irradiation. In other words, the deformation energy of UV cured blends when
conducting adhesive tests was reduced by the curing process. The DCA values of non-UV cured
blends were same as those of UV cured blends. We presumed that the segment mobility of polymer chains on the surface did not influence the reduced peel adhesion and probe tack values.

ABSTRACT1.

INTRODUCTION

Pressure sensitive adhesive (PSA) tapes composed ofacrylic copolymers have been extensively utilized in various industries, such as packaging, printing, electrical insulation, and automobile. In general, the PSA properties(tack, peel adhesion, holding power) of acrylic adhesive
have been controlled by blending tackifiers or dissimilar
polymers, by adjusting molecular weight and its distribution, and also by copolymerization with a polar monomer
and curing system. In recent years, the PSA properties ofacrylic adhesive copolymer have been interpreted by considering various factors, such as dynamic mechanicalproperties, surface tension, and miscibility 1)~6). However,since the values of PSA properties are thought to be influenced by bonding and debonding processes, the PSA
properties could not be clarified.
Incidentally, ultraviolet (UV) curing technology has
already been established in painting and adhesive fields.
A UV cured adhesive can bond to adhere in an instant atroom temperature. UV curing technology is very effective
for controlling the PSA properties of acrylic adhesive,
because mechanical properties, surface tension, and miscibility are dramatically changed by UV irradiation.
**2

Polymer Engineering Center, Ecology & Energy Laboratory
Tape Development Department (AT), Industrial Products
Division

Presumably, the level of PSA properties of UV cured
acrylic adhesive is also controlled by UV oligomers ofmany species, blend ratio, and UV irradiation conditions.
Thus the examination of miscibility among the components is very important for an UV curable PSA.
Recently, various types of PSA tapes have been utilized
in semiconductor processing, such as back-grinding anddicing processes 7)~9). In particular, it is well known that an
UV curable PSA tape is a very effective material for dicing
processes 10)~13). In the cutting process of a dicing process,strong adhesion between PSA tape and wafer is necessary. If a PSA tape adheres weakly to a wafer in the cutting process, the diced chip separates from PSA tape orflies off. On the other hand, weak adhesion between PSA
tape and wafer is desired for the picking-up process of adicing process. Since UV curable PSA is composed ofblends of acrylic adhesive copolymer, UV curable
oligomer, and photopolymerization initiator, the PSA properties can be controlled by means of UV irradiation during
semiconductor processing. In other words, an UV curable
PSA tape is very tacky before UV irradiation, and thistackiness disappears with UV irradiation. The backbone of
the UV curable oligomer is formed out of polyester, epoxy,or urethane, and its functional group is CH2=CHCO- or
CH 2 =C(CH 3 )CO-. The photopolymerization initiator
absorbs UV of 300~450nm. The initiation reaction for UV
curing occurs following a radical reaction from initiator to
UV curable oligomer. Finally, the UV curable oligomer is83polymerized.
In this paper, for acrylic adhesive copolymer / urethane
acrylate oligomer blends, the PSA properties of non-UV
cured adhesive were compared with those of UV cured
adhesive. The effect of urethane acrylate oligomer content
on PSA properties was also investigated for the blends.
Then, the mechanism of reduced PSA properties was
explained via dynamic mechanical properties, DSC, anddynamic contact angle DCA.

2. EXPERIMENTAL
2.1 Materials
The compositions of UV curable adhesives are shown in
Table 1. The acrylate adhesive copolymer is composed of
2-ethylhexyl acrylate and acrylic acid [P(2EHA-AA)]. The
urethane acrylate oligomer [UAO] and 1-hydroxy cyclohexyl phenylketone were used as UV curing oligomer andphotopolymerization initiator, respectively. The peakabsorption of 1-hydroxy cyclohexyl phenylketone is 327
nm. The mixtures of P(2EHA-AA), UAO, and photopolymerization initiator solutions (solvent ; ethyl acetate) were
coated onto a poly(ethylene terephthalate) (PET) baseusing a hand coating bar. After coating, the films weredried at 90°C for 2-3 min and kept at 23±3°C for morethan a week. The blends were 20 µm thick in their dry
state. UV irradiation of 500 mJ/cm2 was carried out with ahigh-pressure mercury lamp.
2.2 Measurements
PSA properties were measured according to JapaneseIndustry Standard (JIS-Z0237). Peel adhesion of the UV
tapes to silicon wafer was tested at a 90°C peel angle andpeel rate of 50 mm/min. The dwell time was 24 h. Probetack of UV tape was measured at 25°C, using a RHESCA
Co. probe tack tester. The probe in the probe tack wasmade of stainless steel. In a holding power measurement,
PSA tape with a 25 mm x 25 mm test area is applied on astainless steel panel. The dwell time was 30 min. The
dead load is 1 kg and test temperature is at 25°C. In conducting their adhesion tests for UV cured adhesive tapes,
UV tapes were adhered to the test panel, then cured andfinally removed.
The glass transition temperatures (Tg) of UV curable
adhesives were determined by a Seiko Instruments Inc.
RDC-220 Robot DSC at a heating rate of 10 K/min in

Table 1
Acrylic adhesivepolymer

nitrogen gas.
The dynamic mechanical properties of the UV curable
adhesive (non-UV and after UV irradiation) were carriedout using Rheometrics Co., Ltd. dynamic mechanical analyzer type RDA II and Seiko Instruments Inc. type
DMA200, respectively. The temperature dependence ofdynamic mechanical properties such as extension storage
moduli E' =3G' or shear storage moduli G' , extension loss
moduli E" =3G" or shear loss moduli G", and dynamic loss
tangent tanδ were measured at 1 Hz.
The dynamic loss factor ∆ of UV curable adhesives wasmeasured by the free decay oscillation method using an
Orientec Co., Ltd. rigid body pendulum-type dynamic
mechanical property apparatus model DDV-OPA III at -100
to 100°C. The heating rate was 5 K/min. The pendulumwith the knife edge put in contact with the sample wasused to perform dynamic mechanical measurements.
The dynamic contact angle DCA was measured with
Orientec Corporation (DCA20) based on the Wilhelmyplate technique. Water was used as the standard liquid.
The dipping velocity was 100 mm/min and the DCA wasmeasured at 23°C. The acrylate copolymer P(2EHA-AA) /ultraviolet curable oligomer UAO 50/50 blend was stirred
overnight in about 10 wt% ethyl acetate solution. The aluminum plates (20 mm wide, 50 mm length, and 0.3 mm
thick) were dipped into the solution and then dried in a
vacuum oven at 40°C for 24 hours. The details were
described elsewhere 14), 15).

3.

RESULTS AND DISCUSSION

Relationship between 90° peel adhesion and UAO content
for P(2EHA-AA) / UAO blends is shown in Figure 1. For
non-UV cured adhesives, the value of peel adhesion
increases with an increase of UAO content. Conversely,for UV cured adhesives, the level of peel adhesion
decreases with an increase of UAO content. The peeladhesion of UV cured adhesive with 40 phr UAO com-

Composition of adhesive

Poly (2-ethylhexyl acrylate-co-acrylic acid)P (2EHA-AA)

100 phr

Mw = 220000, Mw/Mn=7.3
10, 20,

UV oligomer

Urethane acrylate oligomer

30, 40,50 phrInitiator

1-hydroxy cyclohexyl phenylketone

5 phr

Curing agent

Polyisocyanate

4 phr

84

Figure 1

Relationship between 90° peel adhesion and UAO
content for P(2EHA-AA) / UAO blends

Furukawa Review, No. 20

2001pletely disappeared. The probe tack value for non-UV
cured adhesives also increases with an increase of UAO
content, whereas the probe tack of UV cured adhesivewith 40 phr UAO completely disappeared. In other words,
the values of peel adhesion and probe tack for P(2EHAAA) / UAO blends were dramatically reduced by UV irradiation. These blends had a high holding power (non-creep
for 80,000 s). We expect that the differences of PSA properties between the blends and the reduced PSA properties (peel adhesion, probe tack) through UV irradiation areclarified by dynamic mechanical properties, surface properties and miscibility. Thus dynamic mechanical properties(storage moduli G', dynamic loss tangent tanδ) , DSC, anddynamic contact angle DCA were examined for theP(2EHA-AA) / UAO blends.
The relationship between PSA properties and dynamic
mechanical properties for an adhesive polymer has
already been investigated by many researchers 1)~3), 16), 17).
For example, an original analysis of chemical and physicalfactors affecting PSA properties was proposed by
Dahlquist 1 6 ) , who found that the molecular contact
between adhesive and adherend is completely established when compressive creep compliance of the adhesive at 1 s is higher than 10-7 cm2/dyn (Dahlquist criterion). Tse 1) and Yang 2) pointed out that the PSA properties
T are expressed by considering the bonding and debonding processes of adhesive as follows :T = Wa . B . D(1)where Wa is the work of adhesion between adhesive andadherend,B is the function of bonding process anddepends on the plateau moduli Ge of adhesive, and D correlates with debonding process and increases withincreasing loss moduli G" of adhesive. Therefore, we presume that the PSA properties of the P(2EHA-AA) / UAO
blends are influenced by the dynamic mechanical properties of the blends.

Temperature dependence of storage moduli G' of nonUV cured P(2EHA-AA) / UAO blends is shown in Figure 2.
G' decreases as temperature rises. At the high-temperature side (plateau region), G' decreases as UAO content
increases. Presumably, since molecular weight (viscosity)of UAO was remarkably lower than that of P(2EHA-AA),the plateau moduli level was depressed by introducing
UAO. As described above, peel adhesion and probe tackincreased with an increase of UAO content for non-UV
blends. We think that for non-UV blends the function of
the bonding process reflects on PSA properties according
to Tse's equation (1).
Temperature dependence of (a) extensive storage moduli E' =3G' and (b) dynamic loss tangent tanδ for UV cured
P(2EHA-AA) / UAO blends are shown in Figure 3. The
magnitude of E' of P(2EHA-AA) / UAO blend with 20 phr
UAO is similar to that of the blend with 50 phr UAO.
Conversely, the E' values of the blend with 10 phr UAO
are remarkably lower than that of the blend with 20 phr
UAO. The magnitude of tanδ-temperature curve of theblend with 10 phr UAO is significantly larger than that of
the blend with 20 phr UAO. The peel adhesion and probe
tack values are also dramatically reduced for the blend
with 20 phr UAO. Therefore, we judge that for UV cured
blends its PSA properties are influenced by dynamic
mechanical properties.
Secondly, for non-UV and UV cured blends the mechanism of reduced PSA properties was explained throughdynamic mechanical properties, DSC, and dynamic contact angle DCA. For non-UV and UV cured P(2EHA-AA) /UAO blend with 10 phr UAO, temperature dependence ofG' and tanδ are shown in Figure 4. With UV irradiation thevalues of G' of UV cured blend with 10 phr UAO are high-Figure 3
Figure 2

Temperature dependence of G' of non-UV cured
P(2EHA-AA) / UAO blends

Temperature dependence of (a) extensive storage
moduli E' and (b) dynamic loss tangent tanδ for UV
cured P(2EHA-AA) / UAO blends

85

er than that of non-UV cured blend with 10 phr UAO. The
tanδ-temperature curve of the UV cured blend shifted tothe high-temperature side by about 5 K. In particular, theincrease of G' in the rubber plateau region is clearlyobserved for the blend with 10 phr UAO. Since rubber
plateau modulus Gr influences entanglement of polymer
chain, we presume that the molecular weight between
entanglements for P(2EHA-AA) polymer chain decreased
with UV irradiation. Figure 5 shows the temperaturedependence of (a) storage moduli E' and (b) dynamic loss
tangent tanδ for P(2EHA-AA) / UAO blend with 40 phr
UAO. Peel adhesion (P.A.), probe tack (P.T.), and holding
power (H.P.) values are also listed in the Figure. The tanδvalues are lowered by UV irradiation. The temperature oftanδ having maximum (TDmax) also shifts toward the hightemperature side following UV irradiation. At 25°C, the E'
value of non-UV cured adhesive is about 105 Pa, whereas
the E' value of UV cured adhesive is 109 Pa. Dahlquist 16)
suggested that tack appears when the E' value of anadhesive is lower than 106 Pa. In fact, the values of peeladhesion and probe tack of non-cured PSA tape wereremarkably larger than those of UV cured PSA tape.
Therefore, we presume that the reduced adhesion iscaused by the moduli increase and the Tg increase due to
UV irradiation.
The Tg increase due to UV irradiation is confirmed by a
DSC analysis. Figure 6 shows DSC thermograms ofP(2EHA-AA) / UAO blends. The glass transition temperatures Tg of the non-UV and UV cured blends are indicatedin the Figure. For both blends with 10 phr and 50 phr
UAO, the Tg of UV cured blends is higher than that of nonUV blends by about 5 K. Presumably, mobility of the segment in P(2EHA-AA) polymer chain was depressed by UVFigure 486

Temperature dependence of (a) G' and (b) tanδ ofP(2EHA-AA) / UAO blend with 10 phr UAO

cured UAO. All non-UV and UV cured blends films weretransparent. Since the Tg of the blends also exhibited asingle peak, we judge that P(2EHA-AA) is miscible with
UAO.
Tackiness is dominated by dynamic mechanical properties on surface of an adhesive layer. In our previous

Figure 5

Temperature dependence of (a) storage moduli E'
and (b) dynamic loss tangent tanδ for P(2EHA-AA) /UAO blend with 40 phr UAO. Peel adhesion (P.A.),
probe tack (P.T.), and holding power (H.P.) values arelisted in the Figure. N.C.; Non Creep 80000sec

Figure 6

DSC thermograms of P(2EHA-AA) / UAO blends

Furukawa Review, No. 20

2001Figure 7

Temperature dependence of dynamic loss factor ∆for P(2EHA-AA) / UAO blend with 40 phr UAO. a) nonUV, b) after UVTable 2

Dynamic contact angle for P(2EHA-AA) / UAO blend
with 50 phr UAO
Advancing Contact Angle

Receding Contact Angle(deg.)(deg.)

Before UV

92.4

79.9

After UV

92.6

80.0

study18), differences of dynamic mechanical properties on
the surface directly influenced the tack value. It is expected that dynamic mechanical properties on the surface of anon-UV cured adhesive also differs from that of UV cured
adhesive. Thus, the temperature dependence of dynamic
loss factor ∆ for P(2EHA-AA) / UAO blend with 40 phr
UAO was measured by means of the free decay oscillation method as shown in Figure 7. The delineation of thecurve of ∆ and temperature for non-UV cured adhesive
differs remarkably from UV cured adhesive. Therefore, we
considered that the difference of peel adhesion and probe
tack between non-UV and UV cured adhesives corresponded to the differences in dynamic mechanical properties on the surface.
Thirdly, the mobility of the polymer chain on the surface
for P(2EHA-AA) / UAO blend with 50 phr UAO was estimated by dynamic contact angle DCA. The advancing andreceding dynamic contact angles for the blend with 50 phr
UAO are shown in Table 2. A contact angle hysteresis ofabout 12 deg. is observed for non-UV cured and UV cured
blend. In general, the contact angle hysteresis of a polymer occurred because of the following factors: a) surface
contamination, b) surface roughness, c) heterogeneity ofsurface structure, d) reorientation or rearrangement of thesegment, e) swelling, and f) deformation. We presumed
that reorientation or rearrangement of the segment of the
non-UV cured and UV cured blends on the surface influences contact angle hysteresis, because the Tg of theblends is markedly lower than room temperature. In otherFigure 8

Relationship between 90° peel adhesion and loss
moduli E" =3G" for P(2EHA-AA) / UAO blends

words, the blends exhibit reorientation or rearrangementof a surface structure when they are moved from air into
water. These blends possess a large advancing contact
angle and a low receding contact angle. Presumably, theP(2EHA-AA) component can easily exhibit reorientation or
rearrangement at blends-water interface. The dynamic
contact angles of the non-UV cured blend were same asthat of the UV cured blend. This means that the segment
mobility of the polymer chain on the surface was notchanged by UV irradiation. We judged that segment
mobility on the surface did not influence the reduced peeladhesion and probe tack values, although the blends /water system differs from the blends / adherend in termsof interfacial condition.
Thus, we thought that the reduced peel adhesion andprobe tack values were caused by the moduli increase
and the Tg increase due to UV irradiation. Figure 8 shows
the relationship between 90° peel adhesion and loss moduli E" =3G" for P(2EHA-AA) / UAO blends. The 90° peeladhesion value decreases with an increase of E" =3G".
When the function of the debonding process influences
the PSA properties according to Tse's equation (1), the
PSA properties increase while increasing the loss moduli
E" =3G" of the adhesive polymer. However, for P(2EHAAA) / UAO blends peel adhesion value decreased with anincrease of E" =3G". Therefore, we thought that the deformation energy of UV cured blends when conducting adhesive tests was reduced by UV irradiation.

4.

CONCLUSIONSPSA properties were investigated for blends of P(2EHAAA) with UAO. The values of peel adhesion and probe
tack of the P(2EHA-AA) / UAO blends were dramaticallyreduced by UV irradiation. The mechanism of reduced
PSA properties was investigated through dynamic
mechanical properties, DSC, and DCA. Since moduli values and glass transition temperatures Tg of these blends

87after UV irradiation were higher than those of these blends
before UV irradiation, we judged that the reduced peeladhesion and probe tack values were caused by the moduli increase and the Tg increase due to UV irradiation. In
other words, the deformation energy of UV cured blends
when conducting adhesive tests was reduced by the curing process. Conversely, the DCA values of non-UV cured
blend were same as that of UV cured blend. We judgedthat the segment mobility of the polymer chain on the surface did not influence the reduced peel adhesion andprobe tack values. Finally, we point out that the P(2EHAAA) / UAO blends are very effective materials for dicing orback-grinding in semiconductor processes.

ACKNOWLEDGEMENTS
The authors express their thanks to Prof. T. Kasemura ofGifu University for the DCA measurements.

REFERENCES
1) M. F. Tse : J. Adhesion Sci. Technol. 3, 551 (1989)
2) H. W. H. Yang : J. Appl. Polym. Sci. 55, 645 (1995)
3) E. P. Chang : J. Adhesion 34, 189 (1991)
4) Y. Kano, N. Sato, and S. Akiyama : Polym. J. 23, 1489 (1991)
5) Y. Kano and S. Akiyama : J. Appl. Polym. Sci. 63, 307 (1997)
6) H. J. Kim and H. Mizumachi : J. Appl. Polym. Sci. 56, 201 (1995)
7) Y. Morimoto : "Latest Application Technologies of Adhesive
Products II", CMC Press, Tokyo, pp.171-174, 1998 (in Japanese)8) M. Iimi : Converting Technology 23 (5), 18 (1995)
9) H. Yoshikura : Industrial Materials 46 (4), 5 (1998) (in Japanese)10) Furukawa Electric Co. Ltd., United States Patent, 4999242,
514958611) K. Ebe and K. Kondo : Journal of Adhesion Society of Japan 33,
251 (1997) (in Japanese)12) Y. Kano and T. Ozawa : Proceeding of The Third China-Japan
Seminar on Advanced Engineering Plastics, Polymer Alloys andComposites, China, pp.117-119, 1998
13) Y. Kano, S. Ishiwata, and T. Ozawa : Adhesion, 43, 20 (1999) (inJapanese)
14) T. Kasemura : Membrane 21, 197 (1996)
15) Y. Kano, S. Akiyama, T. Kasemura, and S. Takahashi : Polym.
Networks Blends 6, 153 (1996)
16) C. A. Dahlquist : Tack in Adhesion : Fundamentals and Practice,
Ministry of Technology, McLaren, London, 142 (1969)17) M. A. Krenceski, J. F. Johnson, and S.C. Temin : Rev. Macromol.
Chem. Phys. C26, 143 (1986)
18) Y. Kano, S. Akiyama, and T. Kasemura : J. Appl. Polym. Sci. 50,
1619 (1993)

Manuscript received on April 21, 2000.

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Furukawa Review, No. 20

2001