The residual stress behavior of Epoxy/Polyimide thin film

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The residual stress behavior of Epoxy/Polyimide thin film


Won Bong Jang
, Jonghwae Lee, Hyun Soo Chung, and Haksoo Han

Dept. of Chemical Engineering, Yonsei University


Introduction

Advanced composite

materials, surface coatings and electronic circuit
encapsulants are examples of applications involving the cure of a thermoset in contact with a
solid substrate. In such processes the shrinkage of the polymer will be partly constrained by
the substrate, t
hereby generating stresses at the interface between the polymer and the
substrate. High stress levels may greatly reduce the technical performance of the system such
as cracking, interface debonding, and dimensional instability.

Epoxy resins are a versati
le group of crosslinked polymers that have excellent
chemical resistance, good electrical insulating properties, good adhesion to glass and
materials, and can be easily fabricated. The variety of properties helps the epoxy resins to
meet the performance re
quirements of some demanding applications. These include areas as
diverse as construction, electronics, adhesives, and coatings.[1
-
4] However,
The usefulness
of epoxy resins for many applications is often limited due to their inherent brittleness
resulted
from their crosslinked structure.

Development of approaches for toughening the epoxy resins without sacrificing
modulus and glass transition temperature (Tg) would lead to an increase in their applications.

The most studied approach to toughen epoxy resin

was the use of organic rubbers as
toughening additives.[5] While rubbers can be extremely effective as toughening agents,
epoxy resins suffer from some drawbacks such as a reduction in overall resin modulus and in
end use temperatures. A method which has
found increasingly widespread use is the
incorporation of the thermoplastic toughening agents for epoxy resins such as
poly(ethersulfone)[6,7], poly(phenylenether)[8], poly(etherketone)[9], polyester[10], and
poly(etherimide).[11,12]

Polyimides have been w
idely used as protective overcoats and dielectric layers for
semiconductor devices because of their good properties, e.g. excellent thermal stability, high
chemical resistance, good mechanical properties, low dielectric constant, and easy
processability.[1
3,14]
There are many studies which have used polyimides in epoxy systems
intending mainly to improve the thermal stability and toughness. Almost all of the studies are
based on physical blending of unreactive linear polyimides.[15]

In this study, I will ma
ke another approach to prepare epoxy
-
polyimide (EP
-
PI)
composites of high thermal stability and good mechanical properties and use soluble reactive
polyimide containing hydroxyl functionalities as a hardener. This method has an advantage
that the shrinkage

during cure encountered with using poly(amic acid) will be avoided. The
presence of polyimide is capable of exhibiting flexibility characteristics and noticeable
thermal stability as well as being curing agents on their own. An aromatic polyimide
containi
ng pendent hydroxyl groups
ortho

to the heterocyclic imide nitrogen was found to
rearrange to a 2,2
-
bis(3,4
-
dicarboxyphenyl)hexafluoropropane upon heating above 220
o
C in
an inert atmosphere. A hydroxy
-
containing fully aromatic polyimide film based on 2,2
-
b
is(3,4
-
dicarboxyphenyl)hexafluoropropane (6FDA) and 2,2
-
bis(3
-
amino
-
4
-
hydroxyphenyl)
-
hexafluoropropane (AHHFP) was prepared by thermal curing method and then reacted with
Biphenyl epoxy resin. The resulting film was found to be amorphous by wide angle X
-
ra
y
diffraction (WAXD). The film also showed excellent solvent resistance and good thermal
stability by Differential Scanning Calorimeter in nitrogen
occurring

at 500
o
C.


Experimental

A. Reagent
s

2,2
-
Bis(3,4
-
dicarboxyphenyl)hexafluoropropane dianhydride (6FD
A; 99% purity
grade) and
2,2
-
bis(3
-
amino
-
4
-
hydroxyphenyl)
-
hexafluoropropane

diamine (AHHFP; 99%
purity grade) were obtained from CHRISKEV Co. and used without sublimation.
N
-
Methyl
-
2
-
Pyrrolidinone (NMP; 99% purity grade)

purchas
ed

from Aldrich Co. and was

used after
distillation. Epoxy resin (epoxy equivalent weight equals 185) based on Biphenyl was
supplied by EPICLON Co. in Japan


B. Sample preparation

AHHFP (5.00mmol) and NMP were placed into a flask equipped with a nitrogen
inlet and a mechanical stirr
er and then stirred until a clear solution was obtained. Equimolar
amount of
solid

6FDA (5.00mmol) was added to the solution of AHHFP at a concentration of
7wt% solids. Then the obtained brown viscous solution was precipitated in water, filtered,
washed wi
th water, and dried under vacuum at 50
o
C. The overall yield of poly(amic acid)
was 3.76g (92.8%). Poly(amic acid) powder was heated to convert it into polyimide. IR (KBr,
cm

1): 1780 and 1720 (C=O); 1370 (C
-
N); 3400~3500(OH), as shown in Fig. 2.

The epoxy

resin and the polyimide were mixed together into NMP. The mixture
was stirred under nitrogen atmosphere at room temperature until a clear homogeneous
solution was obtained. The solution was cast on
a one
-
side polished
3in(
76.2mm
)

Si (
100
)
wafer as substra
te used in this study.

All solutions were spin
-
coated on silicon wafers and
soft baked at 80
o
C for 30min.



C. Measurement


The curvatures of Si wafer with and without films were measured at room
temperature in nitrogen using He
-
Ne laser beam based home
-
m
ade stress analyzer. The
residual stress of the films was calculated from the radii of the wafer curvatures measured
before and after the film deposition using the following equation (1).
[16]








(1)

W
here,


is the residual stress
in the
epoxy/
polyimide film
. The

subscripts
,

f

and
s

denote the
polyimide film and the substrate.
E
,
v
, and
t

are Young’s modulus, Poisson’s ratio, and the
thickness of the substrate, respectively. R
1

and R
2

are the wafer curvatures measured before
and aft
er the film deposition.
For Si (
100
) wafer, E
s
/(1
-

s
) is 180500Mpa.[17] The ramping
rate was 2.0
o
C/min and the cooling rate was 1.0
o
C/min. The stress behaviors of EP/PI films
were
in
-
situ
measured after conventional thermal imidization from 25
o
C~220
o
C (Fig
.3).

Wide Angle X
-
ray Diffraction (WAXD) measurements were conducted in

the

/2


method over 3~60
o
(2

) using a Rigaku diffractometer with CuK


(

=
1.54Å
) radiation
source. Data were taken every 0.02
o

at a scan speed of 0.4
o
/min. The measured WAXD
patterns w
ere corrected with the background run and then normalized for the film samples.
[18,19] The reflection pattern in which the diffraction vector is in the vertical direction to the
film plane gives the structural information in the direction of film thicknes
s, and the
transmission pattern in which the diffraction vector is in the film plane provides the
structural information in the film plane.

For the other characteristic property, Differential Scanning Calorimetery (DSC)
(Polymer Lab.) was used.


Results a
nd Discussion

Reaction of 2,2
-
bis(3,4
-
dicarboxyphenyl)hexafluoropropane (6FDA) and 2,2
-
bis(3
-
amino
-
4
-
hydroxyphenyl)
-
hexafluoropropane (AHHFP) in
N
-
Methyl
-
2
-
Pyrrolidinone

(NMP) at
ambient

temperature gave the soluble polyamic acid. Attempted thermal solutio
n
imidization converted it into the hydroxy
-
containing soluble polyimide shown in Fig.1.
Epoxy resin (Biphenyl) was blended with the soluble polyimide in various ratios using NMP.
All the blend solutions were clear and transparent in any weight ratio. The
reaction between
phenolic hydroxyl groups on the polyimide and epoxide group results in the formation of
secondary hydroxyl group which further reacts with epoxide groups as shown in Fig. 1.

The residual stress behaviors measured on Si (
100
) substrate were

analyzed
. For
Epoxy/Polyimide (EP/PI) films, as the composition of polyimide increased, the residual
stress drastically decreased in
tension

mode. It is considered that
residual

stress behavior
induced between substrate and EP/PI thin film strongly depend
ed upon the morphological
structure of EP/PI film. The morphological structures of the EP/PI composites from three
components can be obtained from WAXD patterns (Fig. 4). EP/PI chains containing hydroxy
functionalities

and bulky di(trifluoromethyl) group a
re mainly not only amorphorous in the
film plane and out of plane due to the flexible chain nature, but also irregularly packed
together. The incorporation of polyimide segments into the epoxy backbone may influence
easier chain mobility. In other words, t
he content of hydroxy functionalities containing
polyimide segments attached on the EP/PI, it
severely

affected the stress decreasing in the
tension

mode of the resulting EP/PI films. It significantly increased chain mobility in
addition to disturbing the
molecular chain order in the resulting EP/PI films.

With these results, the effect of polyimide
segments

on these residual stress
behaviors might result from some differences in the morphological structures of EP/PI films
prepared from different compositio
n.


Conclusion

With 2,2

-
bis(3,4
-
dicarboxyphenyl) hexafluoropropane dianhydride (6FDA), and
2,2
-
bis(3
-
amino
-
4
-
hydroxyphenyl)
-
hexafluoropropane (AHHFP) the homogeneous polyamic
acids were synthesized and thermally converted to the polyimides containing hydr
oxy
functionalities. In this study, The used method has an advantage that the shrinkage during
cure encountered with using poly(amic acid) could be avoided. The presence of polyimide
was capable of exhibiting flexibility characteristics and noticeable ther
mal stability as well as
being curing agents on their own. For all the EP/PI films, both residual stress and
morphological structure were significantly dependent upon the polyimide composition in the
backbone structure.


Acknowledgement

We would like to th
ank
Electronic Display Industrial Research Association of Korea
(EDIRAK)

for financial support of this work.


Reference

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Fig. 2. IR
-
spectra of Polyimide cured at 150
o
C for 1hr


and 200
o
C for 2hr.






Fig. 1. Thermal imidization
of 6FDA/AHHFP polyimide

and Reaction profile with epoxy resin



Fig. 3. Bending beam experimental layout

Fig. 4.

WAXD pattern
s

for reflection mode of
EP
-
PI

film cured to 220
o
C.