Journal of Photopolymer Science and Technology
Volume 30, Number 2 (2017) 181－185 Ⓒ 2017SPST
Development of Photosensitve Polyimide B-stage
Sheet Having High Cu Migration Resistance
Masao Tomikawa*, Kazuyuki Matsumura, Yu Shoji, Yoshiko Tatsuta,
and Ryoji Okuda
Electronic & Imaging Materials Research Laboratories, Toray Industries
3-1-2 Sonoyama, Otsu-City, Shiga-Ken 520-0842, Japan
Phone: +81-77-533-8420
*Masao_Tomikawa@nts.toray.co.jp
Fan-out type wafer level packaging (FO-WLP) technology is one of promising next
generation semiconductor package. The FO-WLP technology requires fine pattern pitch
re-distribution layer (RDL) with good electrical insulation. In order to meet those
requirements, we examined Cu migration resistance of two types of polyimides and
photosensitive system under high temperature, high humidity, and high electronic field in
order to meet the requirement electrical insulation of fine pattern pitch. We found that there
is no large difference between negative photosensitivity and positive photosensitivity.
Polyimide structure significantly affects Cu migration under bias HAST condition. From
these results, we will describe highly reliable PSPI dry film and coatings. From those
results, we developed a reliable positive tone photosensitive polyimide B-stage dry film.
Keywords: Polyimide, Insulation, Reliability, Migration
1. Introduction
Recently a Fan Out Wafer Level Packaging
(FO-WLP) technology applys for various types of
semiconductor devices, such as power controller,
RF device, and application processor of smart
phone [1]. Because FO-WLP offers low profile of
package with high reliability, so it is desirable for
apply for the package in the smart phone. FO-WLP
was composed of Si chip was placed on
re-distribution layer (RDL) and molded by resin.
In general process flow of FO-WLP is placing a Si
chip in a mold resin, then RDL was formed on the
Si chip and mold resin. So due to thermal stability
limitation of molding resin, RDL process should be
below 200 oC. Photosensitive polyimide or PBO
were good candidates for RDL insulator due to its
good mechanical properties.
However most of polyimides and PBOs were
required rather higher curing temperature. So low
temperature curable photosensitive polyimides and
PBOs with mechanical toughness were desired and
reported [2]. In addition it is desired to show good
insulation properties. Especially the FO-WLP for
an application processor required good insulation
property at fine Cu pattern [3]. In this paper we are
Received
Accepted
April
May
2, 2017
12, 2017
examined the effect of polyimide structure and
photosensitive system (negative and positive) on
Cu migration at high temperature humid condition
under high electronic field.
2. Experimental
2.1. Polyimide preparation
The polyimide resin was obtained by
polycondensation of tetracarboxylic dianhydrides
and diamines. A fixed amount of diamines were
placed in a 4 neck flask with a mechanical stirrer,
thermometer and nitrogen inlet, and dissolved into
N-methyl-2-pyrridone (NMP, Mitsubishi Chem.)
and heated to 60 oC under nitrogen flow. A fixed
amount of tetracarboxylic dianhydrides was added
to the diamines solution with NMP at 30 wt%
concentration. The mixture was stirred for 1 hour
at 60 oC, then heated to 180 oC. The
polycondensation reaction was carried out at 180
o
C for 4 hour.
After cooling the poiyimide solution to room
temperature, the solution was poured into the water
to precipitate the polyimide. The polyimide
precipitate was collected by filtration and washed
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J. Photopolym. Sci. Technol., Vol. 30, No. 2, 2017
by water 3 times. The obtained polyimide was
dried at 80 oC for 72 hour in a convection oven.
2.2. Photosensitive polyimide solution preparation
The photosensitive polyimide solution was
obtained by the following procedure. Polyimide
was measured 10 g then dissolved in a
-butylolactone (GBL, Mitsubishi Chem,) at 35
wt% concentration.
(a)
OR
RO
OR
O
N2
: H = 2.8:0.2
R=
SO2
(b)
H3COH2 C
CH 2OCH 3
N
N
N
H 3COH 2 C
CH2OCH 3
N
CH 2OCH 3
N
N
CH2 OCH3
(c)
(d)
Fig. 1. Chemical structures of (a) TKF-280, (b)
MW-100LM, (c) OXE-02, and (d) BPE-6.
182
A diazonaphthoquinone compound (TKF-280,
Sanbo Chem. 1g) and thermal cross-linker
(MW-100LM, Sanwa-Chem, 0.4 g) were added to
the solution. The obtained solution was filtered by
0.47 m PTFE filter prior to use.
A negative tone photosensitive polyimide
solution was obtained by following procedure.
Polyimide was weighed about 10 g and add GBL
to obtain 35 wt% solution. A photo-iniator
(OXE-02, BASF), 0.3 g and acrylic monomer
(BPE-6E, Shin-nakamura Chemical), 2 g were
added to the solution. The solution was filtered
prior to use by 0.47m PTFE filter.
Chemical structures of those additives are shown
in Fig. 1. UV-vis spectra were taken on a
Shimadzu UV-2400 PC. The patterns of the
resists were observed using a Keyence digital
microscope VHX-200.
Screen printing was
carried out using a screen-printing machine
(LZ-1232, Newlong Seimitsu Kogyo Co., Ltd).
2.3. Photosensitive dry film preparation
Photosensitive dry film was prepared by the
following procedure. The photosensitive polyimide
solution with various types of additive (0.3 g) was
mixed and coated on surface treated PET film at 50
m wet thickness by bar coater. The coated film
was dried in a convection oven at 80 oC for 15 min.
The dried film was used as a photosensitive dry
film.
The photosensitive dry film was laminated on a
Si wafer by pressing the dry film at 120 oC by
hand-roller.
The
laminated
photosensitive
polyimide was exposed at 300 mJ/cm2 (@ 365 nm)
by a contact aligner through a photo mask. The
exposed photosensitive dry film was developed by
2.38% tetramethylammonium solution (TMAH) at
23 oC for a fixed time. It was then cured at 120 oC
for 10 min and heated to 200 oC and stored 200 oC
for 1 hr under nitrogen flow.
2.4. Cu migration measurement
The photosensitive polyimide solutions were
coated on a TEG with a comb-type electrode
(Philtech) to cover the pad area by scotch tape to
coat completely by spin coating method then
heated to 180 to 250 oC for 1 hour under nitrogen
flow.
The electric resistance change (Bias HAST test)
was monitored by SIR-13 system (ESPEC
corporation) under 130 oC 85% HAST condition at
2 MV electric field strength. Test piece structure
was shown in Fig. 2.
J. Photopolym. Sci. Technol., Vol. 30, No. 2, 2017
(a)
Fig. 2. Cu migration test structure.
3. Results and discussion
3.1. Polyimide preparation
The obtained polyimides molecular weights
were measured by GPC to check the reaction. In
this work, the molecular weights of polyimides
were distributed from 20K to 40K at Mw. This
molecular weight range is suitable for
photo-lithographic performance.
We changed the diamine structure and
dianhydride structure from flexible/soft to rigid.
The rigid structure was created with an aromatic
ring or aliphatic ring. The soft structure was
created with alkyene groups.
3.2 Bias HAST test
Results of bias HAST test examples are shown
in Fig. 3 and summarized in Table 1. As shown the
figure, the polyimide having the soft segment
exhibits better reliability. We examined the tested
TEG and found cracks in the TEG using the rigid
polyimide after HAST test. We expected that the
rigid polyimide would show better resistivity
because of the rigid structure. During the HAST
treatment, the rigid polyimide cracked due to poor
Cu adhesion or low elongation to break.
We expected the polyimide having the soft
segment to show lower reliability due to its low Tg.
Because the molecular motion of a low Tg
polyimide is not restricted, the polymer motion
allowed for low break down voltage in general [4].
However, the soft polyimide shows good insulation
reliability. We think following facts are considered.
One is that the 114 oC Tg of the soft polyimide is
still too high for complete molecular relaxation at
HAST condition. The other fact is the soft
polyimide shows intrinsic high resistivity. To
understand further, reliability investigations of
polyimides having a wide range of Tg are required.
3.3. Preparation of photosensitive sheet
In order to obtain a positive tone photosensitive
dry film, we examined various types of additives to
(b)
(c)
(d)
Fig. 3. Resistivity change of (a) Positive-tone, Rigid
Polyimide, (b) Negative-tone, Rigid Polyimide, (c)
Positive-tone, Soft Polyimide, (d) Negative-tone, Soft
Polyimide.
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J. Photopolym. Sci. Technol., Vol. 30, No. 2, 2017
Table 1. Summary of this work
decrease melt viscosity at B-stage. The examined
compounds were epoxy resin, oxetane compound,
methyrol compound.
The positive tone photosensitive polyimide was
obtained by mixing with diazonaphthoquinone
photo active compound and polyimide. In case of
positive tone photosensitive chemistry, unexposed
area remains due to dissolution inhibition by the
diazonaphthoquinone compound.
The diazonaphthoquinone compound is
decomposed to indene carboxylic acid by UV
exposure [5]. So the remaining polyimide pattern
is
composed
of
heat-decomposed
diazonaphthoquinone compound and polyimide.
Hayase et. al. reported that negative tone
photosensitive polyimide pattern was obtained by
mixing with polyimide precursor (Poly(amic acid))
and diazonaphtoquinone compound with relatively
high post bake temperature (140 oC) by diluted
TMAH solution [6].
We
expect
the
decomposition
of
diazonaphthoquinone compound at heat curing
process was initiated to release nitrogen and form a
carbene. The carbene was reactive intermediate
to react with polyimide to form inert resin. But
there is other possibility to form sulfonic acid to
decompose the diazonaphthoquinone compound.
We checked the pH of the cured polyimide resin to
immerse in water. The pH is neutral (about 7), so
we conclude the sulfonic acid was not formed in
this experiment.
In case of negative tone photosensitive
polyimide, radical type photo initiator,, acrylic
monomer and polyimide were used. Muramatsu et.
al. examined the photo reaction of oxime ester type
photo-initiator [7]. In their work, photo initiating
product was secondary decomposed methyl radical.
Then, acrylic monomer was converted to
cross-linked network with polyimide by initiating
radical. After forming the cross linking network,
molecular motion of the polyimide was limited due
to network formation. So, the networked polyimide
shows good electrical insulation.
184
From these results, we successfully developed a
positive high reliable photosensitive polyimide
B-stage dry film. The B-stage dry film covers the
topological surface completely and forms a 10 m
fine pattern resolution in a 22 m thick film (Fig.
4) and shows fine pattern resolution (Fig. 5).
Fig. 4. Cu migration test structure.
Fig. 5. Positive photosensitive polyimide pattern after
Cure (10 m bia with 22 m thick).
4. Conclusion
We investigated the effect of polyimide structure
having soft segment or hard segment on the
reliability of electronic resistance under HAST
condition. A polyimide having soft segments
shows better reliability than polyimides having
hard segments. This is due to the Cu adhesion or
polyimide elongation. From these results, we
developed reliable photosensitive polyimide
B-stage dry film with positive photosensitive.
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