Nano-Scale Conductive Films with Low Temperature Sintering for High Performance Fine Pitch Interconnect

Abstract

In this paper, a novel nano-scale conductive film which combines the advantages of both traditional anisotropic conductive adhesives/films (ACAs/ACFs) and nonconductive adhesives/films (NCAs/NCFs) is introduced and developed for next generation high performance ultra-fine pitch packaging applications. This novel interconnect film possesses the properties of electrical conduction along the z-direction with relatively low bonding pressure (ACF-like) and the ultra-fine pitch (< 100 nm) capability (NCF-like). Unlike typical ACF which requires 1-5 vol% of conductive fillers, the novel nano-scale conductive film only needs less than 0.1 vol% conductive fillers to achieve good electrical conductance in the z direction. The nano-scale conductive film also allows a lower bonding pressure than NCF to achieve a much lower joint resistance (over two orders of magnitude lower than typical ACF joints) and higher current carrying capability. With low temperature sintering of nano-silver fillers, the joint resistance of the nano-scale conductive film could be as low as 10^-5 Ohm, even lower than the NCF and lead-free solder joints. The insertion loss of nano-scale joints are almost the same as the standard ACF or NCF joints, suggesting that the nano-ACF joints are suitable for reliable high frequency adhesive joints in microelectronics packaging. The reliability of the nano-scale conductive film after high temperature and humidity test (85degC/85%RH) was also improved compared to the NCF joints. In order to reduce the silver migration and maintain a good insulation/dielectric property in the x-y plane for the nano-scale conductive film, self-assembled molecular wires (SAM) are used to passivate/protect the silver nano fillers. The protection of silver nano particles with molecular monolayers reduced the silver migration dramatically and no migration was observed upon application of high voltages (up to 500 V) due to the formation of surface chelating compounds between-
the SAM and nano silver fillers. The migration behavior of SAM passivated nano-Ag conductive adhesives was investigated by analyzing the results with the migration model.

Full text

Page 1

Nano-Scale Conductive Films with Low Temperature Sintering
for High Performance Fine Pitch Interconnect

Yi Li, Myung Jin Yim, Kyung Sik Moon and C.P. Wong
Georgia Institute of Technology
cp.wong@mse.gatech.edu

Abstract
In this paper, a novel nano-scale conductive film which
combines the advantages of both traditional anisotropic
conductive adhesives/films (ACAs/ACFs) and nonconductive
adhesives/films (NCAs/NCFs) is introduced and developed
for next generation high performance ultra-fine pitch
packaging applications. This novel interconnect film
possesses the properties of electrical conduction along the z-
direction with relatively low bonding pressure (ACF-like) and
the ultra-fine pitch (< 100 nm) capability (NCF-like). Unlike
typical ACF which requires 1-5 vol% of conductive fillers,
the novel nano-scale conductive film only needs less than 0.1
vol% conductive fillers to achieve good electrical
conductance in the z direction. The nano-scale conductive
film also allows a lower bonding pressure than NCF to
achieve a much lower joint resistance (over two orders of
magnitude lower than typical ACF joints) and higher current
carrying capability. With low temperature sintering of nano-
silver fillers, the joint resistance of the nano-scale conductive
film could be as low as 10-5 Ohm, even lower than the NCF
and lead-free solder joints. The insertion loss of nano-scale
joints are almost the same as the standard ACF or NCF joints,
suggesting that the nano-ACF joints are suitable for reliable
high frequency adhesive joints in microelectronics packaging.
The reliability of the nano-scale conductive film after high
temperature and humidity test (85oC /85%RH) was also
improved compared to the NCF joints. In order to reduce the
silver migration and maintain a good insulation/dielectric
property in the x-y plane for the nano-scale conductive film,
self-assembled molecular wires (SAM) are used to
passivate/protect the silver nano fillers. The protection of
silver nano particles with molecular monolayers reduced the
silver migration dramatically and no migration was observed
upon application of high voltages (up to 500 V) due to the
formation of surface chelating compounds between the SAM
and nano silver fillers. The migration behavior of SAM
passivated nano-Ag conductive adhesives was investigated by
analyzing the results with the migration model.
Introduction
Recently
(ACAs/ACFs)
(NCAs/NCFs) are becoming popular as one of promising
candidates for lead-free interconnection solutions in
microelectronic packaging application due to their technical
advantages such as fine pitch capability (<40 µm pitch), low
temperature processing ability, low cost and environmentally
friendly materials and process, etc [1-6]. ACAs/ACFs consist
of conducting particles (typically 5~10 µm in diameter) and
adhesives which provide both attachment and electrical
interconnection between electrodes. In particular, ACFs are
widely used for high-density interconnection between liquid-
anisotropic
and
conductive
nonconductive
adhesives/films
adhesives/films
crystal display (LCD) panels and tape carrier packages
(TCPs) to replace the traditional soldering or rubber
connectors. In LCD applications, traditional soldering may
not be as effective as ACFs in interconnecting materials
between indium tin oxide (ITO) electrodes and TCP.
Recently, ACFs have also been used as an alternative to
soldering for interconnecting TCP input lead bonding to
printed-circuit boards (PCBs). In addition to ACA/ACF,
NCAs/NCFs have been attractive due to the finer pitch
capability and lowest cost options for interconnection
materials without any conductive filler. As the fine-pitch
capability and low stress in the assembly is becoming hot
issues, ACA/NCA interconnection materials can be used more
frequently in joining materials.
However, there are several issues for ACF/NCF as lead-
free interconnection application. They need particular
bonding pressure for the assembly and interconnection. ACF
normally needs 100 gf per bump with bonding area of
100×100 µm2 for reliable contact resistance. NCF typically
needs more pressure than ACF such as 150 ~ 200 gf/bump,
which is one of limits for their application [7]. Another
limitation of ACF/NCF is the lower electrical properties
compared to solder joints because there is only
mechanical/physical contact of the joints and no metallurgical
contact of interconnects. To ensure low contact resistance and
high current density, interface between conductive fillers and
electrode should be improved [8-10].
One of the approaches to minimize the joint resistance is
to make the conductive fillers fuse each other and form
metallurgical contacts such as metal solder joints. However,
to fuse metal fillers in polymers does not appear feasible,
since a typical organic printed circuit board, on which the
metal filled polymer is applied, cannot withstand the high
melting temperature of conductive fillers. However, our
previous studies showed the low temperature sintering of
nano-sized conductive fillers at the processing temperature of
conductive adhesives [11]. As such, the use of the fine metal
particles would be promising for fabricating adhesives with
high electrical performance through eliminating the interface
between metal fillers. The application of nano-sized particles
can also increase the number of conductive fillers on each
bond pad and result in more contact area between fillers and
bond pads. Therefore, application of nano-sized particles has
potentials to improve the current density of the ACA joints by
distributing current into more conductive paths.
In this paper, to solve the issues (high bonding pressure
and lower electrical performance) of traditional ACF/NCF
while maintaining the advantages of ultra fine pitch and low
cost, a novel ACF/NCF incorporated with very low loading of
nano-scale conductive fillers (nano-silver particles) are
studied for next generation high performance fine-pitch
packaging applications [12]. This novel interconnect film (as
1-4244-0985-3/07/$25.00 ©2007 IEEE1350 2007 Electronic Components and Technology Conference

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illustrated in Fig. 1) combines the electrical conduction along
the z-direction (ACF-like) and the ultra-fine pitch (< 100 nm)
capability (NCF-like). Unlike typical ACF which requires 1-5
vol% of conductive fillers, the novel NCF only needs less
than 0.1 vol% conductive fillers to achieve good electrical
conductance in the z direction. In order to reduce the silver
migration and maintain a good insulation/dielectric property
in the x-y plane, self-assembled molecular wires are used to
passivate/protect the silver nano fillers. The morphology of
nano fillers is characterized using scanning electron
microscopy (SEM). The effects of nano-filler incorporation
and sintering on the electrical performance and reliability of
novel adhesive joints are investigated by comparing the
current-voltage relationship of traditional ACF/NCF and the
novel nano-ACF. Effects of monolayer protection on the
silver migration and electrical properties of nano-conductive
adhesives are evaluated. The high frequency characteristics of
the nano-conductive films are also evaluated before and after
high temperature/high humidity test (85oC/85%RH) and
compared with traditional ACF and NCF.


Fig. 1 Comparison of ACA, NCA, and novel nano particles
incorporated Z-axis conductive adhesive.

Experimental
Nano-scale conductive film formulations
The base formulation of conductive film was prepared with
epoxy, curing agent and silane coupling agent. 0.5 wt% nano
Ag particles were mixed with the base formation using
sonication for 3 hours. The film was pre-bonded on the
substrate at 80oC for 5 seconds. After removal of separator
film, the substrate was aligned and the final bonding was
conducted at 180oC with the application of different bonding
pressure.

Morphology observations
To study the sintering behavior of nano-fillers, the nano Ag
particles were annealed at 180oC and 250oC and the
morphology of the annealed Ag particles was observed by
scanning electron microscopy (SEM, Hitach S-800).

Electrical properties and reliability characterizations of
nano-Ag conductive films
The electrical resistance and current carrying capability of the
joints (contact area: 100 x100 µm2) on Au-finished test
vehicle was measured by a four-point probe method. The
applied currents were varied from 0.5-4.0 A by a power
supply and the voltage of the interconnect joints were
measured by a Keithley 2000.
The reliability of joints was conducted under the environment
of 85°C/85%RH (using an accelerated temperature and
humidity chamber from Lunaire Environmental, model
CEO932W-4). The joint resistance was achieved periodically
using the power supply and multimeter during aging.
The high frequency measurements were conducted with an s-
parameter Network Analyzer (HP Model 8720 ES). For
simple characterization and comparison among three adhesive
materials, the S-parameters were measured and compared.

Migration study of nano-Ag conductive adhesives
The formulated nano-Ag conductive adhesives were stencil
printed on the patterned FR-4 (Flame Resistant 4) printed
circuit boards with a spacing of 1.5mm between electrodes
(Fig. 2). The samples were cured at 150oC for 1 hour. A drop
of DI-water was placed between the electrodes. The leakage
current –voltage relationship of cured nano-Ag conductive
adhesives were measured with a multimeter (Keithley
6517A). The morphology of silver dendrites on the test
boards in the vicinity of nano-Ag conductive adhesives after
high voltage tests was observed with a microscope.


Fig. 2 Schematic illustration of migration test vehicle for
nano-Ag conductive adhesives


(a) (b)
Fig. 3 SEM photographs of nano Ag particles annealed at (a)
180oC and (b) 250oC for 30 minutes.
Results and Discussion
Sintering of nano Ag particles
In order to study the morphology and sintering behavior of
nano silver particles after curing of ACF, the nano Ag
particles were annealed at 180 oC and 250oC for 30 minutes,
respectively. From SEM photographs shown in Fig. 3,
obvious sintering behavior has been observed after annealing.
The particles were fused through their surface and many of
Nano-scale
conductive adhesives
V
DI water
Tips
13512007 Electronic Components and Technology Conference

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dumbbell type particles could be found. The morphology was
similar to a typical morphology of an initial stage in the
typical sintering process of ceramic, metal and polymer
powders. This low temperature sintering behavior of the nano
particles is attributed to the extremely high inter-diffusivity of
the nano particle surface atoms, due to the significantly
energetically unstable surface status of the nano particles, in
particular, their high surface-to-volume ratios. However, there
was not much difference for the particles annealed at 180oC
and 250oC, indicating 180oC (a typical curing temperature)
was sufficient to get the sintered particles.

0.51.01.52.0 2.53.03.5
0.0
0.1
0.2
0.3
0.4
0.5

Voltage (mV)
Current (A)
0.5 wt% Nano Ag-filled ACF @180
0.5 wt% Nano Ag-filled ACF @180
NCF @180
NCF @180
oC/2min
oC/120min
oC/2min
oC/120min

(a)
0.51.01.52.02.53.03.5
0.00
0.04
0.08
0.12
0.16
0.20


Contact Resistance (mΩ Ω)
Current (A)
0.5wt% Nano Ag-filled ACF @180
0.5wt% Nano Ag-filled ACF @180
NCF @180
NCF @180
oC/2min
oC/120min
oC/2min
oC/120min

(b)
Fig. 4 I-V characteristics (a) and I-R relationships (b) of
0.5wt% nano Ag-filled ACF and NCF joints with different
curing time
0.00.51.01.52.02.53.03.5 4.0
10
-3
10
-2
10
-1
10
0
10
1


Current (A)
ACF (Au/polymer filler)
NCF
nano-ACF
lead-free solder
resistance (mΩ)

Fig. 5 Comparison of I-R relationships of typical ACF, NCF
and nano-Ag ACF.
Electrical properties of nano-scale conductive film (nano-
ACF) with sintering
To study the electrical performance of nano-ACF, the
current-voltage (I-V) relationship and correspondingly the
current-resistance (I-R) relationship of the nano-ACF joints
are shown in Fig. 4 with various curing conditions and
compared with NCF joints. As can be seen from the figure,
for the nano-ACF joints cured at 180oC for 2 minutes, a joint
resistance of ~ 0.08 mΩ and current carrying capability of ~
2.4 A were observed. The joint resistance was two orders of
magnitude lower than a typical ACF (with micron-sized Au-
coated polymer fillers). And the current carrying capability
was similar. The reduced joint resistance with nano-Ag ACF
should attribute to the superior electrical conductivity of
silver fillers. When increasing the curing time from 2 minutes
to 60 minutes, the joint resistance could be further reduced by
over one order of magnitude (0.005 mΩ). In addition, the
current carrying capability could also be enhanced from 2.4 to
3.4 A by prolonging curing time. The further improved
electrical performance (lower contact resistance and higher
current density) should be due to the further sintering with
longer time. The more sintering of nano-sized fillers could
enhance the interfacial properties of the ACF joints. As such,
the joint resistance, which is a sum of bulk resistance and
interfacial resistance, would be significantly reduced. In
addition, the thermal properties could also be enhanced by
improving the interfacial properties. The higher thermal
performance could help dissipate the heat more efficiently at
the adhesive joints generated at high current. Therefore, with
more sintering of nano-Ag fillers, the current carrying
capability was also improved. For the NCF joints, although
the joint resistance was much lower than nano-ACF when
cured at 180oC for 2 minutes, furthering prolonging curing
time to 120 minutes did not induce much improvement of the
joint resistance and current carrying capability.
The I-R relationship of nano-ACF was compared with
NCF and typical micron-filler (Au-coated polymer) ACF in
Fig. 5. With sufficient sintering, the nano-Ag ACF could have
a joint resistance over two orders of magnitude lower than the
typical ACF with micron-sized Au-coated polymer fillers. In
addition, the current carrying capability was also dramatically
enhanced due to the superior interfacial properties and thus
the more efficient thermal transport. The joint resistance and
current carrying capability were even better than that of the
NCF and lead-free solder joints.

Effect of bonding pressure on electrical property of nano-
ACF/NCF Joint
The usefulness of nano-Ag-filled ACF is not only to
improve the electrical properties, but also expected for the
reduced bonding pressure to have low contact resistance
compared with conventional NCF joint. In order to study the
required bonding pressure to achieve low resistance for nano-
ACF and NCF, the bonding pressure during thermo-
compression bonding of nano-Ag-filled ACF and NCF joints
were varied and the contact resistance of the joints was
measured. Fig. 6 shows the relationship between the bonding
pressure varied from 50 to 200 MPa and the contact resistance
of nano-Ag-filled ACF and NCF joint.
13522007 Electronic Components and Technology Conference

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As the bonding pressure increases, the contact resistance
of ACF/NCF joint generally decreases due to the contact area
increment. For the adhesives joints, there was a threshold
(minimum) pressure below which the contact resistance
remained a very high value. The result shows that the
minimum bonding pressure for the nano-Ag-filled ACF was
150 MPa, while the bonding pressure for NCF was 200 MPa.
This implies that the bonding pressure can be decreased for
nano-Ag-filled ACF joints.
050
Bonding pressure (MPa)
100150200
0.1
1
10
100

Resistance (mΩ Ω)
NCF
nano-filler ACF

Figure 6 Effects of bonding pressure on the joint resistance of
NCF and nano-ACF

Reliability of nano-Ag-filled ACF joint
The electrical reliability of nano-Ag-filled ACF joint was
evaluated under high temperature and humidity environment
(85oC /85%RH) in comparison with NCF joint. Fig. 7 shows
the contact resistance behaviors of nano-filler ACF and NCF
joints under 85oC/85%RH condition for over 500 hours. The
graph clearly shows that the nano-filler ACF joint has more
stable joint resistance than NCF joints with aging at 85oC
/85%RH, indicating a better reliability. This reliability
improvement with nano-ACF should be attributed to the
stable electrical contact between two Au electrodes by
sintering effect of nano-Ag fillers even under high
temperature and humid condition.
0100200 300400500600
0.00
0.05
0.10
0.15


Resistance (mΩ Ω)
Time in 85
oC/85%RH (hrs)
NCF
nano-filler ACF

Fig. 7 Reliability of NCF and nano-ACF under 85oC/85%RH
Migration behaviors of nano-Ag conductive adhesives with
self-assembled molecular wires (SAMs)
When applying silver as conductive fillers, in particular
for fine pitch interconnects applications, the silver migration
issues need to be addressed and solved. In order to reduce the
silver migration and maintain a good insulation/dielectric
property in the x-y plane, self-assembled molecular wires are
used to passivate/protect the silver nano fillers. Fig. 8 (a)
(dots) shows the I-V relationship of different nano-ACA at
low voltages (0 - 5V). An obvious threshold voltage of
migration was observed at 0.5 V for all the nano-ACA. Below
the threshold voltage, the leakage currents were stabilized at a
near-zero value, however, when the voltage increased over
0.5 V, the leakage current occurred. Although the untreated
nano-Ag ACA showed a dramatic increase with increasing
voltage, the nano-ACA incorporated with SAMs showed
much slower increase in leakage current. In comparison of
two types of acids, the di-acid showed a more significant
improvement than the mono- acid.

Fig. 8 Leakage current-Voltage (I-V) relationship of nano-
Ag ACA at (a) low voltages and (b) high voltages.

The obviously stabilized current leakage and subsequently
the well-controlled electrochemical migration are due to the
protection of silver ions with carboxylic acids by forming the
chelating complexes. The adsorption of carboxylic acids on
silver has been widely studied. The reaction is considered as
an acid-base reaction, and the driving force is the formation of
chelating bond/complex between the carboxylate anion and
the surface silver ion. (Eqn. 1)
Ag - e- → Ag+; Ag+ + COO- → Ag+…COO- (1)

It was also reported that on Ag surfaces, the two oxygen
atoms in carboxylate tend to delocalize and bind to the
surface nearly symmetrically. The bonding can change the
surface properties of silver and control the properties of the
metal-organic interfaces. To reduce the energy, the molecules
of mono-carboxylic acids tend to have an all-trans
conformation on bonding to silver surface. By adopting the
all-trans forms, the molecule may lie close to the surface with
a hydrophobic tail (-CH3), protecting the silver clusters. For
di-acid, it was found that both carboxylate groups were
chelating to silver surface sites. The molecule gains sufficient
stability by bonding two carboxylate groups to the surface
1353 2007 Electronic Components and Technology Conference

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that it is able to adopt less favorable chain conformation. With
the incorporation of SAMs, the interface properties of nano-
ACA could be modified. Unlike typical nano-ACA in which
Ag+ is the major migration species, for SAM-incorporated
nano-Ag ACA, the major migration component becomes the
chelate Ag+…COO-. Ag+…COO- has a lower solubility in
water and higher activation energy (lower driving force) for
migration towards cathode than that of free Ag+, due to the
neutral charge of the Ag+…COO- complex. According to a
semi-empirical model, the migration behavior can be
approximated as Eqn. 2. [13]
ZFDE
ZFDCJ
−−=
))
)/exp(
exp(1 )(/exp(
))exp(1 (
0
0
00
t
RT
EkTHZFD
kTHDZFC
t
RT
D
D
∆−
−−∆−=
(2)
In the equation, Z=valence, F=Faraday’s electrochemical
equivalent, D is the diffusitivity of migration component,
D=D0exp (-∆HD/kT), where ∆HD is activation energy of
diffusion component, C0 is the ionic concentration of
migration component, E=V/d is the electric field, where V is
the applied voltage, d is the distance between electrodes, t is
time, k and R are Boltzmann constant and molar gas constant,
respectively. The lower solubility of Ag+…COO- complex
decreased the C0 value, while the higher activation energy
decreased D value in Eqn. 2. As such, migration became a
kinetically unfavorable reaction and the leakage current of
carboxylic acid incorporated nanocomposites were much
lower than that of untreated composites. Comparing mono-
and di-carboxylic acids, di-carboxylic acid performed better
in terms of migration control, due to the more coverage on Ag
surfaces, consequently, it provided a better protection of
silver from migration.
It is noticed that the curves of leakage current – voltage
relationship can be well fitted into a linear relationship (Fig. 8
(a), lines) by the polynomial fitting of the curves. The linear
relationship can be explained by extracting Eqn. 2. At low
voltages, ZFDE/(RT)t <<1, then Eqn. 2 can be rewritten as
Eqn. 3.
)()())exp( 1 (
000
t
RTd
ZFDV
ZFDCt
RT
ZFDE
ZFDC t
RT
ZFDE
ZFDCJ
=≈−−=
∝ V (3)
Therefore, the leakage current is almost proportional to the
applied voltage.
When the nano-ACAs were tested at higher voltages (5-500
V), the difference of leakage current of nanocomposites was
more apparent (Fig. 8 (b), dots). The SAM incorporated nano-
ACAs showed a more stable leakage current value than the
untreated nano-ACA, which had a dramatically increased
leakage current, in particular, at voltage higher than 200 V.
By using the polynomial fitting of the curves of leakage
current – voltage relationship, it was found that under high
voltage testing, the curve of untreated nano-Ag ACA can not
be simply fitted linearly, instead, a three-order polynomial fits
the curve very well. On the other hand, SAMs incorporated
nano-Ag ACAs still follow a linear relationship (Fig. 8 (b),
lines). The difference can also be explained by analyzing Eqn.
2. For untreated nano-Ag composite at high voltage V,
ZFDE/(RT)t is not a near-zero value and Eqn. 2 cannot be
simplified as Eqn. 3. According to Taylor expansion, Eqn. 2
can be expanded as Eqn. 4.
...))(
6
1
)(
2
1
)(( )) exp(1 (
0
32
0
++−=−−=
t
RT
ZFDE
t
RT
ZFDE
t
RT
ZFDE
ZFDCt
RT
ZFDE
ZFDCJ

(4)
For the SAM incorporated nano-Ag ACAs, however, due to
the much higher activation energy ∆HD and subsequently the
much lower D value, ZFDE/(RT)t << 1, and thus, Eqn. 2 can
also be simplified as a linear relationship as Eq. 3. Although
di-acid showed better protection of nano-silver fillers at low
voltages than mono-acid protection, the difference at high
voltages was not obvious.
Fig. 9 shows the photographs of test boards in the vicinity of
nano-ACAs after high current-voltage tests. Obvious silver
dendrites with several branches were observed for untreated
composite, indicating the severe Ag migration upon high
voltage. For the di-acid incorporated nano-Ag ACA, however,
no obvious dendrites were detected which indicated effective
silver migration control. The dark area around the edge of
nano-Ag ACA is considered from the typical inter-diffusion
between different materials rather than the ionic migration.

Fig. 9 Morphology of Ag dendrites after high voltage
migration tests. (a) untreated nano-Ag ACA showed obvious
dendrite formation and (b) di-acid incorporated nano-Ag
ACA showed no dendrite formation.

High frequency characteristics of nano-ACF/NCF joitns
The high frequency characteristic for nano-ACF joint is
one of important properties for maintaining signal integrity in
high speed flip-chip device interconnection. Therefore, it is
necessary to characterize the high frequency behaviors of
developed nano-ACF in comparison with NCF and
conventional micron-sized conductive filled ACF in the flip
chip structure. We measured the S-parameters, S21, of flip
chip joints using nano-ACF, NCF and micron sized particle-
filled ACF. The test vehicle for the high frequency electrical
characterization consists of two-port GSG 500 µm
13542007 Electronic Components and Technology Conference