IEEE TRANSACTIONS ON ELECTRONICS PACKAGING MANUFACTURING, VOL. 33, NO. 1, JANUARY 2010 65
Investigation of Thermosonic Wire Bonding
Resistance of Gold Wire Onto Copper Pad
Yeau-Ren Jeng, Sang-Mao Chiu, Pay-Yau Huang, and Shiuh-Hwa Shyu
Abstract This paper discusses the electric performance for
thermosonic wire bonding of gold wire onto copper pads. Various
methods normally used to improve bondability were investigated
including the bare copper pads with argon shielding gas and the
copper pads with cupric oxide film, cuprous oxide film, and silver
film. The micro-contact theory was used to determine the effective
contact area. The circuit contact resistance was measured for
each sample and was presented in terms of ultrasound power and
effective contact area. The results show that the increase in the
effective contact area leads to a lower circuit contact resistance
before reaching a minimum value, and further increase in the
effective contact area would not have noticeable effect on the
resistance. Fig. 1. Schematic of a thermosonic wire bonding machine.
Index Terms Chips with copper interconnects, contact resis-
tance, micro-contact theory, thermosonic bonding.
have reported that the oxide layer formed on the surface of
copper pads is mainly cupric oxide. Since cupric oxide is not
a passivation oxide layer, it is unable to protect the sublayers
I. INTRODUCTION
beneath it from excess oxidation. This oxidization phenomenon
IRE bonding has been the mainstay for interconnec-
creates significant bondability problems in the wire bonding
tion to integrated circuits. The thermosonic bonding
W
of copper chips [8] [10]. Several methods [5] [7], [11], [12]
process, as schematically shown in Fig. 1, is a combination of
have been proposed to improve the bondability of gold balls to
ultrasonic bonding with additional heat source. The ultrasonic
copper pads. One method is to establish a shielding environ-
energy helps disburse contaminates on the contact surfaces and
ment during the thermosonic wire bonding process [5], [6], [12],
forces the raw surfaces of the bonding material together with
[13]. Jeng et al. [5] used argon as a shielding gas to facilitate
the help of the thermal energy applied from the heat source.
the thermosonic wire bonding of gold wire onto copper pads.
It offers advantages of automatic operation, lower ultrasonic
The resulting welding quality was analyzed in terms of the ob-
energy, and reliable bonds. Therefore, the thermosonic ball
served interfacial microscopic phenomena [5], [14], [16]. The
bonding is used for the majority of interconnections to inte-
thermosonic bonding strength was found to be related to the in-
grated circuits.
terfacial phenomena between the bonded materials. An alterna-
The needs to improve performance and reduce chip size have
tive approach is to use passivation schemes [17] [19] to prevent
driven copper to replace aluminum interconnection for deep
the copper pad from oxidizing during wire bonding. Wong et al.
submicrometer integrated circuits. Copper has been identified
[2] and Hu and Harper [3] proposed the use of a metallization
as the best candidate to replace aluminum due to its low re-
process to deposit a metallic cap layer on the copper pads to pre-
sistance, high electromigration resistance and likely lower pro-
vent contact between the copper and air, thus suppressing the ox-
cessing cost [1] [4]. However, copper is susceptible to oxidation
idization of the copper pads. Ueno [20] deposited titanium thin
in a higher temperature environment. Previous studies [5] [7]
film on copper pads and examined the effect of the film thickness
on the welding quality. It was established that the film thickness
must lie within an appropriate range if the welding strength is to
Manuscript received April 15, 2009; revised September 04, 2009. Current
be improved. Aoh and Chuang [21] studied the effect of titanium
version published January 07, 2010. This work was supported by the National
Science Council under Grant NSC 92-2212-E-194-003. This work was recom- barriers on the bondability and bonding strength. Jeng et al.
mended for publication by Associate Editor E. Perfecto upon evaluation of the
[22] proposed that the use of a cupric oxide film formed with
reviewers comments.
an appropriate thickness on substrates under specified humidity
Y.-R. Jeng and S.-M. Chiu are with the Department of Mechanical En-
and temperature conditions can facilitate a viable wire bonding
gineering, National Chung Cheng University, Chia-Yi 621, Taiwan (e-mail:
[e-mail]; [e-mail]).
process if the bonding parameters are suitably specified. Jeng
P.-Y. Huang is with the Department of Mechanical Engineering, WuFeng
and Chiu [23] proposed the use of a chemical method to form a
Institute of Technology, Chia-Yi 62153, Taiwan (e-mail: [e-mail].
cuprous oxide layer on the copper pad. The cuprous oxide layer
edu.tw).
S.-H. Shyu is with the Graduate School of Opto-Mechatronics and Materials, is denser than the conventional cupric oxide generally found on
WuFeng Institute of Technology, Chia-Yi 62153, Taiwan (e-mail: sshyu@mail.
the surface of copper pads. They found that a copper pad with
wfc.edu.tw).
a cuprous oxide layer facilitates the wire bonding process and
Color versions of one or more of the figures in this paper are available online
eliminates the requirement for a gas shielding environment or
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TEPM.2009.2034467 the deposition of a metallic cap layer.
1521-334X/$26.00 © 2009 IEEE
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66 IEEE TRANSACTIONS ON ELECTRONICS PACKAGING MANUFACTURING, VOL. 33, NO. 1, JANUARY 2010
Electrical connection must provide a high stable electrical concept of volume conservation to the plastic deformation of
conduction. The usual way to evaluate the electrical conduction asperities during the elastoplastic and plastic stage. A recent
properties of these connections is to measure the contact resis- study by Etsion et al. [34] confirmed that a consideration of
tance. Holm in 1967 [24] proposed the empirical formulation
volume conservation for plastically deformed asperities yields
for contact resistance of the metal contact. Boyer in 2001 [25]
a more accurate solution.
used the simplified Greenwood and Williamson model (G-W
During the bonding process, it is impossible to measure the
model) [26] with the correction based on Holm s empirical cor-
effective contact area of the bonding interfaces experimentally.
relation. Using the finite-element method (FEM), Boyer calcu-
Consequently, the effective contact area can only be estimated
lated the effective contact area between two rough electrode un-
by theoretical model, many studies about the bondability re-
derlining the elastic deformation and measured the contact re-
search have adopted the same technique. [5], [7], [15], [16] In
sistance. Russ [27], Poulain et al. [28], and Crane [29] proposed
the present study, the micro-contact model proposed by Jeng
the theory of contact resistance and the associated measurement
and Wang [33] is adopted to describe the contact of the bonding
method. To evaluate the electrical conduction of various ther-
interfaces. The effective contact area between the bonding inter-
mosonic wire bonding methods, this study measures the circuit
faces is calculated using the micro-contact model that considers
contact resistance for the bare copper pads with argon shielding
the elastic, elastoplastic, and plastic deformations of the con-
gas and the copper pads with cupric oxide film, cuprous oxide
tact surface asperities. In general case, substituting the bonding
film, and silver film. The micro-contact theory was used to de-
process parameters and material characteristics into the micro-
termine the effective contact area. The circuit contact resistance
contact model gives the effective contact area between the
was measured for each sample and was presented in terms of ul-
bonding interfaces as shown in (1) at the bottom of the page.
trasound power and effective contact area. The current study fo-
In these equations, is the nominal contact area, and
cuses on the relationship between the bonding operating param-
are the density and average radius of the asperities, respectively,
eters and the circuit contact resistance (effective contact area) of
is the separating distance between bonding interface, and
gold wire onto copper pad. The bonding strength and reliability
is the distribution function of the asperity heights. Furthermore,
are not discussed in this paper.
, , and are the interference, critical interference at the
point of initial yielding of the asperity, and critical interference
II. THEORY MICRO-CONTACT MODEL
at the point of fully plastic flow, respectively. If is less than
Since all surfaces are microscopically rough, when two
, the deformation of the asperity is classified as elastic. Con-
surfaces are in contact, the load applied on one surface is
versely, if is greater than , the deformation of the asperity
supported by the asperities on the contact surface. Greenwood
is taken to be fully plastic.
and Williamson [30] proposed the classical statistical asperity
The current study also investigates the cases with a soft
micro-contact model (GW model) to describe the phenomena
coated layer on the copper substrate. Under this circumstance,
acting at the contact interface between two surfaces. In devel-
the coated layer is assumed to be under a full plastic deforma-
oping their model, it was assumed that all asperities on the
tion while the substrate may experience elastic, elastoplastic,
contact surface are hemispherical, have an identical contact
and plastic deformation [35], [36]. For those cases, a contact
radius, and experience a deformation which is governed by the
model for a rough surface with a soft coating developed by
Hertzian elastic contact model. Many subsequent researchers
Chang [37] was used to calculate the effective area in the study.
have attempted to verify and improve this asperity micro-con-
tact model. Zhao et al. [31] studied the transition from elastic
III. EXPERIMENTAL DEVICE AND PROCEDURE
asperity deformation to fully plastic flow by adopting the
micro-geometry model proposed by Abbott and Firestone [32] The experiment was performed on a Toshiba thermosonic
to describe the contact pressure and the real contact area for wire bonder (HN-932-FAB). The ultrasound frequency was
the plastically deformed asperities. Specifically, their study 60 kHz, bonding force range was 0 2.5 N, and welding time
considered the continuity and smoothness of variables for range was 0 255 ms. A 25- m diameter gold (containing
different modes of deformation and investigated the elasto- 99.99% gold) wire was used. The test pads, cut to the size of
plastic contact force and the real contact area between two 6mm 6mm 0.525 mm, included the bare copper chips, and
rough surfaces. Recently, Jeng and Wang [33] extended the the chips with cupric oxide layer [22], cuprous oxide [23], or
elastoplastic micro-contact model proposed by Zhao et al. [31] silver layer [7], [10]. For the bare copper, argon gas was injected
to the elliptical contact of surface asperities by applying the over the copper surface to overcome the difficulty of bonding
(1)
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JENG et al.: INVESTIGATION OF THERMOSONIC WIRE BONDING RESISTANCE OF GOLD WIRE ONTO COPPER PAD 67
Fig. 2. Schematics of circuit contact resistance measurement.
Fig. 3. Relation between circuit contact resistance, effective contact area, and
ultrasound power for the bare copper pads with argon shielding.
for the bare copper [5]. In the thermosonic wire bonding, the
wire was guided through a hole in the capillary to the bonding
site, and then an electronic flame-off was used to melt the gold
wire into a gold ball by high-voltage electric discharging [14].
While the gold ball was firmly clamped between the capillary
and the bond pad, a burst of ultrasonic vibration was applied
to the capillary. The combination of pressure, vibration, and
temperature accomplishes a weld between the gold ball and the
pad.
Moreover, a stereomicroscope was used to measure the ap-
parent contact area, and a shear test was used to determine the
bonding force of a thermosonic ball bond. A three-dimensional
surface topography measurement system was conducted to ob-
tain the pad surface roughness. In this system, the surface rough-
ness was measured by profilometer and white light interfer-
ometry. The surface hardness was measured using a Vickers
micro-hardness tester. The measured surface roughness, as well
as the material property, was used to evaluate the effective con-
tact area at the interface between the wire and the pad. The ex-
Fig. 4. Relation between circuit contact resistance, effective contact area, and
perimental conditions that were varied include ultrasonic power
ultrasound power for the copper pads with a cupric film.
and preheat temperature.
The circuit resistance was measured by using Cascade Micro-
power leads to an increase in the effective contact area. As a re-
tech Summit 9000 Analytical Probe Station. An illustration of
sult, lower circuit resistance can be obtained. This is validated
circuit resistance measurement is shown in Fig. 2. The chip
in Fig. 3 in which the circuit resistance decreases with the ef-
was placed on probe station. One probe was located at the first
fective contact area increases. Note that when the effective con-
welding point nearby the pad. Another probe was placed on the
tact area reach 3.23 m , the circuit resistance reaches
second welding point. A Kelvin bridge was formed by using a
power supply to provide a current. A voltmeter was used to mea- a minimum value, 0.0563 , and no longer decreases when the
effective contact area is greater than 3.23 m . For the
sure the voltage difference between the two probes.
cases with a cupric oxide film, it was found that a good bond-
ability can be achieved when the preload is set as 0.5 N, welding
IV. RESULTS AND DISCUSSION
time as 30 ms, and preheat temperature between 225 C 250 C
For the bare copper with argon gas serving as shielding gas, [22]. Fig. 4 shows the relation between the circuit resistance, ef-
it was found that a maximum welding strength can be achieved fective contact area and the ultrasound power. From this figure,
when the preload is set at 0.5 N, welding time of 15 ms, and pre- it is found that, similar to the bare copper with argon gas, both
heat temperature of 150 C 170 C [5]. Therefore, in the cur- the ultrasound and effective contact area improve the electrical
rent study, the parameters were set as the previous study with performance.
the ultrasound power varied. Fig. 3 shows the relation between In the previous study, it has been found that a good bond-
the circuit resistance, effective contact area and the ultrasound ability can be achieved for chips with cuprous film when the
power. The results show that the circuit resistance decreases preload is set as 0.5 N, welding time as 30 ms, preheat tempera-
with the increase in power. This is expected since the increase in ture as 260 C [23]. In this paper, the preheat temperature ranges
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68 IEEE TRANSACTIONS ON ELECTRONICS PACKAGING MANUFACTURING, VOL. 33, NO. 1, JANUARY 2010
Fig. 6. Relation between circuit contact resistance and effective contact area
Fig. 5. Relation between circuit contact resistance and effective contact area
for the copper pads with a silver film.
for the copper pads with a cuprous film.
from 220 C 280 C, and all the other parameters remain the
same as the previous study. Fig. 5 shows the relation between
the circuit contact resistance and effective contact area for the
preheat temperature of 220 C, 240 C, 260 C, and 280 C. The
results show that the preheat temperature is an important param-
eter. The higher preheat temperature would in essence decrease
the circuit resistance. However, when the circuit contact resis-
tance reaches a saturated level around 0.03 , further increases
in the effective contact area would not decrease the circuit con-
tact resistance as shown for the preheat temperature of 260 C
and 280 C. This could be due to the excessive energy at the in-
terface introduced by the excessive preheating temperature that
eventually damages the weld bonding [5].
Previous studies have been found that a silver film can
improve bondability, and that a good welding strength can be
obtained when the preload is set as 0.5 N, welding time as
20 ms, and preheat temperature as 230 C [7], [10]. To investi-
gate the circuit contact resistance of this manufacturing process,
the preload and welding time were set as the previous studies
while the preheat temperature ranged from 190 C 250 C.
The relation between the circuit contact resistance and effective
Fig. 7. Comparison of circuit contact resistance versus effective contact area
for different thermosonic wire bonding methods.
contact area is shown in Fig. 6. The results show that the circuit
contact resistances stay at a lower level and are relatively flatter
than other manufacturing method. The lowest circuit contact
resistance is around 0.03 . Similar to the cases of cuprous film for silver-film process is 230 C. Note that the range of effec-
with preheat temperature of 260 C and 280 C, the damage to tive contact area for bare copper with argon is on the order of
the weld caused by excessive interfacial energy prohibits the m , an order larger than all the other processes. To put
further improvement on the resistance after reaching 0.03 . this case into the comparison, the upper abscissa is scaled from
To compare the electric performance among the various pro- 2 to 3.6 m preserving for the bare copper with argon.
cesses, Fig. 7 shows the curves of circuit contact resistance The results show the existence of minimum circuit contact resis-
against the effective contact area. Each curve represents a type tance. Beyond this minimum point, further increasing the effec-
of manufacturing process with a parameter set that would pro- tive contact area will not decrease the resistance. Fig. 7 shows
vide the optimum bondability. For example, the preheat tem- that the silver-film process has not only the lowest resistance,
perature for the cuprous-oxide-film process is 260 C, and that but also the least influence from the effective contact area.
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JENG et al.: INVESTIGATION OF THERMOSONIC WIRE BONDING RESISTANCE OF GOLD WIRE ONTO COPPER PAD 69
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ACKNOWLEDGMENT
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The authors would like thank MIRL of ITRI for their assis- G335 G337, 2004.
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tance in providing experimental facilities. The authors would
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