High-Temperature-Resistant Interconnections Formed by Using

Kato et al.: High-Temperature-Resistant Interconnections (1/6)
[Technical Paper]
High-Temperature-Resistant Interconnections Formed by Using
Nickel Micro-plating and Ni Nano-particles for Power Devices
Noriyuki Kato, Suguru Hashimoto, Tomonori Iizuka, and Kohei Tatsumi
Graduate School of Information, Production and Systems, Waseda University, 2-7 Hibikino, Wakamatsu-ku, Kitakyushu, Fukuoka 808-0135, Japan
(Received August 6, 2013; accepted November 14, 2013)
Abstract
The improvement of interconnection technology is becoming a top priority for the operation of SiC devices at high temperatures. We proposed a new interconnection method using nickel electroplating to form bonds between chip electrodes
and substrate leads. We also newly proposed low-temperature nickel nanoparticle sintering to form die bonding connections. SiC devices assembled with these new connection methods operated successfully in a high-temperature environment of about 300°C. We confirmed that these methods had adequate potential as an advanced heat resistant package in
comparison with conventional interconnections.
Keywords:High Temperature Resistant Packaging, Power Device, Micro Electro-plating Bonding, SiC Device,
Nano-Nickel
1. Introduction
2. Experimental Procedure
To reduce the size and increase the efficiency of power
2.1 Bonding by nickel micro-plating
converters mounted in hybrid vehicles or electric vehicles,
We used nickel electroplating to form nickel micro-plat-
research on the development of methods to increase the
ing bonds. The plating conditions were as follows: Ni plat-
output power density using technology such as silicon car-
ing bath of Watts solutions, bath temperature of 50°C,
bide (SiC) devices is in progress. High temperature SiC
electrical current density of 5 A/dm2, and substrate and
devices require the materials for packaging capable of
lead material of mainly copper. The growth rate of copper
working at higher temperature than those for Si devices.
plating on a flat plate was ranging from 0.75 μm/min to 0.9
While studies have been conducted on heat-resistant pack-
μm/min.
aging materials that can replace Al bonding wire or solder
To simulate bonding between the chip electrodes and
materials having low melting points, various problems in
the substrate leads of a power device, we brought a copper
alternative methods have been pointed out.[1, 2]
wire (diameter: 172 μm) into contact with a copper plate
Our approach described in this paper is to form the con-
and plated the wire and the plate as shown in Fig. 2 (a). To
ductive connection with Ni at relatively lower temperature.
simulate die bonding, we brought the gold surface of a sili-
[3] For the connection of chip electrodes Ni micro-plating
con chip with vapor-deposited gold into contact with a lead
bonding (NMPB)[4] was used and die attach connection
frame cut into strips, and plated the chip and the lead
was carried out by sintering nickel nanoparticles at a low
frame as shown in Fig. 2 (b)
temperature.[5] An example of the concept for these interconnections is shown in Fig. 1.
A䡃-electrode
A䡈-electrode
A䡃-plang
Ni-plang
Cu-wire Cu plate
Cu-wire
5.0䟚
Cu plate
Ni-plang
Cu plate
Fig. 1 Model of bonded SiC diode specimen.
Copyright © The Japan Institute of Electronics Packaging
2.7䟚
Si-chip
Au-electrode
5.0䟚
㻿㼕㻯㻌㼐㼑㼢㼕㼏㼑㻌
Nano-Ni
lead frame
(a)
(b)
Fig. 2 (a) Copper wire bonded to plate (b) Silicon chip
bonded to lead frame.
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To evaluate the high-heat-resistance reliability of the
2.7 mm × 2.7 mm and (B) 8.0 mm × 8.0 mm. After applying
specimens after micro-plating bonding, we heated them at
the solution to chip (A), electrodes were placed on chip
various temperatures between 100 and 500°C in an argon
(B) with their surfaces facing each other and were heated
atmosphere for 60 min and then measured the shear
and pressurized in atmosphere to form a bond as shown in
strength of the bonds with a shear tester (Rhesca model
Fig. 3(b). The bonding conditions were as follows: the
PTR-01). The specimens were heated to 500°C for acceler-
device stage and the pressurizing head were heated to
ated aging testing to evaluate the interface reliability and
300°C, and a constant load of 147 N was applied by the
the change in resistivity.
pressurizing head. After heating the bonded specimens in
To observe the cross section of the bonded portions we
an argon atmosphere for 60 min (between room tempera-
used a scanning electron microscope (SEM Hitachi model
ture and 500°C), we measured their shear strength.
S-3400N.).
2.3 Chips used for bonding strength evaluation
2.2 Low-temperature nickel nanoparticle sintering
The chips used for bonding were silicon dummy chips
We used a 5 wt.% nickel nanoparticle solution as the
with aluminum film electrodes, and SiC-SBD(Shottky bar-
bonding material. The nickel nanoparticles had a diameter
rier diode) chips (1,200 V, 15 A, 2.7 × 2.7 mm, made by
of 20 nm and were dispersed in a solution of isopropyl
SiCED Electronics Development GmbH) having an alumi-
alcohol (IPA). Since IPA is highly volatile, it evaporated
num electrode film on the top surface and a silver film on
after application, leaving only the nickel nanoparticles.
the back surface. They had a thickness of 365 μm. To
We used an electrostatic atomizer (made by Apic
study bonding on these aluminum electrode surfaces, we
Yamada)[6] to apply the solution. As shown in Fig. 3(a),
used chips having an electroless nickel plating film formed
this device enabled a selective and uniform application of
on an aluminum electrode surface, and chips having vapor-
the solution on the specimen surface by means of electro-
deposited nickel and gold on aluminum electrode films.
static force.
We evaluated nickel nanoparticle bonding strength using
For the evaluation of bonding characteristics with Ni
bonding between pairs of silicon dummy chips (A) and
nanoparticles we prepared Si dummy chips with size (A)
(B). We applied the nickel nanoparticle solution to the
electrode surface and studied on the bonding characteristics between both electrodes.
3. Results and Discussion
3.1 Nickel micro-plating bonding (NMPB) and
evaluation
3.1.1 Plating bonding study
To simulate bonding between the chip electrodes and
the substrate leads of a power device, the copper wire was
into contact with a copper plate and plated the wire and the
plate, as shown in Fig. 2(a). Figure 4 shows the cross sec-
Fig. 3a Schematic diagram for electrostatic atomization.
tion of a copper wire (diameter: 172 μm) bonded to the
147N
Chip(A)
Nano Ni
Chip(A)
Chip (B)
Heating
Stage
Fig. 3b Bonding steps of dummy chips with using Ni nano-particle atomization.
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Cu-wire
Ni-plang
5μm
Cu plate
Fig. 4 Plating cross section of copper wire bonded to copper
plate.
Fig. 6 Heating temperature-shear strength characteristic for
plated bonding specimens of copper wire and copper plate
made using different plating times.
Si-chip
50μm
Ni-plang
lead frame
Fig. 5 Plating cross section of silicon chip with vapor-deposited gold bonded to lead frame.
Fig. 7 Heating temperature-resistance and heating-temperature-resistivity characteristics for specimens of nickel-plated
wire on copper.
interface between copper and nickel layers, indicating that
copper plate by Ni micro-plating.
To simulate die bonding, we plated the chip and the lead
frame as shown in Fig. 2 (b). Figure 5 shows the observed
cross section.
Both cross sections indicate no problematic voids or
other defects generated by the nickel plating.
no deterioration caused by the formation of brittle phases
or intermetallics with voiding occurred in the high-temperature environment.
3.1.3 Copper-nickel alloy resistance measurement
Since the electrical resistance of a copper-nickel alloy
3.1.2 Shear testing
layer formed in a high-temperature environment may
To evaluate whether specimens with plated bonds had
increase, we measured the change in resistance caused by
high heat resistance, we measured their shear strength
alloying. We used the four-terminal method to measure the
after heating. We measured specimens in which a copper
resistance of copper wire specimens (having a length of 30
wire (with a length of 1 mm and diameter of 172 μm) was
mm and diameter of 172 μm) plated for 30 min and then
brought into contact with a copper plate and plated for 15
heated in an argon atmosphere. After plating, the wire
min or 30 min. The specimens were heated in an argon
diameter increased to 200 μm, and the distance between
atmosphere and then shear-tested. We carried out shear
the terminals used for the wire resistance measurement
testing nine times at each heating temperature and found
was 12.5 mm. Figure 7 shows the results. Diffusion and
the average shear strength of the specimens. As shown in
alloying proceeded with an increase in the temperature,
Fig. 6, specimens with plating times of both 15 and 30 min
causing an increase in resistance.[7] The increase in resis-
exhibited high shear strength after being heated to 500°C.
tance even for a heating temperature of 500°C (for 1 h)
The shear strength of both specimen types increased with
was a value of around 10%. The diffusion condition of
an increase in the heating temperature. This finding may
500°C for 1 h is equivalent to that of 350°C for 4,580 h,
have been the result of increasing copper-nickel alloying
assuming that the activation energy for Ni diffusion in Cu
caused by diffusion, resulting in greater adhesion and
is 225 kJ/mol.[8] This indicates that problematic defects
strength at the interface. From the results described in
are not considered to be generated in the interconnection
3.1.4, it appeared that no second phase was formed at the
during practical use up to 350°C.
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(a) Heating condition: no heating (as bonded).
(b) Heating condition: heated for 60 min at 300°C.
(c) Heating condition: heated for 60 min at 500°C.
Fig. 8 SEM images and concentration distribution graphs of
cross section of nickel-plated copper plate specimens.
3.1.4 Analysis of copper-nickel diffusion state
Cu-Ni system. Further, although Kirkendall voids have
To verify the state of copper-nickel diffusion in speci-
formed in the copper plate, the results described in Sec-
mens, after heating, we used a SEM to observe the cross
tions 3.1.2 and 3.1.3 indicate that their effect is not serious.
sections of a copper plate covered by Ni plating. We mea-
3.2 Nickel nanoparticle bonding and evaluation
sured concentration profiles of each element in the vicinity
As described in Section 3.1.1, plating-based die bonding
of interfaces using an energy-dispersive X-ray spectros-
can be carried out by using a strip or lattice substrate
copy (EDX) line analysis. Figure 8 shows the results. The
structure, but it is difficult to use for bonding flat plate
positions of EDX line analysis are indicated within each
surfaces together. It has been shown that flat plates can be
SEM image and the concentration profiles are displayed in
bonded using low-temperature nickel nanoparticle bond-
the graphs.
ing[5, 9]; hence, we studied the application of this method
A comparison of the graphs of unheated specimens and
to die bonding. We brought the electrode surfaces of sili-
specimens heated at 300°C in Fig. 8 (a) and (b) reveals
con dummy chips that are (A) 2.7 × 2.7 mm and (B) 8.0 ×
almost no change in the concentration distribution. On the
8.0 mm in size into facing position, bonded them using
other hand the concentration distribution of the specimen
nickel nanoparticles, and then shear-tested the specimens
heated at 500°C in Fig. 8 (c) shows the formation of an
after heating. We performed shear testing five times at
alloy layer of approximately 5 μm. There were no second
each temperature and plotted the average shear strengths.
phase observed, as predicted from a phase diagram of
Figure 9 shows that a slight drop in the shear strength was
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Kato et al.: High-Temperature-Resistant Interconnections (5/6)
plate at a different constant temperature (ranging between
20 and 325°C). We then measured the diode voltage (VD)
and current (ID), var ying the power supply voltage
(between 0 and 15 V) using the circuit illustrated in Fig.
10. Figure 11 shows the results for different temperatures.
In a high-temperature environment of 325°C, we obtained
the diode V-I characteristics and did not obser ve the
change in the resistivity caused by the bond deterioration
Fig. 9 Heating temperature-shear strength characteristic of
during measurements. With using Ni interconnections
specimens bonded using nickel nanoparticles.
newly introduced in this paper the high temperature operation of the SiC diode was first confirmed.
ID
䡎䠍䠙2䡇Ω
Variable DC
power source
SiC diode
0䡚15V
VD
4. Conclusions
1. Our findings demonstrated that nickel micro-plating
bonding(NMPB) could be used for bonding chips
to substrate electrodes and for die bonding to chip
Fig. 10 Circuit used for measuring diode V-I characteristics.
substrates in power devices. We demonstrated that
low-temperature bonding ensured bond reliability
8
in high-temperature environments of over 300°C.
7
We observed no bond deterioration during acceler-
ID[mA]
6
ated diffusion tests of bonds formed by copper-
5
nickel plating.
4
3
2. We used nickel micro-plating bonding and low-tem-
2
perature nickel nanoparticle sintering to form
1
bonds between SiC power device chips and sub-
0
0
200
400
600
800
1000
VD[mV]
Fig. 11 V-I characteristics of bonded SiC diodes at different
heating temperatures.
strate electrodes and to form die bonding connections, respectively. We tested device operation in a
high-temperature environment of about 300°C.
3. Our findings demonstrated that micro-plating-based
chip bonding technology could be considered to
found when the specimens were heated at 500°C, but
have adequate potential as a practical, simple
since the value still remained high (over 10 N).
mounting technology that is highly heat-resistant
3.3 High-temperature circuit operation testing and
and ensures high reliability and low cost.
evaluation using SiC diode chips
To evaluate whether the bonding technologies that we
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Noriyuki Kato