-
2009 International Conference on Electronic Packaging Technology
& High Density Packaging (ICEPT-HDP)
978-1-4244-4659-9/09/$25.00 2009 IEEE
Low Temperature Cu-Sn Bonding by Isothermal Solidification
Technology
Yibo Rong1, 2, Jian Cai1, 2, Shuidi Wang1, 2, Songliang Jia1, 2
1Tsinghua National Laboratory for Information Science and
Technology (TNList), Beijing 100084, CHN
2Institute of Microelectronics, Tsinghua University, Beijing
100084, CHN Email: [email protected], Tel:
86-10-62794957
Abstract
A low temperature wafer-to-wafer bonding technology for 3D
packaging/integration based on Cu-Sn isothermal solidification (IS)
technology is introduced in this paper. The fluxless bonding
technique using Cu-Sn multilayer composites to produce higher
re-melting temperature bonding layer is presented. The structure of
the intermediate multi-layers and bonding patterns are designed,
and the bonding process is optimized. The microstructure of bonding
layer was investigated by SEM (Scanning Electronic Microscopy) and
EDS (Energy Dispersive X-Ray Spectrometer). The compositions of the
bonding layer show that there are intermetallic compounds (IMCs)
with higher melting points. The bonding layers consist of Cu6Sn5
and Cu3Sn phases. High strength of bonding layer has been detected,
with average shear strength of 37.5MPa.
Introduction With the development of microelectronic technology,
3D
packaging/integration has been paid a lot of attention. As one
of the important interconnect methods, Through Silicon Via (TSV) is
getting more and more attention. The major technologies of TSV
involve via formation (mostly by deep reaction ion etching), via
filling by electroplating and inter-wafer bonding. Wafer-to-wafer
bonding would be critical for TSV technology, especially when there
are multi-layer interconnections. Bonding layer formed should not
melt again in further bonding process. It leads to the requirement
of higher re-melting temperature bonding layer.
As one of the crucial processes for 3D packaging, research and
development of wafer-to-wafer bonding technology have become hot in
both academia and industry. There are different wafer-to-wafer
bonding methods for 3D packaging, including anode bonding, epoxy
bonding, glass frit bonding and solder bonding, etc. All of these
bonding technologies have their own advantages and disadvantages.
For example, solder alloy bonding would have a lower bonding
temperature and has been one of the promising bonding methods.
However, the melting point of traditional solder joints/bonding
layer would have a re-melting temperature no higher than the
bonding temperature. This can not satisfied multiple layer bonding.
Thus, the bonding temperature limited the post-processing
temperature. This also restricts devices applications due to
temperature constraint.
Another solution for wafer-to-wafer bonding is a liquid-solid
inter-diffusion method, which is also called as isothermal
solidification (IS) technology in some literature. Bonding layers
by IS technology could achieve higher re-melting temperature
structure.
IS Technology Mechanism and Bonding Couple Selection The bonding
structure of isothermal solidification (IS)
technology includes a diffusion couple consist of two metals.
One metal is with relatively lower melting point but the other is a
higher melting point metal. At a constant temperature slightly
higher than the melting point of the lower melting temperature
metal, reaction or/and diffusion would take place between the
liquid phase of lower melting temperature metal and the solid phase
of the higher one. The reaction products would be intermetallic
compounds (IMCs) with higher melting point. This means the bonding
layer would have a higher re-melting temperature. So IS process
gives a quantum jump to the post-processing temperature of the
compound or solid solution fabricated. Additionally, as bonding
would be occurred in lower temperature, this could induce lower
stress for bonding system and devices on wafer.
For diffusion couple design, the low melting point metal is the
key issue. Only a few of metals have lower melting temperature.
Considering environmental issues, only Ga, In, Sn, Bi could be used
for bonding. Table 1 shows the melting points of these elements.
The melting temperature of Ga is too close to room temperature and
the melting temperature of Bi is a little bit high. So In and Sn
would be the candidates for diffusion couple. However, In is too
easy to be oxidized. Then, Sn is the final decision for the lower
melting point metal in diffusion couple.
Table 1 Low melting point non-contaminated metals
Element Melting Point(C) Ga 29.8 In 156.6 Sn 232.0 Bi 271.3
For high melting point in diffusion couple, several metals
would be selected. The common metals would be Au, Ag, and Cu.
These metals could form different IMC with Sn and thus would
achieve higher re-melting bonding structure. Compared with Au-Sn
system, Cu-Sn bonding system would be lower costs. And the reaction
rate is also higher. So Cu-Sn system was chosen as bonding media in
this work.
Fundamental of Cu-Sn Bonding To descript the fundamental of
isothermal solidification, it
is necessary to take a look on the Cu-Sn binary phase diagram.
The Equilibrium phase diagram of Cu-Sn System is shown in Fig. 1
[1]. There are many equilibrium phases in this diagram. Among them,
Cu3Sn and Cu6Sn5 are considered to be two most important phases.
Above 227C, with tin composition of 99.3wt.%, reaction occurs
between copper and tin [2]. And above 232C, when tin becomes
liquid, the
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2009 International Conference on Electronic Packaging Technology
& High Density Packaging (ICEPT-HDP)
reaction accelerates. For tin of 61 to 100wt.%, the alloy is a
mixture of Sn and Cu6Sn5 with a melting point of 227C. The alloy
with tin composition of 38 to 61wt.% consists Cu6Sn5 and Cu3Sn with
a melting point of 415C. And if tin composition reduces below
38wt.%, the melting point may even increase to 713C [3].
Fig. 1 Equilibrium phase diagram of Cu-Sn System
In order to realize a stable higher re-melting temperature
of bonding structure, tin has been designed to be exhaust after
reaction, while some copper would be left. The expected structure
after bonding would be as those shown in Fig. 2,
Cu/Cu3Sn/Cu6Sn5/Cu3Sn/Cu or Cu/Cu3Sn/Cu.
Fig. 2 Expected Cu-Sn Bonding Structures
If only Cu3Sn left after reaction, then
3S SCu Cu Sn Sn
W WCu Sn
=
(1) If only Cu6Sn5 left after reaction, then
5 6S SCu Cu Sn Sn
W WCu Sn
=
(2) S: Area of Bonding : Bump Height/Thickness W: Mole volume of
Cu or Sn Hereby, this paper designed the thickness of Cu/Sn/Cu
multilayer as 5m/5m/5m before bonding.
Concerning the requirement of test, the figures of the top wafer
and the bottom wafer are designed separately. Layout of the bonding
pattern is shown in Fig. 3.
Fig. 3 Layout of bonding pattern
Experimental Dummy wafers with Cu/Sn bumps were fabricated
in-
house. After carefully optimizing of fabricating process,
excellent bump plating results are achieved. At a 260C process
temperature, 4 inches wafer-level Cu-Sn bonding, in which joints
were almost void-free, was realized, including an interface with a
melting point of 415C. With further bonding and annealing steps,
re-melting temperature of the joints could increase to 713C.
Details of the whole process are as following.
Firstly, this work prepared the wafers for bonding. TiW/Cu was
sputtered as seed layer. And copper and tin bumps were
electroplated as the diffusion couple for bonding.
When the wafers are ready, they are rinsed in plasma
environment. By this way, impurities and oxides could be removed
from the surfaces. Then, two wafers are aligned in SUSS MA6, and
transferred into SUSS SB6 for bonding.
To avoid bubbles left between bonding patterns, this work select
to process the whole bonding in vacuum, rather than in N2
atmosphere. Heating until 160C firstly and pre-bonding was
preformed for 5 minutes at this temperature. Wafers would be heated
until 260C and bonded for 20 minutes at this temperature. The
temperature of the bonding pair decreased to 200C and inlet N2
cutting down the cooling time. Besides, during the bonding process,
pressure should be kept at 6kgf/cm2. The thermal-pressure curve of
bonding is shown in Fig. 4.
97
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2009 International Conference on Electronic Packaging Technology
& High Density Packaging (ICEPT-HDP)
Fig. 4 Thermal-Pressure curve of bonding
Results and Discussion To evaluate the microstructure of the
bonding layer, SEM
(Scanning Electronic Microscopy) is employed. Fig. 5 shows the
SEM image of the cross-section of bonding intermetallic
inter-layers. From this picture, there are three distinct phases
existing in the joint between two copper phases. The EDS results of
the three phases are listed in Tab. 2. The results show that the
upper part of the bonding layer is Cu3Sn, the middle one is Cu6Sn5,
and the bottom one is Cu3Sn.
Fig. 5 Cross-section of bonding inter-metallic inter-layers
Table 2 EDS results of intermetallic inter-layers IMC Component
wt.% at.%
Cu3Sn(Up) Cu Sn
61.38 38.62
74.81 25.19
Cu6Sn5 Cu Sn
40.61 59.39
55.09 44.91
Cu3Sn(Down) Cu Sn
62.27 37.73
75.51 24.49
Dage Series 4000 Shear Tester was utilized to measure the
shearing strength of the bonding structure. The maximum shearing
strength is as 47MPa and the minimum is as 33MPa. The average
pressure is as 37.5MPa and the variance is 5.1MPa. Fig. 6 shows the
surface of the bond after shearing
test. The image shows that peeling occurred between TiW and Si.
This reveals the high reliability of the multi-layers of Cu-Sn
bonding. Also from Fig. 6, we could figure out that the alignment
accuracy during bonding is about 10m.
Fig. 6 Top-view of sheared bonding structure
Conclusion Based on Cu-Sn isothermal solidification
technology,
bonding structures are designed and processes are optimized,
Good bonding joints are achieved.
SEM results clearly show the microstructures of the joints
consisting of Cu6Sn5 and Cu3Sn inter-metallic phases.
In summary, the fluxless bonding method based on Cu-Sn
isothermal solidification (IS) technology was demonstrated in the
paper. At a 260C process temperature, 4 inches wafer-level Cu-Sn
bonding was realized, including an interface with a melting point
of 415C. Studies on the bonds reveal the basic bonding mechanism,
and demonstrate the feasibility of this theory.
Acknowledgments This work was supported by the National Basic
Research
Program of China, 2006CB302703. The authors would appreciate Ms.
Wenyan Yang, from the
National Key Lab of Tribology, Tsinghua University, for her help
on SEM analysis. The authors would also appreciate for the help
from MEMSensing Microsystems Co., Ltd and Suzhou Institute of
Nano-tech and Nano-bionics, CAS.
References 1. N. Saunders, A. P. Miodownik and etc., Binary
Alloy
Phase Diagrams, ASM International Metals Park, Ohio, USA, 1990:
1481.
2. S. Bader, W. Gust and H. Hieber, Rapid formation of
intermetallic compounds by interdiffusion in the Cu-Sn and Ni-Sn
systems, Acta metal, 1995, 34, pp. 539-557.
3. Chin C. Lee, Yi-Dhia Chen, High temperature tin-copper joints
produced at low process temperature for stress reduction, Thin
Solid Films, 1996, 286, pp. 213-218.
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