European Microelectronics
Packaging Conference
10-13 September 2017, Warsaw, Poland
www.empc2017.pl
Liquid Solid Diffusion (LSD) Bonding
A novel joining technology
Andreas Larsson
TECHNI AS, Applied physics dep.
USN – University College of Southeast Norway,
Dep. of Microsystems
Borre, Norway
andreas.larsson@techni.no
ala@usn.no
Torleif A. Tollefsen
TEGma AS
Oslo, Norway
Knut E. Aasmundtveit
USN – University College of Southeast Norway,
Dep. of Microsystems
Borre, Norway
Ole Martin Løvvik
SINTEF Materials and Chemistry
Oslo, Norway
II. CONCEPT
Abstract—Liquid Solid Diffusion (LSD) bonding is a novel
joining technology forming solid and liquid two-phase field
joints from simple binary systems. LSD creates joints that are
applicable at temperatures above the melting temperature of the
material composition of the initial bond. Liquid and solid interdiffusion transform the joint composition and microstructure.
The final joint is characterized by incongruent re-melting
forming a coherent continuous porous solid solution phase with
liquefied pores. This article briefly describes the methodology
and presents experimental results based on the Au–Ge system.
High quality joints were created. The effective melting point was
increased by more than 200 °C above the initial eutectic melting
point of the system.
Keywords—Bonding, high temperature, Au–Ge
I. INTRODUCTION
Joining of materials or components is an ancient art
emanating many hundreds of years back in time [1], [2]. In
more recent time, joining metals have become an art of
greatest importance. Joining of metals have enabled man to
create extraordinary things and devices such as the space
shuttle and enabling use of the integrated circuit. Some of the
most advanced forms of joining technologies in use today is
within assembly of complex electronic systems (often referred
to as bonding). Such technologies include; soldering, brazing,
welding, contact melting [3], [4], thermo-compression, ultrasonic, solid liquid inter-diffusion (SLID) [5]–[8] / transient
liquid phase (TLP) [9] and sintering [10].
This paper presents a brief description of a novel joining
technology called Liquid Solid Diffusion (LSD) bonding.
Like other comparable joining technologies, such as transient
liquid phase (TLP) bonding [9], LSD creates joints that are
applicable at temperatures above the melting temperature of
the material composition of the initial bond. Unlike TLP and
other similar joints, LSD joints comprise a liquid phase at high
temperatures. This report presents initial experimental results
demonstrating an increase in effective melting temperature of
the joint after bonding.
It is well known that binary eutectic material systems are
partially solid and liquid when in the two-phase field region
between solidus and liquidus in the phase diagram [11].
During solidification of off-eutectic compounds, a mushy
mixture is formed in the aforementioned two-phase field
region, when the temperature is between the liquidus
temperature and the solidus temperature. Much like in a slushy
were solid ice particles float in a liquid. Therefore, it is often
referred to as the mushy region. When reheated after complete
solidification, the same mushy structure normally reforms.
This makes the compound easily reworkable, which is used in
some applications such as soldering pipes with plumbers’
solder. It was envisaged that it was possible to switch place
of the liquid and solid phases forming a continuous and
coherent porous structure with liquefied pores at elevated
temperatures, instead of a mushy state. This could be done by
diluting the compound, preferably close to the maximum
solubility limit, and annealing it such that phase segregation
change the microstructure and morphology of the joint. When
reheated the segregated solute then reacts locally with the
solvent. At the eutectic temperature, a liquid is formed at the
interface between the two phases. The reaction continues until
all the solute has been consumed, forming a eutectic liquid
domain. The surrounding material then comprise a solid single
phase structure. The solid phase may be used to maintain
structural integrity between to adjoined components at a
temperature significantly above the eutectic melting point of
the material system.
III. METHODOLOGY
A. Materials and fabrication
Dummy samples were fabricated within the binary Au–Ge
system. Square chips and substrates, 3.6 mm2 and 15.2 mm2,
were diced of 525 µm thick silicon wafers with a 150 nm thick
TiW, and a 2.8 µm thick Au layer on the bond surface. A 25
µm thick eutectic Au–Ge preform was then sandwiched
between chips and substrates, cf. Fig. 1.
© IMAPS / EMPC 2017
1
European Microelectronics
Packaging Conference
The chip and preform were manually aligned on top of the
substrate forming a symmetrical Si / Au / Au–Ge / Au / Si
structure and placed onto a heater. The stack was clamped
together and the bond process was carried out in vacuum. The
temperature was raised above the eutectic melting temperature
(361 °C) to around 380 °C and then lowered to around 330 °C
and maintained at this temperature for up to 4 hours. The
applied pressure was approximately 2 MPa. The pressure
ensured squeeze-out of excess material from the bond
interface. This minimized the active volume of eutectic
material for the interdiffusion process to form the final joint.
The pressure also secured a thermomechanical contact
between chip and substrate. Reference samples forming a
regular eutectic joint were also fabricated. The annealing step
at 330 °C, transforming the microstructure, was omitted for
the reference samples.
10-13 September 2017, Warsaw, Poland
www.empc2017.pl
Fig. 1 Illustration of the investigated sample components.
B. Characterization
An integrity test of the fabricated joints was performed by
clamping the fabricated samples on a heater (Watlow;
Ultramic 600) in vacuum. A weight was applied to the
structure creating a shear force over the bond in the range of
20-50 kPa. The temperature was then raised at a rate of 30
°C/min up to a maximum of 600 °C or until the chip detached
from the substrate.
Cross-sections were analyzed with use of optical
microscopy (II Neophot 32) and scanning electron
microscopy (SEM) (Hitachi SU8320). Samples were prepared
for cross-section by dicing and Ar ion-milling (Hitachi
IM4000). The joint composition was evaluated by energydispersive X-ray spectroscopy (EDX) (Oxford Silicon Drift
Detector- XmaxN).
IV. RESULTS & DISCUSSION
Fig. 2 Ongoing detachment experiment. Hot plate glowing red hot at ca. 570
°C.
A. Detachment temperature
The fabricated bonds demonstrated structural integrity up
to the maximum temperature of the hot plate of 600 °C, cf.
Fig. 2. This demonstrates that there exists a solid, continuous
and coherent structure adjoining the two components
structurally at temperatures above the eutectic temperature of
361 °C. The reference samples detached around 330 ± 10 °C.
Comparable regular eutectic Au–Ge joints have been reported
to have a significant shear strength of tens of MPa at
temperatures around 300 °C and above [12]–[14]. This
discrepancy is still unclear.
B. Joint quality and composition
Cross-sections of the joint showed high quality joints with
limited voiding. Analysis by SEM and EDX of the joint
showed that it mainly comprised an overall Au rich Au–Ge
mixture with an overall Ge concentration of around 8 at%, cf.
Fig. 3. It was further observed that the desired morphology
had been achieved, cf. Fig. 4. I.e. the β-phase (Ge) had
segregated and was isolated into small domains inside an
otherwise homogenous α-phase (Au with dissolved Ge)
without any significant regions showing a eutectic
morphology.
Fig. 3 The phase diagram of the Au–Ge material system [15]. The dashed
blue line indicates the overall composition of the fabricated LSD joints.
© IMAPS / EMPC 2017
2
European Microelectronics
Packaging Conference
10-13 September 2017, Warsaw, Poland
www.empc2017.pl
bonding : Cu – Sn , Au – Sn and Au – In,” in Electronics
System-Integration Technology Conference (ESTC),
2014, no. Eq 1, pp. 1–6.
[8] A. Larsson, T. A. Tollefsen, O. M. Løvvik, and K. E.
Aasmundtveit, “Ni – Sn solid liquid interdiffusion (
SLID ) bonding – Process , bond characteristics and
strength,” in 6th Electronic System-Integration
Technology Conference (ESTC), 2016.
Fig. 4 SEM micrograph of a cross-section of a fabricated liquid solid
diffusion (LSD) joint. Light grey regions represent the α phase (Au), and the
small and isolated dark grey regions the β phase (Ge).
V. CONCLUSIONS
A brief description of a novel joining method called liquid
solid diffusion (LSD) bonding was presented. Detachment
experiments demonstrated joints with structural integrity at
temperatures significantly higher than the initial melting
temperature of the system. This indicates a solid, continuous
and coherent structure adjoining the two components
structurally at high temperatures. Cross-sectional analysis
showed the joint to be a mixture with about 8 at-% Ge (βphase). The β-phase had segregated into isolated small local
domains in a homogenous matrix of α-phase (Au).
VI. ACKNOWLEDGEMENT
The authors would like to acknowledge the Norwegian
Research Council for supporting this project (Project No.:
244915).
VII. REFERENCES
[1] C. S. Smith, A history of metallography - The
development of ideas on the structure of metals before
1890, 1st ed. Chicago: The University of Chicago Press.,
1960.
[2] C. S. Smith, A search for structure. MIT Pr, 1981.
[3] M. Lacroix, “Contact melting of a phase change material
inside a heated parallelepedic capsule,” Energy Convers.
Manag., vol. 42, no. 1, pp. 35–47, 2001.
[4]
[9] G. O. Cook and C. D. Sorensen, “Overview of transient
liquid phase and partial transient liquid phase bonding,”
J. Mater. Sci., vol. 46, no. 16, pp. 5305–5323, 2011.
[10] V. R. V. R. Manikam and K. Y. K. Y. K. Y. Cheong,
“Die Attach Materials for High Temperature
Applications: A Review,” Components, Packag. Manuf.
Technol. IEEE Trans., vol. 1, no. 4, pp. 457–478, 2011.
[11] H. Okamoto, Desk Handbook: Phase Diagrams for
Binary Alloys, 2nd ed. United States of America: ASM
International, 2010.
[12] A. Drevin-Bazin, F. Lacroix, and J. F. Barbot, “SiC die
attach for high-temperature applications,” J. Electron.
Mater., vol. 43, no. 3, pp. 695–701, 2014.
[13] S. Tanimoto, K. Matsui, Y. Murakami, H. Yamaguchi,
and H. Okumura, “Assessment of Au-Ge Die
Attachment for an Extended Junction Temperature
Range in Power Applications,” in International
Conference and Exhibition on High Temperature
Electronics, 2010, no. HiTEC, pp. 32–39.
[14] W. Sabbah, S. Azzopardi, C. Buttay, R. Meuret, and E.
Woirgard, “Study of die attach technologies for high
temperature power electronics: Silver sintering and
gold-germanium alloy,” Microelectron. Reliab., vol. 53,
no. 9–11, pp. 1617–1621, 2013.
[15] H. Okamoto and T. B. Massalski, “The Au-Ge (GoldGermanium) System,” Bull. Alloy Phase Diagrams, vol.
5, no. 6, pp. 601–610, 1984.
a. P. Kryshtal, R. V. Sukhov, and a. a. Minenkov,
“Critical thickness of contact melting in the Au/Ge
layered film system,” J. Alloys Compd., vol. 512, no. 1,
pp. 311–315, 2012.
[5] L. Bernstein, “Semiconductor Joining by the SolidLiquid-Interdiffusion (SLID) Process,” J. Electrochem.
Soc., vol. 113, no. 12, pp. 1282–1289, 1966.
[6] T. A. Tollefsen, A. Larsson, M. M. V. Taklo, A. Neels,
X. Maeder, K. Høydalsvik, D. W. Breiby, and K.
Aasmundtveit, “Au-Sn SLID Bonding: A Reliable HT
Interconnect and Die Attach Technology,” Metall.
Mater. Trans. B, vol. 44, no. 2, pp. 406–413, 2013.
[7] K. E. Aasmundtveit, T. Luu, A. B. Vardøy, T. A.
Tollefsen, K. Wang, and N. Hoivik, “High-Temperature
Shear Strength of Solid-Liquid Interdiffusion ( SLID )
© IMAPS / EMPC 2017
3