Full text of DOI:10.1557/mrs2001.118

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Among metal alloys, Cu-W has been the
standard for metal heatsinks in micro-
electronics packaging because W added to
Cu provides a lower CTE than Cu alone,
without markedly lowering the thermal
conductivity.3 Cu-Mo alloys have been
developed to reduce the density of the
composites.4 Fe-Ni alloys, examples of
which are 42 alloy, Invar, and Kovar, are
also commonly used for applications re-
quiring low thermal expansion, but these
alloys also have low thermal conductivity.
An Fe-Ni-Ag composite has been devel-
oped through infiltration of molten silver
into a porous Fe-Ni metal preform. The
material has advantages in cost and den-
sity, compared with refractory-based metal
composites.5
Al-SiC composites have been used for
their reduced density. Both liquid-metal
sintering and liquid-metal infiltration
have been employed for their production.
For example, SiC green preforms can be
made using tape casting, fired in air at
1000–1100ꢀC, and molten Al infiltrated
into the fired SiC in N2 at 700–800ꢀC. A
70wt%SiC-30wt%Al composite has a con-
ductivity higher than that of Al, a density
of 40% of Kovar, and a low CTE.6,7
Metallization of
High Thermal
Conductivity
Materials
Yoshihiko Imanaka and Michael R. Notis
Introduction
Great progress in LSI (large-scale integra-
materials with high thermal conductivity
for use in the field of microelectronics
packaging and the metallization technol-
ogy of related materials.
tion) has been made over the past 10 years,
and more rapid progress is expected in the
future. In recent years, the increased circuit
density in LSI devices has led to remarkable
heat generation, thereby creating thermal-
management issues for microelectronics
packaging.1,2 In order to develop high-
performance LSI effectively, packaging and
system-level configuration of the computer
must be modified to handle the heat.
There are two paths for heat dissipation
from an integrated circuit (IC) through the
chip back side and through the substrate,
as shown in Figure 1. With respect to chip
back-side cooling, a heatsink is attached
to the back side of the LSI device to re-
move the heat efficiently, because solders
are damaged by generated heat (see Fig-
ure 1a). The primary requirements for the
sink material are high thermal conductiv-
ity, a coefficient of thermal expansion (CTE)
close to that of Si, and low specific gravity.
When LSI chips are placed directly on the
electrode, heat dissipation is by means of
the substrate (see Figure 1b). In addition
to needing high thermal conductivity, and
a CTE close to that of Si, the substrate then
needs to have high electrical resistance
and a low dielectric constant.
HighThermal Conductivity
A Ag-Cu alloy containing diamond
particles with a thermal conductivity of
Materials for Heatsink Applications
The characteristics of high thermal con-
ductivity materials, including metals, metal
alloys, ceramic/metal composites, and ce-
ramics, are listed in Table I. Cu and Al are
most often used for heatsink materials.
ꢁ1
420 W mꢁ1 K and a CTE of 5.5 ꢂ 10 /ꢀC
has been successfully fabricated.8 The
process of coating fine diamond particles
with Ti, W, or Cr is important for fabri-
cating this material to achieve wettability
of the diamond with the Ag-Cu alloy. In
recent years, optimizing the manufactur-
ing process and interface conditions be-
tween diamond and certain metals has
significantly enhanced the conductivity
ꢁ6
ꢁ1
(ꢀ650 W mꢁ1 K ) and has controlled the
values of thermal expansion (8–10 ꢂ
ꢁ6
10 /ꢀC). These ceramic/metal and dia-
mond/metal composites are expected to
find application as new heatsink mate-
rials, although several problems exist,
such as their high cost, complicated proc-
essing, and poor machinability for indus-
trial applications.
Metallization for Substrates with
HighThermal Conductivity
The processing conditions of metalliza-
tion and the characteristics of metallized
materials strongly control LSI packaging
performance and reliability for microelec-
tronics applications because metallization
provides contact with such advantageous
characteristics as good electrical conduc-
tivity, high thermal conductivity, and strong
adherence to ceramic substrates. Metalli-
zation is therefore the key technology in
microelectronics packaging. In this article,
we describe recent developments involving
A metal layer with low electrical resis-
tivity is used for the wiring circuit on
ceramic substrates. Copper, which has
been widely used for printed circuit
boards, is the best material for conductor
wiring paths in terms of its low resistivity
(1.7 ꢃꢄ cm) and high reliability. However,
small amounts of impurities greatly in-
crease the resistivity of Cu, due to the for-
mation of solid solutions or precipitated
compounds. Thus, to obtain high electri-
cal conductivity for the conductors, im-
Figure 1. Two arrangements of
heat-dissipation paths for large-scale
integration (LSI): (a) chip back-side
cooling and (b) cooling through the
substrate. In chip back-side cooling,
a heatsink and heat spreader are
sometimes placed on the LSI chip to
increase the heat-dissipation efficiency.
Arrows indicate the expected thermal
path out of the LSI chip.
MRS BULLETIN/JUNE 2001
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Metallization of High Thermal Conductivity Materials
Table I: Characteristics of HighThermal Conductivity Materials for Heatsink and
Substrate Applications.
Thermal
Conductivity
(W mꢀ1 Kꢀ1
)
Coefficient of
Thermal Expansion
(10ꢀ6/ꢁC)
Density
(g/cm3)
Dielectric
Constant
Material
Si
Cu
Al
W
Mo
140
398
205
167
138
15
4.2
16.6
23.9
4.5
5.4
5.9
2.3
8.9
2.7
19.2
10.2
8.4
—
—
—
—
—
—
Kovar
(Fe-29Ni-17Co)
Cu-W (10/90)
Cu-Mo (15/85)
Fe-Ni-Ag
Al-SiC (30/70)
Ag-Cu/Diamond
(45/55)
209
180
153
202
420
6.5
6.6
6.5
7.0
5.5
16.4
9.9
8.9
3.0
6.4
—
—
—
—
—
AIN
180
270
270
4.4
3.7
8.0
1.5
3.2
2.9
2.9
3.5
8.9
40
7
SiC (BeO-doped)
BeO
Diamond
2300
5.7
purity contamination in the Cu must be
prevented during the entire metallization
process.
terned on the substrate is densified by fir-
ing, so that it adheres to the substrate.
Using this technique, the thick film can be
formed simply and at low cost, but the
shape and dimension are limited (thick-
ness, ꢀ1–30 ꢃm; line width, ꢅ50 ꢃm).
Table II shows the interfacial behavior
and other features of various conducting
pastes and additives formed by thick-film
metallization on several high thermal con-
ductivity substrates. The conducting pastes
commonly used for AlN substrates are
currently Ag-Pd, which are commercially
used for Al2O3 substrates. However, two
phenomena of concern are observed at the
interface: (1) dewetting and (2) blister for-
mation. To suppress these effects, additives
in the conducting paste have been investi-
gated. In the case of Ag-Pd paste with
Bi2O3 and CdO as additives, dewetting
occurs due to the formation of a glass
layer at the AlN interface. Using a paste
containing glass with a slight PbO con-
tent, no blisters are observed, while blis-
ters are formed at the metallized interface
by the reaction 2AlN ꢆ 3PbO l Al2O3 ꢆ
3Pb ꢆN2 in the case of glass with a high
concentration of PbO.9,10 When a ZnO-
B2O3-PbO glass is used as an additive for
the Ag-Pd paste, ZnAl2O4 spinel and PbO
form as coexisting phases at the interface.
In this case, the formation of coexisting
phases suppresses oxidation of the AlN
substrate, and blistering does not occur.
Li2O-B2O3 glass is also used as an additive;
it provides good wettability to AlN sub-
Metallization on the ceramic substrate
plays an important role in terms of bond-
ing to both active and passive devices.
Two bonding methods are usually used
for microelectronics packaging, solder
bonding and wire bonding (see Figure 1),
and either approach requires an overlay
material on top of the base metallization.
In solder bonding, a metal having good
wettability to the solder, such as Cu, Ni,
Au, Pd, or Pt, is applied for the top layer;
in wire bonding, Al, Al alloys, Au, Au-Pd,
Ag-Pd, Ni, or Pt are used for the top layer.
The best manufacturing method to use
is determined by additional metallization
factors such as shape, dimensional require-
ments, and cost. The methods are clas-
sified into four categories: (1) thick-film
processing, (2) co-firing, (3) brazing, and
(4) thin-film processing, as shown in Fig-
ure 2. The following sections summarize
technical issues relating to various metal-
lization methods for high thermal conduc-
tivity ceramic substrates such as AlN, BeO,
and SiC (see Table I).
Thick-Film Metallization
In this technique, a conducting paste
consisting of metal powder, organic binder,
plasticizer, and solvent is patterned on a
previously fired ceramic substrate, using
a screen-printing method, as shown in
Figure 2a. The conducting paste pat-
Figure 2. Schematic illustrations of
metallization processes applied to
high thermal conductivity substrates:
(a) thick-film processing, (b) co-firing,
(c) brazing, and (d) thin-film processing
(vapor deposition).
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Metallization of High Thermal Conductivity Materials
Table II: Summary ofThick-Film Metallization on HighThermal Conductivity Substrates.
Conducting
Paste
Substrate
AlN
Additive
Bi2O3, CdO
Interfacial Behavior
Remarks
Dewetting
Reference
Ag-Pd
Formation of glass layer with high
concentration of Bi2O3 and CdO
Formation of N2 gas
13
PbO glass
Blister formation
9,10
(2AlN ꢆ 3PbO l Al2O3 ꢆ 3Pb ꢆ N2)
Infiltration to SiO2 layer formed on AlN
Formation of coexistence phase
(Zn Al2O4 ꢆ PbO)
PbO glass
ZnO-B2O3-PbO glass
Good adhesion
Blister-free
(inhibition of
13
10
Al2O3 formation)
Blister-free
Blister formation
Li2O-B2O3 glass
MnO2
Good wettability to AlN
Formation of N2 gas
11
12
Mo
(2AlN ꢆ 3MnO2 l Al2O3 ꢆ 3MnO ꢆ N2)
Infiltration of MnO-based glass to BeO
—
BeO
Mo
W
MnO2
Y2 O3, La2 O3
Good adhesion
Enhancement of
thermal conductivity
Blister formation
Good adhesion
14
15
SiC (BeO-doped)
Ag-Pd, Au
Mo
—
—
Formation of CO and CO2 gases
—
16
16
strates and a blister-free microstructure at
the interface.11 ZnO-B2O3-PbO glass and
Li2O-B2O3 glass are, therefore, effective ad-
ditives for Ag-Pd conducting pastes in the
thick-film metallization of AlN substrates.
Molybdenum with a MnO2 additive also
has been used as a conducting paste for
thick-film metallization on AlN substrates.
Frequently, bubbles form due to the re-
action 2AlN ꢆ 3MnO2 lAl2O3 ꢆ 3MnO ꢆ
N2.12 However, by firing the paste in a wet
H2 and N2 mixed-gas atmosphere with a
controlled dew point, a thick film with
high thermal conductivity and high ad-
hesion with respect to the substrate can
be obtained.
As mentioned earlier, thick-film metal-
lization on AlN substrates has been car-
ried out using conducting pastes designed
for Al2O3 substrates. For these pastes to
work on AlN, it is important to form
oxides on the surface of the AlN substrate.
A porous alumina frame structure can be
formed at the AlN substrate surface by
controlling the oxygen and H2O vapor
pressures at selected firing temperatures.
Furthermore, a dip-coating process, using
a Si alkoxide solution, leads to the forma-
tion of a homogeneous SiO2 layer in the
porous alumina frame structure. Through
these processes, the conducting pastes de-
signed for Al2O3 substrates can be applied
to AlN substrates. When the Ag-Pd paste
containing glass with a small amount of
PbO was metallized on the SiO2 layer-
coated AlN substrate, strong adhesion
resulted between the substrate and the
thick film.13
paste with a MnO2 additive has been
used.14 However, a W paste with Y2O3 or
La2O3 additives has been developed to
improve the thermal conductivity of the
thick film.15
due to the close match in their CTEs. No
chemical reaction layer is formed at the co-
firedAlN/W interface, although W particles
diffusing about 5 ꢃm into the AlN provide
an interlocking mechanism at the inter-
face, as shown in Figure 3.18 The addition
of a slight amount of AlN to the W paste
helps control the shrinkage behavior of
tungsten. We have demonstrated that W
paste with 0.5 wt% AlN can synchronize
the shrinking behavior of an AlN green
sheet, as shown in Figure 4. As a conse-
quence of the shrinkage control, a signifi-
cant increase (ꢀ50%) in adhesion strength
was found in the co-fired 0.5wt%AlN/W
For SiC substrates, commercial Ag-Pd
and Au pastes have been used. Using
these pastes, blistering at the SiC interface
occurs due to CO and CO2 gas generation
during firing, resulting in low adhesion
between the substrate and the metal. It is
important to control the amount of gas
released, in order to suppress blistering.
Molybdenum also has been used as a con-
ducting paste for metallization on SiC
substrates. High adhesion between the
SiC substrate and the Mo thick film is
achieved by controlling the Mo shrinkage
behavior and interfacial reactions through
optimizing firing conditions.16
Co-Firing
Figure 2b shows a schematic diagram of
the co-firing process. A conducting metal
paste is patterned on the ceramic “green
sheet,” consisting of the ceramic powder
and an organic binder, using screen-printing
methods. The printed green sheets are
stacked and pressed at a temperature of
about 100ꢀC, to make a “green body.” The
ceramic and metal conductors are densi-
fied simultaneously by firing this green-
body composite at a high temperature. In
this case, control of shrinkage of the metal
paste and the ceramic green sheet and se-
lection of firing atmosphere are very im-
portant.17 The firing atmosphere is important
to prevent oxidation of the metal paste.
Tungsten is used as the conductor metal
for co-firing with AlN green sheets, mainly
Figure 3. Microstructure of the interfacial
region between W and AlN on a
co-fired multilayer AlN substrate with
W conductors. W particles appear in the
AlN and are distributed in the vicinity
within about 5 ꢃm from the interface.
For thick-film metallization on BeO ce-
ramics, conventionally a Mo conducting
MRS BULLETIN/JUNE 2001
473

Metallization of High Thermal Conductivity Materials
where ꢉG is the Gibbs free energy of re-
Thin-Film Metallization
action, A is the area of the reaction zone,
ꢇss is the reaction product–substrate inter-
facial energy, ꢈ is the contact angle, and
ꢇLV is the liquid–vapor interfacial energy.
According to this equation, the formation
of a thermodynamically stable reactant at
the interface results in a larger work of
adhesion and produces a lower contact
angle. Therefore, the formation of a TiN
layer at the interface provides high adhe-
sion between the Ag-Cu-Ti alloy and the
AlN substrate.
Thin-film metallization is roughly di-
vided into two methods: (1) wet chemical
processes, such as electrolytic and electro-
less plating; and (2) vapor deposition,
including evaporation, sputtering, and
chemical vapor deposition. Thinner films
are obtained by vapor deposition, and
thicker ones are formed by wet chemical
processes. In the wet chemical process,
three factors must be taken into account:
(1) oil and organic removal (dewaxing) at
the substrate surface, (2) adjustment of
surface roughness, and (3) introduction of
a catalyst (PdCl2, etc.) to control nuclea-
tion of the plating. At present, electroless
Cu and Ni-P are applied to AlN substrates
after wet etching with NaOH.24 Good ad-
hesion with SiC substrates is obtained
using XeCl excimer laser irradiation with
electroless Ni and Cu.25
SiC can be bonded using either Ni-Ti or
Cu-Ti alloys.22 Besides these alloys, a Cr
thick-film metallization has also been ap-
plied to improve adhesion strength.23 Are-
acted Cr layer at the interface between SiC
and the brazing alloy serves the purpose
of a barrier layer.
Figure 4. Firing shrinkage curve of an
AlN green sheet, W paste, and W paste
with 0.5 wt% AlN powder. Mismatch of
firing shrinkage between the W paste
and the AlN green sheet, which leads
to cracking and failure, is improved by
adding AlN powder to the W paste.
Direct Bonded Copper Method. The direct
bonded copper (DBC) method is a brazing
technique using a liquid phase formed in
the vicinity of the Cu-O eutectic reaction (see
Figure 514). Strong bonding is obtained by
annealing the bonded couple, which pos-
sesses a hypoeutectic composition at some
temperature between the liquid-phase
temperature and the eutectic temperature.
Atmosphere control (O2 concentration of
0.02–0.08% in N2) and formation of a cop-
per oxide layer on a preheated Cu foil are
key factors in this process.
It is well known that high-purity AlN
substrates show low wettability to Cu. An
alumina layer is formed on the AlN to im-
prove its wettability. A layer 1–3 ꢃm thick
provides the highest bonding strength be-
tween AlN and Cu, while a layer less than
1 ꢃm thick has low strength, indicated by
stress and cracking at the alumina/AlN
interface.
It is difficult to obtain Ti or Ni-Cr thin
films on a ceramic substrate by wet chemi-
cal processes, while Ti or Ni-Cr films ap-
proximately 100–500 nm thick can be
formed by vapor deposition. Such films
are used for the adhesion layer adjacent to
the ceramic substrate. Over this adhesion
layer, a conducting layer and an electrode
layer for bonding are frequently deposited.
material, compared with the co-fired AlN/W
material.
Brazing
This method involves bonding a metal
foil to a ceramic using a liquid-metal phase.
A schematic diagram of the brazing proc-
ess using mechanically applied pressure is
shown as an example in Figure 2c. There
are two methods for forming the liquid
phase: (1) using a braze alloy with a low
melting point, and (2) using a eutectic re-
action between a metal foil and the ceramic.
To obtain a strong bond between the con-
ducting metal and the ceramic, it is im-
portant to find the optimum liquid contact
angle, which significantly influences the
type of metal foil, ceramic, and liquid
phase formed as well as the type of at-
mosphere in the system.
Multilayer Thin-Film Metallization. The
most common multilayer thin-film metal-
lization sequence has been carried out as
follows. First, a bottom layer for adhesion
is deposited on the substrate by sputtering
of Ti or Cr. A seed layer for electrolytic
plating is subsequently formed by sput-
tering of Cu. Electrolytic plating can be
used to form the conducting layer because
of the requirement for a thicker film. Fi-
nally, a surface electrode layer such as Au
is deposited by plating. A barrier layer is
sometimes inserted between the conduct-
ing layer and the surface electrode layer to
prevent interfacial reactions.
Active Brazing. In microelectronics, braz-
ing is applied to AlN and BeO-doped SiC
ceramics. These materials show low wet-
tability with respect to molten metals
because of a low solid–vapor interfacial
energy. To improve wettability, active metals
such as Ti or Zr are added to brazing alloys.
Ag-Cu-Ti alloy and TiH2 powder have
also been used for brazing with AlN
substrates. In this technique, the reaction
AlN ꢆ Ti l TiN ꢆ Al occurs at the inter-
face with AlN.19–21 A wavy TiN layer
provides high adhesion by forming a mi-
croscale interlocked structure at the inter-
face. The work of adhesion (Wad) with
forming a reaction layer at the interface is
expressed as
We have studied Ti/AlN and Cu/Ti/
AlN multilayer thin films intensively, which
are the most interesting materials for mi-
croelectronics packaging applications, and
we have concentrated on their interfaces.
At the interface of sputtered Ti/AlN, the
intermetallic compound Ti2AlN is formed
after annealing at temperatures ranging
from 800ꢀC to 850ꢀC in a vacuum atmos-
phere, as seen in Figure 6.26 When Ti and
Cu are sequentially sputtered on an AlN
substrate, Ti reacts with Cu after anneal-
ing well below 800ꢀC, as shown in Figure 7.
Above 800ꢀC, Al and N from the AlN react
with Cu and Ti from the thin film, and a
TiN layer is formed at the interface and
Al is dissolved in the Cu thin film, as
shown in Figure 8. Because impurity dis-
solution into Cu substantially increases
Figure 5. Phase diagram of the Cu-O
system. The solubility of oxygen at
the eutectic temperature is 0.008 at.%,
and the melting point is lowered to
1065ꢀC at the eutectic composition
of Cu-0.39wt%O.
Wad ꢀ ꢇLV (1 ꢁ cos ꢈ) ꢆ ꢇss ꢁ AꢉG,
(1)
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Metallization of High Thermal Conductivity Materials
the resistivity of Cu, reaction with Cu must
be avoided in order to maintain lower
resistivity.
a Cu/Ti/Ti2AlN/AlN structure. Another
tem, a polyimide (PI) insulating layer and
Cu circuitry (line width, 10 ꢃm; spacing,
10 ꢃm) are formed on the AlN substrate.
method is to insert a W barrier layer be-
tween the Cu and Ti layers. W is not reactive
with either Cu or Ti, and this can produce
a narrow composition gradient at the Cu-W
interface. The final multilayer structure
containing the Cu-W gradient region is
shown in Figure 9, and this is believed to
provide adhesion between Cu and W, as
well as to allow relaxation of the film
stress. Figure 10 shows a multichip module
(MCM) for a mainframe computer using
this metallization technology. For this sys-
Two new metallization processes for
AlN substrates have been developed to
satisfy both the need for a pure Cu layer
having low resistivity and high adhesion
strength with respect to AlN. One approach
is (1) to produce a high-strength adhesion
layer by the formation of Ti2AlN between
the Ti and AlN by heat treatment after Ti
deposition on AlN, and then (2) to deposit
Cu onto the Ti film; this process produces
Concluding Remarks
An ongoing and future need for micro-
electronics packaging is the establishment
of efficient heat-dissipation systems be-
cause improving high-performance LSI
and increasing high-density mounting
both increase the heat flux of the packag-
ing module. Thus, the application of high
thermal conductivity materials is essential
Figure 8. Depth profile of Cu/Ti/AlN annealed at 850ꢀC for 15 min
in vacuum, detected by RBS. Because Cu-Ti interdiffusion is
complete at 600ꢀC, Ti reaches the outside of the film. However,
due to N diffusion from AlN at higher temperatures, Ti moves
back to the original interface adjacent to AlN to form TiN. All of
the Ti is consumed in TiN formation, and a Cu-Al layer is formed
on TiN.
Figure 6. Depth profile of a sputtered Ti film deposited on an
AlN substrate after annealing at 825ꢀC for 5 min in vacuum as
detected by Rutherford backscattering spectrometry (RBS). Note
the layer having Ti, Al, and N with atomic ratio of 2ꢀ1ꢀ1 at the
interface between the AlN and the Ti.
Figure 9. Cross-sectional view of Cu/W/Ti metallization on an AlN
substrate. A W layer is used for the barrier layer, preventing the
interfacial reaction between Cu and Ti. A Cu-W multilayer with a
compositional gradient is also inserted between the Cu and W
layers. AlN particles in the W via form an interlocking structure,
resulting in high adhesion strength.
Figure 7. Depth profile of Cu/Ti/AlN annealed at 300ꢀC for 15 min
in vacuum, detected by RBS. Cu-Ti interdiffusion is observed at
the interface between Cu and Ti on the AlN substrate.
MRS BULLETIN/JUNE 2001
475

Metallization of High Thermal Conductivity Materials
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The authors acknowledge the contribu-
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ꢀ
International Workshop on Device Technology
September 3-5, 2001 • Port Alegre, Brazil
Alternatives to SiO2 as Gate Dielectrics for
Future Si-Based Microelectronics
For more information, contact: Prof. Israel Baumvol, Instituto de Física
Universidade Federal do Rio Grande do Sul
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