Study of Cu Migration Kinetics under Electric and
Thermal Driving Forces
計畫編號：甲 93-EFA04-1-4
執行期限：94 年 4 月 1 日至 95 年 3 月 31 日
主持人：廖建能教授
計畫參與人員：陳冠嘉、劉昆明
interconnects is increasing all the time. A
small voltage or temperature difference
across an interconnecting wire will turn into
a huge electric field or temperature gradient
applied along the wire. Atomic migration
under such substantial electrical and thermal
driving forces becomes a serious reliability
issue for IC industry, which may post a threat
on the advancement of IC technology.
Electromigration (EM) induced voiding
at interconnections is undoubtedly the most
persistent reliability issue in microelectronic
devices.1 Many EM studies were performed
by measuring the resistance change of the
tested structure.2-4 The resistance increase
can be correlated to the void formation in the
metal lines and the kinetics of EM-induced
mass transportation can hence be deduced.
In recent years a number of in-situ EM
studies have been carried out by observing
void and hillock formation on metal lines
using scanning electron microscopy.5-7
These measurements were usually done at
elevated temperatures in order to observe
EM-induced voiding or hillock formation
within a reasonable time frame.
The
question raised would be whether the
extrapolated results from the accelerated
testing can be used to accurately predict the
reliability of microelectronic devices in
normal operating conditions. Indeed, grain
boundary diffusion for Cu is overpowered by
surface diffusion at lower temperatures,
while the grain boundary diffusion becomes
comparable to surface diffusion when the
temperature is at 350 C.8 Thus, Cu EM
mechanisms
deduced
at
elevated
temperatures may not be the same as those at
low temperatures. To clarify the issue we
need a method of detecting minute changes
of the Cu microstructure in the normal
operation temperature regime.
In this
Abstract
In this study atomic-scale migration in
unpassivated copper metal lines under
electric current stressing was directly
observed in ultrahigh vacuum at room
temperature by in situ transmission electron
microscopy. It was found that copper atoms
on a (211) crystal plane vanished
directionally within half an hour when
applying an electric current with a density of
2×106 A/cm2 through the tested Cu line.
The electromigration (EM) induced atomic
migration appeared to be anisotropic, and the
combination of {111} planes and <110>
directions was suggested to be the easiest
electromigration system for crystalline
copper. EM-induced mass transport was
also found to be responsible for the
weakening (111) texture of the Cu lines after
electric current stressing.
Keywords: Electromigration, in situ TEM,
surface diffusion
摘要
本研究利用臨場穿透式電子顯微鏡技
術直接觀測銅導線上銅原子在室溫下之電
遷移行為。結果發現一(211)方向的銅晶粒
在電流密度 2×106 A/cm2 下，其原子影像僅
在 25 分鐘內即消失不見。分析指出銅原子
之電遷移行為具有非均向性，{111}晶面在
<110>方向上對電遷移之抵抗力最低。X 光
繞射分析亦顯示銅薄膜(111)優選方向結構
將隨著電遷移作用而改變。
關鍵詞：電遷移、臨場穿透式電子顯微鏡、
表面擴散
Introduction
Continuous scaling of integrated circuit
elements results in an increase of current
density and associated Joule heating in the
interconnects. Although the driving voltage
of IC devices is decreasing accordingly, the
current density and heat dissipation in IC
1
study, the in situ transmission electron
microscope
(TEM)
observation
of
atomic-scale EM behavior in a Cu line is
reported. The effects of crystal orientation
and anisotropic electric resistivity on
atomic-scale Cu EM characteristics are
discussed. EM-induced texture change in
the Cu line is also a subject of interest.
Experimental
A Cu metal line on a Si3N4 membrane
structure was fabricated for in situ TEM
observations of Cu EM behavior. A SiO2
(40 nm) / Si3N4 (80 nm) bilayer thin film was
deposited in sequence on both sides of a
4-inch double-side-polished Si wafer by a
low pressure chemical vapor deposition
method. The oxide/nitride thin film at the
backside was patterned and dry etched to
become a hard mask for subsequent Si
substrate etching. The wafer was then
placed into a 20% KOH solution that was
maintained at 80 C for 1.5 h. The Si
substrate was etched from the backside until
the nitride layer at the front-side of the wafer
was reached. A silicon nitride membrane
structure on the Si substrate was obtained.
Next, a thin Cu film (30 nm in thickness) was
deposited on the wafer by e-beam
evaporation and patterned using a wet
etching method. After rinsing and drying,
the specimen was annealed at 250 C for 30
min in a vacuum of 10-3 torr to stabilize the
Cu microstructure.
The specimen was
mounted on a specially designed TEM holder
that has two electrodes allowing application
of electric current through the specimen.
After loading the sample into the TEM
system (Model: JEOL 2000V UHV-TEM)
with a base pressure of 3×10-10 torr, the in
situ TEM observation was performed during
stressing the 15 m-wide Cu line at an
average current density of 2×106 A/cm2. In
addition, a long stripe of a Cu thin film
sample of 10 mm × 2 mm was also examined
before and after electric current stressing at a
current density of 8×105 A/cm2 for 3 h by
x-ray diffractometry (Philips: PW3040) to
explore the EM effect on the texture of the
Cu thin film. The x-ray beam was incident
on the Cu sample through a slit of 1 mm in
width. A very strong Si (400) peak at
2=69º from single crystal Si substrate was
used as an internal reference to ensure the
consistent inclination between the incident
x-ray beam and the principal direction of the
Cu film.
Results and discussion
Figure 1 shows a high-resolution (HR)
TEM image of a Cu line under electric
current stressing and its corresponding fast
Fourier transformation (FFT) pattern. Two
intersecting lattice planes were found to be
(1 1 1 ) and ( 1 20) planes with a zone axis in
the [211] direction from the analysis of the
FFT pattern shown in fig 1. The interplanar
spacing of the lattice plane in the neighboring
grain was also identified to correspond to
d ( 220) . By applying an electric current
through the Cu metal line, the evolution of
the Cu atomic structure on the examined (211)
grain was video recorded in real time.
d
d (22¯ 0)
(1 ¯
11¯
)
FFT
d( ¯
120
 [211]
)
2nm
FIG. 1 HRTEM image of a thin Cu line under
electric current stressing and its corresponding
FFT pattern.
Figure 2 reveals the atomic images of Cu
grain along the [211] direction at different
stages during electric current stressing.
Interestingly, the Cu atomic images appeared
to fade gradually from the lower left of the
examined grain and disappeared completely
after stressing the Cu line for 25 min. It is a
strong indication of EM-induced atomic
movement in the Cu grain. The Joule
heating induced temperature rise at the
sample was also estimated by measuring the
resistance change of the tested Cu line. Our
measurement showed that the increase in the
Cu line resistance was less than 5% under a
current density of 2×106 A/cm2, indicating
the Joule heating induced temperature rise is
less than 20 C according to the thermal
coefficient of resistance of the Cu line. A
2
further test was conducted for in situ
observation of the Cu film without applying
any current.
No change was detected.
Therefore, direct observation of Cu
electromigration phenomenon at such low
temperatures within a short period of time
has been achieved.
e-
[110], and [111] at room temperature and
found them in the order of 111] > 100] >
110]. It implies that electrons flowing in
the <110> directions would experience less
resistance than in the <111> directions. By
applying a voltage across a crystalline copper,
the partial current density in the <110>
directions is expected to be higher than that
in the <111> directions. Based on the
anisotropic diffusion and electrical resistivity
of crystalline copper, the combination of
{111} planes and <110> directions is
suggested to be the most favorable
electromigration system inside a grain, which
is well supported by our in situ TEM
observations of atomic-scale Cu EM
behavior.
[0
11¯
]
e-
(a)
(b)
e-
e(c)
(d)
e-
e-
(e)
(f)
FIG. 2 Real-time HRTEM images of a thin Cu
line under electric current stressing recorded at (a)
6, (b) 9, (c) 12, (d) 15, (e) 21, and (f) 25 min.
By examining the sequential HRTEM
images carefully, the atomic images of the
Cu grain did not seem to shrink equally in all
directions. Instead, the (211) lattice image
faded away from the edge of the
(1 1 1 ) lattice plane along the [01 1 ] direction.
It is noted that Cu surface diffusion on
different crystal planes has distinct migration
energies.
Face-center-cubic
crystal
structure, in general, has the lowest migration
energy on {111} planes among the major
low-index planes.9-10 Figure 3 shows the
top view of the (211) crystal plane which
has –A–B–C–A–B–C– layer configuration.
It can be found that the path along the
[01 1 ] direction, one of the close-packed
directions in the (1 1 1 ) plane, turns into a
diffusion channel for the surface (1st layer)
atoms on the (211) plane. In addition,
electrons flux inside a crystal may not be
uniformly distributed due to anisotropic
electrical property. Magnaterra11 presented
a theoretical calculation of the partial
resistivities of Cu in the directions of [100],
FIG. 3 A schematic diagram of atomic
arrangement on (211) planes for FCC crystals
and the migration channel of surface atoms along
[01 1 ] direction.
Since Cu grain boundary diffusivity
(Dgb=3×10-15 cm2/s) at 100 C is lower than
Cu surface diffusivity (Ds=10-12 cm2/s) by
three orders of magnitude,8 the atomic
diffusion along grain boundaries has little
impact on Cu EM at a temperature below 100
C.
Our in-situ TEM experiment was
performed at temperatures around 40-50 C
when the Joule heating effect is taken into
account. It is rather reasonable to assume
the predominance of Cu surface diffusion
under such testing conditions. Although we
cannot measure the atomic-scale Cu thinning
quantitatively, we did observe some Cu
grains vanishing in the stressed Cu line by
TEM, which is not shown here. A global
thinning of Cu grains, especially for Cu (111)
grains, is expected when the Cu line is
3
subjected to a high current density stressing.
Thus, we speculated that the relative
distribution of Cu grain orientation in the Cu
line may change due to preferential EM on
Cu (111)-oriented grains. Figure 4 shows
the x-ray diffraction results of the annealed
Cu thin film stripe before and after applying
a current density of 8×105 A/cm2 for 3 h.
The relative intensity of the (111) peak to
other major indices peaks was used to gauge
the degree of preferred crystal orientation in
the Cu thin film. The ratio of I(111)/I(200) and
I(111)/I(311) was found to change from 12 to 8.3
and 4.7 to 3.6, respectively, indicating
weakening of the (111) texture in the Cu thin
film after electric current stressing. Similar
results were also reported that the (111)
texture of the unpassivated Cu lines was
weakened after EM testing.7 These findings
lead to the conclusion that Cu (111) oriented
grains are more susceptible to EM damage
than others when surface diffusion is a
dominant migration mechanism.
voiding in the metal lines. The highly
textured Cu line has more uniform grain
boundary properties and hence less flux
divergence sites for void nucleation than the
randomly oriented one.
Therefore, Cu
interconnections with strong textured
microstructure may have superior EM
lifetimes. In the present study, we observed
the intragranular atomic surface diffusion
driven by electric currents. The role of
grain
boundary
in
atomic-scale
electromigration and void nucleation will be
a subject of future study
Conclusions
In conclusion, in situ TEM observation
of atomic-scale electromigraion in a thin Cu
line was successfully performed at a
temperature below 100 C.
The
combination of {111} planes and <110>
directions was found to have the least EM
resistance
due
to
anisotropic
diffusion/electrical properties of crystalline
Cu. The Cu (111) texture of copper metal
lines was degraded after electric current
stressing because EM induced mass transport
is highly preferred on Cu grains with (111)
orientation.
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FIG.4 X-ray diffraction spectra for a thin Cu
stripe (a) before and (b) after stressing at a
current density of 8×105 A/cm2 for 3 h.
This conclusion seems somewhat
contradictory to our general understanding of
EM reliability. It is reported that Cu metal
lines with improved (111) texture have
longer EM lifetimes by minimizing high
angle grain boundaries that can serve as fast
diffusion paths.12 Nevertheless, Vanasupa’s
studies13 revealed that Cu metal lines with
strong (111) texture have inferior EM
performance. The discrepancy may result
from the different Cu deposition methods,
barrier materials, and sample fabrication
processes employed in the above studies.
Typical EM induced failures are related to
4