Study of Cu Migration Kinetics under Electric and
Thermal Driving Forces
計畫編號：甲-91-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
Abstract
Introduction
Electromigration (EM) in unpassivated
copper lines at room temperature has been
investigated in ultra-high vacuum by in-situ
transmission electron microscopy (TEM).
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 EM induced
atomic migration appeared to be anisotropic,
and the combination of {111} planes and
<110> directions was suggested to be the
easiest EM system for crystalline copper.
Moreover it was found that some unusual
diffraction patterns of Cu (111) crystal plane
appeared within 20 min when applying an
electric current with a density of 2×106
A/cm2 through the tested Cu line. It is
believed that the EM-induced Cu twining
may be held responsible for the extra
diffraction patterns.
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。此外
實驗結果發現一(111)方向的銅晶粒在電流
密度 2×106 A/cm2 作用下 20 分鐘，其電子
繞射圖案出現特殊的繞射點。分析指出銅
原子之非均向性電遷移行為，在電流作用
下造成銅雙晶形成，使得額外的電子繞射
點產生。
關鍵詞：電遷移、臨場穿透式電子顯微鏡、
表面擴散
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
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 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
1
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
study, the in situ transmission electron
microscope
(TEM)
observation
of
atomic-scale EM behavior in a Cu line is
reported.
different time interval during electric current
stressing. Two crystallographic lattice planes
were found to be (1 1 1) and ( 1 20) planes
with a zone axis in [211] direction as shown
in the inset of figure 1(a). It is noted that the
direction of electron flux is from the right to
the left of the micrographs. It is noted that
the atomic images of Cu grain did not seem
to shrink equally in all directions. Instead, the
Cu atomic images appeared to fade gradually
from the lower-left of the grain inspected.
Moreover, the atomic images of the grain
disappeared completely within 25 min. It is
known that surface diffusion is the dominant
mechanism for Cu EM at low temperatures.
Face-center-cubic (FCC) crystal structure, in
general, has the lowest migration energy on
{111} planes among the major low-index
planes.9,10 It is reasonably expected that the
EM-induced atomic migration has to be
closely related to the crystal orientation of
the Cu grains under stressing.
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 specimens were annealed at 250 0C for 1
min and 450 0C for 2.5 min, respectively, 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 Cu line at an average current density of
2×106 A/cm2.
e-
e-
(a)
(11¯
1¯)
d
[0
11¯
]
ed(1
¯20
)
 [211]
(c)
e-
e-
(b)
(e)
(d)
e(f)
Fig. 1 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 (f) 25 min.
Figure 2 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], [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 other directions. Based
Results and discussion
Figure 1 shows the high resolution TEM
(HRTEM) images of Cu (211) grain at
2
on the anisotropic diffusion and electrical
resistivity of crystalline copper, the
combination of {111} planes and <110>
directions is suggested to be the easiest EM
system inside a grain.
Our observation in the electric current
stressed Cu film agrees well with the
theoretical predictions as shown in Fig. 3(b).
(a)
(a)
(c)
022
(b)
1
( 2 4 2)
1
P ( 2 2 4)
3
3
1
( 4 22)
3
Fig. 2 A schematic diagram of atomic
arrangement on (211) planes for FCC crystals
and the migration channel of surface atoms along
[01 1 ] direction.
Figure 3 shows the TEM images and the
corresponding selective area electron
diffraction patterns (SAEDP) for an
individual Cu grain with (111) orientation
before and after applying a current density of
2×106 A/cm2 through the Cu line annealed at
250 0C for 1 min. We found that the
non-integral indices such as 13 2 2 4  (point P)
appeared after stressing the sample for 20
min as shown in Fig.3(d).
These
non-integral diffraction spots (DPs) is
speculated to be related to the Cu twinning
structure. The twin planes for the facecenter-cubic (FCC) crystal structure are {111}
planes.12 The primary twin planes of the
{111} plane can be obtained from the matrix
lattice by a rotation of 180 0C about the
<111> twin axes. For example, the twin
plane of the 1 11 is 13 5 1 1  , and the twin
plane of 1 1 1  is 13 1 5 1 with respect to the
[111] twin axis.
Moreover, the double
diffraction spot may also appear in the matrix
diffraction patterns. Pashley et al suggested
that the 13 2 2 4  DP observed in the
(111)-textured Au films results from the
double diffraction of 1 11 and 13 1 5 1 .12
Bedsides, the other 13 422 DP is attributed
to the 1 1 1   13 1 1 5 double diffraction.
3
Fig. 3 The TEM micrographs and the electron
diffraction patterns of the Cu (111) grain (a)(b)
before and (c)(d) after stressing the sample at a
current density of 2×106 A/cm2 for 20 min.
Figure 4 shows TEM electron diffraction
patterns of the respective Cu (210) and (110)
grains after current stressing for the samples
annealed at 400 0C for 2.5 min. We found
that there are some usual ED spots for the Cu
(110) grain, but not for the Cu (210) grain as
shown in Fig. 4. It is expected that some
diffraction spots such as 13 1 15 result from
twinning crystals and some are double
diffraction spots.
Figure 5 shows the
reciprocal lattice for a face-center-cubic
crystal with the extra spots which arise from
{111} twining and those from double
diffraction at twin region. Nevertheless, the
electron diffraction pattern for the Cu (210)
grain did seem to change with electric current
stressing as shown in Fig.4 (a). Since the
Cu electromigration is dominated by the
surface diffusion, which is highly dependent
on the crystal orientation as indicated in Fig.
1. If the Cu twining structure can be
enhanced by electric current induced atomic
migration, the formation of Cu twin crystals
should depend on the crystal orientation of
the individual grain as well.
The
mechanism regarding to the EM-induced Cu
twinning and its dependence on the crystal
6. E. liniger, L. Gignac, C. K. Hu and S. Kaldor,
J. Appl. Phys., 92, 1803 (2002).
7. T. G. Koetter, H. Wendrock, H. Schuehrer, C.
Wenzel and K. Wetzig, Microelectron. Reliab.,
40, 1295 (2000).
8. K. N. Tu, J. Appl. Phys., 94, 5451 (2003).
9. C. L. Liu, J. M. Cohen, J. B. Adams and A. F.
Voter, Surf. Sci., 253, 334 (1991).
10. M. Karimi and T. Tomkowski, Phys. Rev. B,
52, 5364 (1995).
11. A. Magnaterra, Phys. Lett., 44A, 63 (1973).
12. D. W. Pashley and M. J. Stowell, Phil.
orientation would require a further through
investigation.
(a)
(b)
1
( 1 15)
3
Double diffraction
1
(1 1 5 )
3
Mag. 8, 1605 (1963).
Fig.4 Electron diffraction patterns of the Cu (a)
(210) grain and (b) (110) grain after stressing the
sample at a current density of 2×106 A/cm2.
Fig.5 The reciprocal lattice for a FCC crystal
with the extra spots which arise from {111}
twining planes and those from double diffraction
at twin region.
Conclusions
In conclusion, the in situ TEM
observation of EM 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. Electric current induced Cu
twining may be held responsible for the
unusual diffraction patterns.
References
1. C. K. Hu and J. E. Harper, Mat. Chem. Phys.,
52, 5 (1998).
2. B. H. Jo and R. W. Vook, Thin Solid Films,
262, 129 (1995).
3. D. Padhi and G. Dixit, J. Appl. Phys., 94, 6463
(2003).
4. M. Hayashi, S. Nakano and T. Wada,
Microelectron. Reliab., 43, 1545 (2003).
5. K. L. Lee, C. K. Hu and K. N. Tu, J. Appl.
Phys., 78, 4428 (1995).
4