Impact of Low k Dielectrics on Electromigration Reliability
for Cu Interconnects
Paul S. Ho*, Ki-Don Lee*, Ennis T. Ogawa+, Sean Yoon*, and Xia Lu*
*Laboratory for Interconnect and Packaging, University of Texas at Austin, Austin, TX 78712-1063
^Silicon Technology Development, Texas Instruments, Inc., MS 3737, Dallas TX 75243
Abstract. Multi-link statistical test structures were used to study the effect of low k dielectrics on EM reliability of Cu
interconnects. Experiments were performed on dual-damascene Cu interconnects integrated with oxide, CVD low k, porous
MSQ, and organic polymer ILD. The EM activation energy for Cu structures was found to be between 0.8 and 1.0 eV,
indicating mass transport is dominated by diffusion at the Cu/SiNx cap-layer interface, independent of ILD. Compared with
oxide, the decrease in lifetime and (jL)c observed for low-k structures can be attributed to less dielectric confinement in the low k
structures. An effective modulus B obtained by finite element analysis was used to account for the dielectric confinement effect
on EM. For all the ILDs studied, (jL)c showed no temperature dependence.
INTRODUCTION
DIELECTRIC CONFINEMENT ON EM
Electromigration (EM), a major reliability concern
for on-chip interconnects, has been extensively studied
in Cu structures with oxide interlevel dielectrics (ILD)
in the past several years [1-3]. Compared with
Al/oxide interconnects, Cu/oxide structures have
distinct EM characteristics due to the dual-damascene
architecture introducing different transport path, flux
divergence and damage mechanisms.
Statistical
studies have revealed multi-mode failures in the
Cu/oxide structures with early failures dominated by
void formation at the via bottom interface [4,5]. With
device scaling continuing beyond the 130nm node, low
k ILD is being implemented to replace oxide in Cu
interconnects. Compared with oxide, low k dielectrics
are softer, expand more and conduct less heat. The
weak thermomechanical properties cause significant
concern on EM reliability of Cu/low k interconnects
and its impact has to be investigated. The purpose of
this paper is to investigate the effects of low k
dielectrics on EM reliability of Cu interconnects. This
paper discusses first the effect of dielectric
confinement on EM reliability. This is followed by a
summary of recent results from our laboratory on
Cu/low k interconnects focusing on the EM lifetime
and the threshold current density-length (JL)C product.
Both are sensitive to the dielectric confinement and the
implications on EM reliability will be discussed.
Consider a line/via element in a Cu dual-damascene
structure as shown in Fig.l. Under a current driving
force, the drift velocity (v^) of Cu ions induced by EM
can be expressed as [6],
= JU (Z*epj - OAcr/L)
(1)
where Z*e is the effective charge, j is the current
density, I? is the atomic volume and Ao/L is the EM
induced stress gradient along the line. In a confined
structure, the current induced mass transport VEM is
opposed by a back flow vBr induced by the Blech
stress gradient as a result of mass transport from the
cathode to the anode. According to Eq. 1, a threshold
product (jL)c can be defined when vd = 0 as:
(2)
With n and Z ep being material constants, (jL)c is
proportional to Acr. For a Cu damascene structure, the
value of Acr that can be sustained depends on the
confinement on the Cu structure imposed by the ILD
and the surrounding barrier and cap layers. Thus the
(jL)c product provides an effective measure to evaluate
the back-flow stress and the dielectric confinement
effect on EM.
CP683, Characterization and Metrology for VLSI Technology: 2003 International Conference,
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533
will decrease the back-flow stress Aa resulting in an
increase of the net drift velocity and a reduction of the
EM lifetime. The confinement effect on EM can be
further examined by considering the kinetics of stress
evolution and void formation under EM, which have
distinct characteristics for Cu interconnects comparing
with Al interconnects and have different implications
on EM reliability [10,11]. In Fig.2, we show the initial
state of a Cu line with a uniform tensile stress at time to
before EM. Upon EM, mass transport builds up a
small tensile stress at the cathode at ti with a
corresponding compressive stress at the anode. With
continuing EM at t2, the tensile stress at the cathode
can reach a critical value to induce void formation.
Different from the Al interconnect, void can nucleate
and grow more readily at the Cu interface near the
cathode end under a moderate tensile stress of about
100 MPa [11]. Void formation releases the local
tensile stress but continues to build up the compressive
stress in the Cu line at t3 until reaching the steady state
at t4 as shown. With void formation, a large steadystate compressive stress can occur at the anode and
causes the Cu interconnect more prone to metal
extrusion at the anode. Based on void growth kinetics,
Korhonen et al. deduced a critical void volume at the
steady state as [10],
FIGURE 1. Mass transport under EM in a confined
structure. The drift velocity driven by the current VEM is
opposed by a stress-induced back flow VBF.
The confinement effect can be quantified using an
effective elastic modulus as first formulated by
Korhonen et al. for Al interconnects [7]. The symbol
B in eq. (3) is used to represent the stiffness response
of the interconnect structure to the stress generated by
mass transport under confinement.
dC/C=-da/B
(3)
Here dC/C is the volumetric strain induced by EM, and
da is the corresponding amount of stress generated as
measured by the effective modulus B of the metal
structure. Defined in this way, B depends on the
elastic properties and geometry of the metal line and
surrounding materials. It also depends on the mass
transport mechanism because the mechanical response
of the interconnect structure depends on how mass
transport is distributed along different directions. In
Al interconnects, mass transport is primarily through
the grain boundaries, resulting in an isotropic mass
distribution and da. For Cu interconnects, mass
transport is dominated by diffusion at the cap layer
interface [8], which results in an anisotropic mass
distribution mainly along the normal to the line
direction. Using a finite element analysis, Hau-Riege
calculated B for AlCu interconnects under EM
assuming an isotropic mass transport [9]. He found
that as the elastic modulus of ILD decreases from 72
GPa for oxide to below 10 GPa for low k materials, the
value of B is reduced but to a significantly less degree,
from 25 GPa to about 10 GPa. This indicates that the
Ti/TiN upper and lower layers of the Al line and the W
via contribute substantially in additional to that from
ILD to metal confinement. When an Al line was
embedded in a damascene structure with additional
side-wall barriers, B of low k ILD was found to
increase about 50%. This demonstrated that the barrier
and cap layers in the damascene structures make
significant contributions to metal confinement,
particularly for low k dielectrics.
The confinement effect on EM reliability can be
readily observed from Eq.l where a weak confinement
V =
B
2QS
(4)
Here <? is the initial thermal stress and L is the length
of the metal line and both the initial thermal stress and
EM contribute to the void volume. Assuming that the
EM lifetime is determined by the critical void volume
Vc, Eq.4 provides a relationship to correlate EM
lifetime to the confinement effect via the effective
modulus B and the line length L under test conditions
ofy and d.
FIGURE 2. Stress evolution in confined Cu line under EM
as a function of time, ob is the initial thermal stress.
534
In deducing Fc, the EM induced stress at the steady
state is assumed to be built up with a uniform stress
gradient over the length of the Cu line and void
formation occurs only at the cathode end. The
assumption on stress distribution may be reasonable
for Cu structure since mass transport in Cu is
dominated by interfacial diffusion, so "blocking" grain
boundaries [10], even if they exist, would not disrupt
the stress buildup over the line length. However, voids
are often formed at other locations near the cathode,
this will release the local stress at the void site. As the
back-flow stress reaches the steady state in the
remaining part of the line, the stress gradient will be
the same but the maximum compressive stress at the
anode will be reduced. This will reduce the driving
force for metal extrusion at the anode end, making EM
failure more likely to occur as a result of void
formation.
For Cu/low k structures with weak adhesion
strength at the cap/etch stop interface, interfacial
delamination can occur prematurely to cause line
failure by metal extrusion before the steady-state is
reached. In this case, the EM lifetime will be lower
than that estimated from B based on the confinement
effect. Another factor affecting EM reliability concerns
void evolution where recent Cu EM results showed
that the evolution of void morphology may be
important in determining the EM failure statistics [12].
This can be particularly relevant regarding early
failures induced by void formation at via bottom as a
result of void evolution from the line at the cathode
end to the via bottom.
Based on these discussions, EM reliability and the
confinement effect for Cu/low k interconnect seem to
be a complex phenomenon depending on material
properties, interconnect geometry and processing
defects. The effect of dielectric confinement was first
investigated using x-ray diffraction to measure thermal
stresses under thermal cycling in Cu damascene line
structures with oxide and low k dielectrics [13]. The
results confirmed that the cap and barrier layers are
important and have to be considered in order to
account for the observed triaxial stress characteristics,
particularly for the stress component normal to the line
direction. In this paper, we summarize the results of
EM studies on lifetime and (jL)c for Cu damascene
structures with oxide and low k ILDs using a statistical
approach. The results show a good correlation of the
confinement effect on EM with the effective elastic
modulus calculated using finite element analysis for
most of the Cu/low k structures investigated. For
organic polymer low k ILD, the discrepancy from the
general correlation can be traced to premature failure
due to interfacial delamination. The results and
experimental details are presented in the following
sections.
EXPERIMENTAL DETAILS
Experiments were performed using Cu/low k
damascene structures to examine the effect of low k
dielectrics on EM reliability. For this study, a critical
length (LC) test structure was used consisting of a
collection of serially connected line/via interconnects.
As shown in Fig.3a, this test structure contains 6
repeating sets of 14 interconnects where the M2 length
varies from 10 to 300 jam and with a line width of 0.5
jam. The length of Ml remains constant at about 5 jam
in order to drive the failure to occur above the Ml
level at either the via or the M2 trench to facilitate
failure analysis. Designed in this way, the serial
connection of test structures provides an ensemble of
line/via elements with varying line lengths to measure
the dielectric confinement effect on EM failure
statistics and threshold (jL)c product.
FIGURE 3. (a) Schematic overview of LC test structure,
(b) Stacking layers of dual-damascene Cu/low k
interconnects
The low k ILDs used included CVD low k, porous
MSQ, and organic polymer. Their EM behaviors are
compared with oxide. The material properties of the
dielectric materials are listed in Table 1. Test
structures were designed at UT-Austin and fabricated
at International Sematech and LSI Logic. The samples
were prepared using 200 mm wafers and consisted of
two-level interconnect structures based on a Ta/low
temperature PVD seed Cu/ electroplated (EP) Cu stack
[14]. The metal lines in the test structures show an
apparent "near bamboo" microstructure with a
significant amount of twinning that is associated with
535
Cu film growth. In Fig.3b> we show the schematic
structure of the Cu/low k dual-damascene interconnect
where the low k material was fully implemented in all
levels. EM experiments were performed in a high
vacuum chamber filled with 20 torr of N2 gas to reduce
oxidation and to improve temperature uniformity for
all test samples. More experimental details have been
described previously [5, 15].
B is significantly less than that of E indicating that the
barrier and cap layers are important in addition to the
ILD in confining the Cu lines. Except for the organic
polymer, the EM lifetime seems to be proportional to
B, which is consistent with void-induced failure at the
steady state. For the organic polymer, the EM lifetime
is lower than that estimated from B. This can be
attributed to premature failures caused by interfacial
delamination at the cap/etch stop interface and will be
further discussed in the failure analysis section.
TABLE 1. Dielectric constant (k), coefficient of temperature
expansion (CTE) and Young's modulus (E) for different
dielectric ILDs and simulated effective bulk modulus (B) and
(jL)c for dual-damascene Cu interconnects are summarized.
k
Oxide
3.9
CTE
(ppm/°C)
0.51
71.4
8
(Gpa)
13.7
CVD low k
2.7
25
6
7.6
2000
Porous MSQ
2.2
7.3
3.6
7.3
2500
7.2
Org. Polymer
2.7
66
2.5
p
These values were determined at 1.0 MA/cm2.
1200
E
(Gpa)
jLc
(A/cm)
3700
Cu/CVD Low k
Q = 0.86eV
j = 1.0 MA/cm2
10 2 :
Cu/Oxide
Q = 0.81 eV
j = 1.0 MA/cm2
I
s
Cu/Org. Pol.
Q = 0.97 eV
1.0 MA/cm2
RESULTS AND DISCUSSION
17
18
19
20
Cu/Porous MSQ
Q = 0.93 eV
= 1.0 MA/cm2
21
22
1/KT
EM Lifetime and Threshold (jL)c Product
FIGURE 4. Graph shows t50 vs. l/KT for Cu/oxide,
Cu/CVD Low k, Cu/Porous MSQ, and Cu/Organic Polymer.
The activation energies determined from the slope of
Arrhenius plot were found to be between 0.8 and 1.0 eV.
EM experiments were performed on LC test
structures at temperatures between 190 and 360°C.
The first abrupt change or instability in resistance trace
during EM stressing was considered as an "onset" EM
failure and was used to determine EM lifetime. In
Fig.4, Arrhenius plots of t5o, vs. 1/kT obtained at 1.0
MA/cm2 are plotted for Cu/oxide, Cu/CVD low k,
Cu/porous MSQ, and Cu/organic polymer structures
integrated with Ta barrier and SiNx cap layer. The
activation energies of Cu structures were found to be
between 0.8 and 1.0 eV. This is commonly associated
with mass transport at the Cu/SiNx interface,
suggesting that interfacial diffusion dominates EM
mass transport [15]. The test structures had the same
Cu/Ta and Cu/SiNx interfaces, so EM occurred via the
same mechanism independent of ILD.
In Fig.4, the EM lifetimes of Cu/low k structures
are generally shorter than that of Cu/oxide structures
under similar test conditions, which can be attributed
to the confinement effect. As shown in Eq. 1, the lower
dielectric confinement by low k ILD reduces the backflow term vBh^ resulting in an increase in the net mass
transport and a reduction of EM lifetime.
To estimate the confinement effect, B was
calculated using finite element analysis for the Cu test
structures in this study assuming mass transport only at
the cap layer interface. The results are listed in Table
1. The values of B for all low k ILDs are very similar,
equal to about half of that of oxide. The difference of
After a long EM stressing, individual lines in LC
structures were examined using a focused-ion beam
(FIB) microprobe and an optical microscope to
identify EM induced damages. In this way, the failure
statistics as a function of line length can be
determined. As shown in Fig.5, the failure probability
drops sharply as the line length decreases. The critical
length (Lc) for a given current density (/) can be
determined from the intercept of the regression line
generated by assuming that failure probability is
proportional to the net drift velocity and thus the line
length [16]. In Fig.5, Lc was found from the intercept
to be 10 jam giving an (jL)c of 3000 A/cm.
536
Failure Analysis
Focused ion beam (FIB) microprobe was used to
identify the EM failure characteristics together with
FIB-induced contrast (FIBIC) technique to locate the
locations of interconnect failures in the Cu lines as
shown in Fig.Ta for Cu/porous MSQ structure. Here
we found that voiding at the cathode end was large
enough to stop electric current flow causing a failure.
In most cases, test structures were found to fail by
cathode voiding in the Cu/porous low k structures.
Some of the Cu/porous MSQ structures showed lateral
Cu extrusion near the anode under the SiNx cap layer,
followed by interfacial delamination, as shown in
Fig.Tb.
FIGURE 5. Failure distribution of Cu/Porous MSQ LC test
structures tested at 190 °C and 3.0 MA/cm2 is shown as a
function of line length. Failure probability drops as line
length decreases. Lc = 10 um. (jL\ = 3000 A/cm.
The results of (JL)C measured for Cu/oxide and
Cu/low k structures are summarized in Fig.6.
Compared with oxide, low k ILDs have smaller
threshold (jL)c products, corresponding to 3700, 2000,
2500 and 1200 A/cm with error limits of 250-500
A/cm for oxide, CVD low k, porous MSQ and organic
polymer, respectively.
In general, there is no
temperature dependence for (jL)c under our test
conditions. There is a good correlation between the
lifetimes in Fig.3 and (jL)c since both are proportional
to Aa, reflecting the dielectric confinement effect on
EM characteristics. Similar to the lifetime, the
extrinsic effect of interfacial delamination reduces (jL)c
for the organic polymer.
FIGURE 7. (a) void formation at the cathode end (b)
extrusion at the anode end with interfacial delamination
observed in Cu/porous MSQ structures.
uuuu
4000 -
•
•
A
•
Cu/Oxide
Cu/Porous MSQ
Cu/CVD Low k
Cu/Org. Polymer
T
T
T
?
* T
? 3000 -
I
g.
o 2000 -
if
1000-
While CVD low k and porous MSQ structures
failed mainly by cathode void formation, organic
polymer structures failed by cathode void formation
followed by anode extrusion [17]. In both cases, EM
lifetime depends on the amount of void formation at
the cathode, so regardless of anode extrusion, voiding
will fail the line. If anode extrusion occurs prematurely
due to low adhesion strength at the anode interfaces,
such an extrinsic failure effectively reduces the back
stress and accelerates the cathode void formation. This
mechanism can reduce the EM lifetime significantly as
observed in the organic polymer structures.
FIB was used to examine the failure mode at the
anode of organic polymer low k interconnects and the
results are shown Fig.8. Here the top view (Fig. 8a)
shows an extrusion failure near the anode end. The
h
o .
200
240
280
320
360
400
Temperature (°C)
FIGURE 6. Graph shows (jL\ vs T. There was no
temperature dependence in our test conditions. (jL\ data
were obtained from EM tests at 1.0 MA/cm2, except for
Cu/CVD low k which was tested at 0.5 MA/cm2.
537
EM reliability of Cu interconnects. Experiments were
performed on dual-damascene Cu interconnects
integrated with oxide, CVD low k, porous MSQ, and
organic polymer ILD. The EM activation energy for
Cu structures was found to be between 0.8 and 1.0 eV,
indicating mass transport is dominated by diffusion at
the Cu/SiNx cap-layer interface, independent of ILD.
Compared with oxide, the decrease in lifetime and
(jL)c observed for low-k structures can be attributed to
less dielectric confinement in the low k structures. An
effective modulus B obtained by finite element
analysis was used to account for the dielectric
confinement effect on EM. For all the ILDs studied,
QL)C showed no temperature dependence.
A number of interesting questions remain to be
answered. These include the contribution of plastic
deformation to the confinement effect, the effect of
premature failure before reaching the steady state on
failure statistics, and the effect of residual thermal
stress and stress relaxation on EM lifetime and
threshold product. EM experiments will be performed
on Cu/low k interconnects under various conditions in
our laboratory to address these issues.
cross-sectional FIB micrograph in Fig. 8b shows that
the extrusion was initiated at the top corners of the Cu
line beneath the oxide etch stop. (See schematic
drawing in Fig. 8c). This suggests that the corner at
the barrier, low k ILD and cap/etch stop layer
intersection is a mechanical weak point where failure
can occur by interfacial delamination induced by a
large compressive back-flow stress at the anode. This
was confirmed by high resolution SEM observations
which identified interfacial delamination and anode
extrusion along the low k and oxide etch stop interface,
as shown in Fig. 9. Additionally, atomic force
microscopy (AFM) was used to examine the surface
topology, revealing that SiNx at the anode end was
lifted by EM by about 180 nm [17]. Thus the weak
adhesion of low k/etch stop interface causes the
interfacial breakdown and premature EM failure. In
this case, the Cu/low k structure was able to sustain
less back-flow stress than that estimated from the
effective elastic modulus.
ACKNOWLEDGEMENT
The authors would like to thank the International
SEMATECH and LSI Logic Corporation for providing
test structures for this study. They also gratefully
acknowledge the partial support from the state of
Texas, and the SRC Center for Advanced Interconnect
Science and Technology.
FIGURE 8. (a) Top down view of anode extrusion and (b)
A-A1 cross-section of anode of Cu/organic polymer test
structures, (c) Schematic view of FIB cutting.
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FIGURE 9. Cross-section of Cu/organic polymer
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anode extrusion and interfacial delamination due to high
hydrostatic compressive stress at anode which makes SiNx
layer lifted up.
[4]
SUMMARY
[5]
In summary, multi-link statistical test structures
were used to study the effect of low k dielectrics on
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