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In situ X-ray Microscopy Studies of Electromigration
in Copper Interconnects
G. Schneider1^, M.-A. Meyer2, E. Zschech2, G. Denbeaux3,
U. Neuhausler4, P. Guttmann5
1
BESSYm.b.H., Albert-Einstein-Str. 15, D-12489 Berlin, Germany
e-mail: [email protected]
2
AMD Saxony LLC & Co. KG, Materials Analysis Department, P.O. Box, 11 01 10, D-01330 Dresden, Germany
3
Lawrence Berkeley National Laboratory, Center for X-ray Optics,
One Cyclotron Rd, MS 2-400, Berkeley, CA 94720
4
European Synchrotron Radiation Facility (ESRF), X-ray microscopy beamline ID21,
BP 220, F-38043 Grenoble Cedex, France
5
Institute for X-ray Physics, c/o BESSYm.b.H., Albert-Einstein-Str. 15, D-12489 Berlin, Germany
Abstract. Real-time X-ray microscopy is applied for degradation studies to understand electromigration-induced
transport processes in on-chip copper interconnects. The material transport in inlaid Cu line/via structures is observed
with about 40 nm lateral resolution. The image sequences show void formation, migration and nucleation processes.
Correlation of the real-time X-ray images with post-mortem SEM micrographs is used to discuss degradation
mechanisms in inlaid copper interconnects. Due to the high penetration power of X-rays through matter and its high
spatial resolution, X-ray microscopy (XRM) overcomes several limitations of conventional microscopic techniques. It
utilizes the natural absorption contrast between the structures of interest, i.e. for on-chip copper interconnects embedded
in dielectrics. Due to their different X-ray absorption characteristics at 0.52 keV, even different silicon compounds like
Si, SiO2, and Si3N4 can be distinguished in X-ray images of thinned layers as demonstrated. For failure analysis of
thicker layers, phase contrast microscopy in the multi-keV photon energy range is proposed.
are not well understood so far, but they are probably
exacerbated by normal stresses at the copper/barrier
and copper/capping layer interfaces as well as
hydrostatic stress components. Particularly, fast
diffusion paths have to be identified and failure
mechanisms based on directed transport of atoms have
to be understood. This knowledge is the basis for
process and materials changes which improve
interconnect reliability.
In passivated interconnects, the mass flow due to
EM is constrained by the embedding dielectrics. This
leads to a high mechanical stress in the interconnect
which significantly influences the material transport.
Therefore, EM phenomena should be studied in situ at
fully embedded interconnect structures using an intact
layer system including barriers and passivation layers.
For this purpose, an imaging technique is required
which maintains high spatial resolution when
penetrating through several microns of dielectrics.
INTRODUCTION
Advanced process technologies and new combinations
of materials bring new reliability challenges: Different
microstructure of the metallic interconnects, other
types of interfaces and new degradation phenomena.
Electromigration (EM), stress-induced degradation and
mechanical weakness in case of low-k materials are
reliability concerns for inlaid copper interconnects.
The current generation of highly integrated
microprocessors, requiring dense interconnects and
increased current densities, has highlighted the EM
issue. Formation of voids in copper lines induced by
EM during normal microprocessor operation will
cause an interconnect open or high resistance resulting
in malfunction or speed degradation. Void formation,
growth and movement, and consequently degradation,
depend on the interface bonding and on the copper
microstructure of individual vias [1]« Stress-induced
degradation phenomena and later catastrophic failures
CP683, Characterization and Metrology for VLSI Technology: 2003 International Conference,
edited by D. G. Seiler, A. C. Diebold, T. J. Shaffner, R. McDonald, S. Zollner, R. P. Khosla, and E. M. Secula
© 2003 American Institute of Physics 0-7354-0152-7/03/$20.00
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X-RAY IMAGING OF COPPER
INTERCONNECTS AND DIELECTRICS
E
Fresnel zone plates are used as imaging lenses in
X-ray microscopes. The numerical aperture NA of a
zone plate depends on its outermost zone width drn and
the used diffraction order m, i.e. NA=A, m/(2 dr^,
where 1 is the wavelength of the radiation. At present,
mainly the first diffraction order (m=l) is used as
standard scalar diffraction theory predicts a decay in
diffracted flux proportional to 1/m2. Therefore, the size
of drn which determines the achievable resolution
needs to be minimized. To generate the zone plate
pattern, electron beam lithography is applied.
The diffraction efficiency, i.e. the fraction of
radiation that actually contributes to the diffraction
order used for imaging should be as high as possible in
order to improve the throughput of the experiment.
The efficiency depends strongly on the height and the
shape of the diffracting structures. In general, the
required structure heights to obtain acceptable
efficiencies are much larger than the outermost zone
width drn resulting in the need to fabricate nanostructures with high aspect ratios. The necessary
height strongly increases with the X-ray photon
energy, which is the reason why smaller structure
widths (and thus better spatial resolution) can be
realized in the soft X-ray range. With current
technologies for nanostructuring, zone structures with
about 20 nm width have been manufactured [2,3]. In
the soft X-ray range, spatial resolution values of below
30 nm have been obtained. In the multi-keV range,
values of about 60 nm are the present limit [4,5].
X-rays are able to penetrate integrated circuits with
layer stacks on top of the silicon substrate, that are
many micrometers thick. Additionally, due to the
unique interaction with matter, X-rays provide a
natural image contrast between different elements
which we exploit to image Cu interconnect lines
embedded within isolating SiO2. Copper interconnects
embedded in dielectrics which consist of low-Z
elements provide a high absorption contrast in X-ray
microscope images. However, as shown in Fig. 1, at
the low-energy side of the K-absorption edge of
oxygen at 0.52 keV, it is even possible to distinguish
different dielectric materials which are all Si
compounds like Si, SiO2, Si3N4 due to their different
attenuation lengths.
The interconnect structures used for the
experiments are located within the scribelines of
production wafers. Fig. 2(a) shows a schematic crosssection of the test structures used in the experiments
with a two-level copper interconnect. The focused ion
beam (FIB) technique was used to thin the region of
-a
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! 0.1
o 0.05
100
500
1000
photon energy / eV
FIGURE 1. Attenuation length, the depth into the material
measured along the surface normal where the intensity of Xrays drops to 1/e of its value at the surface for Si, SiO2, Si3N4
and Cu layers as a function of photon energy. Relevant layer
thicknesses for interconnect studies are in the micrometer
range.
interest to a thickness of about 2 \im (see Figs. 2(b))
and 2(c)). Material was removed from both sides of
the copper structures under test, so that all neighboring
metal lines were removed from the microscope's field
of view. A 50 \im wide trench was cut, leading to the
area of interest. The X-ray beam is able to penetrate
the sample through this trench (see Fig. 2(b)). Fig. 2(d)
shows an X-ray micrograph of the prepared
interconnect stack imaged at 1.8 keV photon energy
using the X-ray microscope XM-1 at the ALS [6]. Fig.
3 shows an X-ray micrograph of a similarly prepared
sample which was imaged at 0.52 keV photon energy
with the novel BESSY II X-ray microscope [7]. It
demonstrates the possibility to visualize clearly
different dielectric layers by absorption contrast.
REAL-TIME ELECTROMIGRATION
STUDIES
The mass flow due to electromigration in on-chip
interconnects of ICs is constrained by their
encapsulation. Depending on the dielectric material
used to isolate inlaid copper interconnects, a certain
mechanical stress is obtained in the interconnect. Since
the interfaces are main transport paths for copper
interconnect systems and since the stress influences
the material transport significantly, degradation studies
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to understand the mechanisms of electromigrationinduced material transport should be performed in situ
with an intact layer system including all barrier,
capping layers. The full- field X-ray microscope, XM1, installed at the ALS was used to study the dynamics
of void development in passivated Gu line/via
structures [6,8]. Figs. 4(a)- 4(f) show a sequence of Xray micrographs of the copper via/line interconnect
structure, which were captured during an in situ EM
experiment at 1.8 keV photon energy. The electron
flow was from left to right in the image, and upwards
through the via. Fig. 4(a) shows the initial state of the
interconnect structure without any voids. During the
experiment, void formation (Fig. 4(b)), movement
(Figs. 4(c) - 4(d)) and agglomeration (Fig. 4(f)) were
seen in the via. The image sequence indicates that
initial voids are formed in the copper bulk structure,
probably at grain boundaries or grain boundary triple
points, additionally to void formation, growth and
movement along interfaces.
FIGURE 2. (a) Schematic of the cross-section of dual-inlaid
Cu interconnects, (b) Fully prepared sample for electromigration test, (c) SEM micrograph of the cross-section of
the copper metallization system which was thinned locally
using FIB. Note that the passivation layer of about 500 nm
thickness significantly reduces the obtainable resolution
when imaging buried Cu line/via structures, (d) X-ray
micrograph recorded at 1.8 keV photon energy showing the
sample region as Fig. 2(c).
FIGURE 4. Selected X-ray images ((a) - (f)) from an image
sequence taken at successive times showing void formation,
movement and agglomeration inside the passivated Cu via.
To obtain detailed structural information of the via
after the EM experiment, a FIB cross section of the via
was prepared and imaged in a SEM, which shows a
grain boundary leading to the large void (see Fig. 5).
Small voids are visible along this grain boundary,
giving evidence for the grain boundary diffusion
mechanism. By correlating the information showing
the material transport visualized by X-ray microscopy
with the copper microstructure obtained from electron
FIGURE 3. X-ray micrograph imaged at 0.52 keV photon
energy with the new BESSY II transmission X-ray
microscope. Note that different dielectric materials can be
clearly distinguished at this photon energy.
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micrographs, the dominant copper diffusion pathways
can be identified. This study gives evidence for
theoretical models postulating that voids grow at sites
of flux divergences like grain-boundary triple points
and interfaces [9]. Void formation, growth and
movement, and consequently degradation, depend
clearly on the microstructure of individual vias.
To detect the exact location (bulk or interface) of
voids in interconnects, real-time X-ray tomography
will be used for future experiments.
(see Fig. 6) creates an image field free of zero order
radiation from the zone plate objective. At the same
time, it ensures the required phase shift of the
undiffracted light while leaving the phase of the light
diffracted by sample structures nearly unaltered [10].
Thereby, the phase information of the object is
transformed into an intensity modulation in the image
plane. In the Zernike-type phase contrast setup, even
the image contrast of weakly phase shifting samples
can be tuned to almost any value by employing
optimized phase rings which also attenuate the
undiffracted light.
m
FIGURE 5. SEM image of a FIB cross-section of the
sample shown in Figure 4(f). A grain boundary leading to
the large void as well as small voids are visible along this
grain boundary, giving evidence for the grain boundary
diffusion mechanism.
FIGURE 6. Schematic of the optical setup of the X-ray
microscope (TXM) in Zernike phase contrast mode at the
ESRF. A zone plate condenser focuses the monochromatized
undulator beam onto the object and a micro zone plate
objective forms a magnified image on a phosphor-coupled
CCD camera. The phase ring in the back-focal plane of the
objective selectively phase shifts the undiffracted light of the
object [10].
PHASE-CONTRAST MICROSCOPY
FOR PHYSICAL FAILURE ANALYSIS
Physical failure analysis in copper interconnects
using X-ray microscopy techniques requires several
micrometer thick samples. To penetrate such samples,
photon energies in the multi-keV range are necessary.
However, the absorption contrast provided by copper
interconnects embedded in dielectrics decreases with
increasing photon energy and penetration power of the
X-rays.
In general, phase shift is dominating over
absorption in the multi-keV photon energy range.
According to Zernike, phase contrast images are
obtained in a full-field microscope by inserting a phase
plate in the back focal plane of the objective. In the
novel Zernike-type phase contrast X-ray microscope
installed at the insertion device ID21 at the ESRF, a
hollow-cone illumination provided by a ring-shaped
condenser in combination with a central beam stop
The high resolution phase contrast imaging
technique in Zernike mode was applied to investigate
on-chip copper interconnects. Phase contrast X-ray
micrographs of test structures with inlaid copper
structures are shown in Fig. 7. As demonstrated with
these images, phase contrast X-ray microscopy is a
powerful new technique for interconnect process
development and failure analysis. In addition, due to
the large depth of field of X-ray objectives operating
in the keV photon energy range, high resolution
tomography based on X-ray microscope images is
possible.
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Energy, office of Basic Energy Science under Contract
No. DE-AC03-76SF00098 and the Deutsche
Forschungsgemeinschaft under Contract No. SCHN
529/1-1.
REFERENCES
1. Zschech, E., Geisler, H., Zienert, I., Prinz, H., Langer, E.,
Meyer, A. M., Schneider, G., Proc. of the Advanced
Metallization Conference, San Diego (2002)
2. Peuker, M., Appl Phys. Letters 78, 2208 (2001)
3. Anderson, E.H., Olynick, D.L., Harteneck, B., Veklerov,
E., Denbeaux, G., Chao, W., Lucero, A., Johnson, L. and
Attwood, D., J. Vac. Sci. Technol. B18 (6), 2970 (2000)
4. Schneider, G., Neuhausler, U., Ludwig, W., Hambach,
D.: "Zernike phase contrast microscopy with 60 nm
resolution in the multi-keV photon energy range", in
preparation (2003)
5. Neuhausler, U., Schneider, G., Ludwig, W., Hambach
D., Proceedings of the 7th International Conference on X-ray Microscopy, Journal de Physique IV, in press (2003)
FIGURE 7. Different copper interconnect structures, imaged
in positive phase-contrast mode at the ID21 transmission
microscope at the ESRF. Irregularities in the square
structures - as indicated by white circles - can be seen (a).
Furthermore, an irregularity within the copper interconnect is
indicated (b) [ 10].
6. Schneider, G., Denbeaux, G., Anderson, E.H., Bates, B.,
Pearson, A, Meyer, M.A., Zschech, E., Stach, E.A.,
Appl. Phys. Lett. 81, 2535 (2002).
7. Guttmann, P., Niemann, B., Thieme, J., Hambach, D.,
Schneider, G., Wiesemann, U., Rudolph, D., Schmahl,
G., Nucl. Instr. Meth. A, 467-468, 849 (2001).
CONCLUSIONS
8. Schneider, G., Meyer, M.A., Denbeaux, G., Anderson,
E., Bates, W., Pearson, A., Knochel, C., Hambach, D.,
Stach, E.A., Zschech, E., J. Vac. Sci.Tech. B 20(6), 3089
(2002).
In situ X-ray microscopy is a powerful technique to
study degradation mechanisms like electromigration
and stress voiding in on-chip copper interconnect
structures. It is a complementary technique to SEM for
physical failure analysis at interconnect structures,
particularly to study void formation, migration and
agglomeration mechanisms in inlaid copper interconnects. Depending on the photon energy used at
synchrotron radiation sources, a spatial resolution up
to 30 nm has been achieved, and several material
combinations and defects have been visualized.
Laboratory tools using X-ray tubes can be used for
structural investigations with a spatial resolution of
about 80 nm [11].
9. Spolenak, R.: PhD Thesis, Universitat Stuttgart (1999)
10. Neuhausler, U., Schneider, G., Ludwig, W., Meyer,
M.A., Zschech, E., Hambach, D., ESRF Highlights 2002,
84 (2002).
11. www.xradia.com
ACKNOWLEDGMENTS
We greatfully acknowledge the staff of ALS, BESSY
and ESRF for providing excellent working conditions.
This work is funded in part by the U.S. Department of
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