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Improving the Quality of PVD Cu Seed Layer
A. Dulkin, E. Ko, L. Wu, I. Karim, K. Leeser, and K.J. Park
Novellus Systems, Inc.
L. Meng, D.N. Ruzic
Center for Plasma-Material Interactions, University of Illinois at Urbana-Champaign
ABSTRACT
The quality of PVD deposited barrier/seed interface in copper interconnect metallization was significantly
improved by enhancing Cu nucleation on the Ta surface through filtering of non-energetic species from
the deposition flux, increasing the fraction of Cu ions, improving metal ion flux uniformity, and
minimizing gas ion bombardment of growing film. The self-sputtering ability of Cu was combined with
magnetically confined high density plasma in the Novellus HCM® PVD source. Spatial profiles of plasma
density and temperature, as well as ion flux, ion metal fraction, and ion energy, were measured by planar
Langmuir probes, quartz crystal microbalance, and gridded energy analyzer, all located at the wafer level.
Multiple criteria, such as seed step coverage and roughness, its resistance to agglomeration, and its
stability in the plating bath, have been used to evaluate interface quality. As a result, a new and improved
Cu PVD process which demonstrated superior stability during subsequent process steps and ensured
successful electrofill performance with a more than 50% reduction in minimal sidewall thickness
requirement has been developed.
I. INTRODUCTION
The state-of-the-art interlevel metallization process in VLSI manufacturing employs the so-called
damascene process in which Cu lines are inlaid in the previously etched trenches and vias in the dielectric
layers. The lines are formed by electroplating on barrier and seed layers typically formed by a metal
sputtering process also known as PVD. The most common barrier consists of a TaN/Ta bilayer, and Cu is
used as the seed layer.
The reliability of the interconnect metallization depends on the quality of different interfaces 1,
especially the barrier/seed 2 interface. Failure to ensure continuous seed coverage inside the recessed
features with good adhesion to the barrier may result in voiding during electroplating, post-CMP defects,
and stress/electromigration failures. Several factors which are inherent to the PVD process complicate the
task. The neutral deposition flux has a wide angular distribution resulting in deposition build up on the
feature corners called overhang, which shadows the sidewall leading to a poor step coverage. Thin
sidewall film tends to be discontinuous due to island formation during the film growth. Ta and Cu are
immiscible metals with respect to each other and do not form alloys at any temperature. This makes
Ta(N) a good diffusion barrier for Cu. At the same time the absence of chemical bonds makes the wetting
of Cu on Ta difficult. The resulting rough film can be easily attacked by electrolytes during the initiation
phase in the plating bath, leading to seed dissolution and subsequent plating voids. To increase nucleation
density and avoid seed agglomeration, Cu deposition on Ta underlayer is usually conducted at low
temperatures to minimize atom mobility. The key factor in building a stable film is the presence of
energetic metal species in the deposition flux. It had been demonstrated that Cu forms a much stronger
interface with the barrier for sputter deposited films compared to evaporated films 3. Sputtered Cu atoms
leave the target with relatively high energy but lose it rapidly due to collisions with the background gas 4.
The fraction of Cu+ ions in the deposition flux is usually relatively low, but it is a very important
component. While their energy in the plasma is similar to the energy of the neutrals, ions are accelerated
by the wafer sheath potential. Thus the presence of Cu+ ions can increase the nucleation density and
adhesion of Cu to Ta due to their surface penetration. The sheath electric field gives high directionality to
the ion flux, thereby allowing for a much better step coverage compared to neutral only flux.
A number of production-worthy techniques have been proposed to increase the ionization of depositing
metal species via increased plasma density in magnetron PVD sources 5, 6, 7
. They are mainly based on (i)
increasing the power density in the target area by increasing magnetic confinement, (ii) applying short
high power pulses, and/or (iii) supplying additional RF or microwave power to the discharge. Utilizing
inductively coupled plasma (ICP), up to 40% of Cu atoms in the sputtered flux can be ionized, but they
have to be slowed down through thermalization for effective ionization to occur5. The remaining 60% of
the deposition flux is then comprised of slow neutrals, similar to those in evaporation systems.
The main subject of this paper is improving the quality of the barrier/seed interface by addressing the
issues of film continuity, adhesion, and stability, via control of the species and their energies in the
deposition flux in the HCM source of the Novellus PVD INOVA® system during Cu seed deposition.
Plasma and deposition flux parameters have been measured at the wafer level to assist with process
development.
The baseline deposition process used a magnetic field optimized for target utilization and film thickness
uniformity while maintaining a high level of Cu ionization for good step coverage. Despite relatively low
Ar pressure of 0.5 mTorr, the electron confinement in the source was very effective. The exponent n in
the source current-voltage characteristic
I=kVn (1)
was as high as 22, higher than typical magnetron sputtering values (between 3 and 15) 5.
It is desireable to optimize the baseline process to handle aggressive feature geometries associated with
the transition to advanced design nodes in ULSI manufacturing. For these applications, an extremely thin
sidewall seed has to be used to electrofill high AR vias. The goal of the process optimization was to
optimize the kinetics of the film formation, so that even a very thin seed would be continuous and stable to
ensure void free electroplating and reliability in subsequent process steps. Special attention was paid to
achieve the desirable quality uniformly across the wafer.
II. EXPERIMENTAL DETAILS
The experiments have been conducted on the 300mm HCM Cu PVD source which is described
elsewhere 8. A schematic diagram of the source is presented in Fig. 1. One of the critical distinguishing
characteristics of the source is its magnetic field. It exists not only in the source area, as in standard planar
magnetrons, but also in the wafer area 8. For unbalanced magnetrons for which the outer magnetic pole
has a stronger field than the inner pole, there exists a magnetic separatrix between the target and substrate.
The boundary separates these two areas, allowing to a certain extent independent manipulation of the
sputtering plasma at the target and the depositing plasma at the wafer level. The dome shaped target
enhances electron confinement due to the hollow cathode effect. Manipulation of the magnetic field at the
target allows for the modulation of the target sputtering area, thereby controlling the sputtered atoms to
have a desirable angular distribution and line of sight to the wafer. Shifting the erosion groove upwards in
the source as shown in Fig. 2 results in filtering out species having low deposition angles with respect to
the plane of the wafer by the combination of long throw and collimation effects. Another mechanism that
helps to eradicate low energy atoms from the deposition flux is the process of ionization. Slower atoms
naturally spend more time in the plasma, increasing their chances of getting ionized via electron impact.
Tight confinement of the erosion area also results in increased DC power density which, in turn, helps to
maintain high plasma density.
Very high plasma density (>1013
cm-3
) 6 inside the source makes electron impact the dominant ionization
mechanism for metal species. The high self-sputtering ability of Cu makes it possible to have an ionized
flux of Cu species depositing on the wafer with minimum gas ion bombardment. Self-sputtering
conditions are achieved by igniting the discharge at <1 mTorr of Ar and immediately reducing the pressure
to <0.1 mTorr upon ignition. At these pressures, there is no thermalization of the sputtered Cu atoms as
they travel to the wafer virtually collision free, so the neutral component of the depositing flux is more
energetic compared to the baseline process. Even at such low pressures the exponent n in equation (1) is
still about 14 when the DC power is high (55-70kW). Sputtered Cu atoms can be ionized inside the HCM
source, close to the target, or in the second area of high plasma density located below the separatrix center,
where a magnetic null (zero magnetic field) is formed 6. Plasma beam escapes from the source through
the magnetic null to the wafer following the magnetic field lines created by downstream magnets. The
interaction of the source and downstream magnetic fields shapes the separatrix, affecting both the plasma
density and beam profile.
Several analytical techniques can be used to measure plasma and flux parameters in the deposition
chamber. We have chosen the ones which allowed us to do the measurements at the substrate level. A
complex analytical module consisting of planar Langmuir probes, quartz crystal microbalance (QCM), and
gridded energy analyzer (GEA), all with spatial resolution, was developed and used to optimize the
deposition process. The details of the analytical module construction and operation were discussed
elsewhere 9. The planar Langmuir probes have been used to measure electron density and temperature, ion
density, and plasma and floating potentials, from which the ion energy was estimated. QCM and GEA
measured the ionization fraction of Cu flux, Cu+/Ar
+ ratio, and ion energy distribution.
We evaluated the barrier/seed interface quality using multiple criteria. The emphasis was placed on the
study of the film inside the recessed features, particularly on the sidewalls, where electroplating and
reliability problems occur. The film forming fluxes to the recessed feature sidewalls are very different
from those to the horizontal surfaces. The common techniques for film characterization have generally
been developed for flat, relatively large surfaces. We therefore had to employ indirect and/or qualitative
techniques for feature sidewall film characterization. The most important metric was void-free
electrofilling of high aspect ratio vias with minimal seed thickness. The plating bath is a hostile
environment for the seed since it will quickly dissolve in the bath if the initiation of plating is delayed.
We consequently used the time for which the seed can survive in the bath before plating began as a metric
for seed quality. It is well known that even a thick seed on the sidewalls will not ensure void-free plating
if the film is not continuous. Film continuity can be gauged by its roughness. Rough films usually form
when the nucleation density is low and the growth occurs predominantly by island coalescence
mechanism, which leaves behind open areas until the film is relatively thick. It is difficult to do roughness
measurements on the sidewalls of small vias, so we used SEM images of the area of interest for visual
analysis of film smoothness. Evaluation of film adhesion was done by annealing the thin sidewall film for
90s at 300C in a hydrogen atmosphere. The degree of subsequent film agglomeration was used as a
measure of seed stability. We have used for our studies patterned wafers produced by the Novellus
Customer Integration Cente, as well as commercially-available wafers from IBM.
III. RESULTS AND DISCUSSION
The optimized seed process was evaluated using the abovementioned techniques and compared with the
baseline process, as well as with some modified processes to unconvolute the impact of different factors.
To unconvolute the film thickness factor, we deposited the same amount of material on the sidewalls with
both processes. According to TEM measurements the sidewall coverage was similar between baseline and
optimized processes while the bottom coverage of the optimized process improved significantly, up to
70% for high aspect ratio vias.
High aspect ratio vias (AR=6) were seeded with different field thicknesses with the baseline and
optimized processes, and subsequently electroplated. The corresponding SEM micrographs of FIB cross-
sections are presented in Fig. 3. Since the sidewall coverage was similar, we annoted the field thickness to
compare the results. The minimum field thickness required for void-free fill for the baseline process was
145nm. At 135nm, field thickness voids were observed at the edge of the wafer. The corresponding
threshold thicknesses for the optimized process were 65nm and 55nm, respectively, with voids appearing
simultaneously at the center and edge.
Degraded seed quality at the wafer edge for the baseline process was confirmed during hydrogen anneal
tests, as shown in Fig. 4. The baseline seed severely agglomerated on both the trench and via sidewalls at
the wafer edge with the chosen anneal conditions. On the contrary the optimized seed resistance to
agglomeration was similar at the wafer center and edge, which confirmed the improved uniformity in
sidewall film formation with the optimized process.
The optimized seed formed a more continuous layer than the baseline seed, as confirmed from SEM
images of via sidewall seed roughness. The baseline process formed a rough film with distinctive grains
while the optimized process yielded a much smoother film with weakly defined grain boundaries, as
shown in Fig. 5. This proves that nucleation density was increased, explaining how continuous coverage
with void-free electroplating was achieved with much less film thickness. As mentioned before, low
deposition temperatures are usually required to enhance Cu nucleation. We nevertheless were able to
deposit the films at a higher temperature (pedestal temperature +20C compared to the usual -40C) without
any detectable degradation in the roughness, continuity, or fill performance with the optimized process. In
conjunction with the anneal data, the superiority of the optimized process with respect to film adhesion
and interface stability is evident.
Seed stability was also evaluated with “cold” plating tests, where seeded wafers were subjected to an
intentional delay in the plating bath before plating was initiated. The thinnest area of the film deposited by
PVD is usually at the middle of a high AR via sidewall. Rough films with well-defined grain boundaries
as in Fig. 5a were more prone to be dissolved faster than smooth ones, like that shown in Fig. 5b. As soon
as the film became discontinuous, a void formed in the mid-sidewall area during subsequent
electroplating, as revealed in the FIB-SEM cross-sectional images in Fig. 6. The optimized seed withstood
50% longer exposure to the “cold” bath than the baseline seed.
We have attributed the improvement in the Cu seed film characteristics to the changes in the deposition
flux composition and energies of the species forming the film. This assumption is confirmed by the data
obtained with the plasma diagnostic module which was placed inside the deposition chamber in place of a
regular wafer.
The plasma sheath potential drop, defined as the difference between the plasma and floating potentials,
and the ion density were measured by the Langmuir probe. As shown in Fig. 7, the ion energy, which is
proportional to the sheath potential, almost doubled for the optimized process and its distribution across
the wafer improved. The ion density, which is shown in arbitrary units, increased by almost an order of
magnitude. The ion energy distribution as measured by the GEA was in good agreement with the
Langmuir probe measurements 9.
A significant change has been observed in the flux composition between the baseline and optimized
processes as measured by the QCM/GEA. While the flux of the baseline process was dominated by Cu
neutrals, the optimized process had comparable fluxes of ions and neutrals impinging on the wafer, as
shown in Fig. 8. The ionization fraction of Cu increased more than two-fold while Ar+/Cu
+ ion ratio
decreased.
IV. CONCLUSIONS
From the film characterization inside the recessed features and the plasma / deposition flux
measurements, we conclude that the critical factor for forming a thin, continuous, and stable Cu seed layer
on a Ta underlayer is the contribution of the energetic Cu species to the film growth. Their high energy
allows them to penetrate the barrier surface and implant themselves, thereby creating multiple stable
nucleation sites on the feature sidewalls.
We were able to filter out the low energy Cu species from the deposition flux and increase its metal ion
fraction by simply changing the magnetic field topology in the HCM source and switching from gas
discharge to essentially metal discharge plasma. The long throw and collimation effects of the HCM
target geometry and the increased power density resulted in a deposition flux dominated by energetic
neutrals and metal ions with increased energy. The improved plasma uniformity due to changes in the
magnetic field topology at the wafer level led to uniform seed performance across the wafer.
It was shown that there was a significant improvement in the film quality when it was predominantly
grown from an energetic Cu species.
The optimized seed layer agglomerated less when subjected to vacuum anneal, did not degrade when
deposited at an elevated temperature, was able to withstand longer delay times during electroplating, and
ensured void-free electrofill performance at half the thickness of what was required for the baseline seed.
The latter is particularly important for small feature sizes, since overhang limits the maximum seed
thickness.
As a result, copper interconnect metallization with PVD deposited seed layer can be extended to the next
device generations by enabling the formation of very thin but continuous films on the feature sidewalls.
ACKNOWLEDGMENTS
The authors would like to thank K. Ashtiani for useful discussions on the Langmuir probe design and data
interpretation and S. Fields for his contribution to QCM and GEA designs.
REFERENCES
1 M.Y. Yan, K.N. Tu, A.V. Vairagar, S.G. Mhaisalkar, A. Krishnamoorthy, ”Confinement of
electromigration induced void propagation in Cu interconnect by a buried Ta diffusion barrier layer”,
Appl. Phys. Lett. Vol. 87, pp. 261906-1-3, 2005
2 K. Ishikawa, et al., Impact of Metal Deposition Process upon Reliability of Dual-Damascene Copper
Interconnects, Proceedings of the IEEE 2003 International Interconnect Technology Conference, pp. 24 –
26, 2-4 June 2003
3 S. Kamiya, S. Suzuki, K. Yamanobe, M. Saka, ”Quantitative evaluation of bonding energy for the
interfaces in Cu metallization systems”, J. Appl. Phys. Vol. 99, pp. 034503-1-7, 2006
4 N. Baguer, A. Bogaerts, ”Study of the sputtered Cu atoms and Cu
+ ions in a hollow cathode glow
discharge using a hybrid model”, J. Appl. Phys. Vol. 98, 033303-1-3, 2005
5 U. Helmersson, M. Lattemann, J.Bohlmark, A.P. Ehiasarian, J.T. Gudmundsson, “Ionized physical vapor
deposition (IPVD): A review of technology and applications”, Thin Solid Films, 513, 1, 2006
6 J.A. Hopwood, ed., Ionized Physical Vapor Deposition, Academic Press, London, 2000
7 P. Ding et al., “New PVD Source for Copper Technology at 65nm and beyond”, Nanochip Technology
Journal, No 1, pp.47-52, 2005
8 J.Q. Lu, T. Yu, L. Stenzel, J. Tobin, “Apparatus and method for improving film uniformity in a physical
vapor deposition system”, United States Patent 6471831
9 L. Wu, E. Ko, A. Dulkin, K.J. Park, S. Fields, K. Leeser, L. Meng, and D.N. Ruzic, Flux and Energy
Analysis of Species in Hollow Cathode Magnetron Ionized Physical Vapor Deposition of Copper, Rev.
Sci. Instrum. 81, 123502, 2010
FIGURE CAPTIONS
Fig.1. Schematic diagram of the Novellus HCM source
Fig.2. Erosion groove of the baseline and optimized sputtering processes
Fig.3. Electroplating results of 6:1 AR vias with the (a) baseline and (b) optimized Cu seed processes with
different field thicknesses at the center and edge of the wafer
Fig.4. SEM micrographs of annealed seed on the via and trench sidewall for the (a) baseline and (b)
optimized processes
Fig.5. Seed roughness. Blowup images of the Cu film from the area defined by the dashed rectangle. SEM
micrograph of Cu seed on the sidewall of the via structure for the (a) baseline and (b) optimized processes
Fig.6. Seed Stability. FIB-SEM cross-sections of “cold” electroplated dual damascene structures with
delay times of 4s and 6s for the (a) baseline and (b) optimized processes
Fig.7. Radial distribution of the sheath potential drop (Vp-Vf) and ion density across the 300mm wafer for
the (a) baseline and (b) optimized processes
Fig.8. Radial distribution of neutrals and ions in the deposition flux for the (a) baseline and (b) optimized
processes
FF
Fig.1. Schematic diagram of the Novellus HCM source
Rotating Magnet
EM Coils
Wafer
EM Coils
Deposition flux with high ion fraction
Target
Fig.2. Erosion groove of the baseline and optimized sputtering processes
Optimized
Baseline
Predominant Erosion Area
(a) (b)
Fig.3. Electroplating results of 6:1 AR vias with the (a) baseline and (b) optimized Cu seed processes with
different field thicknesses at the center and edge of the wafer
Center
1488A
1388A 1367A
665A 643A
542A 543A
1453A
Edge Center Edge
(a) (b)
Fig.4. SEM micrographs of annealed seed on the via and trench sidewall for the (a) baseline and (b)
optimized processes
center
edge
center
edge
(a) (b)
Fig.5. Seed roughness. Blowup images of the Cu film from the area defined by the dashed rectangle. SEM
micrograph of Cu seed on the sidewall of the via structure for the (a) baseline and (b) optimized processes
Area of interest
4s 6s 4s 6s
(a) (b)
Fig.6. Seed Stability. FIB-SEM cross-sections of “cold” electroplated dual damascene structures with
delay times of 4s and 6s for the (a) baseline and (b) optimized processes
center
center
edge
edge
(a) (b)
Fig.7. Radial distribution of the sheath potential drop (Vp-Vf) and ion density across the 300mm wafer for
the (a) baseline and (b) optimized processes
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