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

(a) (b)

Fig.8. Radial distribution of neutrals and ions in the deposition flux for the (a) baseline and (b) optimized

processes