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Electrodepositionofcopper–tinalloythinfilmsformicroelectronicapplications

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Electrodeposition of copper�/tin alloy thin films for microelectronicapplications

Deenesh Padhi *, Srinivas Gandikota, Hoa B. Nguyen, Chris McGuirk,Sivakami Ramanathan, Joseph Yahalom 1, Girish Dixit

Applied Materials Inc., 3303 Scott Boulevards, M/S 1148, Santa Clara, CA 95054, USA

Received 15 July 2002; received in revised form 18 October 2002

Abstract

The continuing shrink in device size has generated great interest to create interconnects with low resistivity and superior resistance

to electromigration (EM) and stress migration (SM) in comparison to the existing Al or Al-alloy interconnections. Copper has

become the metal of choice to meet the needs of present and future generation devices. In order to improve the intrinsic resistance of

copper to EM/SM induced failure, alloying elements can be added into copper metallurgy. In the present investigation, we discuss a

method to co-deposit an alloy of copper and tin in sub-microscopic features with high aspect ratio using a sulfate bath. It is observed

that a small amount tin begins to co-deposit at potentials smaller than the equilibrium reduction potential. Under activation control

regime, the composition is not affected by current density. The results of this study conclude that substantial tin deposition occurs

upon onset of mass-transport limitation. It is found that a finite amount of time is required before electrolysis is controlled by mass-

transfer. The transition time and hence, the composition of the plated film is affected by the hydrodynamic conditions, current

density, and electrolyte composition. These factors must be taken into account in order to control the composition profile of tin in

vias and trenches.

# 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Copper�/tin alloy electrodeposition; Alloy thin film co-deposition; Electromigration in interconnects; Alloy deposition under mass-

transfer control; Under potential deposition

1. Introduction

The continuing shrink in device size has generated

great interest to create interconnects with low resistivity

and superior electromigration (EM) and stress migra-

tion (SM) lifetimes in comparison to the existing Al or

Al-alloy interconnections. Copper has become the metal

of choice to meet the needs of present and future

generation devices. In order to further improve the

intrinsic resistance of copper to EM/SM induced failure,

researchers have attempted to introduce various alloying

elements into copper lines. Among the Cu-alloy systems

studied, the Cu�/Sn system has shown significantly

higher EM lifetimes and activation energy than pure

Cu. Hu et al. [1] studied the impact of addition of Mg,

Sn, and Zr on the EM of sputtered copper lines. They

concluded that the drift velocity of copper increased by

addition of Mg, while addition of Sn and Zr resulted in

reduction of drift velocity. Activation energy for EM of

copper increased from 0.75 to �/1.3 eV with addition of

1 wt.% Sn, while the resistivity increased to 4.1 mV/cm2.

Furthermore, due to the low solid solubility of Sn into

Cu, the Cu�/Sn system is amenable to heat treatment to

produce precipitation of intermetallic phases [2] and

segregation of solute species to grain boundaries [1],

thereby, reducing the aggregate resistivity of the alloy.

In addition to improved resistance to EM induced

voiding, Cu�/Sn alloy films have superior resistance to

corrosion, which is very desirable in the processing of

multilevel Cu-wiring [1]. Although copper�/tin alloys

can be electrodeposited using cyanic and phosphate

baths, due to the toxic nature of these electrolytes,

* Corresponding author. Tel.: �/1-408-563-9151; fax: �/1-408-563-

8305

E-mail address: deenesh_padhi@amat.com (D. Padhi).1 Present address: Technion-Israel Institute of Technology.

Electrochimica Acta 48 (2003) 935�/943

www.elsevier.com/locate/electacta

0013-4686/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.

PII: S 0 0 1 3 - 4 6 8 6 ( 0 2 ) 0 0 7 7 4 - 0

sulfate or sulfamate based baths have been the subject of

several investigations in the past [3�/5].

Electrochemical deposition is widely accepted today

as a preferred method of depositing copper for inter-connect applications in the 0.13 mm device node and is

being evaluated for extension to future generations of

devices. Void-free gap-fill of sub-micrometer vias and

trenches can be achieved through electroplating copper

from an acidic sulfate bath [6]. To the authors’ knowl-

edge, most of the research has been focused towards

understanding the impact of Sn-incorporation in the

sputtered Cu-seed on the interconnect performance [1,7]and there has not been any study to investigate the effect

of Sn alloying in the electroplated copper film. In the

present investigation, we discuss a method to co-deposit

a very dilute Cu�/Sn alloy inside very high aspect ratio

features. In addition, we discuss the issues involved with

the transient phenomena in mass-transport control

regime, which has significant implications on deposition

of alloy inside the vias and trenches.

2. Experimental details

Alloy deposition was studied under galvanostatic and

voltametry conditions. Two hundred millimeter Si

wafers having 1500 A of Cu seed layer on 250 A of

TaN/Ta diffusion barrier layer were used for galvano-

static studies. The barrier/seed layers were deposited byphysical vapor deposition (PVD) using self-ionized

plasma (SIP) technique. Electroplating was carried out

in an experimental tool. A schematic of the plating

reactor is shown in Fig. 1. Wafer is held by a ‘contact

ring’ and head-assembly such that the copper surface

faces downward. The chamber is designed to provide a

laminar flow*/a gap between the wafer edge and the

chamber wall ensures that turbulence is minimized.Electrical energy is supplied to the wafer surface

through a ‘contact ring’ that contacts the copper surface

at the edge of the wafer. A saturated calomel reference

electrode (SCE) was used to monitor the cathodic

potential of the wafer during plating. Electrolyte from

a reservoir is pumped around the anode assembly

towards the wafer as shown in Fig. 1. The electrolyte

consisted of CuSO4, SnSO4, H2SO4, and Cl�. The

concentrations of Cu2� and Sn2� were varied while

the concentrations of H2SO4 and Cl� were keptconstant at 30 grams/liter (g/l) and 50 PPM, respectively.

In addition, a commercial additive package comprising

of an accelerator, suppressor, and leveler were added to

the plating chemistry in order to obtain ‘supper-fill’

inside vias and trenches with sub-micrometer diameter.

The temperature of the bath was maintained constant at

20 8C throughout the study. Current density was varied

between 5 and 60 mA/cm2 and plating rotation speedwas varied between 0 and 40 rpm. The flow rate of

electrolyte in the plating chamber was kept constant at

4.5 gallons per minute (GPM). For transient study, the

electrolyte was kept stagnant. Composition of plated

film on blanket wafer was measured by dissolution of

the film into a liquid medium and its analysis by

Inductively Coupled Plasma�/Mass Spectrometry

(ICP�/MS). Compositional profiles of copper and tinin deposit were characterized using Secondary Ion Mass

Spectroscopy (SIMS) technique. Gap-fill was checked

using SEM cross sections. Voltametric study was con-

ducted using a Solartron SI 1287 Electrochemical Inter-

face having a copper counter electrode, saturated Ag/

AgCl reference electrode, and platinum rotating disk

electrode (RDE). The potential scanning rate was fixed

at 20 mV/s.

3. Results and discussions

The composition of plated alloy is determined by the

partial current densities of the depositing species, which

depend on the cathode potential and the mass-transport

conditions during plating. In the ensuing section, we

discuss the effects of these parameters on co-deposition

of copper�/tin alloys.

3.1. Effect of the total current density (ITotal)

Four different combinations of Cu2� and Sn2�

concentrations were tried for the plating bath. These

Fig. 1. Schematic of the electroplating apparatus.

D. Padhi et al. / Electrochimica Acta 48 (2003) 935�/943936

are: (A) 43.5 g/l Cu2� and 3.3 g/l Sn2�; (B) 39.1 g/l

Cu2� and 7.9 g/l Sn2�; (C) 36.8 g/l Cu2� and 12.4 g/l

Sn2�; (D) 32.1 g/l Cu2� and 16.6 g/l Sn2�. The wafers

were rotated at a constant rotational speed (v ) of 20

rpm and the electrolyte flow rate was maintained

constant at 4.5 GPM throughout this study. A summary

of the results from this study is presented in Fig. 2(A and

B), in which the amount of co-deposited tin in the alloy

is shown at various current densities and cathode

potentials. There is obviously no clear correlation

between the current density in the range of 10 and 40

mA/cm2 or the corresponding cathode potential on the

content of tin in the deposit. In all cases the amount of

co-deposited tin is very small (�/0.1 to 0.2 wt.%).

However, the amount of tin in the deposit increases

with the increase in the Sn2�/Cu2� ratio in the

electrolyte. At a given ITotal, the wafer potential (Dfc)

remained practically constant during electrolysis. It is

interesting to note that it was possible to deposit tin at a

potential as high as �/0.17 VSCE. However, the amount

of tin did not increase even at a Dfc of �/0.29 VSCE for

the electrolyte with a) 43.5 g/l Cu2� and 3.3 g/l Sn2�

and �/0.35 VSCE for the electrolyte with 36.8 g/l Cu2�

and 12.4 g/l Sn2�. In order to resolve these issues, a

voltametric study was conducted on electrolytes con-

taining (1) only Sn2� (2) only Cu2� and (3) both Sn2�

and Cu2�. The concentrations of Cl�, H� and organic

additives were maintained constant at levels used to fill

sub-0.25 mm vias and trenches. The polarization curves

(Fig. 3) in various solutions reveal that reduction of tin

begins at a potential of about �/0.4 V vs. SCE. It is

further seen in Fig. 3 that with a lower concentration of

Sn2�, the electrolysis is diffusion limited at a higher

potential. The reduction potential of Cu2� is 0 V vs.

SCE (Fig. 4). Fig. 4 further illustrates the effect of

substrate rotational speed on the polarization character-

Fig. 2. Effects of primary (A) current density and (B) cathodic

potential on alloy composition.

Fig. 3. Polarization behavior of electrolytes containing only Sn2� and additives.

D. Padhi et al. / Electrochimica Acta 48 (2003) 935�/943 937

istics of Cu2� reduction. Polarization behaviors of the

electrolytes containing both cations in addition to other

additives are summarized in Fig. 5. It appears that

addition of Sn2� does not significantly alter the

polarization characteristics of the mixed electrolyte

from that of electrolytes containing only Cu2�. This

may indicate that the incorporation of tin in the copper

deposit is in fact physical*/in the form of tin oxide.

Small amounts of tin can also co-deposit at a potential

higher the equilibrium potential via an under potential

deposition (UPD) mechanism [5]. Fig. 6 showing the

polarization curve of an electrolyte containing only 17 g/

l Sn2� (and other additives) reveals a finite current

density (in the order of 0.05�/0.1 mA/cm2) at very low

cathodic potentials (i.e. B/�/0.3 VSCE). This indicates

that UPD of Sn may be possible with sulfate based

electrolytes similar to the results of Horkans et al. [5].

Further investigation is needed to fully understand this

aspect of Cu�/Sn co-deposition.

An attempt was made to characterize the concentra-

tion profile of tin across the depth of film by using SIMS

technique. Accurate measurement of tin concentration

could not be obtained due to lack of standard dilute

Cu�/Sn alloy specimens. However, the depth profile

Fig. 4. Polarization behavior of electrolytes containing only Cu2� and additives. The rotational speed of the disk electrode is shown in the

parenthesis.

Fig. 5. Polarization behavior of an electrolyte containing Cu2�, Sn2�, and additives. Polarization curves of electrolytes containing only Cu2� and

only Sn2� are shown for comparison.

D. Padhi et al. / Electrochimica Acta 48 (2003) 935�/943938

revealed that the concentration of tin was uniformacross the thickness precluding the possibility of surface

segregation of tin.

3.2. Effect of hydrodynamics

The effect of changes in ‘v ’ on the composition of the

alloy when plated with electrolytes containing differentproportions of Cu2� and Sn2� is shown in Fig. 7. At

low total current density, the effect of ‘v ’ is negligible

(in the range of 5�/40 rpm) for the electrolytes tested as

shown in Fig. 7(A). However, at 40 mA/cm2, rotation of

the wafer has a pronounced effect on the alloy composi-

tion. This can be seen in Fig. 7(B) revealing the

increasing trend of the amount of tin in the plated film

with decreasing ‘v ’. The extent of this increase is afunction of the electrolyte composition; the biggest

effect of rotational speed on tin composition was

observed for the electrolyte with the lowest [Cu2�]/

[Sn2�] ratio. While plating at 10 mA/cm2 in different

electrolytes, Dfc was in the range of �/0.17 to �/0.30

VSCE (Table 1), suggesting that co-deposition occurred

under activation control. As ITotal is increased from 10

to 40 mA/cm2, the cathodic potential decreased to �/

0.28 VSCE or lower depending on electrolyte composi-

tion. At such high overpotentials, the deposition of

copper occurs, at least partially, under mass transfer

control, depending on the concentration of Cu2� in the

electrolyte and the rotational speed of the substrate.

This can be clearly seen in Fig. 4 where the electrolysis

of a solution containing 34.5 g/l Cu2� (and other

additives) becomes mass-transfer controlled at approxi-mately �/0.34 V vs. SCE when the rotational speed of

RDE was decreased from 400 to 0 rpm. For electrolytes

with (A) 43.5 g/l Cu2� and 3.3 g/l Sn2� (B) 39.1 g/l

Cu2� and 7.9 g/l Sn2�, the cathodic potentials remained

practically constant during plating at 40 mA/cm2 for 75

s at different rotational speeds and a variation in ‘v ’ did

not change Dfc and hence the deposit concentration.

Fig. 6. Polarization behavior of an electrolyte containing 17 g/l Sn2� and standard additives. An electrode with copper surface was used for this

study. The sweep rate was fixed at 20 mV/s.

Fig. 7. Effect of rotational speed of wafer on alloy composition at (A)

10 and (B) 40 mA/cm2.

D. Padhi et al. / Electrochimica Acta 48 (2003) 935�/943 939

For electrolytes with (C) 36.8 g/l Cu2� and 12.4 g/l

Sn2� (D) 32.1 g/l Cu2� and 16.6 g/l Sn2�, the amount

of tin in the film decreased from 3.82 to 0.11 wt.% and

from 20. to 0.21 wt.%, respectively, with increasing ‘v ’

from 5 to 20 rpm (Table 1). Based on the above

observations, it is evident that electrodeposition of tin

begins to occur substantially once the partial copper

deposition current is controlled by mass-transfer. The

higher the rotational speed of the wafer, the higher is the

limiting current density for copper deposition, and

hence, lower is the amount of co-deposited tin. Similar

to the effect of changing rotational rate, the amount of

co-deposited tin was found to increase with decreasing

the rate of electrolyte flow, as demonstrated in Fig. 8.Since the deposition of tin is influenced by the

hydrodynamics, its concentration profile across the

Table 1

Summary of Cu�/Sn alloy plating chemistries and process parameters

Electrolyte Sn2� (g/l) Cu2� (g/l) Current density (mA/cm2) rpm Flow rate Potential (VSCE) Sn (wt.%) Film surface roughness (nm)

A 3.3 43.5 10 5 4.5 0.230 0.04

A 3.3 43.5 10 10 4.5 0.200 0.06

A 3.3 43.5 10 20 4.5 0.175 0.08

A 3.3 43.5 10 40 4.5 0.185 0.13

A 3.3 43.5 20 20 4.5 0.260 0.04

A 3.3 43.5 40 5 4.5 0.286 0.04

A 3.3 43.5 40 10 4.5 0.281 0.02

A 3.3 43.5 40 20 4.5 0.286 0.02

A 3.3 43.5 40 40 4.5 0.281 0.02

A 3.3 43.5 60 20 4.5 0.291 0.02

B 7.9 39.1 10 5 4.5 0.225 0.09

B 7.9 39.1 10 10 4.5 0.225 0.08

B 7.9 39.1 10 20 4.5 0.225 0.13

B 7.9 39.1 10 40 4.5 0.225 0.09

B 7.9 39.1 20 20 4.5 0.235 0.06

B 7.9 39.1 40 5 4.5 0.351 0.21

B 7.9 39.1 40 10 4.5 0.326 0.08

B 7.9 39.1 40 20 4.5 0.321 0.09

B 7.9 39.1 40 40 4.5 0.321 0.07

C 12.4 36.8 10 5 4.5 0.290 0.11

C 12.4 36.8 10 10 4.5 0.290

C 12.4 36.8 10 20 4.5 0.295 0.13

C 12.4 36.8 10 40 4.5 0.305 0.13

C 12.4 36.8 20 20 4.5 0.320 0.14

C 12.4 36.8 40 5 4.5 0.38�/0.43 3.82

C 12.4 36.8 40 10 4.5 0.371 0.43

C 12.4 36.8 40 20 4.5 0.351 0.14

C 12.4 36.8 40 40 4.5 0.331 0.11

D 16.6 32.1 10 5 4.5 0.215 0.15

D 16.6 32.1 10 10 4.5 0.175 0.14

D 16.6 32.1 10 20 4.5 0.175 0.15 7.3

D 16.6 32.1 10 40 4.5 0.175 0.18

D 16.6 32.1 40 5 4.5 0.24�/0.48 20.70 23.2

D 16.6 32.1 40 10 0.5 0.261 9.86 22.7

D 16.6 32.1 40 10 4.5 0.251 5.33

D 16.6 32.1 40 10 7.5 0.251 1.37 11.8

D 16.6 32.1 40 20 4.5 0.211 0.18 12.3

D 16.6 32.1 40 40 4.5 0.201 0.21 9.9

D 16.6 32.1 60 20 4.5 0.231 15.14

Fig. 8. Effect of rate of electrolyte flow on alloy composition at 40

mA/cm2 and 10 rpm.

D. Padhi et al. / Electrochimica Acta 48 (2003) 935�/943940

thickness of the film must be a function of the transient

mass-transport conditions. In other words, the time to

form a cation-depleted ‘diffusion layer’ and the time

dependence of its thickness govern the local tin compo-

sition in the film. This poses a significant concern for

electrodeposition of thin alloy film for metallization of

ULSI circuits where the vias and trenches are filled in

the initial 1000�/2000 A of metal deposition. To the

authors’ knowledge, this aspect of Cu�/Sn alloy deposi-

tion has not received adequate attention. In order to

study the transient effects, wafers were plated at 40 mA/

cm2 for different durations, with different rotational

speeds in a stagnant electrolyte having 32.1 g/l Cu2�

and 16.6 g/l Sn2�. The cathodic potential transients

during this study are plotted in Fig. 9 which reveal that

electrolysis begins with a potential in the range of �/�/

Fig. 9. Effect of rotational speed of wafer on cathodic transients at 40 mA/cm2.

Fig. 10. Variation of alloy composition (as measured by ICPMS) with film thickness.

D. Padhi et al. / Electrochimica Acta 48 (2003) 935�/943 941

0.2 VSCE that decreases very slowly for some finite time

prior to dropping abruptly. The time for which the

potential remains practically constant (t ) is found to

depend on ‘v ’. At v�/0 rpm, the potential begins to

sink after 30 s while the same event takes place after 45 s

when ‘v ’ was increased to 5 rpm; with further increase

in ‘v ’ to 10 rpm, the potential practically remained

constant without any abrupt change till 65 s. Fig. 10

shows the amount of tin in the films plated for different

duration as a function of ‘v ’. From the data shown in

Fig. 10, it is evident that during an initial period when

the potential of the wafer is greater than �/0.4 VSCE very

little amount of tin is co-deposited. However, as the

potential begins to decrease sharply, the partial tin

deposition current increases. This phenomena is clearly

illustrated by the depth profile of tin concentration in a

film deposited at 40 mA/cm2 at 5 rpm (Fig. 11), showing

a region with negligible amount of tin near the PVD

Seed/Electroplated alloy interface followed by a regionrich in tin. These observations can be easily explained by

the theory of electrodeposition under mass transfer

control regime. When a cathodic substrate is polarized

galvanostatically, assuming that that the charge-transfer

reaction has a high exchange current density, the

instantaneous cathodic potential can be legitimately

described by the Nernst equation as [8]:

Dfc�Df0�RT

nFln cx�0 (1)

where, Dfc�/instantaneous cathodic potential, Df0�/

equilibrium cathodic potential, R�/universal gas con-

stant, T�/temperature of electrolysis (K), C�/concen-

tration of the cation, x�/distance from the cathode/

electrolyte interface. As the cations are consumed at the

interface by the charge transfer reaction, its concentra-

Fig. 11. Depth profile of alloy composition of a film plated under mass-transfer control regime.

Fig. 12. Effect of ITotal on transition time (t ) illustrating the linear relationship between ‘t ’ and (ITotal)�2.

D. Padhi et al. / Electrochimica Acta 48 (2003) 935�/943942

tion at the interface departs from the initial value (i.e.

the bulk concentration) and the cathodic potential starts

diminishing. Because of the logarithmic nature of Eq.

(1), the decrease in potential is very gradual in the

beginning, until the concentration of the electron

acceptors approaches zero, when a rapid negative surgein potential is observed. In the present case, deposition

of tin begins to occur as Dfc dips below the reduction

potential of Sn2� (i.e. �/0.4 VSCE), as revealed from

Figs. 9 and 10. After lapse of the transition time (t), the

amount of the tin in the film is determined by ITotal and

the partial limiting current density of copper reduction.

The transition time is a function of current density and

is given by Sand’s Equation [9] as follows:

t�pD

�c0nF

2iCu

�2

(2)

Fig. 12, a plot of ‘t ’ vs. (ITotal)�2, shows a very good

agreement of the experimental data to the prediction

based on Eq. (2). Similarly, ‘t ’ was seen to decrease withincreasing ‘v ’ (Fig. 9) because of more efficient convec-

tion which counteracts the tendency of the interfacial

concentration of Cu2� to reduce to zero.

Table 1 summarizes the film surface roughness data

(measured by AFM technique) for films plated under

different conditions. The alloy films exhibited rougher

morphology than a plated copper film (e.g. 3.7 nm).

Films plated under activation control had a surfaceroughness of about 7�/13 nm. As expected, the surface

roughness increased (to about 22�/23 nm) when the

deposition was controlled by mass-transfer.

Based on the results of the present study, the

chemistry and plating parameters were chosen to

electro-fill sub-micron interconnect features by co-

deposition of a dilute Cu�/Sn alloy. Fig. 13 is a SEM

cross section illustrating void-free gap-fill in vias with adiameter of 0.25 mm and aspect ratio of 4.5. The absence

of seams in the micrograph suggests that the bottom-up

growth [10] is not disturbed by the addition of an

additional cation species to the Cu2� containing elec-

trolyte. The nominal composition of the deposit was Cu-0.15 wt.% Sn.

4. Conclusions

In the present investigation, a method to electroche-mically co-deposit an alloy of copper and tin in sub-

microscopic features with high aspect ratio using a

sulfate bath is presented. It is observed that a small

amount tin begins to co-deposit at potentials higher

than the equilibrium reduction potential. Under activa-

tion control regime, the amount of tin in the dilute alloy

increases with [Sn2�]/[Cu2�] ratio, but is unaffected by

the current density. This may indicate that the incor-poration of tin in the copper deposit is due to either

physical incorporation of tin oxide or UPD of tin.

Preliminary studies indicate that UPD of tin may be

possible with sulfate electrolyte but more work is needed

to understand this behavior. The results of our investi-

gation reveal that substantial tin deposition occurs upon

onset of mass-transport limitation. It is found that the

transition time required before electrolysis is controlledby mass-transfer decreases with increasing current

density and [Sn2�]/[Cu2�] ratio and decreasing rota-

tional speed of wafer. In contrast to the homogeneous

tin deposition under activation control regime, the

transient behavior affects the composition profile of

deposits. The morphology of the film became rougher

with the transition from activation control to mass

transfer control deposition.

References

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Fig. 13. SEM micrograph showing void-free gap-fill using Cu�/Sn co-

deposition in vias with a diameter of 0.25 mm and aspect ratio of 4.5.

The electrolyte had 40 g/l Cu2� and 15 g/l Sn2�. The wafer was plated

with a current density of 10 mA/cm2 for about 100 s at a rotational

speed of 20 rpm. The flow rate was maintained at 4.5 GPM.

D. Padhi et al. / Electrochimica Acta 48 (2003) 935�/943 943