Acta Materialia 52 (2004) 2435–2440

www.actamat-journals.com

Stress-induced surface damage and grain boundary characteristicsof sputtered and electroplated copper thin films

Hyun Park, Soo-Jung Hwang, Young-Chang Joo *

School of Materials Science and Engineering, Seoul National University, Seoul 151-742, Republic of Korea

Received 20 October 2003; received in revised form 26 January 2004; accepted 27 January 2004

Abstract

The morphology of the stress-induced surface damage and its relationship with the microstructure in electroplated and sputtered

copper thin films is discussed. After annealing at 435 �C for 1 h, two types of surface damage were observed. In some films, grooves

along the grain boundaries were formed, whereas in other films, voids at the grain boundary triple junctions were observed. In films

of similar thickness, the triple junction voids were deeper than the grain boundary grooves. It was found that the high energy grain

boundaries (HEGBs) or their triple junctions are the sites where damages are generated by thermal stress. The area ratio of the

HEGBs to the surface area per grain, which is a function of both the grain size (d) and the film thickness (h), as well as the fraction of

HEGB (f ), determines the morphological equilibrium between the two types of damage. It is suggested that, in general, a micro-

structural parameter, d=hf , can be used to predict the damage morphology in Cu films.

� 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Copper; Thin film; Stress-induced surface damage; Grain boundaries

1. Introduction

In integrated circuits, the difference in thermal ex-

pansion between the metal film or line and the neigh-

boring materials gives rise to high stress during the

fabrication process, resulting in stress-induced damage,

such as voids or grooves [1–21]. Furthermore, since Cuis used in highly enhanced integrated circuits, these

stress-induced phenomena have attracted many re-

searchers� attention.It is well known that the stress-induced damage ob-

served in metal thin films is formed as a result of a mass-

diffusion process along special diffusion paths. Yost [1]

claimed that voids in Al thin films grow as a result of a

stress-driven grain boundary diffusive process, in whichthe grain boundary acts as a reservoir for the mass

diffusion from the voids. Thouless [19] and Genin et al.

[20,21] suggested that thermal stress is relaxed by the

diffusion of mass from the surface to the grain bound-

aries, resulting in grooves that are deeper than thermal

* Corresponding author.

E-mail address: ycjoo@plaza.snu.ac.kr (Y.-C. Joo).

1359-6454/$30.00 � 2004 Acta Materialia Inc. Published by Elsevier Ltd. A

doi:10.1016/j.actamat.2004.01.035

grooves. Recently, Weiss [18] and Weiss et al. [17]

claimed that void formation in a Cu–Al alloy film that

was oxidized at temperatures above 500 �C was due to

the interaction of the grain boundaries under tensile

stress with the surface oxide.

Meanwhile, some other important factors which play

a role stress-induced damage formation were also re-ported [2–16]. Kobrinsky et al. [16] proposed that the

capping layer of the Cu line restrains the diffusional

creep caused by matter transport from the surface to the

grain boundaries, and plays a crucial role in the reli-

ability of Cu interconnects. Nucci et al. [7–9] claimed

that the grain boundary structure, as determined by the

film texture, influences both the nucleation and growth

of voids, illustrating the importance of the local micro-structure on stress voiding reliability. Koike et al [13]

and Sekiguchi et al. [14] attributed void formation to the

concentration of thermal stress induced by the difference

in orientation of the two grains.

Although these previous works provide valuable in-

formation about stress-induced damage in Cu thin films,

its nature is still not fully understood. In particular,

quantitative analyses are required of the relationship

ll rights reserved.

2436 H. Park et al. / Acta Materialia 52 (2004) 2435–2440

between the microstructure of the film and the formation

of the damage, if our understanding of this matter is to be

improved. If stress relaxation and damage formation are

controlled by atomic transport caused by a diffusion

process along certain diffusion paths, such as the surfaceand the grain boundary [16–22], then it would be useful to

investigate their effects on atomic transport.

In order to identify the microstructural factors asso-

ciated with stress-induced damage formation, which can

be applied to Cu films in general, various copper films

were deposited, and the damage caused to their surface

after annealing was analyzed. Based on the results of our

investigation, the roles of the microstructural factors,the high energy grain boundaries, the grain size and film

thickness at the damage site and the morphology are

discussed. Finally, a parameter that can be used to

predict the type of damage most susceptible to be caused

to various kinds of Cu film is suggested.

2. Experimental procedure

Copper films with thicknesses ranging from 0.3 to

1.5 lm were produced by ionized metal plasma sput-

tering, conventional sputtering, and electroplating on a

TaN barrier (450 �A)/oxidized Si wafer, with the excep-

tion of the 1.5-lm thick sputtered (SP) film, which was

deposited on an oxidized Si wafer without a TaN bar-

rier. The electroplated (EP) films were also producedwithout any additives, in order to investigate the effect

of the additives on the EP films. The details of all of the

samples are listed in Table 1.

All films were annealed at 435 �C for 1 h in a vacuum

(<3� 10�7 Torr) with heating and cooling rates of 3 �C/min. Some films were detached from the substrate and

annealed, in order to investigate the effect of the sub-

strate on the film. Since the surface of the as-depositedEP film without any additive was too rough for mea-

surements to be made on it, ion milling was performed

before annealing.

The surfaces of the films, both before and after an-

nealing, were observed by means of a scanning electron

Table 1

Summary of damage morphologies and microstructural characteristics in co

Sample type [film

thickness, h (lm)]

Damage morphology Film t

EP film (0.3) Grain boundary groove h111iEP (1.0) Grain boundary groove h111iEP (1.5) Grain boundary groove h111iEP without additive (1.1) Triple junction void h111iIonized SP film (0.3) Triple junction void h111iIonized SP (0.8) Triple junction void h111iIonized SP (1.0) Grain boundary groove h100iNormal SP (1.5) Triple junction void Rando

EP and SP stand for ‘‘electroplated’’ and ‘‘sputtered’’, respectively. f �:boundaries with a misorientation angle of more than 15 �, except for the R3

microscope with a field emission gun (FE-SEM, JEOL

JSM-6330F). The grain sizes, grain orientations, and

grain boundary misorientation angles were measured by

high-resolution electron back-scattering diffraction

(HR-EBSD, JEOL JSM 6500F equipped with EBSDsystem of Oxford INCA Crystal). The grain size was

determined by measuring the grain boundaries, includ-

ing the twin (R3 and R9) boundaries. The surface pro-

files of the films were measured by means of an atomic

force microscope (AFM).

3. Results

Fig. 1 shows the surface damage produced in the Cu

films after annealing. All films were smooth and

undamaged before annealing, but severe damage was

observed after annealing, except in the case of the free-

standing film. In the freestanding film, which was de-

tached from the substrate before annealing, grooves

were observed, but they were less outstanding than thoseobserved on the films annealed on the substrate.

(Fig. 1(a) shows the surface morphology of the free-

standing 1.5-lm thick EP film and Fig. 1(d) shows the

same film annealed on the substrate.) Therefore, the

damage in the films annealed on the substrate, which is

shown in Figs. 1(b)–(i), was caused by thermal stress,

which originated from the thermal expansion mismatch

between the Cu film and the substrate.Two types of stress-induced damage were observed.

One type of damage consisted of severe grooves along

the grain boundaries. This was mainly observed in the

EP films with additives (Figs. 1(b)–(d)) and in the SP

films with a very large grain size (Fig. 1(h)). This kind of

damage is referred to as stress-induced grooves (as dis-

tinguished from the thermal grooves shown in Fig. 1(a)).

These stress-induced grooves were formed along thegrain boundaries and were interconnected. The other

type of damage consisted of isolated voids formed at

grain boundary triple junctions. These were mainly ob-

served in the SP films (Figs. 1(f), (g), (i)) and the EP

films without additive (Fig. 1(e)). This type of damage is

pper films

exture Median grain size,

d (lm)

f � (%) d=h d=hf

0.351 42.67 1.17 2.74

1.120 31.86 1.12 3.52

1.220 31.44 0.81 2.59

0.382 54.94 0.35 0.64

0.328 48.41 1.09 2.26

0.371 49.15 0.46 0.94

7.700 42.09 7.70 18.29

m 0.360 51.56 0.24 0.47

fraction of high energy grain boundaries, which refers to those grain

and R9 twin boundaries.

Fig. 1. Surface images of Cu films after annealing at 435 �C for 1 h: (a) free-standing 1.5-lm thick EP film, (b) 0.3 lm EP film, (c) 1.0 lm EP film,

(d) 1.5 lm EP film, (e) 1.1 lm EP film without additive, (f) 0.3-lm thick SP film, (g) 0.8 lm SP film, (h) 1.0 lm SP film and (i) 1.5 lm SP film on

the substrate.

0 10 20 30 40 50 600

10

20

30

40

50

60

Per

cent

age

of b

ound

arie

s [%

]

Misorientation angle [Degree]

Grain boundary without groove Grain boundary with groove

Fig. 2. Misorientation angle distributions of grain boundaries, with

and without grain boundary grooves, in 1.5-lm thick EP film with

additive.

H. Park et al. / Acta Materialia 52 (2004) 2435–2440 2437

referred to as triple junction voids. In some films (Figs.

1(b), (g) and (i)), both grooves and triple junction voids

were observed. In these cases, if the grooves were not

interconnected, then the damage type was considered tobe triple junction voids.

Line scanning by AFM showed that the triple junc-

tion voids were deeper than the grain boundary grooves

in films of similar thickness. For example, the depth of

the triple junction voids in the 0.8-lm thick SP film was

about 1700 �A, while that of the grain boundary grooves

in the 1.0-lm thick EP film was about 800 �A.

The grain boundary misorientation angles weremeasured by HR-EBSD. It was observed that the grain

boundary misorientation angle determined the location

of the damage. As shown in Fig. 1(b), stress-induced

grain boundary grooves were formed only at certain

grain boundaries. Fig. 2 shows the misorientation angle

distribution of the grain boundaries, with and without

stress-induced grooves, in the 1.5-lm thick EP film.

These grooves were only observed along random highangle grain boundaries, having a misorientation angle of

greater than 15�. On the other hand, no grooves were

formed along the low angle grain boundaries, which

have a misorientation angle of less than 15�, or along

the first order twin (R3) and second order twin (R9)boundaries, whose misorientation angles were approxi-

mately 60� and 39�, respectively [23].

The misorientation angle distributions of the grainboundaries in the different films are compared in Fig. 3.

These distributions can be classified into two types.

First, in the 1.5-lm thick EP film with additives and the

1.0-lm thick SP film with a very large grain size, a very

high proportion of grain boundaries was observed with

a misorientation angle of around 60�, with these mainly

being twin boundaries (Fig. 3(a)). Second, in the SP

films with a relatively small grain size and the EP film

with no additive, no clear peak corresponding to twin

boundaries (Fig. 3(b)) was observed. It was determined

that the damage type depends on the misorientation

angle distribution. Grain boundary grooves were ob-served in the first type of film having a higher fraction of

twin boundaries, while triple junction voids were ob-

served in the second type of film.

The grain sizes, textures and the fractions of the high

energy grain boundaries, i.e. the random high angle

grain boundaries except for the low angle and twin (R3and R9) boundaries, over the total grain boundaries on

(a)

10 20 30 40 50 600

10

20

30

40

50

60

Per

cent

age

of b

ound

arie

s [%

]

Misorienation angle [Degree]

1.1 µm EP film without addtive 0.8 µm SP film

10 20 30 40 50 600

10

20

30

40

50

60

Per

cent

age

of B

ound

arie

s [%

]

Misorientation Angle [Degree]

1.5 ϖm EP film with additive 1.0 µm SP film (large grain)

(b)

Fig. 3. Misorientation angle distributions in films with stress-induced (a) grain boundary grooves and (b) triple junction voids.

2438 H. Park et al. / Acta Materialia 52 (2004) 2435–2440

the film surface, as measured by HR-EBSD, are sum-

marized in Table 1. No texture dependence on the

damage type was observed. The dependences of the film

thickness and grain size and the fraction of high energygrain boundaries will be discussed in the next section.

4. Discussion

The analysis of the misorientation angle distribution

of the grain boundaries, with and without stress-induced

grooves, in the EP film with a thickness of 1.5 lm(Fig. 2) showed that grooves were formed only along the

random high angle grain boundaries. The low angle and

twin boundaries have lower grain boundary energy, and

thus lower diffusivity, than the random high angle grain

boundaries. The higher energies, and thus higher diffu-

sivities, to be found in the random high angle grain

boundaries lead to the formation of surface damage, as

shown in our results. Nucci et al. [9] and Weiss et al.[17,18] reported similar phenomena, in that voids were

formed at the high angle grain boundaries, except in the

case of the twin boundaries, in electron-beam evapo-

rated Cu lines and sputtered Cu films, respectively. Our

results confirm that the grooves observed in EP Cu films

are also the result of stress-induced damage, as in the

case of the voids reported by other researchers.

It is interesting that both voids and grooves wereobserved, and that the relative proportion of these two

morphological features depended on various micro-

structural factors of the film, in particular the fraction of

high energy grain boundaries. Table 1 shows the exis-

tence of a trend, such that when the fraction of high

energy grain boundaries is high, triple junction voids

form, while in the opposite case, grain boundary

grooves form. It seems that at the intersection of thesurface and the high energy grain boundary, an in-

equality in atomic transport arises, and the magnitude

of this inequality determines the morphological equi-

librium between these two types of damage.

If damage formation is controlled by atomic diffusion

from the surface to the grain boundaries under tensile

stress [16–21], then the surface and the grain boundaries

may play a role in the atomic source and sink sites [1],respectively. Therefore, it is necessary to consider the

film thickness and grain size, as well as the fraction of

high energy grain boundaries, in order to quantify the

inequality of atomic transport. If the diffusivities on the

surface and along the grain boundaries, and the driving

forces for the surface and grain boundary diffusions, are

constant during damage formation, then the source and

sink areas, which are dependent on the film thicknessand grain size, determine the atomic amounts incoming

from the surface and outgoing toward the grain

boundaries at the intersection.

If the film consists of columnar grains, then the areas

of the surface and of the grain boundaries per grain, AS

and AGB, are functions of the film thickness, h, and grain

size, d, and can be expressed by AS / d2, AGB / dh, re-spectively. Then, the ratio of the surface area to thegrain boundary area, R, can be expressed by

R ¼ dh: ð1Þ

The ratio, R, or the value of d=h, refers to the ratio of the

atomic amount diffusing from the surface to the atomic

amount diffusing into the grain boundary.

Thouless [19] suggested the use of the same parameter

to indicate the contribution of the surface and grain

boundary diffusions to creep in films with a uniform,

sintered array of cylinders. In his model, the creep rate is

defined as d�1h�2 for grain boundary controlled diffusion,and d�2h�1 in the surface-diffusion controlled regime.

Therefore, d=h determines the dominant mechanism for

creep. The effects of surface diffusion become more im-

portant as the film thickness decreases. The d=h values ofour films are summarized in Table 1 and the relationship

between the d=h value and the damage type is plotted in

Fig. 4. However, the relationship between the d=h value

and the damage morphology is not clear.

0

4

8

12

16

SP1.0µm*

Film with triple junction void Film with grain boundary groove

SP1.0µm

EP0.3µm

EP1.0µm

SP0.3µm

EP1.5µm SP0.8µm

SP1.5µm

d /

hf

EP1.1µm

Fig. 5. Relationship between d=hf and damage type. Data for SP

1.0 lm* is taken from [18]; h ¼ 1:0 lm, d ¼ 2:4 lm, f ¼ 66:2%.

0

2

4

6

8

Film with triple junction void Film with grain boundary groove

EP1.1µm

EP 0.3µmSP 0.3 µm

SP 1.0µm

EP1.0µmEP1.5µm

SP 0.8µmSP 1.5µm

d /

h

Fig. 4. Relationship between d=h and damage type.

H. Park et al. / Acta Materialia 52 (2004) 2435–2440 2439

In Eq. (1) and Thouless� model, all of the grain

boundaries are considered to be identical. However,stress-induced damage is formed only in those high en-

ergy grain boundaries in which atoms diffuse rapidly.

Therefore, the roles of the low angle grain boundaries

and the twin boundaries, in which atoms have much

lower diffusivity, may be negligible, and the grain size in

Eq. (1) should replaced by the ‘‘effective’’ grain size,

which only takes into consideration the high energy

grain boundaries. The effective grain size, d 0, is pro-portional to the grain size, d, taking all of the different

kinds of grain boundaries into consideration, and is

inversely proportional to the fraction of high energy

grain boundaries over the total grain boundaries, f , thatis d 0 / d=f . f is the fraction of high energy grain

boundaries, which refers to those grain boundaries with

a misorientation angle of more than 15�, except for theR3 and R9 boundaries, over the total grain boundaries.For example, if all of the grain boundaries are of the

high energy type, d 0 is the same as d. However, if half of

the boundaries are of the high energy type, the effective

grain size will be doubled. Therefore, Eq. (1) should be

modified to represent R0, which can be expressed as

R0 ¼ dhf

: ð2Þ

The d=hf values were calculated for all of the films, and

are summarized in Table 1, and the relationship between

d=hf and the damage type is shown in Fig. 5. It was

found that the d=hf value can act as a parameter in the

determination of the morphology of the surface damage.

When d=hf is greater than 2.5, stress-induced grain

boundary grooves are formed.

Our model suggests that when the film is thin or the(effective) grain size is large, grooves tend to be formed.

This is because a large portion of the surface area can

provide enough atoms or vacancies from the surface for

damage to occur, and this damage is extended along the

grain boundaries. In the opposite case, where the surface

area is small, there is less transport from the surface.

Therefore, the damage will be propagated deeply into

the film, especially toward the grain boundary triple

junction, which is known to be a higher energy site than

the grain boundary.

Our model can be applied to other researcher�s data.Weiss [18] observed a triple junction void in surface pas-

sivated Cu films, whereas grooving was observed in thenon-surface passivated Cu films. The term, ‘‘surface

passivated film’’, indicates that there is no diffusion from

the surface,which is similar to the casewhere the grain size

is extremely small, because the contribution made by the

surface to the atomic transport is small. This implies that

the d=hf value is practically 0, and so the surface damage

should consist of triple junction voids. For the other case,

we calculated the value of d=hf , and it fell within the rangeof the grain boundary groove case (Fig. 5).

It should be noted that the type of damage can in-

fluence the reliability of the film or interconnect. The

triple junction void is deeper than the grain boundary

groove. In the fabrication of damascene Cu intercon-

nects, a deep triple junction void can survive as a surface

defect even after the process of chemical–mechanical

polishing (CMP), subsequently causing more seriousreliability issues than a shallow grain boundary groove.

Therefore, our model is very important, in that it offers a

practical means of controlling the damage morphology

and increasing the reliability of Cu interconnects.

We believe that our model can be used for the predic-

tion of damage morphologies in other metal films, where

both surface and grain boundary diffusions exist. This

model can be applied to access other reliability issues,such as current leakage when the damage is profound or is

propagated throughout the entire thickness of the film.

5. Conclusion

1. After annealing the Cu films at 435 �C for 1 h,

two types of stress-induced surface damage were

2440 H. Park et al. / Acta Materialia 52 (2004) 2435–2440

observed. In some films, stress-induced voids were

formed at the triple junction, whereas in other films,

stress-induced grooves were formed along the high

energy grain boundaries.

2. Surface damage occurred solely at the high anglegrain boundaries (>15�), with the exception of the

twin boundaries and triple junctions, which are

known to be higher energy sites compared to the

grain boundaries.

3. It was found that the d=hf value, which is determined

by three microstructural factors, the grain size (d),film thickness (h), and the fraction of high energy

grain boundaries (f ), can act as a parameter in theprediction of the type of surface damage that is likely

to be sustained. Those films with a large d=hf value

tended to form grain boundary grooves, while those

films with a small value tended to form triple junction

voids.

4. The fact that the d=hf value acts as a parameter indi-

cates that the damage type is controlled by the com-

petition between the surface and grain boundarydiffusions, and that the major portion of the high en-

ergy grain boundary area, which acts as an atomic

sink site, gives rise to the formation of grain bound-

ary grooves, while the major portion of the surface

area, which acts as a source site, leads to the forma-

tion of triple junction voids. It is believed that our

model is very important, in that it offers a practical

means of controlling the damage morphology and in-creasing the reliability of Cu interconnects and other

metal films.

Acknowledgements

The authors grateful acknowledge EBSD measure-

ments by Prof. K.H. Oh and Dr. J.-H. Cho. This workwas supported by the Center for Electronic Packaging

Materials of the Korea Science and Engineering

Foundation.

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