Residual Stress and In-situ Thermal Stress Measurements

Materials Science Research International, Vol.7, No.1 pp. 54-60 (2001)

General paper

Residual Stress and In-situ Thermal Stress Measurements

of Copper Films on Glass Substrate

Takao HANABUSA* and Masayuki NISHIDA***Faculty of Engineering, Tokushima University Minamijosanjima, Tokushima 770-8506, Japan

**Kobe City College of Technology Gakuen-Higashi, Nishi-ku, Kobe 651-2194, Japan

Abstract: The present study investigates the behavior of residual stress and thermal stress in thin copper films

with X-rays. The copper films were deposited on a glass substrate by radio frequency (RF) sputtering. In-situ

thermal stress measurement was also made on copper films during heat cycles. The residual stress of

as-deposited state was tensile regardless of the conditions of film preparation. Early in the cooling stage, thermal

stress behaves in an elastic manner so as to reduce the tensile stress until it stabilizes close to zero. In the cool-

ing stage, little increase in the thermal stress was observed above 100•Ž and the hysteresis of thermal cycle was

very small.

Key words: Cu film, Residual stress, In-situ thermal stress, X-ray stress measurement

1. INTRODUCTION

In thin films deposited on various kinds of sub-strates by physical vapor deposition (PVD) or chemical vapor deposition (CVD), residual stresses are developed by extrinsic and intrinsic reasons [1-5] . The former re-fers to a thermal residual stress which occurs because of the difference in coefficients of thermal expansion be-tween the film and the substrate. The latter originates in a film during deposition by the introduction of vacan-cies and interstitial atoms and for other reasons depend-ent on several conditions of deposition.

These stresses directly influence the reliability of the film itself or the film/substrate system. For example, if the residual stress is very large, micro cracks may de-velop in the film and delamination of the film may occur [6]. Furthermore, a temperature change of the film/substrate system develops a thermal stress in the film due to the difference in coefficients of thermal ex-pansion between the film and the substrate [3-5] . This thermal stress will give rise to plastic yielding and creep deformation of the film at high temperatures.

In recent LSI technologies, downsizing of aluminum lead lines in devices has opened the problem of stress migration and electromigration; breaking the lines and short-circuiting between the adjacent parallel lines. Early efforts to overcome these migrations were cen-tered on giving an antimigrating ability by alloying [7]. Much attention, however, has been paid to the ap-plication of copper, instead of aluminum, to improve electric resistibity and resistance to migration [8]. Knowledge of internal stresses in thin copper film structures is essential in understanding the film proper-ties, such as stress migration.

The purpose of the present study is to gain a basic understanding of internal stresses in copper films depos-

ited on a glass substrate. The crystal orientation of

copper films deposited by radio frequency (RF) sputter-

ing was investigated by X-ray diffraction. Residual

stresses after deposition and in-situ thermal stresses dur-

ing the thermal cycles were then observed by the

two-tilt X-ray stress measuring method, and surface

morphological structure after heat cycles was observed

by scanning electron microscope (SEM).

2. EXPERIMENTAL METHOD

2.1. Deposition of Copper Films

Table 1 shows conditions under which copper films

were deposited onto a Corning 7059 (Borosilicate glass)

substrate with thickness of 0.5mm. Deposition was

made by RF sputtering in an argon gas atmosphere. The

gas pressure, sputtering power and sputtering time were

controlled in accordance with the conditions given in

Table 1. The thickness of copper films in the present

investigation was between 0.5 and 3ƒÊm which de-

pended on the deposition conditions. No external heat-

ing was applied and the substrate temperature was not

measured in this experiment.

Table 1. Deposition of copper films.

Received April 12, 2000

Accepted January 9, 200154

In-situ Thermal Stress Measurement of Copper Films

2.2. X-ray Measurement of Crystal Orientation and

Residual Stress in Copper Films

Crystal orientation and residual stresses in the pre-

pared copper films were investigated by X-ray diffrac-

tion with CuKa characteristic radiation. As will be

shown later, the copper films have strong {111} texture;

therefore, the conventional sin2ƒµ method [9] cannot

be applied to determine residual stresses. In the present

case, the two-tilt method [10] was used to measure the

331 diffraction peaks appearing at ƒÕ1=22,0•K and

ƒÕ2=48.5•K. The formula calculating the stress values is as

follows:

(1)

where S44 is the elastic compliance of copper single

crystal and values 1.328•~10-5/GPa, ƒÃ(ƒÕ1) and ƒÃ(ƒÕ2)

are lattice strains at the angles of ƒÕ1 and ƒÕ2, respective-

ly. Further, the equiaxial plane stress state was assumed

throughout this experiment, similar to the case of alu-

minum and TiN films [11, 12].

The position of the diffraction line was determined

by the half-value-breadth method for the Ka1 component

after separating the Ka doublet. Repeated measurements

showed that the error of the determination of peak posi-

tion was below 0.012•K in 2ƒÆ.

In order to measure in-situ thermal stresses in the

film, a high temperature vacuum furnace was mounted

on an ƒ¶-diffractometer. The sample was heated in the

heat cycles from room temperature to adjusted maxi-

mum temperatures up to 500•Ž. The thermal stress was

measured at 100•Ž intervals. After reaching the desired

temperature, the specimen was kept at this temperature

for 1.2ks before measurement was started. The rate of

heating and cooling was 1.67•Ž/s. Table 2 shows the

conditions of X-ray stress measurement used in the

present investigation.

X-ray penetration depth for the ƒ¶-diffractometer

method is expressed as follows:

(2)

where ƒÆ is the Bragg angle, ƒÕ is the inclination

angle of the diffraction relative to the normal of the

specimen surface, and ƒÊ is the linear absorption factor.

Table 2. Conditions of X-ray measurement.

In the case that CuKa characteristic X-rays penetrate

into copper material, ƒÊ is 472.192cm-1. At the angles of

ƒÕ p=22.0•K and 48.5•K, T is estimated to be 8.73 and 4.89

ƒÊm, respectively, for the 331 diffraction. Since the cop-

per film was 0.5•`2.4 ƒÊm in thickness in the present in-

vestigation, information on the lattice strain is the aver-

age throughout the entire thickness of copper films.

There may be a residual stress gradient in the film. If

the lattice strains can be measured at any ƒÕ-angle as in

the sin2ƒµ method, some information about the gradient

can be deduced from the non-linear relation in the

e-sin2ƒÕ diagram. In the present study, however, no in-

formation on the stress gradient could obtained because

the lattice strains were measured only at two ƒÕ-angles.

3. EXPERIMENTAL RESULTS AND DISCUSSION

3.1. Deposition of Copper Films

Figure 1 shows the dependence of the sputtering

power and the argon gas pressure on the thickness of copper films deposited for 1.2ks. The film thickness

increases almost linearly with sputtering power and is

weakly depend on the argon gas pressure. Another ex-

periment showed that the film thickness increases lin-

early with the sputtering time; the rate of thickness de-

velopment was 1.7•~10-3ƒÊm/s under the sputtering con-

ditions of 0.26Pa and 400W. In the following, the

films deposited at 0.26Pa and 400W were used in the

experiments. The deflection of the substrate was ignored

because the thickness of the films was only a few hun-

dredth that of the substrate.

3.2. Crystal Orientation of Copper Films

Figure 2 shows the diffraction profile obtained from

the copper film deposited under the conditions of argon

gas pressure of 0.26Pa, RF power of 400W and sput-tering time of 1.2ks. The thickness of this film was 2.0

m. Large (111) type diffraction and small (100) type

diffraction are observed in this figure. The feature is

Fig. 1. Dependence of RF sputtering power and argon

gas pressure on film thickness.

55

Takao HANABUSA and Masayuki NISHIDA

Fig. 2. Diffraction profile obtained from a copper film deposited under 0.26Pa, 400W and 1.2ks.

Fig. 3. Rocking curve for 311 diffraction: The specimen is the same with Fig. 2.

qualitatively the same as for the other films deposited

under different sputtering conditions. These results mean

that the copper film deposited on a glass substrate by

RF sputtering has a strong [111]-texture and weak

[100]-texture, i.e., [111] and [100] crystal directions

orient themselves preferably along the normal of the

film surface.

When [111] orientation lies parallel to the surface

normal, {331} crystallographic planes appears at angles

22.0•K and 48.5•K according to the cubic crystal structure.

Figure 3 shows a rocking curve for 331 diffraction,

where 2ƒÆ-axis was fixed at 137•K and ƒÆ-axis (ƒÕ-axis)

was scanned independently. The positions of four inten-

sity maxima which are observed in this figure almost

coincide with the above two crystallographic angles,

which indicates [111]-texture exists in the film.

3.3. Residual Stress in As-Deposited Films

Residual stress of as-deposited state was measured

for the specimens deposited under various conditions.

Three measurements were made at the same position for

each sample: the maximum difference in the results

among them did not exceed 20MPa throughout the pre-

sent experiment.

Figure 4 shows the results of residual stresses

against RF sputtering power at four argon gas pressures.

Average values of each three measurements are plotted

in this figure. Residual stresses in as-deposited state are

Fig. 4. Dependence of RF sputtering power on residual stress in copper films.

Fig. 5. Dependence of argon gas pressure on residual stress in copper film.

tensile in all cases, ranging from 100 to 250MPa.

When the gas pressure is low (0.26Pa), tensile stress is

fairly large and exhibits a small RF power dependence;

increasing with the increasing RF power. On the other

hand, tensile residual stresses show a very large RF

power dependence at higher gas pressures.

Figure 5 shows the dependence of argon gas pres-

sure on residual stress in the films. When the RF power

is 400W, residual stresses exhibit fairly high value of

about 250MPa and are almost independent of argon gas

pressure. When the RF power is 100W, however, re-

sidual stresses are low and tend to decrease with in-

creasing argon gas pressures.

From these two figures, it is observed that the resid-

ual stresses after deposition are tensile but their value is

influenced by the sputtering conditions such as RF sput-

tering power as well as argon gas pressure. Additional

experiments showed that the residual stress is little in-

fluenced by sputtering time, i.e., it was almost constant

around 200•`230MPa for the O.6•`1.8ks of sputtering,

corresponding to 0.5•`3.1ƒÊm in thickness, at RF power

56


of 400W and argon gas pressure of 0.28Pa.

3.4. In-situ Thermal Stress Measurement of Copper

Films

In-situ thermal stress was measured for the copper

film during the heat cycles between room temperature

and desired temperatures. The deposition conditions of

the films used in this experiment were as follows: argon

gas pressure 0.26MPa, sputtering power 400W and

sputtering time 1.2ks. Figure 6 shows a series of results

measured in the film which was deposited for 0.3ks.

First, the sample was heated to 100•Ž and then cooled

down to room temperature. The thermal stress became

almost zero at 100•Ž and then increased to 200MPa

after cooling. In the second heat cycle to 200•Ž, the

stress rapidly decreased in the initial stage of heating to

100•Ž and then stayed close to zero level up to 200

•Ž. In the cooling stage, the thermal stress changed al-

most the same way as in the heating stage but was

slightly greater. Further measurements of thermal stress

change were sequentially made during each heat cycle

to 300, 400 and 500•Ž. Typical features are as follows:

the hysteresis in the variation of thermal stress in the

copper film is smaller than that in the aluminum film

and there is no significant thermal stress development

above 200•Ž. Figure 7 shows similar results for the

copper film deposited for 1.2ks.

The main reason for the thermal stress development

is the difference in the coefficients of thermal expansion

of the film, afilm, and the substrate material, asub. If there

is a temperature change of ƒ¢T, the thermal stress devel-

opment is calculated by the following equation:

Δσ(3)

where Efilm and vfilm are Young's modulus and Poisson's

ratio of the film, respectively. In the present calculation,

the bulk data were used for these parameters: afilm=

16.8•~10/•Ž, asub=4.6•~10-6/•Ž, Efilm=128GPa and vfilm=

0.308.

The calculated value of ƒ¢ƒÐ/ƒ¢T based on these data

is drawn in Figs. 6 and 7 as an initial slope in the ther-

mal stress change. A fairly good coincidence between

experimental and the theoretical data means that the

copper film behaves in an elastic manner in the temper-

ature range below 100•Ž. On the other hand, the ex-

periment shows no significant increase in thermal

stresses in the film above 100•Ž up to the maximum

temperature in the heating processes.

It is shown that the yield strength of oxygen-free

copper abruptly decreases in the temperature range of

Fig. 6. In-situ thermal stress measurement of the specimen deposited for 0.3ks.

Fig. 7. In-situ thermal stress measurement of the specimen deposited for 1.2ks.

57


200•`300•Ž, from 400MPa of in room temperature to

below 50MPa at 300•Ž [13]. This may explain the

very small thermal stress in the film above the anneal-

ing temperature of 200•Ž. A creep mechanism by

atomic diffusion is another way to relax the stress in the

films. If the temperature is high enough, the atoms can

be transferred through the surface, grain boundaries, and

grains themselves. Also, the high temperature creep may

occur by dislocation climb controlled by atomic diffu-

sion. Softening of the substrate may be another cause of

stress relaxation in copper films. However, we do not

have the data on the strength of Corning 7059 at high

temperatures. These findings show that the film behaves

in such a way as to easily decrease internal stresses by

transforming elastic strain into plastic strain, particularly

in high temperature regions.

It seems strange that the thermal stresses at the ini-

tial cooling stage at higher temperatures stayed in a

compressive state. In the present study, the sample was

heated from the back surface of the substrate, with the

heat in the sample radiating from the film surface. This

results in the temperature gradient from the substrate to

the copper film, which may cause a deflection of the

substrate so as to develop a compressive stress in the

film.

Another typical feature is that the thermal stress in

the copper film behaves almost the same in the heating

and cooling processes, so that the hysteresis observed is

insignificant compared with the results of aluminum

films deposited on a silicon substrate [10]. The reason

of this behavior is considered to be the relaxation of

elastic strain due to several sources described above.

3.5. Residual Stresses after Heat Cycles and Mor-

phological Observation of Film Surface

Figure 8 shows the change in residual stresses after

heat cycles. The residual stress in the films which were

deposited for 0.3, 0.6, and 1.2ks increases with anneal-

ing temperatures up to 200•Ž and then decreases there-

after. However, the residual stress of the film deposited

for 1.8ks remains almost constant at about 150MPa.

The main reason of residual stress development is also

Fig. 8. Change in residual stress after heat cycles.

the thermal stress estimated by Eq. (3), if the film de-

forms only in an elastic manner. In the case of copper

film, however, an atomic migration will occur in several

ways in a high temperature range, changing the surface

morphology and structure of films and relaxing residual

stresses in the films as a result.

A surface morphological structure was observed in a

sequence of annealing temperatures by scanning electron

microscopy. Figure 9 shows the observed surfaces of

the films deposited for (a) 0.3, (b) 1.2 and (c) 1.8ks

followed by the annealing at 300•Ž. There was no sig-

nificant change from the as-deposited film surfaces, in-

dicating that heating up to 300•Ž has no effect on the

film surface as far as we can see in the SEM photo-

graphs. The film surface after 0.3ks deposition is very

smooth and a correct focusing was attained only from

the existence of a trace of scratches. As the depositing

time increases, the surface becomes rougher and tiny

crystals are detectable.

Figure 10 shows a change in the surface of the film

deposited for 0.3ks, after different annealing tempera-

tures. Although the annealing at 300•Ž showed no sig-

(a) (b) (c)

Fig. 9. Surface observation of the specimen annealed at 300•Ž: Deposited for (a) 0.3ks, (b) 1.2ks , (c) 1.8ks.

58


(a) (b) (a) (b)

Fig. 10. Surface observation of the specimen deposited

for 0.3ks: Annealed at (a) 400•Ž, (b) 500•Ž.

Fig. 11. Surface observation of the specimen deposited

for 1.2ks: Annealed at (a) 400•Ž, (b) 500•Ž.

(a) (b) (c)

Fig. 12. Enlarged photographs for the specimens annealed at 500•Ž: Deposited for (a) 0.3ks, (b) 1.2ks, (c) 1.8ks.

nificant difference from the as-deposited state, small

hillocks began to appear on the surface when the film

was heated to 400•Ž and these hillocks increased in

number and size as the annealing temperature was

raised to 500•Ž.

Figure 11 shows a morphological change at different

annealing temperatures for the film deposited for 1.2ks.

Compared with the former case, it can be observed that

voids or cavities appeared in the grain boundaries.

These cavities grew to form grain boundary cracks

when annealed at 500•Ž. Magnified photographs in Fig.

12 show clear evidence of the grain boundary cracks. In

the case of the film deposited for 0.3ks (a), hillocks are

visible on the surface and grain boundary voids are rare,

but in thicker films which were deposited for 1.2ks (b)

and 1.8ks (c), grain boundary voids and cracks are

clearly seen.

4. CONCLUSION

Residual and in-situ thermal stresses were measured

by X-ray diffraction in copper films deposited on a

glass substrate. The results obtained are as follows:

(1) Tensile residual stresses ranging from 100 to 250

MPa are developed in the copper films deposited on a

glass substrate by RF sputtering; they are dependent on

sputtering conditions such as argon gas pressure and RF

sputtering power.

(2) In-situ thermal stress measurement has revealed that

the initial tensile stress decreases elastically according to

the predicted thermal stress development in the heating

stage leading to 100•Ž, and then stays close to zero

level of the thermal stress up to the maximum tempera-

ture of the cycles. In the cooling stage, only a slight in-

crease in the thermal stress is observed in the tempera-

ture range above 100•Ž, exhibiting a very small

hysteresis in the whole thermal cycle.

(3) It is found from SEM observations that hillocks and

grain boundary cracks are generated after the heat treat-

ments above 400•Ž.

Acknowledgment-Financial support of Grant-in-Aid

for Scientific Research (C) from The Ministry of Edu-

cation, Science, Sports and Culture is greatly acknowl-

edged. The authors also express their gratitude to Mr.

Yuh Yamasaki, undergraduate student of The Faculty of

Engineering, Tokushima University, now at Tokushima

City Office, for the preparation of specimens.

59


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Residual Stress and In-situ Thermal Stress Measurements