~ Pergamon

0008-4433(95)00015-1

Canadian Metallurgical Quarterh', VoL 34, No. 3, pp. 28%292, 1995 Copyright ~C 1995 Canadian Institute of Mining and Metallurgy

Printed in Great Britain. All rights reserved 00084433/95 $9.50 + 0.00

C O R R E L A T I O N BETWEEN SPECIAL G R A I N B O U N D A R I E S A N D E L E C T R O M I G R A T I O N B E H A V I O R OF

A L U M I N U M THIN FILMS

K. T. LEE, '~ J . A. S Z P U N A R , t A. M O R A W I E C , t D. B. K N O R R ~ :

and K. P. RODBELL§ "['Department of Mining and Metallurgical Engineering, McGill University, 3450 University Street,

Montreal, Quebec, Canada, H3A 2A7 :~Materials Engineering Department, Rensselaer Polytechnic Institute, Troy, NY 1218~3590,

U.S.A, §IBM, T. J. Watson Research Center, P.O. Box 218, Yorktown Heights, NY 10598, U.S.A.

(Recewed 4 Auclust 1994; in ret,isedJorm 15 December 1994)

Abstract---The texture in thin films develops during processing steps such as deposition and annealing. Recent studies show that texture plays an important role in stress voiding, thermal hillock formation, grain collapse and electromigration failure. Specifically, electromigration failure depends on the grain misorientation distribution, which describes the probability of different grain boundaries and, therefore, links the grain boundary structure to the mass transport that takes place primarily along the grain boundaries. To understand the relationship between the grain misorientation and electromigration lifetime in aluminum thin films, the texture was measured on three sets of films from different manufacturing conditions. The frequency of occurrence of coincidence site lattice (CSL) grain boundaries, which represent special misorientations between grains, was obtained, and electromigration tests were done for all three conditions. Experimental results show that the lifetime of patterned films increases as the amount of { 111 } texture and the frequency of CSL boundaries increased.

R6sum6~La texture des couches minces se d6veloppe pendant des 6tapes de traitement telles que le d6p6t et le recuit. Des 6tudes r~centes demontrent que la texture joue un r61e important dans l'annulation de la fatigue, la formation de pic thermique, la d6formation des grains et la rupture par 61ectromigration. Sp6cifiquement, la rupture par 61ectromigration d6pend de la distribution de m6sorientation des grains laquelle d6crit la probabilit6 de differents joints de grains et, donc, relie la structure des joints de grains au transport massif qui a lieu le long des joints de grains. Pour comprendre la relation entre la m6sorientation des grains et la dur6e de vie de l'61ectromigration dans des couches minces d'aluminium, on mesure la texture de trois couches fabriqu6s dans des conditions diffdrentes. Nous avons obtenu une fr6quence de l'occurence de joints de grains ~. r6seau de coincidence (CSL) qui repr6sente des m6sorientations sp6ciales entre les grains, et nous avons fait des tests sur l'61ectromigration dans les trois conditions. Les r6sultats exp6rimentaux ddmontrent que la dur6e de vie des couches modelis6s augmente quand la quantit6 de texture {1 11 } et la fr6quence des joints CSL augmentent.

1. I N T R O D U C T I O N

Because of the increasing sophistication in multilayer inter- connect structures of silicon integrated circuit chips, thin film multilayers are becoming smaller and more complex, which induces various types of failure. One of the important mech- anisms of failure in thin films is electromigration failure. Many factors influence the electromigration lifetime of thin films, including texture [1-4]. Texture in thin films develops during deposition processes such as evaporation, sputtering, chemical vapor deposition, and electrochemical deposition. In fact, it is rather difficult to deposit a sample without texture. Recent studies show that the activation energy for electromigration is

texture dependent and that a strong { 111 } texture in aluminum thin film improves the electromigration lifetime [5, 6].

During the last few years, the research on properties of coinci- dence site lattice (CSL) grain boundaries has intensified, because these boundaries are linked to special physical proper- ties of materials [7-9]. Few systematic studies have investigated the effect of CSL boundaries on electromigration failure of thin films. Fionova [10] showed that a large number of special grain boundaries (CSL boundaries) in annealed films may in- crease the electromigration lifetime of thin films; however, it is not clearly understood what role the CSL boundaries play in electromigration failure. The main objective o f this study is to improve the understanding of the influence of

287

288 K. T LEE et a/.: ELECTROMIGRATION BEHAVIOR OF Al THIN FILMS

4, 0” -90’

c&=0:

l&=20’

0

4&30’

(a> $,=lO”

La_

I I 2 30 60 2

II----

2\

0 -

I- I , I

42-Qo” ODF Max :

0 1

c$,=80” 42-w ODF Max :

25.32 Cmicws :

1, 10, 20

90”. I , ,

4,=20’ 1

I 42=800 4,-90” ODF Max :

5.94 Contours :

1.3, 5

Fig. I. Orientation distribution function (ODF) for specimens (a) PIB-2/I. (b) PIB-212, (c) SP-2.

K. T. LEE el al.: ELECTROMIGRATION BEHAVIOR OF AI THIN FILMS 289

CSL boundaries in electromigration failure in aluminum thin fihns.

2. EXPERIMENTAL PROCEDURE

Pure a luminum films were deposited on oxidized silicon by partially ionized beam (PIB) deposition [11] and by sputtering. During PIB deposition, a small fraction of evaporated alumi- num atoms was ionized while a potential was applied to the substrate. Both neutral a luminum atoms and selfqons were deposited on the substrate simultaneously. The energetic ions provided enhanced surface mobility during deposition and effective in-situ cleaning. The substrate bias potential (ion energy) was fixed at 2 kV. Two specimens were deposited using the PIB technique with two different ion contents: PIB-2/1 with 1% ion content and PIB-2/2 with 2%. The substrate was held at room temperature, background pressure was 10 4 Pa during deposition, and deposition rate was approximately 10 A/s. The sputtered film, SP-2, was deposited at 2 kV. The a luminum films were 1 /tm thick. The samples obtained using the three different processing conditions were annealed at 40OC for an hour in forming gas (90% N2-10% Hz)

The texture of the films was measured using a Siemens texture measuring system, consisting of a computer controlled hori- zontal diffractometer with an Eulerian cradle. A molybdenum x-ray source was used. Texture data were collected by step- scanning every 5 degrees over the range 0 ~< ~ ~< 85 and 0~< fl ~< 355' for the {11l} pole figure. Counting time for adequate counting statistics was 2 s/step. The raw data were corrected for defocusing, background, and absorption effects, and then plotted in the form of a pole figure. The corrected data were used as input to calculate the coefficients of the orientation distribution function (ODF).

The O D F data were used to calculate the distribution of CSL boundaries. The computer program for this calculation picks two orientations with probability determined by the ODF, cal- culates their misorientation and checks if this misorientation is close to given sigma misorientation. The accuracy of coinci- dence is defined by Brandon's criterion [12],A0 = 15 2 -~ ~-. The calculation of the frequency of occurrence of CSL bound- aries is based on the assumption that the material is spatially disordered, i.e. features such as correlation between orientation

of neighbouring grains, inhomogeneities0 clustering and grain size effects are not taken into account. The only fact that influ- ences this calculation is the orientation distribution. The cal- culation is also carried out under the assumption that all of the grains are same size. according to which the frequency obtained represents either the numbers of grain boundaries of a certain misorientation or the total area of such boundaries. Details of this method are described in another paper [13]. Ten thousand pairs of orientations were generated from the ODF and their misorientations were classified as CSL or non-CSL boundaries. The frequency of occurrence of 23 -227 CSL boundaries was calculated.

For an electromigration analysis, thin films were patterned into test structures by standard silicon processing techniques. At least ten unpassivated lines 1.8/~m wide were tested at a cur- rent density of 1 x l0 b ,~/cm 2 and at temperatures of 150, 175, 200, 225 and 253' C. Data for 225'~C tests are used in this study.

3. RESULTS AND DISCUSSION

The results of the texture measurements for the PIB-2/1, PIB- 2/2 and SP-2 are shown in Fig. 1. All three conditions have a strong { l I I I fibre texture as seen along the 7 fibre (~b = 55.4 ~, 0 < 4q < 90 , /(y) = ~h2 = 45) . To clarify the shape of the ~t I 1 ] I fibres, the ~ = 45 sections of the ODF are presented in Fig. 2 for each specimen. Figures 1 and 2 show that PIB-2/1 has the sharpest [1111 fibre texture and that the maximum intensity, which is 73 times random, is located in the centre of the fibre. PIB-2/2 also has a maximum intensity of 25 in the centre, although this maximum is much weaker than that for specimen PIB-2/1. For specimen SP-2, the fibre texture is weak and spread out where the maximum intensity of 6 is located about 10 from the centre of the {1ll} fibre (@=55 .4 °, ~2 = 45 ). This type of texture is referred to as near (111) [14].

Figure 3 illustrates the distribution of CSL boundaries up to 227 for all three specimens. In all specimens, the frequency of occurrence of the 23 and 27 boundaries is relatively higher than other CSL boundaries. In addition, the thin films deposited by the partially ionized beam technique (PIB-2/1, PIB-2/2) have higher frequencies of 23, 27, 213b, 219b and E21 a boundaries than the sputtered film (SP-2). PIB 2/1 has a higher frequency of these specific CSL boundaries than PIB-2/2, especially for

0 o 90 °

90

qb1=45 °

60

30 ~ 2

dp1=45"

20

~t=45 °

5

3

PIB-2/1 PIB-2/2 SP-2

Fig. 2. (9~ = 45 sections of the orientation distribution ['unction (ODF) for each specimen.

290 K.T. LEE et al.: ELECTROMIGRATION BEHAVIOR OF AI THIN FILMS

g

0 - J ? - = - , ~ 3 5

I ~ PIB 2/1

Pm z/2

C--] sP2

7 9 11 1 3 a 1 3 b 15 1 7 a l T b 1 9 a 1 9 b 2 1 a 2 1 b 23 2 5 a 2 5 b 2 7 a 2 7 b

value

Fig. 3. Distribution of £3 £27 CSL boundaries in each specimen.

the Z7 and Z19b boundaries. Specimen SP-2 has a higher fre- quency of E9, 5'15, E17a, Zl7b and Z21b boundaries.

Figure 4 shows the frequency of occurrence of the most prevalent CSL boundaries as a function of the maximum ODF intensity found for the { 111 } texture component of each speci- men. Figure 4(a}-(c) shows the changes in frequency of occur- rence of CSL boundaries having a common ( 111 ), (110) and

100) rotation axis, respectively. The frequency of the ( I l l ) - type grain boundaries (£3, Y~7, Z l3b, 5" 19b and 5`21 a) increases as the intensity of the { 111 } fibre texture increases [Fig. 4(a)]. The increase of the 5`7 boundaries is the most significant. The frequency of the 23 boundaries in the specimen PIB-2/2 was much higher than that in specimen SP-2; however, there is only a small difference between specimens PIB-2/1 and P1B-2/2. In specimen SP-2, which has the weakest { l 11 } texture, there is a higher amount of ( 110)-type CSL boundaries, such as 5`9, Z 11, 5`1% and 5`27a boundaries [Fig. 4(b)]. In the specimen PIB-2/2, the frequency of these boundaries decreases as the maximum intensity of { 111 } fibre texture increases. However, it increases slightly for the specimen PIB-2/I. The frequency changes of (100)-type CSL boundaries are shown in Fig. 4(c). As the maximum intensity of the { 111 } texture increases, the frequency of the Z5 CSL boundaries increases, while the frequencies of some other boundaries decrease.

The results of the electromigration tests are shown as a cumu- lative failure distribution in Fig. 5 and the frequency of occur- rence of prevalent CSL boundaries having (111), (110) and (100) rotation axis is illustrated as a function of the median time-to-failure (ts0 value) in Fig. 6(a)-(c). The frequency change of CSL boundaries as a function of ts0 value is very similar to the frequency change as a function of intensity of { 111 } texture

component (Fig. 5). The results of the electromigration test are compared to the total frequency of CSL boundaries in Fig. 7. Specimen PIB-2/1 has the highest percentage of CSL bound- aries, 14.9%, as well as the longest median time-to-failure, t~0 = 736 h. Specimen PIB-2/2 has 11.9% of CSL boundaries and a ts0 value of 235 h. Specimen SP-2 has the shortest lifetime, t50 = 106 h, and the lowest percentage of CSL boundaries, 10.4%. Several studies [12, 15, 16] have calculated the frequency of CSL boundaries in materials with a random grain orientation If(g) = 1]. A total of 9.2% of the boundaries were found to be Z3-Z27. This result indicates that the grain boundary texture in SP-2 deviates only slightly from the distribution expected in a random material. Modelling of { 111 } fibre textures [17] shows that the proportion of CSL boundaries continues to increase as the {111} distribution tightens beyond the levels in P1B-2/1. The trend of an increasing fraction of E3, Z7, E13, £19 and Z21 boundaries develops further. Electromigration results on very tightly textured aluminum alloy films [18] demonstrate the continuing trend of substantially longer times to failure with increasing fraction of CSL boundaries.

It is clear from this comparison that there is an association between the time-to-failure and the frequency of occurrence of CSL boundaries; a higher percentage of CSL boundaries correlates with longer lifetime in patterned lines. It is known that the diffusivity along the large-angle grain boundaries depends on the misorientation of grains [19, 20] and that the diffusivity of grain boundaries with special misorientation (CSL boundaries) is less than that for general grain boundaries by several orders of magnitude [19]. It seems that a relatively large frequency of CSL boundaries in the specimen PIB-2/1 has con- tributed to improvement of its electromigration lifetime. The

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K. T. LEE et al.: ELECTROMIGRATION BEHAVIOR OF A1 THIN FILMS

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Fig. 4. Frequency of selected CSL boundaries as a function of ODF maximum for: (a) (l I 1)-type CSL boundaries; (b) ( 110)-type CSL boundaries; (c) (100)-type CSL boundaries.

¢n

.c: v

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Fig. 5. Electromigration results for three specimens.

specific role of particular grain boundaries is not known. Detailed analyses of the grain orientation from many failure sites and the diffusivity along CSL boundaries are required.

4. C O N C L U S I O N S

The role of special grain boundaries in electromigration behaviour of aluminum thin films was investigated for three thin film specimens with different textures.

The following conclusions are drawn from this study:

1. Thin films deposited using the PIB technique have a stronger { 111 } fibre texture and their median time-to-failure is longer than in the thin film deposited by sputtering.

2. The frequency of occurrence of CSL boundaries for speci- men PIB-2/1 is higher than that in specimen PIB-2/2. and lowest in specimen SP-2. It seems that the lifetime of thin films increases as the number of special boundaries and the strength of { 111 } texture increases.

3. As the intensity of the { 111} texture increases, the frequency of occurrence of CSL boundaries increases. The elec- tromigration failure in aluminum thin films is correlated with

e3

g ¢.

(a)

f.l.,

~ < I I I >

K. T. LEE et al.: ELECTROMIGRATION BEHAVIOR OF AI THIN FILMS

/ ' / J /

o, , 2 j

/,

200 400 600

t~ value (hours)

,2~- < I I 0 >

~" 1 ",,

+ Sigma 3 ~ ',,

- - ~ Sigma 7 ~ o.a- ',,,

-~i-- Sigma lab ~ o.e~ %\ " , . / ~ '~ J -

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- ~ S i g m a 21a 0.4~ ; ~ .

0,2 ~ 5 ~ . ~ = ~ _ _ / ~ 4 ~ 3

800 1000 200 400 600 800

i o

(b) t~ value (hours)

S i g m a 9

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o ~ : - ~ Sigma 5

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I -3~ Sigma 17a ! o4 t @ Sigma 25a

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0 200 400 600 800 1000

(c} ts0 value (hours)

Fig. 6. Frequency of CSL boundaries as a function of median time-to-failure (tso value) for: (a) (111 )-type CSL boundaries; (b) (1 l O>-type CSL boundaries; (c) (100>-type CSL boundaries.

3° i 25

I

2 0 -

15

10

!

PIB z/1

292

Total CSL (%)/looo

tso (h) ~ j S00 m

O

600 .~ [--

400

Lti°° o PIB 2 / 2 SP 2

Fig. 7. Comparison of the total frequency of CSL boundaries with the ts~j value of each specimen.

the f requency o f occur rence o f CSL bounda r i e s and the s t reng th o f { 111 } texture.

R E F E R E N C E S

1. M. J. Attardo and R. Rosenberg, J. Appl. Phys. 41, 2381 (1970).

2. E. Nagasawa, H. Okabayashi, T. Nozaki and K. Nikawa, Pro- ceedings of The 17th Reliability Physics Symposium, p. 64. IEEE, New York, 1979.

3. K. P. Rodbetl, D. B. Knorr and D. P. Tracy, Proe. Mat. Res. Soc. Syrup. 265, 107 (1992).

4. H. Taniguchi, T. Ushiro, Y. Okamoto, Y. Akagi and M. Koba, Proc. Mat. Res. Soc. S.vmp. 280, 523 (1993).

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7. Tadao Watanabe, Mater. Sci. Forum 46, 25 (1989). 8. G. Palumbo, P. J. King, K. T. Aust, U. Erb and P. C. Lichtenberger,

Scripta. Metall. 25, 177 (1991). 9. J. Harase and R. Shimizu, Acta Metall. 40, 1101 (1992).

10. L. K. Fionova, O. V. Kononenko and V. N. Matveev, Thin Solid Fihns 227, 54 (1993).

11. S. N. Niu and T. M. Lu, J. Vac. Sci. Tech. A6, 9 (1988). 12. D. G. Brandon, Acta Metall. 14, 1479 (1966). 13. A. Morawiec, J. A. Szpunar and D. C. Hinz, Acta Metall. 41, 2825

(1993). 14. D. B. Knorr, Proc. Mat. Res. Soc. Syrup. 309, 75 (1993). 15. D. H. Warrington and M. Boon, Acta Metall. 23, 599 (1975). 16. A. Garbacz and M. W. Grabski, Acta Metall. 41,469 (1993). 17. A. Garbacz and M. W. Grabski, Acta Metall. 41,475 (1993). 18. H. Toyoda, T. Kawanoue, M. Hasunuma, H. Kaneko and M.

Miyauchi, 32nd Annual Proceedings Reliabili O, Physics, p. 178. IEEE/IRPS, NY, 1994.

19. H. Gleiter and B. Chalmers. Progr. Mater. Sci. 16, 142 (1972).

20. R. W. Balluffi, Metall. Trans. A 13, 2069 (1982).