Realistic electromigration lifetime projection of VLSI interconnects

ELSEVIER

Thin Solid Films 253 (1994) 508 512

Realistic electromigration lifetime projection of VLSI interconnects Hisao Kawasaki, Charles Lee, Tat-Kwan Yu Advanced Products Research and Development Laboratory, Motorola, 3501 Ed Bluestein Blvd., Austin, TX 78721, USA

Abstract

Tungsten plug contacts/vias electromigration experiments have been performed using a variety of test structures under different stress conditions. It was found that electromigration failures, failure mechanisms of tungsten plug contacts/vias, were strongly influenced by the test structures and stress conditions. This paper discusses the effect of test structures and stress conditions on electromigration failure for realistic lifetime projection of VLSI interconnects.

Keywords:Electromigration; Metallization;

Stress; Tungsten

1. Introduction

99.99 99.9

Through our extensive studies, electromigration of the tungsten plug contact/via is found to be the most critical factor in defining metallization reliability. Understanding of the reliability of the tungsten plug contact/via is essential in assuring the reliability of devices of 0.5 ~tm and smaller. Several new tungsten plug contact/via test structures have been developed for realistic projection of electromigration lifetime of circuits. In this paper, results obtained from a series of experiments are presented. In particular, the effects of test structures and stress conditions on electromigration failure are discussed.

2. Lifetime prediction/maximum current density calculation

Data obtained from accelerated tests are used to predict the lifetime of metallization, or circuits determined by electromigration. Also, for new devices with new metallization technologies, electromigration data are used to calculate the maximum current density which can be applied to the circuit design. Fig. l shows a typical failure distribution due to electromigration, which follows a lognormal distribution. Data are then analyzed if the metallization satisfies a reliability goal such as 1 FIT (failures in ten-to-the-ninth hours). The most commonly used model for lifetime prediction is Black's model [1] 0040-6090•94]$7.00 © 1994 - - Elsevier Science S.A. All rights reserved SSDI 0 0 4 0 - 6 0 9 0 ( 9 4 ) 04663-Q

99

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95

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~

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1000

105

107

109

Time to failure (hours)

Fig. 1. Typical electromigration failure distribution. Calculated failure distribution under normal operating conditions, Jt and T 1, is also shown.

Q

MTF=~sexp(~-~) where MTF is median

(1)

time to fail, A is a proportionality constant, J is current density, Q is activation energy for electromigration mass transport, k is Boltzmann's constant and T is temperature. Using this model, the lifetime of the metallization under operating conditions, which are typically at lower current density and lower temperature than accelerated conditions, will be calculated. The reliability goal of < 1 FIT indicates, for example, that it should take more than 105h to reach a cumulative failure of 0.01%. (time to 0.01% cumulative failure) is obtained using the Black's model and lognormal statistics as

TTFo.ov./,,

509

H. Kawasaki et al. / Thin Solid Films, 253 (1994) 508-512

Jo

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One O.6x0.6pm2 tungsten plug contact

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e x P L l k (~-~l T~)}

~,e"

+ sN~do.ov,/o[ > 105 h

(2)

where MTFacc is median time to fail obtained from an accelerated test, Jo and To represent current density and temperature used in the accelerated test, J1 and T~ represent current density and temperature for normal operation, s is a sample sigma of the failure distribution, and Nsd0.Ol % is the number of standard deviations for 0.01% cumulative probability. In Fig. 1, the expected failure distribution under normal operating conditions, at current density J~ and ambient temperature Tj, is also shown. With 105 h as TTFo.o~o/,,,the maximum current density, Jm~x can be calculated: .]max 2 = Jo 2

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Two 0.6xO.6pm2 tungsten plug contacts

Four 0.6x0.6pm 2 tungsten plug contacts

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Fig. 2. N e w l y designed tungsten p l u g contact electromigration structures w i t h one, two, and f o u r contacts. 70

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Fig. 3. Resistance change as a function of measuring current for Kelvin contact and new structure.

where t(a, b) is a factor of Students t distribution with one-sided confidence level a and degree of freedom b (N = number of samples), ~ is probability, z2(a, b) is a factor of Z2 distribution with one-sided confidence level a and degree of freedom b (N = number of samples).

3. Effect of test structures on electromigration failure

It is clear that data from accelerated tests have been used to determine reliability performance of the metallization, and also maximum current density applied for the new device design. In this section, we discuss how electromigration failures are influenced by the test structures used in the accelerated experiments. The samples used in the experiments were 0.6 ~tm x 0.6 ~tm Kelvin contact and new tungsten plug contact electromigration structures (the new structures shown in Fig. 2) fabricated using a 0.5 ~tm CMOS technology which utilizes titanuim salicide, tungsten plug contact/via, and the A1-Cu-based triple level metallization. Prior to the electromigration test, Joule

heating effect in the structures was evaluated by measuring the resistance for different measuring currents. Fig. 3 shows the total resistance of the structures (A1 line + W plug + n + junction) as a function of measurement current. While the new structure shows constant resistance up to 30 mA, the Kelvin contact with narrow junction width (2.0 lam) shows a large increase in resistance due to Joule heating. This results in a large temperature gradient in the Kelvin structure during electromigration, which is know to enhance a flux divergence in electromigration mass transport. Finally, the resistance shows an abrupt drop when the intrinsic carrier concentration exceeds the extrinsic carrier concentration in the junction [2]. This limits the stress current in the case where higher stress conditions are preferred. Fig. 4 shows a cross-sectional scanning electron microscopy (SEM) image for the failure site of the Kelvin contact stressed at a current of 15 mA and an ambient temperature of 200 °C. As seen in the figure, the failure occurs at the interface between the top of the tungsten plug and the A1. Conversely, as shown in Fig. 5, for the new structure that was tested under the same stress conditions as the Kelvin contact, the failure occurs in the AI line near the edge of the tungsten plug contact where the current crowding is found to be maximal, based on the finite element simulation as shown in Fig. 6(a). It was also found that the activation energy obtained using the Kelvin contact was different from

510

H. Kawasaki et al. / Thin Solid Films, 253 (1994) 508-512 4 • Tungsten plug contact A Kelvin tungsten plug

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1

Fig. 4. Cross-sectional SEM photograph for the failure site of the Kelvin contact stressed at a current of 15mA and an ambient temperature of 200 °C.

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Fig. 7. Arrhenius plots for Kelvin structure and the new structure.

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Fig. 5. SEM photograph of failure site for the one-contact structure stressed at a current of 15 mA and an ambient temperature of 200 °C. 4E6

A/cm

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A/cm 2

a)

O A/cm 2

b)

c)

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Fig. 6. Contour plot of current crowding in the AI line of the test structures obtained by finite element simulation for one contact (a), two contacts (b), and four contacts (c). that e x t r a c t e d f r o m the d a t a using the new structure. Fig. 7 shows the A r r h e n i u s plots for the Kelvin c o n t a c t a n d the new structure. The a c t i v a t i o n energies o b t a i n e d for the Kelvin c o n t a c t a n d the new structure are 1.1 a n d 0.7 eV respectively. The difference in the a c t i v a t i o n energy is u n d e r s t o o d as the difference in the failure

i i lO lOO Metal straplength (Urn)

lOOO

Fig. 8. Electromigration MTF as a function of metal strap length between the tungsten plug vias in the via chains for Via-1 (between metal-1 and -2) and Via-2 (between metal-2 and -3). mechanisms. T o f o r m the void at the interface between the tungsten plug a n d A1, A1 has to be t r a n s p o r t e d by lattice diffusion. The a c t i v a t i o n energy o f AI lattice diffusion is m e a s u r e d to be 1.2 eV [3], which is in g o o d a g r e e m e n t with the result f r o m the Kelvin contact. M e c h a n i c a l stress-induced A1 b a c k flow is a n o t h e r issue in the test structures. A n e x p e r i m e n t was perf o r m e d using the 0.6 ~tm x 0.6 lam tungsten plug via chain with v a r i o u s m e t a l s t r a p lengths between vias to investigate the effect o f the AI b a c k flow on electromig r a t i o n lifetime. Fig. 8 shows e l e c t r o m i g r a t i o n M T F as a function o f metal s t r a p length between the tungsten plug vias for the via-1 chains ( b e t w e e n metal-1 a n d -2) a n d the via-2 chains ( b e t w e e n metal-2 a n d -3). Lifetimes for the structures with a s h o r t e r strap length were f o u n d to be longer t h a n that with a longer s t r a p because o f the AI b a c k flow. The results suggest that e l e c t r o m i g r a t i o n failure is significantly affected by the test structures used in these experiments,

4. Effect of stress conditions on electromigration failure D e p e n d e n c e o f e l e c t r o m i g r a t i o n failure on stress conditions was investigated using the new structures with one, two a n d four c o n t a c t s (see Fig. 2), T h e test structures were designed to investigate the effect o f

H. Kawasaki et al. / Thin Solid Films, 253 (1994) 508 512

511

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I

Two contacts

go

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.....

-.-,

..........

, I,,,,

2

3

Time to Fail (arb.unit)

(a)

Fig. 9. Electromigration failure distribution of the tungsten plug contact structures with one, two, and four contacts stressed at a current of 15 mA and ambient temperature of 200 °C. 4.5 m

-- (~ -Area/Perimeter rule II ----X--- Experiment ~

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Fig. 10. Normalized maximum current based on the area/perimeter rule and experimental data. additional contacts on electromigration failure time. Fig. 9 shows failure distribution of the tungsten plug contact structures with one, two and four contacts (see Fig. 2). The test was performed at a current of 15 m A and an ambient temperature of 200 °C. Fig. 10 compares the m a x i m u m current that can be applied to circuits based on the traditional area/perimeter rule with experimental data. The traditional area/perimeter rule allows the m a x i m u m current in the ciruit to increase proportionally to the increase in either the contact area or the contact perimeter. It is clear from the results that this rule cannot be applied. This is attributed to the current crowding, as the finite element simulation results suggest (Fig. 6). After the experiment, failure analysis on the multiple tungsten plug contact structures was performed. There was found to be a difference in the failure mechanism between one-contact structures and two-/four/contact structures. Figs. 1 l(a), (b) and (c) show how A1 depletion due to electromigration occurs for one-, two- and four-contact structures, respectively. For the one-contact structure, A1 depletion occurs in the metal line. In contrast, AI depletion starts from the edge of the A1 line for two- and four-contact structures. This phenomenon was further investigated by performing another set of experiments using the structure with four contacts. The samples were tested at two different stress current

(c) Fig. 11. Optical photographs showing how AI depletion due to electromigration occurs for one*contact (a), two-contact (b), and four-contact (c) structures. values, 15 m A and 25 mA. The ambient temperature was kept the same, 200 °C, for both cases. Figs. 12(a) and (b) show the resistance change of the four contact structures during the electromigration test for the samples subjected to higher current stress (25 mA) and lower current stress (15 mA) along with optical photographs of the stressed samples. It was noticed that the sample subjected to the higher stress current showed catastrophic failure and the A1 depletion was located in the interior of the metal line, as observed for the one-contact structure. Conversely, the sample subjected to the lower stress current shows a step increase in resistance and A1 depletion from the edge of the metal line. Fig. 13 shows an SEM micrograph of the failure site after the electromigration test for the four-contact structure which has been stressed at a current of 15 mA. AI was completely swept away from the contact area.

512

H. Kawasaki et al. / Thin Solid Films, 253 (1994) 508 512 22 20

--~" 18 e

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Stress Time (arb. unit)

Fig. 14. SEM photograph of the four-contact structure after electromigration stress, showing AI depletion at the edge of the metal line.

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,~ 14

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Fig. 12. Resistance change as a function of electromigration stress time for the four-contact structure stressed at 25 mA (a) and 15 mA (b) along with optical photograph of failure site.

Fig. 13. SEM photograph of failure site for the four-contact structure stressed at lower current ( 15 mA). AI is completely swept away from contact area.

ing in void formation in the metal line. Conversely, when a lower current is applied, generated vacancies are fed by A1 diffused from the edge of the line, resulting in A1 depletion from the edge of the metal line. This can explain the step increase in resistance for the sample stressed at the lower current. As Fig.12(b) shows, the contacts failed one by one as A1 depletion proceeded. The results clearly suggest that the stress conditions can change the failure mechanism, and also that this phenomenon could not have been observed if the structures had a semi-infinite AI source for feeding, as is the case with the Kelvin structure.

5. Summary Electromigration failures of tungsten plug contacts have been shown to be strongly influenced by the test structures and stress conditions. For realistic lifetime prediction of the circuit at operating conditions, data have to be taken using test structures designed to minimize structural effects on lifetime and failure mechanisms. Also, the test conditions have to be set so that the failure occurs under the mechanism that is expected to be responsible for device failure under operating conditions.

Acknowledgements To investigate the failure process, an SEM micrograph has been taken from a sample which was subjected to the electromigration stress but did not fail. Fig. 14 shows an SEM micrograph of A1 depletion due to electromigration at lower current stress. Unlike the failure for the sample stressed at higher current, A1 depletion does not occur at the highest current density area but at the edge of the A1 line. The change in the failure site for different stress current is controlled by two processes, namely electromigration-induced vacancy generation and "feeding" of generated vacancies by A1 diffusion [4]. When a higher current is used, the feeding cannot keep up with vacancy generation, result-

The authors would like to thank the High-End MPU design group for test structure layout. Encouragement by Fabio Pintchovski and Bob Yeargain is greatly appreciated.

References [1] [2] [3] [4]

J. R. Black, IEEE Trans. Electron. Devices, ED-16 (1969) 338. S.-W. Sun et al., IEEE Electron. Device Lett., I0 (3), 1989, R. E. Hummel et al., J. Phys. Chem. Solids, 37 (1976) 73. C. K. Hu et al., 2nd Int. Workshop on Stress Induced Phenomena in Metallization 1993, to be published.