Electromigration-induced strain relaxation in Cu conductor lines

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ectromigration-induced strain relaxation in Cu conductor lines H. Zhanga) Cascade Engineering Services Inc., Redmond, Washington 98052

G.S. Cargill, III Department of Materials Science and Engineering, Lehigh University, Bethlehem, Pennsylvania 18015 (Received 5 November 2010; accepted 28 December 2010)

Strain evolution in 0.45-m-thick, 2-m-wide, and 100-m-long Cu conductor lines with a passivation layer has been investigated using synchrotron x-ray microdiffraction. A moderate electromigration-current density of 2.2  105 A/cm2 was used to minimize Joule heating in the Cu conductor lines. After 120 h of current flowing in the Cu lines at 270 °C, measurements show strain relaxation and homogenization occurring in the Cu lines with current flowing, but not in Cu conductor lines without current. Stronger interaction between electrons with Cu atoms in areas with higher strains was proposed to explain the observation.

Electromigration (EM) is the diffusion of atoms, usually in the direction of electron flow, due to the transfer of momentum from electrons to atoms in the conductor line.1 EM in Cu conductor lines has been widely studied due to the importance of Cu materials in the microelectronic industry.2–4 EM has caused the formation of voids and extrusions in the Cu conductor lines, leading to reliability concerns. However, direct measurement of EM-induced strains in the Cu conductor lines with line width in the range of micrometer or submicrometer levels is challenging with the traditional x-ray diffraction (XRD) techniques. Synchrotron microbeam XRD provides enough brightness to investigate strain evolution in Cu conductor lines during EM with spatial resolution of micrometer or submicrometer levels.5,6 Strain evolution in Al conductor lines with line width at the micrometer level has been reported, and a strain gradient along the Al lines was found when the EM was in the steady-state stage.7,8 Valek et al.9 found that plastic deformation and grain rotations occurred in sputtered Al (0.5 wt% Cu) conductor lines during EM. Similar behavior in Cu conductor lines is expected according to the EM model proposed by Blech.10 Budiman et al.11 used synchrotron white beam x-ray microdiffraction to study the evolution of Laue spots during EM in damascene Cu conductor lines. They found that EM-induced plastic deformation occurred before the formation of voids and hillocks in the Cu conductor lines. However, there have been no reports of direct x-ray measurements of strain evolution in Cu conductor lines a)

Address all correspondence to this author: e-mail: [email protected] DOI: 10.1557/jmr.2011.2 498

J. Mater. Res., Vol. 26, No. 4, Feb 28, 2011

http://journals.cambridge.org

Downloaded: 14 Mar 2015

during EM. In this study, we use synchrotron microbeam x-rays to investigate strain evolution of Cu conductor lines during EM. The Cu lines used in this study are 0.45 m thick, 2 m wide, and 100 m long, with dielectric and liner materials. Figure 1(a) shows a scanning electron microscope (SEM) image of the Cu lines. The SiO

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