Concentration and Stress Evolution During Electromigration in Passivated Al(0.25 at. % Cu) Conductor Lines
- PDF / 220,782 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 40 Downloads / 205 Views
CONCENTRATION AND STRESS EVOLUTION DURING ELECTROMIGRATION IN PASSIVATED Al(0.25 at. % Cu) CONDUCTOR LINES H.-K. Kao*, G. S. Cargill III* and C.-K. Hu** *Lehigh University, Bethlehem, PA 18015, [email protected] **IBM Research Division, T. J. Watson Research Center, Yorktown Heights, NY 10598. ABSTRACT We have used x-ray microbeam fluorescence and diffraction for in-situ measurements of electromigration-induced Cu diffusion and stress evolution in passivated, polycrystalline 10µmwide, 200µm-long Al(0.25 at.% Cu) conductor lines. Cu migration is in the direction of the electron flow and is determined by the direction and magnitude of the current and by the temperature during electromigration. The effective charge and diffusivity of Cu in Al(Cu) have been obtained from analysis of the Cu concentration profiles. The evolution of electromigrationinduced strains normal to the sample surface has been monitored by x-ray microbeam diffraction. A linear strain profile developed after about 9 hrs of electromigration with 1.5×105 A/cm2 at 300oC, corresponding to 3MPa/µm equi-biaxial stress. From the Cu profile measured at the same time, the critical Cu concentration for significantly slowing down Al grain boundary diffusion is estimated to be ∼0.15 at. %. These data also confirm that downstream Cu transport is accompanied by a counter flow of Al in the upstream direction. INTRODUCTION The role of copper in reducing the rate of electromigration (EM) damage in Al(Cu) conductor lines has been studied for many years [1-9]. Additions of a few percent of copper increase EM lifetimes by one or two orders of magnitude [2]. Cu atoms tend to segregate to the Al grain boundaries and apparently reduce the grain boundary mobility of Al atoms [4,9,10]. Most models for EM in Al(Cu) conductor lines [4,7,10] predict that an incubation time is needed, after the start of current flow, before the onset of significant Al migration. This delay in Al migration is attributed to the Cu concentration having to drop below some threshold level within a current-dependent, critical length of the cathode end of the conductor line. This threshold concentration has been estimated to be ∼0.1 at.% [9,11], but no direct measurements have been reported. During EM in an Al(Cu) conductor line with blocking boundaries, a stress gradient is expected to develop in the part of the line where EM of Al occurs, but not in the part of the line where the Cu concentration remains above the threshold value [4,9,10] which drastically reduces Al migration. These issues are addressed in this paper, which describes synchrotron based realtime x-ray microbeam measurements of the dynamics of Cu motion and of stress development during EM in passivated Al(Cu) polycrystalline conductor lines. EXPERIMENTAL The samples and instrumentation used in the present experiments are described in ref. 15, and the experimental data shown in Fig. 2 of this paper are the same as those which were discussed in ref. 15. Measurements of Cu-Kα fluorescence show that the Cu concentration is uniform along the
Data Loading...
