Available on-demand - F.SF08.06.04
A Comparative Study of Electromigration in Nanowires with Twin Boundaries
Mohammad Waliullah1,Rodrigo Bernal1
The University of Texas at Dallas1
Electromigration is a critical failure mode of metallic interconnects in the semiconductor industry, and in the emerging fields of flexible and wearable electronics. This failure can be attributed to the diffusion of metallic atoms due to momentum transfer of electrons flowing through the interconnect when a high current density is applied. The diffusion eventually leads to formation of voids and hence a time-dependent increase of electrical resistance of the interconnect. Since it is a diffusion process, the microstructure of the interconnect is one of the controlling factors of electromigration. Twin boundaries, in contrast to grain boundaries, can have the potential to reduce diffusion of atoms and consequentially improve electromigration performance both in electronic chips and in flexible electronics. Note that in nanostructured flexible electronics, it is common to have nanowires with twin boundaries, acting as interconnects. Unfortunately, very few studies have been conducted about the diffusion performance of these interconnects. No literature is available about the collective effect of twin boundaries throughout the interconnect. Moreover, some of the studies said that twinning slows down the diffusion and hence void growth ,  and some said that twinning does not necessarily hinders void growth . Given these conflicting reports, there is clearly a need for a systematic study of diffusion through twinned interconnects.
In this work, we investigated electromigration failure of penta-twinned silver nanowires, with diameters ranging from 50 nm to 80 nm to understand the diffusion process during electromigration of twinned interconnects. Samples were prepared by random deposition of nanowires on a substrate containing gold electrodes fabricated by photolithography. The nanowires were connected with the electrodes by e-beam lithography. The electrodes were then connected to a printed circuit board (PCB) through a wirebonding package. Under a fixed current density and temperature, the resistance was measured using 4-point measurement method so that it can be measured continuously without the influence of contact resistances. The currents were applied in an automated fashion to test a large number of nanowires. The failure criterion was set as 10% increase in resistance which is widely used in the electronics industry. Resistance vs. time plots were obtained in different temperatures, which were fitted to a mathematical model combining the theory of electromigration diffusion flux and the theory of void growth. The fit produces a diffusivity product, independent of temperature, for twin boundaries and the activation energy required for atomic fluxes through twin boundaries. These parameters enable a comparison between the diffusion through twinned interconnects and the published data for diffusion through other microstructures of interconnects currently used in the electronics industry. The results of this study should also allow reliability calculations to be made for novel electronic devices that utilize twinned metallic interconnects, such as wearable electronics that use nanowires as flexible conductors.
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