Grain structure effect on electromigration reliability of Cu interconnects with CoWP capping

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Oliver Aubel and Christian Hennesthal GLOBALFOUNDRIES Dresden Module One LLC & Co. KG, 01109 Dresden, Germany

Ehrenfried Zschech Fraunhofer Institute for Non-Destructive Testing IZFP, Dresden, Germany (Received 17 April 2011; accepted 31 August 2011)

This article investigates the effect of grain structure on electromigration (EM) reliability of dual-damascene Cu interconnects with a CoWP capping layer, including the lifetime and statistics. Downstream EM tests were performed on two sets of CoWP-capped Cu interconnects with different grain sizes. Compared to Cu interconnects with the standard SiCN cap layer, the CoWP capping clearly improved the EM lifetime by ;24 for the small grain structure and by another ;14 for the large grain structure. Here, the effect of grain structure on EM lifetime was attributed to the grain boundary contribution to mass transport. The lifetime improvement, however, was accompanied with an increase in the statistical deviation, increasing from 0.27 for the SiCN cap to 0.53 for the small grain structure and to 0.88 for the large grain structure with the CoWP cap. This was attributed to the effect of grain structure in changing the statistical distribution of flux divergence sites and thus the failure statistics.

I. INTRODUCTION

Cu has replaced Al as Back End of Line (BEOL) interconnects due to its low electrical resistivity and superior electromigration (EM) reliability. Compared with Al, Cu has a distinct EM failure mechanism due to its different mass transport characteristics. While the mass transport in Al interconnects comes mainly from grainboundary diffusion, in submicron Cu interconnects, it is dominated by diffusion along the Cu/SiNx cap interface.1 Although scaling will degrade the EM lifetime of both Al and Cu interconnects since less void volume will be required to induce EM failure, the effect will depend on the specific mechanism dominating the mass transport. For submicron Cu interconnects, the EM lifetime degrades by half for each new technology node, even with the same current density.2 This has stimulated great interests in modifying the Cu/SiNx interface by using a metal cap layer3–7 to suppress the interfacial mass transport and improve EM reliability. The CoWP cap was found to be particularly effective, yielding two orders of magnitude improvement in the EM lifetime at the test temperature.4,7 However, the EM lifetime statistics was found to degrade with the application of the CoWP capping, and the EM a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.321 J. Mater. Res., Vol. 26, No. 21, Nov 14, 2011

improvement seemed to vary with the manufacturing process,7–9 suggesting a metallization with great potential for future interconnects but difficult to implement. A recent study by Hu et al. showed that, for Cu interconnects narrower than 90 nm, small grains agglomerated and mixed with large bamboo grains.10 The EM lifetime was found to be degraded further due to an increase in the grain boundary contribution to mass transp

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