Title: Electrodeposition of Epitaxial Metals for the Fabrication of Single Crystal Interconnects
Description: The continued miniaturization of interconnects results in performance and reliability issues for integrated circuit (IC) chips. As the critical dimension of Cu interconnects approaches dimensions near the mean free path of the metal (39.9 nm at room temperature), a rise in resistivity is observed. This phenomenon, termed resistivity size effect, is the result of electron scattering at grain boundaries and surfaces. Cobalt (Co) and ruthenium (Ru) are considered promising candidates to replace Cu as an interconnect metal because these metals exhibit a lower product of mean free path times bulk resistivity when compared with Cu. Additionally, given that electron scattering from grain boundaries is a major contributor to the resistivity-size effect in nanoscale interconnects, we investigate the electrodeposition of epitaxial Co, Ru, and Cu and demonstrate the capability of electrodeposition to fabricate epitaxial, single crystal metal films. Co, Ru, and Cu have a misfit strain that is tensile, zero, and compressive with the epitaxial Ru(0001) seed layer, respectively.

This allowed for the study of every kind of misfit relationship that is possible in epitaxial film growth. Ultimately, the successful electrodeposition of Co and Cu epitaxial to a single crystal, conductive seed layer suggests the plausibility of electrodeposited, single crystal interconnects in future IC chips. Co electrodeposited as an epitaxial, single crystal film onto the Ru(0001) seed layers to finite thicknesses relevant for interconnect fabrication. Electrodeposition onto polycrystalline Ru seed layers, however, resulted in the growth of a rough, polycrystalline Co film with faceted growth. Despite a large misfit strain of 7.9%, the epitaxial electrodeposition of planar Co was achieved up to a thickness 75x beyond the calculated critical thickness for defect formation before a transition to island growth was observed. Thus, the importance of a conductive, single crystal seed layer, preferably with a minimal misfit strain with the depositing layer, is demonstrated. Metallic Ru was found to electrodeposit onto Ru(0001) as a porous layer comprised of (0001) oriented Ru crystallites.

The presence of a porous Ru deposit was found to be independent of the seed layer, Ru metal ion source, and deposition mode used. An optimization of the deposition electrolyte to improve Ru atomic mobility is necessary to achieve the epitaxial electrodeposition of single crystal Ru. Finally, Cu demonstrated epitaxial growth on the Ru(0001) seed layer, with an out-of-plane epitaxial orientation relationship of Cu(111) | Ru(0001). The hexagonal close packed Ru(0001) seed layer allows Cu to deposit with two equivalent in-plane orientations; thus, the electrodeposited Cu film was determined to be a bicrystal, not a single crystal. While epitaxial deposition of Cu was achieved, a seed layer that permits only one orientation of Cu is required for a significant reduction in electron grain boundary scattering, hence, resistivity.