New-Tech Europe Magazine | Q4 2021

Interest for interconnect applications comes as no surprise. Graphene exhibits a high intrinsic carrier mobility (up to 200,000cm2V-1s-1) and a large current carrying capacity (up to 108A/cm2). In addition, graphene has a high thermal conductivity and competitive robustness against electromigration. It can also be made atomically thin, which helps alleviating the thickness contribution to the RC delay. Despite these interesting properties, graphene has one major drawback: intrinsically, it does not hold enough charge carriers to be useful as a local interconnect. The lack of charge carriers severely reduces its electrical conductivity, a key metric for interconnect performance that is proportional to both the mobility and the carrier concentration. For this reason, several layers of graphene will be needed to cross-over Cu for example, for (local) interconnect applications – as confirmed by modelling. The number of layers will be a trade-off between the material’s overall contribution to resistance and capacitance. Fortunately, there are ways to further modulate graphene’s conductivity. This has driven the research of so-called graphene nanoribbons – graphene layers patterned into narrow strips. The specific angular orientation of the graphene layers with respect to their underlying layer provides another knob for improvement. Finally, the conductivity of graphene can be boosted by doping, this way providing graphene with extra electrons or holes to carry the current. Doping can be performed in several ways, for example by metal-induced doping – enabled by bringing graphene in direct in graphene

Figure 1: A view on the imec FEOL (top) and BEOL (bottom) technology roadmaps.

as a potential alternative for Cu metallization. But the concepts presented here are expected to be expandable towards other ‘interconnect’ metals. Capping Ru with graphene For this study, the imec researchers realized Ru/graphene hybrid structures by transferring a multilayer graphene film (grown by chemical vapor deposition (CVD)) onto a thin Ru film (typically 5nm) that was grown by physical vapor

contact with metals like Cu or Ru. These hybrid metal/graphene schemes bring together the best of both ‘worlds’: the high carrier concentration of the metal and the high mobility of graphene. This article looks into the feasibility of using hybrid metal/graphene structures for sub-2nm interconnect applications. Two different structures are being examined: graphene-capped metal and metal- capped graphene devices. The study focuses on Ru as a metal of interest, as it has recently emerged

Figure 2: Comparing properties of graphene (carbon nanotube (CNT), single layer graphene (SLG) and few layer graphene (FLG)) with other interconnect materials of interest (tungsten (W), copper (Cu) and ruthenium (Ru).

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