New-Tech Europe Magazine | Q4 2021

deposition (PVD). After transfer, graphene was found to adhere well to the large area PVD Ru film. Of interest for interconnect applications is the metal-induced doping of graphene that is expected to happen at the interface with Ru. To understand and be able to control the doping, the charge transfer at the Ru/graphene interface was systematically investigated. The two main observations can be summarized as follows. First, the researchers found that the sheet resistance of Ru dropped by an average of 15% after encapsulation with graphene. Second, they observed a downward shift of graphene’s Fermi level into the valence band by ~0.5eV compared to intrinsic graphene, corresponding to a hole concentration of 1.9E13cm-2. This observation is an indication of metal-induced doping that happens at the interface, causing graphene to become p-doped when added as a capping layer to Ru. From this study, it has become clear that encapsulating Ru with graphene can boost the electrical performance of Ru as an interconnect. Yet, more fundamental insights are needed to determine the exact conduction mechanism taking place within the capped structure. Either Ru remains the main conductor, with graphene helping to reduce its resistivity by suppressing scattering mechanism(s) in the metal. Or the two conductors now act in parallel, with a higher conductivity for graphene (with respect to intrinsic graphene) because of the charge transfer. Modelling work is currently ongoing to get a better understanding. It should also be noted that the Ru lines are observed to be less sensitive

Figure 3: Experimentally measured sheet resistance of bare Ru (black) and graphene-capped Ru (red) devices for different thickness of Ru thin film substrate [as presented at IITC 2019].

A metal/graphene sandwich structure In the longer term, researchers at imec are looking into stacking alternating layers of graphene and metal to further boost the electrical conductivity. In such a metal/ graphene/metal/... sandwich-like structure, a second and different interface will now play an equally important role: the interface that results from depositing a layer of metal on top of graphene. Just like in the above study, the nature of the graphene/metal interaction at the interface can modify the physical properties of graphene. And its electronic band structure can also be significantly altered by the charge distribution at the interface. Engineering the graphene/metal

to temperature fluctuations when encapsulated with graphene. This could be due to the high thermal conductivity of graphene, providing an alternative/additional path for efficient heat dissipation. This observation is of interest for future interconnect applications, as the self-heating in highly scaled IC wires and an insufficient heat dissipation to the surrounding dielectric can degrade the interconnect’s thermal reliability. Overall, the researchers conclude that these graphene-capped metal hybrid structures provide an answer to the RC delay problem for future interconnects. Imec envisions their introduction in the BEOL technology roadmap for the 1nm node and beyond.

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