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it has potential to fulfill diverse roles in
interconnect applications. The material
has, for example, been considered as
an oxidation barrier and as an ultrathin
diffusion barrier for metals. Researchers
have also investigated the feasibility
of using multilayer graphene wires or
nanoribbons as an alternative conductor.
Graphene, however, comes with 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 Figure 3: Transmission electron microscope (TEM) image of a graphene-capped Ru structure.
carrier concentration. For this reason, transferring a multilayer graphene film shift of graphene’s Fermi level into the
several layers of graphene will be needed (grown by chemical vapor deposition valence band by ~0.5eV compared to
to cross-over Cu for example, for (local) (CVD)) onto a thin Ru film (typically intrinsic graphene, corresponding to a
interconnect applications – as confirmed 5nm-thick) that was grown by physical hole concentration of 1.9E13cm . This
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by modeling. The number of layers will be vapor deposition (PVD) (Figure 3). After observation is an indication of metal-
a trade-off between the material’s overall transfer, graphene was found to adhere induced doping that happens at the
contribution to resistance and capacitance. well to the large area PVD Ru film. interface, causing graphene to become
Fortunately, there are ways to further The two main observations can p-doped when added as a capping layer
modulate graphene’s conductivity. This be summarized as follows. First, the on Ru [2-4].
has driven the research of so-called researchers found that the average Furthermore, the Ru lines were
graphene nanoribbons – graphene layers resistivity of Ru dropped by more than obser ved to be less sensit ive to
patterned into narrow strips. The specific 15% after encapsulation with graphene, t e m p e r a t u r e f l u c t u a t ion s w h e n
angular orientation of the graphene layers accompanied by a significant decrease encapsulated with graphene. This could
with respect to their underlying layer in contact resistance (Figure 4). Second, be due to the high thermal conductivity
provides another knob for improvement. internal photoemission spectroscopy of graphene, providing an alternative/
Finally, the conductivity of graphene experiments indicate a downward additional path for eff icient heat
can be boosted by doping, thereby
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
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.

Exploring hybrid graphene/metal
interconnects
Below, we discuss 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.
Graphene-capped ruthenium. 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. Interfaces were formed after Figure 4: Experimentally measured hybrid film resistivity of bare Ru (black) and graphene-capped Ru (red)
devices for different thicknesses of Ru thin-film [4].`

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