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Introducing hybrid graphene/metal structures in the

BEOL technology roadmap

By Swati Achra, Inge Asselberghs, Zsolt Tokei [imec]
A dvanced scaling in the

front-end-of-line (FEOL)
needs to go hand in hand
with innovations in the back-end-of-line
(BEOL)—the network of interconnects
t hat con ne ct s sea m le ssly t o t he
underlying device structures. This BEOL
is organized in different metal layers
(local, intermediate, semi-global and
global) that are vertically-interconnected
by means of via structures – filled
with metals. Today, Cu-based dual-
damascene is the workhorse process flow
for making the interconnects. But with
each new technology generation, routing
congestion and a dramatic signal delay
(resulting from an increased resistance-
capacitance (RC) product) become
more and more problematic, forcing Figure 1: A grasp of new alloys screened for future interconnects [1].
chipmakers to consider new integration
2
schemes and materials for fabricating the The promise of graphene for to 108A/cm ) (Figure 2). In addition,
interconnects. Imec, for example, foresees interconnect applications graphene has a high thermal conductivity
the introduction of alternative integration Interest in graphene for interconnect and competitive robustness against
schemes such as hybrid via metallization, applications comes as no surprise. electromigration. It can also be made
a semi-damascene process and hybrid Graphene exhibits a high intrinsic carrier atomically thin, which helps alleviate the
height with zero vias for the nodes to mobility (up to 200,000cm V s ) and thickness contribution to the RC delay.
2
-1 -1
come. These innovations promise to a large current-carrying capacity (up Because of these exceptional properties,
address the challenges that come along
with metal pitches moving towards 21nm
and beyond.
In parallel, alternative conductors
with better figures of merit are being
investigated to be used in combination
with these advanced process schemes.
The figure of merit is defined as the
product of the bulk resistivity and
the mean free path of the carriers
in the metal. Of interest are cobalt
(Co), ruthenium (Ru), tungsten (W)
and ordered binar y inter metallic
compounds such as AlNi or RuV 3
(Figure 1) [1]. Researchers also look
intensely at graphene, which, thanks
to its remarkable properties, is making
its way into many interesting fields of
application such as (bio)sensing, energy
storage, photovoltaics, optoelectronics
a nd complement a r y met al- ox ide
semiconductor (CMOS) scaling.
Figure 2: Comparing properties of graphene (single-layer and few-layer) with other interconnect materials of interest.

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