New-Tech Europe Magazine | Q4 2021

The promise of hybrid graphene/metal structures for advanced interconnects Swati Achra, Inge Asselberghs, Zsolt Tokei, IMEC

Advancing the chip’s back-end-of-line While chipmakers continue to make advancements with transistor technologies in the front-end-of-line (FEOL), the interconnects within the chip’s back-end-of-line (BEOL) are challenged to keep pace. This BEOL is organized in different metal layers (local, intermediate, semi-global and global) which are vertically interconnected by means of via structures – filled with metals. 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 chipmakers to consider new integration schemes and materials for fabricating the interconnects. For the 5nm technology node – the most advanced chip generation

that has entered volume production – critical local interconnects have metal pitches as tight as 28nm. Cu-based dual damascene still is the workhorse process flow for making the interconnects. But with metal pitches soon moving towards 21nm and beyond, chipmakers may gradually move away from mainstream technologies. Imec, for example, foresees the introduction of alternative integration schemes such as hybrid via metallization, a semi-damascene process and hybrid height with zero via for the nodes to come. In parallel, alternative conductors with better figure of merit are being investigated to be used in combination with these advanced process schemes. This 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 binary intermetallic compounds such as AlNi or RuV3. Inaddition, researchers 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, opto- electronics and CMOS scaling. Why graphene? In recent years, graphene has been intensively studied for interconnect applications, where it has potential to fulfill diverse roles. 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.

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