“Our work shows that doped-ML-GNR is an attractive alternative to copper interconnects, which were introduced by the semiconductor industry around two decades ago but are now reaching their fundamental limits,” explains team leader Kaustav Banerjee. “This is because copper’s current carrying capacity diminishes as its width reduces due to sharp increases in its resistivity. Ours is the first demonstration of a carbon-based material suitably designed to function as a VLSI interconnect that is better in terms of both performance and reliability with respect to copper interconnects.”

VLSI stands for very-large-scale integration and it is the process of creating an integrated circuit by combining millions of transistors into a single chip.

The researchers made their ML-GNRs by first synthesizing high-quality multilayer graphene using a technique called chemical vapour deposition. They were able to control the thickness of the material thanks to a specially-designed alloy catalyst. They then patterned the multilayer graphene into ML-GNRs by electron beam lithography with feature sizes as small as 20 nm. Finally, they intercalation-doped the structure with ferric chloride (FeCl3).

Similar to semiconductor doping

“The process of intercalation doping is done at a relatively low temperature of 360 °C,” explains Banerjee. “It is similar to semiconductor doping in the sense that it increases the carrier concentration in a GNR and thereby its electrical conductivity thanks to transfer of charges (electrons) from the GNR to the dopants (FeCl3).”

The team then made metal contacts and pads to the ML-GNRs by electron-beam lithography to be able to measure their electronic properties.

“The most important property of doped ML-GNRs is that they are very reliable with respect to copper thanks to their significantly higher current carrying capacity that exceeds 200 MA/cm2,” Banerjee tells nanotechweb.org. “In contrast, that of a copper interconnect is less than 5 MA/cm2. This implies that we can reduce the doped ML-GNRs' aspect ratio (thickness to width) without encountering the reliability issue of copper interconnects.”

Smaller parasitic capacitance

“As a result, doped ML-GNR interconnects have much smaller parasitic capacitance compared to copper ones and this helps significantly improve an integrated circuit’s performance and its energy efficiency. Again, these benefits stem from our efficient intercalation doping method that enhances the electrical conductivity of nanoscale GNRs tremendously.”

The researchers, reporting their work in Nano Letters DOI: 10.1021/acs.nanolett.6b04516, say they are now busy working on integrating their intercalation-doped GNR interconnects in standard CMOS technology. “We are also looking into novel patterning methods to demonstrate sub-20 nm width ML-GNRs, wherein the benefits of the technology should be even greater.”