Ruthenium shows way to 2nm
imec in Belgium has demonstrated metal interconnects built using ruthenium (Ru) that could be used for 2nm process nodes.
The electrically functional 2-metal-level interconnects were built with a ruthenium semi-damascene and airgap technology and shown for the first time at the 2020 International Interconnect Technology Conference in XX this week.
The 2-metal-level semi-damascene module was built on 300mm wafers using Ru for the metallization with a 30nm pitch. These showed over 80 percent reproducibility with no evidence of shorting and a lifetime of more than 10 years. The mechanical stability of the Ru air-gapped structures was found to be comparable to today’s dual-damascene copper interconnects. A metal pitch of 24nm is seen as necessary for a 2nm process technology.
An analysis of a 12-metal-layer analysis using a 64bit ARM processor core showed system-level benefits in terms of RC, power consumption and IR drop. Ru was also shown as a promising alternative for contact plugs in the middle-of-line of advanced nodes.
Alternative metallization materials such as Ru and alternative metallization approaches such as semi-damascene are a key area of research for back-end-of-line (BEOL) and middle-of-line (MOL) technologies for the 2nm technology node and beyond. For the BEOL, imec proposes a semi-damascene integration as an alternative to traditional dual-damascene integration. This needs metals other than copper or cobalt with a high bulk resistivity to be deposited without a diffusion barrier that can be patterned using a subtractive etch lithography.
This allows for interconnect height increase which, in combination with using an airgap as a dielectric, promises to reduce the resistance-capacitance (RC) delay which is a major bottleneck for BEOL scaling.
“The results show that semi-damascene in combination with airgap technology not only outperforms dual-damascene in frequency and area, it also provides a scalable path for further enhancements,” said Zsolt Tokei, program director nano-interconnects at imec. “Airgap shows the potential to improve performance by 10 percent while reducing the power consumption by more than 5 percent. The use of high-aspect-ratio wires can reduce the IR drop in the power network by 10 percent to improve reliability.”
Next: 2nm semi-damascene mask
“In the near future, a newly developed mask set for the semi-damascene module will allow us to further improve the semi-damascene integration
and to experimentally validate the predicted performance improvements,” said Tokei.
The work showed that ruthenium can also be used for MOL contact plugs as vias as a replacement for tungsten or cobalt in a 2nm process. It is mined in South Africa, Russia and Zimbabwe and is a sustainable supply, unlike cobalt.
“Alternative metals such as barrier-less Ru have the potential to further reduce the contact resistance that results from shrinking the contact area. In a benchmark study, imec evaluated both Ru and Co. The results indicate that Ru is a promising candidate for replacing Co in narrow MOL trenches,” said Naoto Horiguchi, director CMOS device technology at imec.
The resistance of a ruthenium-filled via on a 0.3nm TiN liner (without barrier) was shown to outperform the Co filled equivalent process (with 1.5nm TaN barrier). It has also been used as a source/drain contact material for transistors, with low contact resistivity in the order of 10-9Ωcm-2 on both p-SiGe and n-Si.
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