November 6th, 2020 — At the 2020 International Interconnect Technology Conference, imec, a world-leading research and innovation hub in nanoelectronics and digital technologies, demonstrated for the first time electrically functional 2-metal-level interconnects fabricated with Ru semi-damascene and airgap technology showing a long lifetime and good mechanical strength. A complementary 12-metal-layer analysis confirms the system-level benefits of this semi-damascene technology 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 intensively researched to scale the back-end-of-line (BEOL) and middle-of-line (MOL) towards the 2nm technology node and beyond.
For the BEOL, imec proposes a semi-damascene integration as an alternative to traditional dual-damascene integration. To fully leverage the potential of the semi-damascene technology, metals other than Cu or Co are required that can be deposited without diffusion barrier, have a high bulk resistivity and can be patterned using e.g. subtractive etch. This allows for interconnect height increase which, in combination with airgaps as dielectrics, promises to reduce the resistance-capacitance (RC) delay ? a major bottleneck for BEOL scaling.
Imec has for the first time fabricated and characterized a 2-metal-level semi-damascene module on 300mm wafers using Ru for the metallization. Devices with 30nm metal pitch line test structures showed more than 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 traditional Cu dual-damascene structures. A complementary 12-metal-layer analysis revealed for the first time the system-level benefits of the semi-damascene approach in sub-3nm nodes ? using a 64-bit ARM CPU as a benchmark design. Zsolt Tokei, program director nano-interconnects at imec: ?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. 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. 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.?
Imec also demonstrated the beneficial impact of using Ru as alternative metal in advanced MOL contact plugs ? as a replacement for W or Co. Naoto Horiguchi, director CMOS device technology at imec: ?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.? The resistance of a Ru filled via on a 0.3nm TiN liner (without barrier) was shown to outperform the Co filled equivalent process (with 1.5nm TaN barrier). Ru as a source/drain contact material was also demonstrated, with low contact resistivity in the order of 10-9?cm-2 on both p-SiGe and n-Si.