(Invited) Innovations to Enable Extension of Cu Interconnects and Shift to Alternative Conductor
Authors: Nogami Takeshi 
Published: July 2021

Cu/low-k interconnect is facing challenges in its extension beyond 32nm metal pitch and impacting circuit RC delay adversely. The difficulties lie in securing (#1) Cu gap fill capability in scaled via in the dual damascene process flow due to the limited step coverage of PVD Cu seed layer, (#2) Vx-Mx breakdown voltage and TDDB reliability due to scaled spacing and misalignment in lithography, (#3) Electromigration reliability when the barrier/liner is scaled, (#4) Low line resistance due to decreased Cu volume when the barrier/liner scaling is limited, (#5) Low via resistance when the barrier/liner scaling is limited, (#6) low-k dielectrics patterning due to low-k wiggling in scaled metal pitch and insufficient Young modulus of low-k and (#7) Low interconnect parasitic capacitance when thickness of the plasma-induced damage of the intra-layer low-k dielectric becomes significant comparing to the inter-metal spacing per scaling.
Because of these difficulties in extension of dual damascene Cu interconnection scheme, various alternative metallization unit processes, materials (conductors and dielectrics) and schemes (alternative to dual damascene) have been investigated. These new processes and materials include (a) those for extension of Cu technology such as hybrid interconnect where wires stay with Cu and vias are pre-filled with alternative conductor or Cu, and (b) those for entire replacement of Cu to other conductors. However, extension of Cu interconnects is the preference in the industry rather than entire replacement to alternative conductors and/or metallization schemes, partly because of poor competitiveness in line resistance of fine lines. In addition, even when fine Cu lines are replaced to alternative conductors such as Co for fine lines, wide power rails need to stay with Cu for the required low line resistance. Cu/Co composite metallization has been investigated to resolve the problem f high line resistance of power rail. Also, Buried Power Rail (BPR) and Back Side Power Distribution Network (BS-BPN) are potential solution which can support replacement of Cu to alternative conductors.
In this talk, process candidates which have potential to resolve the difficulties listed in Table 1 and enable extension of Cu interconnection scheme are reviewed Then, the possibility and challenges to shift from Cu to alternative conductors are reviewed.
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