Patterning challenges for direct metal etch of ruthenium and molybdenum at 32 nm metal pitch and below

Ruthenium and molybdenum are candidate materials to replace Cu as the back-end-of-line interconnect metal for the tightest pitch features for future technology nodes. Due to their better figure of merit ρ0 × λ (ρ0 bulk resistivity, λ electron mean free path), it is expected that the resistance of <10 nm wide Ru and Mo metal lines can be significantly reduced compared to Cu. An important advantage for Ru and Mo is that both materials, in contrast to Cu, can be patterned by means of so-called direct metal etch, through reactive ion etching or atomic layer etching and can potentially be implemented without barrier. An integration scheme with direct metal etch instead of damascene patterning could simplify the overall patterning flow and eventually opens the possibility for exploring new integration concepts and patterning approaches. However, the learning on direct metal etch of Ru and Mo in the literature is scarce, especially at the relevant dimensions of today's interconnects. In this work, we will focus on the major patterning challenges we have encountered during the development of direct metal etch processes for Ru at 18 nm pitch and Mo gratings at 32 nm pitch. We have observed that the direct metal etch of Ru at these small dimensions is impacted by the growth of an oxidized layer on the sidewalls of the hard mask, which originates from the sputtering of the hard mask in combination with the O2-based Ru etch chemistry. This results in a narrowing of the trenches to be patterned and can easily lead to an etch stop in the smallest features. We will discuss several mitigation mechanisms to remove this oxidized layer, as well as to avoid the formation of such a layer. For patterning Mo with a Cl2/O2-based chemistry, the major patterning challenges we encountered are the insufficient sidewall passivation and the oxidation of the patterned Mo lines. The sidewall passivation issue has been overcome with an in situ thin SiO2-like deposition after partial Mo etch, while a possible mitigation mechanism for the Mo oxidation could be the in situ encapsulation immediately after Mo patterning.