Bottomless Barrier Strategy for Next-Generation Cu Interconnects Using Phenylethyl Mercaptan-Assisted Area-Selective ALD for Scalable Ru/ZnO Bilayer Integration

Abstract

Self-assembled monolayers (SAMs) consist of long alkyl chains of large molecular size and low volatility, which require conventional solution-based processes that are incompatible with bottomless barrier structures, especially for sub-10 nm 3D semiconductor fabrication, leading to nonuniform inhibition and poor selectivity. Here, we introduce a bottomless barrier architecture strategy that selectively deposits barrier material on the SiO2 via-sidewall surfaces using area-selective atomic layer deposition (AS-ALD), while leaving the Cu bottom surface exposed. A phenylethyl mercaptan (PEM) small-molecule inhibitor (SMI) is employed as a Cu-selective passivating agent, exploiting strong Cu–S chemisorption and π–π stacking interactions to form a densely packed inhibition layer on Cu surfaces, thereby directing ZnO ALD exclusively onto the SiO2 sidewalls with superior selectivity compared to bulkier SAM-based approaches. ZnO is decided as the barrier material due to its low-temperature ALD capability (<150 °C), strong interfacial adhesion with Si-based dielectric, and effective suppression of Cu diffusion. The PEM inhibitor exhibited sufficient vapor pressure for a stable vapor-phase ALD-ZnO process that selectively adsorbs approximately 90% during a 50-cycle ZnO film with a thickness of about 9.1 nm. Notably, both H2 molecular and H2 plasma treatments are found to be efficient in removing PEM inhibitor from Cu surfaces before Ru deposition. ALD-Ru is subsequently deposited controllably as a low-resistivity (∼23 μΩ·cm) liner and seed layer with a thickness of ∼15 nm, further maintaining a low resistivity of ∼38.1 μΩ·cm even at a thickness of ∼6 nm, as well as nearly 100% step coverage in dual-trench structures with an aspect ratio of ∼6.3. The resulting Ru/ZnO bilayer on SiO2 sidewalls and the Ru layer on the Cu bottom surface could be incorporated into dual-damascene patterns prior to Cu filling for modern-day interconnects.