Nucleation Enhancement and Area-Selective Atomic Layer Deposition of Ruthenium Using RuO₄ and H₂ Gas

Inherent substrate selectivity is reported for the thermal RuO4 (ToRuS)/H-2 gas atomic layer deposition (ALD) process on H-terminated Si (Si-H) versus SiO2. In situ spectroscopic ellipsometry (SE) detected Ru growth from the first cycle on blanket Si-H, whereas on blanket SiO2, 60 cycles were needed to detect growth. Area-selective growth was evaluated on a patterned substrate with 1-10 mu m wide Si H lines separated by 10 mu m wide SiO2 regions. Ex situ planar scanning electron microscopy and cross-sectional high resolution transmission electron microscopy measurements showed that a smooth, continuous Ru film of 4.5 nm could be deposited on Si-H, with no Ru detected on SiO2. The proposed mechanism behind the inherent substrate selectivity is the oxidation of the Si-H surface by RuO, which was confirmed by in vacuo X-ray photoelectron spectroscopy (XPS) experiments. A methodology to enhance the nucleation of the RuO4/H-2 gas process on oxide substrates is also reported. In situ SE and in vacuo XPS experiments show that the nucleation delay on SiO2 can be completely removed by exposing the surface to trimethylaluminum (TMA) just before the start of the ALD process. We found evidence that the TMA pulse makes the oxide surface reactive toward RuO4, by introduction of surface methyl groups, which can be combusted by RuO4. As TMA is known to be reactive toward many oxide substrates, this methodology presents a way to achieve Ru metallization of virtually any surface. Therefore, one can either (i) use the RuO4/H-2 gas process to coat nonoxidized surfaces selectively with Ru or (ii) use TMA-priming by which one can bypass the selectivity and coat a wide variety of surfaces nonselectively with Ru.