Ruthenium is a candidate to replace copper in future sub-10 nm interconnects. At these dimensions the resistivity of Ru lines is expected to be lower compared to Cu due to the lower sensitivity to size effects.¹ In addition, it is likely that Ru interconnects won’t require a diffusion barrier, and will show a better electromigration performance.² At feature sizes below 10 nm it will be difficult to align subsequent lithography steps, and the conformality of the deposition method is increasingly important, such that area selective atomic layer deposition (ALD) of ruthenium is of high interest.³

We first report inherent area selective ALD of Ru on H-terminated Si (Si-H) versus SiO₂, using the thermal RuO₄ (ToRuS^TM)/ H₂-gas ALD process.⁴ In situ spectroscopic ellipsometry (SE) on blanket substrates shows that Ru growth initiation occurs from the first cycle on Si-H, while on SiO₂ the growth is delayed, resulting in a substrate selectivity window of ~ 70 cycles. Area selective Ru ALD was evaluated using a patterned substrate of 1-10 µm wide Si-H lines separated by 10 µm wide SiO₂ regions, and exposing it to 20 cycles of the RuO₄ / H₂-gas ALD process. Ex situ scanning electron microscopy (SEM) and cross section high resolution transmission electron microscopy (HRTEM) measurements show that a 4.5 nm Ru film could be deposited on the Si-H, with no Ru detected on the SiO₂. In vacuo X-ray photoelectron spectroscopy (XPS) experiments showed that exposure of Si-H to a single RuO₄ pulse leads to the oxidation of the Si surface, together with the deposition of RuO₂. On SiO₂ however, the surface is already oxidized, and in vacuo XPS shows that for the same exposure to RuO₄ no Ru is deposited on the surface. Therefore, we propose that the mechanism behind the inherent substrate selectivity is the oxidation of the Si-H surface by RuO₄. Secondly, we report for a methodology to enhance the nucleation of the RuO₄ / H₂-gas process on oxide substrates. In vacuo XPS and in situ SE experiments show that a single exposure of SiO₂ to trimethylaluminum (TMA) makes the surface reactive towards RuO_4, which allows for Ru growth initiation from the first cycle. We propose that this is due to the combustion of surface CH₃-groups by RuO₄. As TMA is known to be reactive towards many oxide substrates, this methodology presents a way to achieve Ru metallization of virtually any surface.

[1] S. Dutta et al. IEEE Elec. Dev. Lett. 2017, 38, 949.

[2] O. V. Pedreira et al. 2017 IEEE IRPS, Monterey, CA, 6B-2.1.

[3] P. C. Lemaire et al. J. Chem. Phys. 2017, 146, 052811.

[4] M. M. Minjauw et al. J. Mater. Chem. C. 2015, 3, 132.