Studying the efficacy of hydrogen plasma treatment for enabling the etching of thermally annealed ruthenium in chemical solutions

Ruthenium (Ru) is a noble metal and is known to be resistant to many common chemicals and mixtures. We report in this study a controlled etching/recessing of Ru via wet processing and a combination of dry and wet process using metal-free chemical mixtures. The wet etching study was performed using both as-deposited and annealed atomic layer deposited ruthenium (Ru) films. The etching rate of Ru film in periodic acid mixtures was found to decrease substantially after being annealed at elevated temperatures (300 and 420 °C). X-ray photoelectron spectroscopy results indicated that this behavior could be explained by the formation of a thin surface ruthenium oxide layer induced by the thermal anneal. A short treatment in H2-based atmosphere at elevated temperature or in H2 plasma rendered the annealed Ru film etchable, explained by the reduction of RuOx film at the surface. The increase in surface roughness after wet etching was most likely caused by the gradual formation of oxidized Ru at the surface. Similarly, non-uniform etching of Ru was also observed for damascene Ru structure with the critical dimension ranging from ∼12 to 35 nm. An alternative approach using O2-based plasma achieved a good control of the etching (recessing) depth but also generated a thin layer of “residues” close to the interface between Ru and the liner (TiN). The results from this study have demonstrated that a short H2 plasma treatment prior to the wet process using the same chemical mixtures was able to etch the modified Ru surface layer together with the interfacial residues selectively to bulk Ru.