Reactive Ion Etching of Silicon Dioxide Using Molybdenum as a Hard Mask

Various metals such as Aluminum (Al) and aluminum nitride (AlN) [1] have been used as hard masks for etching thick silicon oxide (SiO₂) layers. This study explores the efficacy of using Molybdenum (Mo) as a hard mask for Reactive Ion Etching (RIE) of SiO₂. Molybdenum offers the advantage of straightforward removal methods. In our experiments, a 30 nm thin film of Mo was employed as a hard mask to etch 300 nm of SiO₂ using a CHF₃ (60 sccm) and O₂ (3 sccm) gas mixture in an RIE system (30 mTorr and 100 W). The etch rate of SiO₂ was determined to be approximately 25 nm/min based on ellipsometry measurements. The etch rate of Molybdenum in the same process was estimated to be less than 0.5 nm/min based on electrical resistance measurements. SEM was used to analyze the surface morphology of the films after etching and revealed that the Mo thin film remains smooth and highly conductive after the etch, making it an attractive choice for microfabrication processes. Post-etching, the Mo layers were successfully removed either through wet etching with hydrogen peroxide (H₂O₂) or deionized (DI) water [2], or via a secondary RIE step using CF₄ and O₂ gases. Molybdenum can be used as a hard mask in CHF₃/O₂ RIE for applications requiring deep etching and clean removal processes.<br/>[1] Bliznetsov, V., Lin, H. M., Zhang, Y. J., & Johnson, D. (2015). Deep SiO₂ Etching with Al and AlN masks for MEMS devices. Journal of Micromechanics and Microengineering, 25(8), 087002. https://doi.org/10.1088/0960-1317/25/8/087002<br/>[2] Yin, L., Cheng, H., Mao, S., Haasch, R., Liu, Y., Xie, X., Hwang, S., Jain, H., Kang, S., Su, Y., Li, R., Huang, Y., & Rogers, J. A. (2013). Dissolvable metals for transient electronics. Advanced Functional Materials, 24(5), 645–658. https://doi.org/10.1002/adfm.201301847