Etching of molybdenum via a combination of low-temperature ozone oxidation and wet-chemical oxide dissolution

Abstract

Etching of molybdenum was demonstrated in two steps. Mo was first oxidized in an ozone gas ambient to form molybdenum oxide. It is shown that comparable oxide thicknesses can be obtained in ozone and oxygen but at lower temperatures for the former. Initial oxide growth is fast but then considerably slows down due to its diffusion-limited character. The metal-oxide thickness can be controlled by temperature and defines the amount of metal etch per cycle (EPC). XPS analysis showed that the thermally grown oxide is MoO3. In the second, wet-chemical step, MoO3 was dissolved selectively toward the Mo metal using an aqueous solution. The dissolution rate of amorphous MoO3 formed in O3 at temperatures below ∼230 °C is fast, but the dissolution of MoO3 formed at Tox > 230 °C was shown to be incomplete. Cross-section TEM showed a matrix of amorphous oxide with crystallized MoO3 islands, the latter more difficult to dissolve. However, the crystalline phase could be completely and selectively removed using a more concentrated NH4OH solution at an elevated temperature (70 °C). The EPC was determined for temperatures between 150 and 290 °C. The etch rates increased with temperature from 1–2 nm/cycle at 150 °C to 5–6 nm/cycle at 290 °C. This hybrid thermal-wet etching sequence is well suited for vertical and lateral recess etching as it shows a controlled and isotropic dissolution of polycrystalline Mo at the nanoscale. Furthermore, the process shows a progressive surface smoothening upon increasing the number of etching cycles.

Thermal Atomic Layer Etching of Molybdenum Using Sequential Oxidation and Deoxychlorination Reactions

“…42 Mo has also been targeted by the semiconductor industry as a possible candidate for sub-10 nm interconnects because of its small electron mean free path compared with other metals. 43,44 Etching processes for Mo can involve high-temperature sublimation, 45 wet etching, 46 or plasma-based etching. 47,48 Plasma-based Mo ALE processes have also been developed recently using oxidation and chlorination or fluorination to form volatile oxychlorides or oxyfluorides.…”

“…Molybdenum (Mo) is a refractory metal used to form metal alloys that have high strength and resistance against corrosion . Mo has also been targeted by the semiconductor industry as a possible candidate for sub-10 nm interconnects because of its small electron mean free path compared with other metals. , Etching processes for Mo can involve high-temperature sublimation, wet etching, or plasma-based etching. , Plasma-based Mo ALE processes have also been developed recently using oxidation and chlorination or fluorination to form volatile oxychlorides or oxyfluorides. , However, there have been no previous reports of a Mo thermal ALE process. Thermal ALE processes are desirable because they avoid the ion surface damage that can occur during plasma processing.…”