| AVS 64th International Symposium & Exhibition | |
| Plasma Science and Technology Division | Friday Sessions |
| Session PS+NS+SS+TF-FrM |
| Session: | Atomic Layer Etching II |
| Presenter: | Nicholas Johnson, University of Colorado at Boulder |
| Authors: | N. Johnson, University of Colorado at Boulder S.M. George, University of Colorado at Boulder |
| Correspondent: | Click to Email |
Atomic layer etching (ALE) of metals is important for the controlled removal of many valuable semiconductor materials such as conductors (e.g. W, Cu), metal gates (e.g. Ta, Ti) and metals in magnetic multilayers (e.g. Co, Fe). However, few reports exist for metal ALE using either plasma or thermal processes. Conventional thermal ALE that has defined recent work on metal oxide [1] and metal nitride [2] materials does not work for metals. New reaction pathways are required to etch metals. This study targets W ALE and examines both WO3 ALE and W ALE as W oxidation to WO3 is needed to define self-limiting reactions for W ALE.
WO3 ALE was demonstrated using an AB exposure sequence with boron trichloride (BCl3) and hydrogen fluoride (HF). BCl3 and HF etch WO3 by a “conversion-fluorination” mechanism. The BCl3 converts the WO3 surface to a B2O3 layer while forming volatile WOxCly. HF then spontaneously etches the B2O3 layer producing volatile BF3 and H2O products. WO3 films were formed by oxidizing W ALD films with an oxygen plasma at 280oC. In situ spectroscopic ellipsometry (SE) studies determined that the BCl3 and HF reactions were self-limiting versus exposure. WO3 ALE etch rates increased with temperature from 0.55 Å/cycle at 128°C to 4.19 Å/cycle at 207°C. W served as an etch stop because BCl3 and HF could not etch the underlying W film.
W ALE was performed using a three-step “oxidation-conversion-fluorination” mechanism. This is an ABC exposure sequence that where the W surface is first oxidized to a WO3 layer and then the WO3 layer is etched with BCl3 and HF. SE could simultaneously monitor the W and WO3 thicknesses and conversion of W to WO3. Oxidation of the W surface was performed using O3. SE measurements showed that the W film thickness decreased linearly with number of ABC reaction cycles. W ALE was shown to be self-limiting with respect to each part of the ABC process. The etch rates for W ALE were 2.4 Å/cycle at 207°C. An oxide thickness of ~20Å remained during W ALE, but could be removed with BCl3/HF without affecting the W layer.
[1] Younghee Lee, et al., “Trimethylaluminum as the Metal Precursor for the Atomic Layer Etching of Al2O3 Using Sequential, Self-Limiting Thermal Reactions”, Chem. Mater. 28, 2994-3003 (2016).
[2] Nicholas R. Johnson, et al., “Thermal Atomic Layer Etching of Crystalline Aluminum Nitride Using Sequential, Self-Limiting HF and Sn(acac)2 Reactions and Enhancement by H2 and Ar Plasmas”, J. Vac. Sci. Technol. A 34, 050603 (2016).