| AVS 64th International Symposium & Exhibition | |
| Plasma Science and Technology Division | Thursday Sessions |
| Session PS+NS+SS+TF-ThM |
| Session: | Atomic Layer Etching I |
| Presenter: | Jonas Gertsch, University of Colorado Boulder |
| Authors: | J.C. Gertsch, University of Colorado Boulder V.M. Bright, University of Colorado Boulder S.M. George, University of Colorado Boulder |
| Correspondent: | Click to Email |
Thermal atomic layer etching (ALE) is based on sequential self-limiting thermal reactions [1]. Thermal ALE offers a precise and gentle etching procedure and has been demonstrated for many materials including Al2O3, HfO2, ZnO, ZrO2, SiO2 and AlN [1,2]. This study developed thermal ALE processes for vanadium oxide (VO2). VO2 has a metal-insulator transition at ~68 °C and is useful for thermochromic films and heat-switching devices. The initial VO2 films were deposited using VO2 atomic layer deposition (ALD) with tetrakis(ethylmethylamino) vanadium(IV) (TEMAV) and H2O as the co-reactants at 150 °C.
The VO2 films were etched using sequential exposures of sulfur tetrafluoride (SF4) and either tin(II) acetylacetonate (Sn(acac)2) or boron trichloride (BCl3) at temperatures ranging from 150-250 °C. In situ quartz crystal microbalance studies were used to monitor film growth and etching during the ALD and ALE reactions. The VO2 etching mechanism using SF4 and Sn(acac)2 is observed to occur by fluorination and ligand-exchange reactions [1]. The SF4 exposures yielded mass gains that were consistent with fluorination of VO2 to VF4. The Sn(acac)2 exposures then led to mass losses that were attributed to ligand-exchange reactions that produced volatile acetylacetonate reaction products. VO2 ALE etch rates increased with temperature from 0.04 Å/cycle at 150 °C to 0.27 Å/cycle at 225 °C.
A different reaction mechanism was observed for VO2 ALE using SF4 and BCl3. The SF4 exposures yielded mass losses at all temperatures that were not consistent with simple fluorination of VO2 to VF4. The BCl3 exposures produced mass losses at higher temperatures and slight mass gains at 150 °C. The etching of VO2 by SF4 and BCl3 is believed to occur by a “conversion-etch” mechanism [2]. In the “conversion-etch” mechanism, BCl3 converts the surface of VO2 to a thin B2O3 layer. SF4 can then remove the B2O3 layer to produce volatile BF3 and SO2. The VO2 etch rates increased with temperature from 0.06 Å/cycle at 150 °C to 1.9 Å/cycle at 250 °C.
[1] Y. Lee, C. Huffman and S. M. George, “Selectivity in Thermal Atomic Layer Etching Using Sequential, Self-Limiting Fluorination and Ligand-Exchange Reactions”, Chem. Mater. 28, 7657 (2016).
[2] D. R. Zywotko and S. M. George, “Thermal Atomic Layer Etching of ZnO by a “Conversion-Etch” Mechanism Using Sequential Exposures of Hydrogen Fluoride and Trimethylaluminum”, Chem. Mater. 29, 1183-1191 (2017).