Thermal oxide (or similar pure SiO2) can be removed in vapor HF from 49% concentrated HF acid. Sample wafer(s) can be warmed by a light bulb to remove moisture on the surface to keep the process dry.
Etchrate seems to be almost equivalent to that of BHF (buffered HF) for thermal oxide. Careful trial & error is needed for your best results.
Maximum 1 millimeter long cantilever has been successfully released (with stress-controlled smartcut SOI, 1.5 microns SOI / 1.0 micron BOX).
#ref(): File not found: "VHF_jjap.pdf" at page "Research/Vapor HF Sacrificial Oxide Release with Lab Utensils"
#ref(): File not found: "vhf0.pdf" at page "Research/Vapor HF Sacrificial Oxide Release with Lab Utensils"