Plasma etching technology has been widely used since the 1980s in the fabrication of ultra-large-scale integrated (ULSI) circuits. However, charged particles such as ions and electrons generate ion implantation to Si, have charge up damage to SiO@sub 2@ film and have low selectivity of the photoresist. On the other hand, HF gas is also widely used in a removal process of native SiO@sub 2@ films and in a cleaning process of a wafer as a dry process. Only chemical reactions are dominant in the reactions of HF gas etching and Si is not etched by HF gas, consequently, non-damaged, highly selective etchings of SiO@sub 2@ film and quartz plate can be achieved in HF gas etching. However, almost all reports about HF gas etching were isotropic etchings. When I investigated planarization technology employing anhydrous HF gas, very unique etching characteristics were observed when the etched depth of AHF gas etching was measured employing OFPR-800 as a photoresist. The stage temperature was set at 20° C and AHF gas was introduced to the chamber at a pressure of 30 Torr. After 15 min etching, AHF gas selectively etched the quartz surface just below the OFPR resist. In this etching, neither plasma nor ultraviolet light was used. A SiO@sub 2@ film below the photoresist, OFPR-800, was selectively etched with AHF gas. However, etch-stop was observed at the depth of 1.5 µm after 15 min. I found that the flow rate of HF gas and a high-pressure process were very effective with regard to this issue. When the flow rate of HF gas was increased to 4300 sccm, etching never stopped over 40 min and 20 µm in depth. The etched depth increased linearly with the increase in etching time. Both ZEP-520-22 and SAL-602-SR2 gave the same results. The etch-stop problem was considered to be mass-balance between the condensed layer below the resist and the gas phase. When AHF etched SiO@sub 2@ film on Si substrate, notching profile was observed during the over-etching process.