Reduced Pressure Etching of Thermal Oxides in Anhydrous HF/Alcoholic Gas Mixtures

Reduced pressure etching of thermal oxide in anhydrous HF gas with three different alcoholic solvent vapors is studied. Thermal oxide etch rates as functions of temperature, pressure, time, and HF partial pressure are presented for methanol, ethanol-water azeotrope (95.6% ethanol, 4.4% water), and 2-propanol (isopropyl alcohol). The etch rates are interpreted in terms of alcohol vapor pressure, HF ionization, and reaction product desorption. The efficient desorption of reaction products compared to vapor HF/H₂O is believed to be responsible for both the wider process window for alcoholic solvents and the alleviation of the solid residue formation problem. Among the alcoholic solvents studied, methanol has the best potential while 2-propanol can also be useful in selected applications.