Thermal etching of AlF3 and thermal atomic layer etching of Al2O3

Abstract

Thermal etching of AlF3 with dimethyl-aluminum chloride (DMAC) and thermal isotropic atomic layer etching (ALE) of Al2O3 with alternating anhydrous hydrogen fluoride (HF) and DMAC steps were studied. DMAC vapor etches AlF3 spontaneously at substrate temperatures above 180 °C. The thermal etching reaction of AlF3 with DMAC exhibited no self-limitation and showed a linear dependence on DMAC pressure. The authors determined an activation energy of 1.2 eV for this reaction. When Al2O3 is fluorinated, DMAC removes the fluorinated layer partially. The etch amount per cycle (EPC) in thermal isotropic ALE of Al2O3 with HF/DMAC is primarily determined by the fluorination step placing significant importance on its design. Fluorination with HF gas was found to be more effective and repeatable than with NF3. Plasma fluorination is faster and provides higher EPC, but the selectivity to Si3N4 or SiO2 mask materials is compromised. For pressures between 10 and 110 mTorr and a substrate temperature of 250 °C, thermal ALE of Al2O3 with HF/DMAC was found to have a very high selectivity to SiO2 and amorphous silicon. HfO2, however, etched with similar EPC as Al2O3.