Thermal atomic layer etching of silicon nitride using an oxidation and “conversion etch” mechanism

Abstract

Thermal atomic layer etching (ALE) of silicon nitride was achieved using sequential exposures of oxygen (O2) or ozone (O3), hydrofluoric acid (HF), and trimethylaluminum [TMA, Al(CH3)3]. Thermal Si3N4 ALE will be useful to etch Si3N4 in semiconductor, optoelectronic, and MEMS devices. Thermal Si3N4 ALE was performed with Si3N4 thin films deposited on silicon wafers using low pressure chemical vapor deposition. In situ spectroscopic ellipsometry (SE) was employed to monitor the changes in the Si3N4 film thickness as well as the SiO2 layer thickness. The SE results at 290 °C yielded an Si3N4 etch rate of 0.25 Å/cycle with an O2-HF-TMA reactant sequence using partial pressures of 250, 0.65, and 1.2 Torr for O2, HF, and TMA, respectively. The O2, HF, and TMA reactants were held statically at the indicated partial pressures for 10, 5, and 5 s, respectively. Larger etch rates were observed using O3 instead of O2 as the oxidant. A higher Si3N4 etch rate of 0.47 Å/cycle was measured at 290 °C using an O3-HF-TMA reactant sequence at the same partial pressures and static exposure times as the O2-HF-TMA sequence. The Si3N4 etch rate was observed to decrease at lower temperatures. An Si3N4 etch rate of 0.07 Å/cycle was measured at the lowest temperature of 210 °C using an O3-HF-TMA reactant sequence. The Si3N4 surface roughness was reduced after Si3N4 ALE. The SiO2 layer on Si3N4 could be removed using sequential HF and TMA exposures. These sequential HF and TMA exposures could also very slowly etch the Si3N4 substrate. The Si3N4 etch rate was dependent on the reaction sequence. When an O3-TMA-HF sequence was employed with reactant partial pressures of 250, 0.65, and 1.2 Torr for O3, HF, and TMA, respectively, the Si3N4 etch rate was 0.20 Å/cycle at 290 °C. Thermal Si3N4 ALE adds to the growing list of materials that can be etched with atomic layer control using thermal chemistry.