Atomic Layer Etching of HfO2 Using Sequential, Self-Limiting Thermal Reactions with Sn(acac)2 and HF

Abstract

The atomic layer etching (ALEt) of HfO2 was performed using sequential, self-limiting thermal reactions with tin(II) acetylacetonate (Sn(acac)2) and HF as the reactants. The HF source was a HF-pyridine solution. The etching of HfO2 was linear with atomic level control versus number of Sn(acac)2 and HF reaction cycles. The HfO2 ALEt was measured at temperatures from 150–250°C. Quartz crystal microbalance (QCM) measurements determined that the mass change per cycle (MCPC) increased with temperature from −6.7 ng/(cm2 cycle) at 150°C to −11.2 ng/(cm2 cycle) at 250°C. These MCPC values correspond to etch rates from 0.070 Å/cycle at 150°C to 0.117 Å/cycle at 250°C. X-ray reflectivity analysis confirmed the linear removal of HfO2 and measured an HfO2 ALEt etch rate of 0.11 Å/cycle at 200°C. Fourier transform infrared (FTIR) spectroscopy measurements also observed HfO2 ALEt using the infrared absorbance of the Hf-O stretching vibration. FTIR analysis also revealed absorbance features consistent with HfF4 or HfFx surface species as a reaction intermediate. The HfO2 etching is believed to follow the reaction: HfO2 + 4Sn(acac)2 + 4HF → Hf(acac)4 + 4SnF(acac) + 2H2O. In the proposed reaction mechanism, Sn(acac)2 donates acac to the substrate to produce Hf(acac)4. HF allows SnF(acac) and H2O to leave as reaction products. The thermal ALEt of many other metal oxides, as well as metal nitrides, phosphides, sulfides and arsenides, should be possible by a similar mechanism.