Thermal atomic layer etching of germanium-rich SiGe using an oxidation and “conversion-etch” mechanism

The thermal atomic layer etching (ALE) of germanium-rich SiGe was demonstrated using an oxidation and “conversion-etch” mechanism with oxygen (O₂) or ozone (O₃), hydrofluoric acid (HF), and trimethylaluminum [TMA, Al(CH₃)₃] as the reactants. The crystalline germanium-rich SiGe film was prepared using physical vapor deposition and had a composition of Si_0.15Ge_0.85. In situ spectroscopic ellipsometry was employed to monitor the thickness of both the SiGe film and the surface oxide layer on the SiGe film during thermal ALE. Using a reactant sequence of O₂-HF-TMA, the etch rate of the SiGe film increased progressively with temperatures from 225 to 290 °C. At 290 °C, the SiGe film thickness decreased linearly at a rate of 0.57 Å/cycle with a surface oxide thickness of 18–19 Å. This etch rate was obtained using reactant pressures of 25, 0.2, and 0.4 Torr and doses of 1.5, 1.0, and 1.0 s for O₂, HF, and TMA, respectively. The TMA and HF reactions were self-limiting and the O₂ reaction was reasonably self-limiting at 290 °C. Using an O₃-HF-TMA reaction sequence, the SiGe ALE etch rate was 0.42 Å/cycle at 290 °C. This etch rate was obtained using reactant pressures of 15, 0.2, and 0.4 Torr and dose times of 0.5, 1.0, and 1.0 s for O₃, HF, and TMA, respectively. The O₃, TMA, and HF reactions were all self-limiting at 290 °C. Atomic force microscopy images revealed that thermal ALE with the O₂-HF-TMA or O₃-HF-TMA reaction sequences did not roughen the surface of the SiGe film. The SiGe film was etched selectively compared with Si or Si₃N₄ at 290 °C using an O₂-HF-TMA reaction sequence. The etch rate for the SiGe film was >10 times faster than Si(100) or Si₃N₄ that was prepared using low-pressure chemical vapor deposition. This selectivity for the SiGe film will be useful to fabricate Si nanowires and nanosheets using SiGe as the sacrificial layer.