Thermal Atomic Layer Etching of Zinc Oxide from 30–300 °C Using Sequential Exposures of Hydrogen Fluoride and Trimethylgallium

Thermal atomic layer etching (ALE) of zinc oxide (ZnO) was demonstrated over a large temperature range from 30–300 °C using sequential exposures of HF (hydrogen fluoride) and Ga(CH₃)₃ (trimethylgallium (TMG)). In contrast to earlier studies of thermal ZnO ALE using sequential exposures of HF and trimethylaluminum (TMA), ZnO ALE with sequential HF and TMG exposures occurred without competing GaF₃ atomic layer deposition (ALD) or ZnO conversion. Quartz crystal microbalance (QCM) studies during ZnO ALE revealed a stepwise mass increase during fluorination by HF exposures and a larger mass decrease during ligand-exchange by TMG exposures. The mass changes per cycle (MCPC) were self-limiting versus HF and TMG exposures at 100 °C. Spectroscopic ellipsometry measured etch rates over a wide temperature range. The etch rates varied from 0.24 Å/cycle at 30 °C to 3.82 Å/cycle at 300 °C. The temperature-dependent etch rates were consistent with an activation barrier of E_a = 3.3 kcal/mol. TMG exposures were also compared with TMA exposures at 100 °C on fresh ZnO surfaces grown by ZnO ALD. TMG exposures led to a mass gain consistent with TMG adsorption. In contrast, TMA exposures produced a mass loss consistent with the conversion of ZnO to Al₂O₃. Previous studies showed that conversion of ZnO to Al₂O₃ prevented ZnO ALE using HF and TMA exposures at temperatures less than 205 °C. Etching at <205 °C was restricted because HF adsorption on fluorinated Al₂O₃ led to competing AlF₃ ALD. In contrast, ZnO ALE at temperatures as low as 30 °C is possible because no competing GaF₃ ALD occurs using HF and TMG exposures. Quadrupole mass spectrometry (QMS) experiments were also performed to identify the etch products during ZnO ALE. The QMS experiments support fluorination and ligand-exchange reactions without conversion during ZnO ALE using HF and TMG exposures. The HF and TMG exposures were selective for ZnO ALE compared with HfO₂, ZrO₂ or Al₂O₃ ALE. ZnO ALE could also smooth ZnO surfaces progressively versus number of ZnO ALE cycles.