Thermal Atomic Layer Etching of Al2O3, HfO2, and ZrO2 Using Sequential Hydrogen Fluoride and Dimethylaluminum Chloride Exposures

Atomic layer etching (ALE) of Al2O3, HfO2, and ZrO2 was accomplished using sequential exposures with hydrogen fluoride (HF) as the fluorination reagent and dimethylaluminum chloride (DMAC, AlCI(CH3)(2)) as the metal reactant for ligand exchange. DMAC could provide either CH3 or Cl ligands for the ligand-exchange reaction. The presence of the CI ligand on DMAC led to efficient HfO2 and ZrO2 etching attributed to the formation of stable and volatile chloride species. In situ quartz crystal microbalance (QCM) measurement observed mass changes during Al2O3, HfO2, and ZrO2 ALE reactions at 200-300 degrees C. Al2O3, HfO2, and ZrO2 were etched linearly versus number of HF and DMAC sequential exposures. The QCM analysis confirmed that the HF and DMAC reactions were self-limiting versus reactant exposure. The QCM studies observed mass changes per cycle (MCPC) of -12.2 ng/(cm(2) cycle), -94.1 ng/(cm(2) cycle), and -75.6 ng/(cm(2) cycle) for Al2O3, HfO2, and ZrO2 ALE, respectively, at 250 degrees C. These MCPC correspond to Al2O3, HfO2, and ZrO2 etch rates of 0.39 angstrom/cycle, 0.98 angstrom/cycle, and 1.33 angstrom/cycle, respectively. In comparison, the AlF3, HfF4, and ZrF4 surface layers were estimated to have thicknesses of 3.0 angstrom, 3.3 angstrom, and 4.4 angstrom on Al2O3, HfO2, and ZrO2, respectively. The magnitudes of these fluoride thicknesses have the same ordering as the etch rates for Al2O3, HfO2, and ZrO2 ALE, respectively. X-ray reflectivity (XRR) and spectroscopic ellipsometry measurements verified the etch rates for Al(2)O(3)ALE. XRR analysis also confirmed smoothening of the etched Al2O3 film. The etch rates for Al2O3, HfO2, and ZrO2 ALE increased with temperature from 200 to 300 degrees C. A comparison of Al2O3, HfO2, and ZrO2 ALE using either HF and DMAC or HF and trimethylaluminum (TMA, Al(CH3)(3)) revealed that higher etch rates were observed for DMAC for all three materials. Cross-sectional transmission electron microscopy (TEM) studies also revealed that Al2O3 ALE on masked Al2O3 substrates was isotropic and selective in the presence of SiN and SiO2. Because of the ability of DMAC to provide either CH3 or Cl ligands during the ligand-exchange reaction, DMAC should be a very useful metal reactant for the thermal ALE of various materials.