Effect of crystallinity on thermal atomic layer etching of hafnium oxide, zirconium oxide, and hafnium zirconium oxide

Thermal atomic layer etching (ALE) can be achieved using sequential, self-limiting fluorination and ligand-exchange reactions. Previous studies have demonstrated thermal ALE of amorphous HfO2 and ZrO2 ALD films. This study explored the differences between thermal ALE of amorphous and polycrystalline films of hafnium oxide, zirconium oxide, and hafnium zirconium oxide on silicon wafers. HF, XeF2, or SF4 were used as the fluorination reactants. Titanium tetrachloride or dimethylaluminum chloride (DMAC) was employed as the metal precursor for ligand exchange. The spectroscopic ellipsometric measurements revealed that the amorphous films had much higher etch rates per cycle than the crystalline films regardless of the fluorination reactants or metal precursors for ligand exchange. The differences were most pronounced for HfO2. Using HF and TiCl4 as the reactants at 250 degrees C, the etch rates were 0.36 angstrom/cycle for amorphous HfO2 and 0.02 angstrom/cycle for crystalline HfO2. In comparison, the etch rates were 0.61 angstrom/cycle for amorphous ZrO2 and 0.26 angstrom/cycle for crystalline ZrO2. The etch rates were 0.35 angstrom/cycle for amorphous HfZrO4 and 0.04 angstrom/cycle for crystalline HfZrO4. When HF and DMAC were used as the reactants, the etch rates were higher than with HF and TiCl4 for every material. Using HF and DMAC as the reactants at 250 degrees C, the etch rates were 0.68 angstrom/cycle for amorphous HfO2 and 0.08 angstrom/cycle for crystalline HfO2. In comparison, the etch rates were 1.11 angstrom/cycle for amorphous ZrO2 and 0.82 angstrom/cycle for crystalline ZrO2. The etch rates were 0.69 angstrom/cycle for amorphous HfZrO4 and 0.16 angstrom/cycle for crystalline HfZrO4. SF4 as the fluorination reactant resulted in higher etch rates than for HF when using TiCl4 as the metal precursor for ligand exchange. XeF2 as the fluorination reactant resulted in even higher etch rates than for SF4. The differences in the etch rate with the fluorination reactant can be partially attributed to differences in thermochemistry for fluorination by HF, SF4, and XeF2. The differences in etch rates between amorphous and crystalline films may be caused by the greater degree of fluorination and subsequent ligand-exchange reaction for the amorphous films. The amorphous films have a lower density and may be able to better accommodate the large volume expansion upon fluorination.