Thermal Atomic Layer Etching of Aluminum Nitride Using HF or XeF₂ for Fluorination and BCl₃ for Ligand Exchange

Thermal atomic layer etching (ALE) of amorphous and crystalline aluminum nitride was performed using sequential exposures of hydrogen fluoride (HF) or xenon difluoride (XeF 2 ) as the fluorination reactant and boron trichloride (BCl 3 ) as the ligand-exchange reactant. The expected fluorination reactions are AlN + 3HF(g) → AlF 3 + NH 3 (g) and AlN + 1.5XeF 2 → AlF 3 + NF 3 (g) + 1.5 Xe(g). The expected complete ligand-exchange reaction is AlF 3 + BCl 3 (g) → BF 3 (g) + AlCl 3 (g). The HF fluorination reactant together with BCl 3 could etch amorphous AlN films deposited using AlN atomic layer deposition (ALD). The XeF 2 fluorination reactant together with BCl 3 was required to etch crystalline AlN. Fourier transform infrared (FTIR) spectroscopy was utilized to study the growth of AlN ALD using tris(dimethylamido)aluminum and ammonia as the reactants. Thermal AlN ALE of the AlN ALD films with HF and BCl 3 as the reactants was then monitored using FTIR spectroscopy. The etching of the AlN ALD films occurred at temperatures greater than 200 °C. The change of the spectral features versus HF and BCl 3 exposures was consistent with a ligand-exchange etching mechanism for AlN thermal ALE. In situ spectroscopic ellipsometry (SE) was used to measure the thermal ALE of crystalline AlN using XeF 2 and static exposures of BCl 3 . The crystalline AlN displayed etch rates that varied with temperature from 0.19 Å/cycle at 212 °C to 0.93 Å/cycle at 298 °C. The etch rates were dependent on both the static BCl 3 exposure and the XeF 2 exposure. Consistent etch rates could be obtained by controlling the time or number of reactant exposures. X-ray photoelectron spectroscopy (XPS) studies were consistent with aluminum fluoride (AlF 3 ) being formed during the XeF 2 exposure and subsequently removed after the BCl 3 exposure. Quadrupole mass spectrometry (QMS) was also used to identify the volatile etch products during the reaction of BCl 3 with AlF 3 . These QMS investigations observed AlCl 3 as the etch product and BCl x F y ligand-exchange products. The temperature dependence of the AlCl 3 etch products and BCl x F y ligand-exchange products revealed that the ligand-exchange products appeared at temperatures below the onset of the AlCl 3 etch products at 200 °C. H 2 O was also utilized together with XeF 2 and BCl 3 to attempt to remove Cl and F from the AlN surface either during or after AlN ALE. XPS studies revealed that a final large H 2 O exposure after AlN ALE was most effective at Cl and F removal.