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

Abstract

Thermal atomic layer etching (ALE) of amorphous and crystalline aluminum nitride was performed using sequential exposures of hydrogen fluoride (HF) or xenon difluoride (XeF2) as the fluorination reactant and boron trichloride (BCl3) as the ligand-exchange reactant. The expected fluorination reactions are AlN + 3HF(g) → AlF3 + NH3(g) and AlN + 1.5XeF2 → AlF3 + NF3(g) + 1.5 Xe(g). The expected complete ligand-exchange reaction is AlF3 + BCl3(g) → BF3(g) + AlCl3(g). The HF fluorination reactant together with BCl3 could etch amorphous AlN films deposited using AlN atomic layer deposition (ALD). The XeF2 fluorination reactant together with BCl3 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 BCl3 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 BCl3 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 XeF2 and static exposures of BCl3. 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 BCl3 exposure and the XeF2 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 (AlF3) being formed during the XeF2 exposure and subsequently removed after the BCl3 exposure. Quadrupole mass spectrometry (QMS) was also used to identify the volatile etch products during the reaction of BCl3 with AlF3. These QMS investigations observed AlCl3 as the etch product and BClxFy ligand-exchange products. The temperature dependence of the AlCl3 etch products and BClxFy ligand-exchange products revealed that the ligand-exchange products appeared at temperatures below the onset of the AlCl3 etch products at 200 °C. H2O was also utilized together with XeF2 and BCl3 to attempt to remove Cl and F from the AlN surface either during or after AlN ALE. XPS studies revealed that a final large H2O exposure after AlN ALE was most effective at Cl and F removal.