Thermal Atomic Layer Etching of Aluminum Nitride Using HF orXeF2 for Fluorination and BCl3 for Ligand Exchange

Thermal atomic layer etching (ALE) of amorphous and crystallinealuminum nitride was performed using sequential exposures of hydrogenfluoride (HF)or xenon difluoride (XeF2) as thefluorination reactant and boron trichloride (BCl3)asthe ligand-exchange reactant. The expectedfluorination reactions are AlN + 3HF(g)-> AlF3+NH3(g) and AlN + 1.5XeF2 -> AlF3+NF3(g) + 1.5 Xe(g). The expectedcomplete ligand-exchange reaction is AlF3+ BCl3(g)-> BF3(g) + AlCl3(g). The HFfluorination reactant together with BCl3could etch amorphous AlNfilms depositedusing AlN atomic layer deposition (ALD). The XeF2fluorination reactant together withBCl3was 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 ALDfilms with HF and BCl3as the reactants was then monitored using FTIR spectroscopy. Theetching of the AlN ALDfilms occurred at temperatures greater than 200 degrees C. The change of the spectral features versus HF and BCl3exposures 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 XeF2and static exposures of BCl3. The crystalline AlN displayed etchrates that varied with temperature from 0.19 A/cycle at 212 degrees C to 0.93 A/cycle at 298 degrees C. The etch rates were dependent on boththe static BCl3exposure and the XeF2exposure. Consistent etch rates could be obtained by controlling the time or number ofreactant exposures. X-ray photoelectron spectroscopy (XPS) studies were consistent with aluminumfluoride (AlF3) being formedduring the XeF2exposure and subsequently removed after the BCl3exposure. Quadrupole mass spectrometry (QMS) was also usedto identify the volatile etch products during the reaction of BCl3with AlF3. These QMS investigations observed AlCl3as the etchproduct and BClxFyligand-exchange products. The temperature dependence of the AlCl3etch products and BClxFyligand-exchangeproducts revealed that the ligand-exchange products appeared at temperatures below the onset of the AlCl3etch products at 200 degrees C.H2O was also utilized together with XeF2and BCl3to attempt to remove Cl and F from the AlN surface either during or after AlNALE. XPS studies revealed that afinal large H2O exposure after AlN ALE was most effective at Cl and F removal.

Automated summary

Thermal atomic layer etching (ALE) of amorphous and crystalline aluminum nitride was investigated using different reactants. The etching mechanism and etch rates were analyzed using Fourier transform infrared spectroscopy and in situ spectroscopic ellipsometry. X-ray photoelectron spectroscopy and quadrupole mass spectrometry studies provided further insights into the chemical reactions observed in the experiments.