Surface reaction kinetics of metal β-diketonate precursors with O radicals in radical-enhanced atomic layer deposition of metal oxides

The surface reaction kinetics of Er(TMHD)3 and Y(TMHD)3 with O radicals in radical-enhanced atomic layer deposition (ALD) of Er2O3 and Y2O3 was investigated in situ using a quartz crystal microbalance (QCM). The adsorption isotherms were fitted with the Langmuir-Hinshelwood adsorption model and the extracted adsorption rate coefficient was found to decrease with increasing temperature, exhibiting a negative apparent activation energy of −0.24 ± 0.09 eV for Er(TMHD)3 and −0.14 ± 0.05 eV for Y(TMHD)3. The corresponding activation energies for desorption were determined to be 0.29 ± 0.03 and 0.16 ± 0.03 eV. Exposing the adsorbed Er(TMHD)3 precursors to O radicals at 533 K resulted in a rapid mass decrease followed by saturation, indicating that the reactions proceeded in a self-limiting manner. The critical O radical exposure needed to reach this saturation increased with increasing adsorbed mass and approached approximately 2 minutes as the adsorbed mass increased towards the saturation level. The net mass change ratio per cycle decreased with increasing temperature and reached 0.27 at 603 K for deposition of pure Er2O3. In addition to effectively removing the β-diketonate ligands, the O radicals were found to create reactive sites for precursor adsorption. Specifically, when the O radical pulse time was shorter than the critical oxygen radical exposure, the removal of β-diketonate ligands by the O radicals was incomplete, and consequently, less reactive sites were created. This ultimately led to a decrease in adsorption during the subsequent precursor pulse. Finally, radical-enhanced ALD of Er2O3 thin films was achieved at temperatures ranging from 473 to 573 K, using alternating pulses of metal β-diketonate precursors and O radicals.