The mechanism for ion-enhanced chemical etching of hafnium aluminate thin films in Cl-2/BCl3 plasmas was investigated in this work, specifically how the film composition, ion energy, and plasma chemistry determine their etch rates. Several compositions of Hf1-xAlxOy thin films ranging from pure HfO2 to pure Al2O3 were etched in BCl3/Cl-2 plasmas and their etch rates were found to scale with E-ion in both Cl-2 and BCl3 plasmas. In Cl-2 plasmas, a transition point was observed around 50 eV, where the etch rate was significantly enhanced while the linear dependence toE(ion) was maintained, corresponding to a change in the removal of fully chlorinated to less chlorinated reaction products. In BCl3 plasma, deposition dominates at ion energies below 50 eV, while etching occurs above that energy with an etch rate of three to seven times that in Cl-2. The faster etch rate in BCl3 was attributed to a change in the dominant ion from Cl-2+ in Cl-2 plasma to BCl2+ in BCl3, which facilitated the formation of more volatile etch products and their removal. The surface chlorination (0-3 at. %) was enhanced with increasing ion energy while the amount of boron on the surface increases with decreasing ion energy, highlighting the effect of different plasma chemistries on the etch rates, etch product formation, and surface termination.