The defect densities controlling the LIDT of three HfO2 films with different underlying interfaces were measured using STEREO-LID. This technique measures the actual damage fluence during a 1-on-1 test. The films were tested with pulses of ~10 ns duration at 1064 nm. The 30-nm HfO2 films were prepared by ion-beam sputtering: the first was deposited directly on a fused silica substrate; the second was deposited after first laying down a half-wave buffer layer of SiO2; the third was deposited on a half-wave SiO2 buffer with a gradual transition to HfO2. The buffer layer reduces the density of defects triggering damage at low fluence by more than a factor of two, but the gradual interface slightly adds to the defect density. The implications of these results are compared to the damage behavior of a thicker (quarter-wave) HfO2 film.