We present a systematic study on effective passivation of p-type crystalline silicon (c-Si) by Al2O3 and Al2O3/SiNx stacks. Ozone-based atomic layer deposition (ALD) Al2O3 was deposited with varied thickness, ozone concentration and deposition temperature. Thermal stability of fabricated samples was investigated systematically varying post-deposition thermal treatments, such as forming gas annealing (FGA) and fast firing. Blister free Al2O3/SiNx stacks were obtained for Al2O3 thicknesses between 3.5-10 nm yielding effective charge carrier lifetimes (tau(eff)) above 1.5 ms, which correspond to an implied open-circuit voltage (iV(oc)) above 730 mV. The best performing Al2O3 layers in terms of their passivation quality, deposition uniformity and firing stability after being capped by SiNx were obtained when a low ozone concentration was utilized in the ALD process. As no additional out-gassing process is necessary between ALD Al2O3 and PECVD SiNx deposition, these ozone-based ALD Al2O3 layers have the potential to simplify the rear-side passivation of PERC solar cells in mass production.