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The present generation of AC plasma display panels (AC PDP) requires further improvement because of poor voltage characteristics and high production cost. The voltage characteristics of AC PDPs are greatly affected by a protective layer because it is directly exposed to the gas plasma. To replace the conventional MgO protective layer in order to improve the voltage characteristics, panels with various protective layers of an MgO-CaO system were prepared by an e-beam evaporation method. Among the oxides of various composition of the MgO-CaO system, a panel with an Mg0.9Ca0.1O protective layer had the lowest firing and sustaining voltages. But, it exhibited poor aging characteristics, compared to the conventional MgO protective layer. A post annealing process was introduced so as to improve the surface properties and aging characteristics of the Mg0.9Ca0.1O protective layer in this work. The annealing process was performed in nitrogen gas as a function of holding time at 400 °C. From XRD analyses, the (111) orientation formed in the as-deposited Mg0.9Ca0.1O thin film remained unchanged after the annealing process. However, the surface morphology of the Mg0.9Ca0.1O protective layer, which is closely related to its aging characteristics was greatly affected by the annealing conditions. The refractive index of the Mg0.9Ca0.1O thin film was increased by the annealing process at 400 °C for 5 h, which is probably due to an increase in film density. The sustaining voltage of the annealed Mg0.9Ca0.1O film is lower in the initial aging period but higher than that of the as-deposited Mg0.9Ca0.1O film after obtaining a stable voltage. However, the annealing in nitrogen gas made the aging process much shorter, compared to that of the as-deposited Mg0.9Ca0.1O film. The aging characteristic of the Mg0.9Ca0.1O protective layer was found to be greatly improved by the annealing process in nitrogen gas at 400 °C for 5 h.