The effect that the base pressure achieved prior to deposition has upon the electrical, optical, structural, and chemical properties of ZnO:Al (aluminum-zinc oxide, AZO) thin films was studied. The thin films were obtained at room temperature on glass substrates by direct-current (DC) magnetron sputtering with a power of 120 W (corresponding to a power density of 2.63 W/cm2) and a total deposition pressure of 4.0 mTorr (0.53 Pa) with only argon gas. It was observed that all AZO characteristics and properties varied with base pressure without following a simple tendency, although some correlation was found between the crystallinity and deposition rate, with the latter depending directly upon the achieved base pressure. However, three distinct vacuum zones were identified, each of which produced AZO thin films that exhibited similar characteristics and properties. Among the base pressures studied, the lower base pressure zone [best vacuum level: under ~5.7×10-7 Torr (~7.6×10-5 Pa); dark yellow zone] produced dark yellow AZO thin films with moderate transmittance, but the highest resistivity and the worst stability to the environment. The middle base pressure zone [~5.7×10-7 Torr (~7.6×10-5 Pa) to ~7.3×10-7 Torr (~9.7×10-5 Pa); opaque zone] produced dark gray or brown AZO thin films with good conductivity, but that exhibited the worst transmittance. The higher base pressure zone [bad vacuum level: higher than ~7.3×10-7 Torr (~9.7×10-5 Pa); transparent zone] produced highly transparent AZO thin films with moderate resistivity. The best AZO thin film was deposited after achieving this transparent base pressure zone, but specifically in the transparent-opaque border [~7.35×10-7 Torr (~9.80×10-5 Pa)]. It was found that the films deposited near this transparent-opaque border within a wide margin in the direction of the transparent zone (to higher base pressure) exhibited the best transmittance, conductivity, and stability to the environment. The use of this moderate vacuum level is therefore recommended for the deposition of AZO thin films.