Amorphous Al_2O_3 is an attractive material for integrated photonics. Its low losses from the UV till the mid-IR together with the possibility of doping with different rare-earth ions permits the realization of active and passive functionalities in the same chip at the wafer level. In this work, the influence of reactive gas flow during deposition on the optical (i.e., refractive index and propagation losses) and material (i.e., structure of the layer) characteristics of the RF reactive sputtered Al_2O_3 layers is investigated and a method based on the oxidation state of the sputtering target is proposed to reproducibly achieve low loss optical guiding layers despite the continuous variation of the condition of the target along its lifetime.