A method for manufacturing a display panel substrate has the steps of flowing a working gas along and out of a slit defined between two sputtering targets of Mg and toward the substrate thereby selecting the purity of the Mg material of the targets to be at least 99% and thereby blowing sputtered-off material out of the slit and toward the substrate. Introduction, in an area between the slit and the substrate, of a reactive gas containing oxygen, follows, and reacting the sputtered-off material with the reactive gas results in depositing on the substrate, an MgO layer. The method also includes setting the temperature of the substrate during the coating process.