3.3. Reactive process
In the oxide mode, the mass deposition rate is about 8 times lower than when sputtering in pure Ar as a result of target oxidation [31]. This corresponds to about 13 times lower sputtering yield of Ti, considering that stoichiometric TiO2 is deposited in the oxide mode. Therefore, the measurements were performed with a gridless sensor where the electron repelling grid is replaced by a magnetic field and bias is applied directly to the top electrode of the quartz crystal. In order to mitigate the formation of an insulating layer on the sensor during measurements in the oxide mode, the surface was periodically coated with fresh Ti as described in the Experimental section.