The solid-state electrolyte UPON is synthesized using reactive magnetron sputtering. The correlation between plasma conditions and chemical composition of the layers is studied by X-ray photo electron spectroscopy, accompanied by electrochemical measurements of the ionic conductivity of the films. It is found that the plasma conditions in the vicinity of the substrate affect the lithium and nitrogen contents in a reciprocal manner. The target sputter-age and an additional bias potential to the substrate are identified as important parameters to optimize the performance of the layers. Based on this information, bilayer samples are prepared, consisting of UPON and a cathode of LiFePO4. Chronopotentiometry at these bilayers reveals the importance of a low interface resistance, and moreover, it identifies an insufficient lithium content at the UPON/LiFePO4 interface as a reason for high interface resistance. Nitrogen is also essential to increase the conductivity and hence, a lack of nitrogen at the interface is suggested to have the same consequence. It is shown how the interface resistance can be controlled by the preparation conditions to achieve an optimized performance.