Superconducting radio-frequency cavities made out of niobium form the fundamental block of modern particle accelerators. A model proposed by Gurevich [1] suggests the use of a superconductor-insulator-superconductor (SIS) structure to achieve higher accelerating fields and a reduced surface resistance beyond the thermodynamic limits of Nb. As a first step to pursue this approach, a single-cell cavity was coated with a thin Al2O3 film via atomic layer deposition (ALD) to create an insulating layer [2] and baked for 3h at 300°C (mid-T heat treatment) [3]. In parallel, a mechanically polished two-grain-Nb sample was treated and coated analogically to the cavity. To further understand the RF performance of the coated and annealed cavity, an XRR analysis of the sample was carried out at each processing step to follow the changes in the niobium native oxides at process conditions (120°C) and throughout the chemical deposition and show that the coating technique and the resulting structure form a viable way for a further tailoring of cavity properties.