Carrier-selective and passivating SiOx/TiOy heterocontacts are an attractive alternative to conventional contacts due to their high efficiency potentials combined with relatively simple processing schemes. It is widely accepted that post deposition annealing is necessary to obtain high photovoltaic efficiencies, especially for full area aluminum metallized contacts. Despite some previous high-level electron microscopy studies, the picture of atomic-scale processes underlying this improvement seems to be incomplete. In this work, we apply nanoscale electron microscopy techniques to macroscopically well-characterized solar cells with SiOx/TiOy/Al rear contacts on n-type silicon. Macroscopically, annealed solar cells show a tremendous decrease of series resistance and improved interface passivation. Analyzing the microscopic composition and electronic structure of the contacts, we find that partial intermixing of the SiOx and TiOy layers occurs due to annealing, leading to an apparent thickness reduction of the passivating SiOx. However, the electronic structure of the layers remains clearly distinct. Hence, we conclude that the key to obtain highly efficient SiOx/TiOy/Al contacts is to tailor the processing such that the excellent chemical interface passivation of a SiOx layer is achieved for a layer thin enough to allow efficient tunneling through the layer. Furthermore, we discuss the impact of aluminum metallization on the above mentioned processes.