The interface formation between ferroelectric HfO₂ layers and TiN bottom electrodes was studied by hard X-ray photoelectron spectroscopy and directly correlated to the electric polarization characteristics of the TiN/HfO₂/TiN capacitors. We consistently deduced the interface chemistry from HfO₂- and TiN-related core levels, dependent on the oxygen flow in supplied before and during physical vapor deposition (PVD) growth of HfO₂. The results underline the critical, twofold impact of oxygen supply on HfO₂ and interface properties. When supplied before growth, the supplied oxygen stabilizes the TiN/HfO₂ interface by oxidation and formation of a self-limiting (noninsulating) TiO₂ intralayer. When supplied during growth, on the other hand, oxygen flows above a critical threshold reduce the oxygen vacancy concentration within the HfO₂ film. We reveal a direct relation between the maximum ferroelectric remanent polarization and a critical threshold PVD oxygen exposure flow rate. The results allow for advancement of the PVD growth process in terms of a more flexible design of the ferroelectric HfO₂ films with chemically stable TiN interfaces.