Emerging communication technologies like 5G or Near Field Communication call for voltage tunable ferroelectric (FE) film capacitors to work at higher frequencies or lower voltage, thus requiring the reduction of the FE thickness to push away the piezoelectric resonance frequency and maintain high tunability. Unfortunately, two interface-related phenomena, the FE dead layer and leakage current, impede this evolution. Recent ab initio calculations showed the importance of the chemical bonding, polar discontinuity and rumpling at electrode / (Ba,Sr)TiO3 (BST) FE perovskite interfaces for polarization stabilization and Schottky barrier height (SBH) adjustment.1,2 In this work, a systematic electrode / BST interface engineering has been performed with the insertion of a thin La1-xSrxMnO3 Interface Control Layer with continuous composition variation (LSMOx ICL 0≤x≤1) by combinatorial Pulsed Laser Deposition. In the epitaxial (00l) STO(subs.)/La0.7Sr0.3MnO3/LSMOx (3uc)/SrTiO3 (0-4nm) heterostructure, the La3+/Sr2+ ratio controls the charge discontinuity at LSMOx/STO interface. As x↘ the nominal LSMOx terminal plane changes from Mn+4O2-2 to Mn+3O2-2 corresponding to a surface charge per unit cell variation from 0 to -1. Consequently, the LSMOx surface work-function increases leading to a transition from negative to positive band bending at LSMOx/STO interface as seen by UV photoemission spectroscopy (Figure a).3 Local I(V)s through LSMOx/STO interaces were acquired by AFM in contact mode. The SBH extracted from Fowler-Nordheim tunneling model fitting quantitatively agreed with UPS measurements (Figure b). DFT calculations reproduced the observed UPS band bending as a function of Sr content and underlined its correlation with therumpling in STO. Furthermore, to implement this knowledge in industrial BST varactors, we deposited polycrystalline Pt/LSMOx (5uc)/BST (200nm)/Au heterostructure on Sapphire. This varactors library was investigated by impedance spectroscopy (IS) to understand the effect of the LSMOx ICL on the tunability and losses (Figure c). The Sr content has an antisymmetric effect on the permittivity and the tunability, pointing towards a strong ICL influence on BST interface polarization. IS and Ferroelectric local characterizations will be pres