In this work, we study the modification of the piezoelectric surface after immersion in a methane plasma, the adhesion of DLC films without a buffer layer and the patterning of the DLC films needed for opening windows for deposition of other materials or access for metallization. Rutherford backscattering spectroscopy and elastic recoil detection analysis are used to determine the composition of the films and X-ray photoelectron emission spectroscopy allows the determination of the binding state of carbon on the LiTaO substrate. From nano-scratch tests we evaluate the adhesion by measuring the critical loads for first elastic cracking 3 and first delamination. X-ray reflectometry is used to obtain electron density profiles and in particular it yields knowledge of the interfacial layer created during deposition (thickness, electron density, roughness). Better adhesion is obtained at low pressure when competitive sputtering occurs, creating an interfacial layer between LiTaO and carbon.