Ultrathin aluminum electrodes on polyethylene terephthalate (PET) enable lightweight self-healing film capacitors, but the large surface-energy mismatch between Al and PET promotes Volmer-Weber island growth. At thicknesses of only several tens of nanometers, this growth mode leaves discontinuities, raises sheet resistance, and weakens interfacial adhesion. This work links radio-frequency plasma-induced surface states with the nucleation, microstructure, electrical response, and adhesion of vacuum-evaporated Al films. Biaxially oriented PET was treated in Ar, O₂, N₂, and O₂/Ar plasmas. Water and diiodomethane contact angles were combined with Owens-Wendt-Rabel-Kaelble surface-energy analysis, atomic force microscopy, and X-ray photoelectron spectroscopy. Evaporation rate (0.5-2.0 Å s⁻¹), substrate temperature (5-20 °C), and base pressure (8.5×10^-4 - 3.5×10^-3 Pa) were then optimized before evaluating Al films on untreated and plasma-treated PET. All plasmas increased wettability, but their mechanisms differed. O₂ plasma gave the strongest response: treatment at 20 W for 80 s reduced the water contact angle from 85.9° to 3.7°. O₂ treatment increased the O/C ratio and introduced C=O and COOH groups, whereas N₂ produced a moderate response through N-C=O groups. Ar acted mainly through ion bombardment and physical etching. Across the treatment atmospheres, the polar component of surface energy governed the water contact angle, while the surface-area difference correlated with the diiodomethane contact angle by a power law (R² = 0.987). For evaporation, 2.0 Å s^-1, 5 °C, and 8.5 × 10^-4 Pa provided the best electrical condition. Increasing oxygen partial pressure raised the sheet resistance of 20 nm Al films to 38.42 and 96.42 Ω sq^-1 at 2.5 × 10^-3 and 5.0 × 10^-3 Pa, respectively, confirming the sensitivity of Al conductivity to oxygen-related high-resistance phases. O₂ pretreatment produced more uniform Al grains, introduced an Al-O-C interfacial component (18.69%), and increased the peel force from approximately 4 to 82 gf. It also lowered the threshold for continuous film formation: at 2 nm, untreated PET showed only 30.6% Al coverage, whereas the O₂-treated surface supported a continuous film. At 20 nm, all films were already continuous and their sheet resistance remained similar (5.44-5.82 Ω sq^-1). These results show that plasma chemistry controls interfacial activation, while topography provides a complementary contribution. Coupling low-power O₂ treatment with controlled vacuum evaporation promotes early Al coalescence and strong PET/Al bonding without an interlayer, supporting thinner and more uniform metallized electrodes for reliable film capacitors.
 

polyethylene terephthalate; RF plasma; vacuum evaporation; ultrathin Al film; surface energy; interfacial adhesion