Amorphous hydrogenated carbon (a‐C:H) deposited on steel with plasma enhanced chemical vapor deposition can be used as elongation tolerant oxygen barrier. However, the elongation tolerance of the a‐C:H film is lost if deposited on a poly(ethylene terephthalate) (PET) for reasons unknown. To assess this phenomenon, a‐C:H was deposited on PET, silicon substrates, and silicon micro‐cantilevers, and the stress was determined by measuring the radius of curvature. a‐C:H deposited on PET showed lower compressive stress than on silicon. This difference is not due to the formation of a gradient layer or plastic deformation of PET. Instead, the most probable explanation is that energetic ions cause a partial release of biaxial orientation within the PET, thereby reducing the compressive stress.