Nanoscale thinning of metal-coated polypropylene films by Helium-ion irradiation

Polypropylene (PP) films have a wide range of applications, e.g. as dielectric materials for metallized film capacitors. In this article, we present a method for thickness reduction of PP films by ion irradiation, which has a direct effect on device capacitance. We show that the thickness of PP layers can be reduced by irradiation with He+ ions and controlled on the nanometer scale by the irradiation dose. The effect of different thin metal film coatings on PP surface was also investigated. The metal coatings were used for two reasons: they function as one of the metal electrodes in the capacitor structure, and they minimize sample charging during ion irradiation. Three different metallization materials were investigated: 5 nm Pt60Pd40, 5 nm Au, and 15 nm Al. We studied two technologically relevant PP films: the thinnest commercially available biaxially oriented polypropylene (BOPP) and spin-coated polypropylene (SC-PP) thin films. The irradiation was done with a focused Helium-ion beam (He-FIB) in a Zeiss Orion NanoFab Microscope at a landing energy of 30 keV with doses in a range of 5.4 × 10–5 nC/μm2 to 8.1 × 10–3 nC/μm2. An atomic force microscope (AFM) was used to analyze the details of surface modification: the surface height of the irradiated regions and surface morphology changes caused by the irradiation. For all applied doses, the Al-coated samples demonstrated smaller surface-height reduction compared to the Pt60Pd40 and Au-coated samples. We speculate that the possible factors responsible for this effect include differences in the thickness and the crystalline-grain orientation (texture) of the metallization films. Both BOPP and spin-coated PP presented surface ridges at the borders between the irradiated and non-irradiated regions. It can be attributed to the mechanical strain induced by the material modification.