Charging effects in the ion beam analysis of insulating polymers

We have studied the energy shift introduced by surface charging during the course of ion beam analysis of an insulating material (a Kapton thin film) using Rutherford Backscattering Spectrometry (RBS). The potential that develops was found to be proportional to both the incident ion energy and the sample thickness. This correlation suggests a conductance mechanism that is based on the ion beam induced carriers that are generated in the material. A simple relationship between the mobility of the induced carriers and the built-up surface potential has been derived. It provides a semi-quantitative explanation for the observed spectral offset. This relationship also provides important insight into the electrical characteristics of the insulating material being irradiated. In the specific case of Kapton, we observed a split of the high energy edge in the RBS spectra, which we suggest is most likely a reflection of the presence of both amorphous and crystalline forms of Kapton. Some contribution from partial discharge of the surface potential to the surrounding is also considered.