Intensity calibration for monochromated Al Kα XPS instruments using polyethylene

We describe a method for the intensity calibration of XPS instruments using polyethylene as the reference material. Previous methods for have employed noble metals, such as gold, silver and copper. Polyethylene has a number of advantages over these. It has far fewer photoelectron and Auger electron peaks than such metals, i.e. the spectrum largely comprises inelastic background over a wide and continuous range of kinetic energies. The XPS spectrum can be described by a mathematical function enabling simple and noise-free implementation of the reference spectrum. Polyethylene can be cleaned ex-situ using a sharp knife or razor blade to remove trace oxygen and, due to its chemical composition, should not be affected by adventitious carbon contamination. Thus, an ion source for sputter cleaning is not required, although an electron flood source for charge compensation is required. The drawback to using polyethylene is that the photoelectron yield is far lower than gold or silver and this necessitates longer acquisition times and removal of dark noise. Longer acquisition times carry the risk of damaging the polyethylene surface and we show that, even if damage does occur, it has negligible effect on the XPS background intensity. The reference spectrum is valid for monochromated Al Kα XPS instruments with a monochromator-sample-analyser angle close to 60°.