Abstract
There is a growing demand to obtain XPS analyses from increasingly smaller sample areas. The development of a Transmission X-Ray Photoelectron Spectrometer (TXPS) will allow spectrum acquisition and imaging with a lateral resolution approaching 1 mum2. The principle is based on back-foil excitation where thin samples are placed on an aluminium foil or have an evaporated source backing. Electron irradiation of the foil produces characteristic X-rays locally and this, in turn, gives rise to a small source of photoelectrons from the opposite side of the sample. Rastering the electron beam scans the X-ray interaction volume which allows imaging. Photoelectron lateral resolution is dependent on the sample and foil thicknesses and is determined by the width of the X-ray excitation envelope and on the electron beam diameter. In this work a VG Scientific MA500 is modified to give the ideal 180° geometry for TXPS. A hemispherical analyser with an extended high magnification transfer lens ensures a large solid angle of photoelectron collection, and the hemispherical analyser gives the (previously unobtainable) energy resolution necessary to obtain chemical state information. Aspects unique to TXPS spectral and image interpretation are described. The analysis conditions where sample damage is likely are investigated. Photon induced damage is manageable, but is shown to be far more of a technique drawback than Joule heating. Specimen charging presents no particular problems, although it can be both positive and negative in TXPS. It is believed that TXPS is the ideal way of examining ultramicrotomed sections through interfaces and soft composite materials. Harder materials, such as ceramics and metals, require ion beam thinning in a similar way to TEM specimens. Specimen production needs are addressed as well as the problem of making the TXPS technique far more routine to the analyst. A magnesium/aluminium alloy is ion beam thinned to demonstrate TXPS from a harder material. The interlayer between a chlorine containing latex and mild steel is also analysed by TXPS, following the removal of the bulk substrate and ultramicrotomy. Data are examined and an iron valence state change across the interface allows conclusions to be drawn about the continued growth of the protective interphase under environmental exposure.