Influence of surface defects on hydrogen outgassing of stainless steel

M. LEISCH, A. JUAN

Daresbury

Conferencia; 1st Vacuum Symposium UK; 2010

British Vaccum Council

Stainless steel (SS) is one of the most commonly used constructional materials for vacuum chambers and components. Special applications like accelerator, storage ring facilities or advanced semiconductor device processing make need for extreme high vacuum (XHV). In the XHV regime a reduction of the outgassing rates of the materials used in the construction of the vacuum system is essential [1]. Beside surface treatment to reduce the surface roughness high temperature vacuum firing became an alternate method and widely accepted practice of reducing the amount of hydrogen dissolved in SS. For the description of the outgassing rate basically two models common as diffusion limited model (DLM) and recombination limited model (RLM) have been discussed [2-5]. Surface states which may influence the outgassing kinetics significantly are not considered in the DLM. The hydrogen atoms approaching the surface from the bulk are desorbing in a second-order process. It is well established that the rate of recombination depends strongly on the atomic structure of the surface and is e.g. generally higher on stepped surfaces than on flat close packed planes. In order to gain atomic level information on the real morphology of a surface after common bake-out and vacuum firing SS samples were imaged in the atomic force microscope (AFM) and the scanning tunnelling microscope (STM). The main experimental work has been carried out on a combined STM – atom probe field ion microscope (AP-FIM) apparatus [6]. A unique feature of the particular combined instrument is that it allows a fully-predictive preparation of STM probe tips in situ by FIM which is important for a reliable imaging of complex surfaces.