Signal enhancement strategies for angular profile measurements of gas injected in ultrahigh vacuum

Abstract

With current trends in physical chemistry and various ultrahigh vacuum (UHV) based fabrication techniques, such as thin film deposition and epitaxy, there is a growing need to develop a precise and reliable simulation platform to predict the angular distribution of gas molecules injected in vacuum through a nozzle of a given geometry. Such models need to be validated through a systematic experimental study that clearly isolates the contributions of each parameter. For this purpose, a test platform dedicated to the measurement of the molecular beam angular profiles produced by a nozzle in UHV has been designed and built. Its main features are discussed, especially regarding its ability to produce precise and reproducible data. In order to reduce the contribution from the molecules scattered by the chamber walls to the measured flux densities and thus achieve a better signal-to-background ratio, several strategies are considered and evaluated. In particular, an innovative approach based on a direction selection tube is presented with a quantitative evaluation of its effect on the measurements. Finally a rule of thumb is proposed for the choice of the tube’s dimensions allowing a maximum background reduction while keeping the impact on the signal as small as desired.