Paper VT-WeA2
Background Reduction Strategies for Angular Profile Measurements of Gas Injected in Ultra-High Vacuum

Wednesday, November 11, 2009, 2:20 pm, Room J1

Ultra-high vacuum (UHV) based deposition techniques, such as molecular beam epitaxy (MBE) and chemical beam epitaxy (CBE), have stringent requirements on layer thickness and composition uniformity. Concurrently, the source use efficiency is usually very low and needs to be improved while maintaining the same level of uniformity. There is therefore a need for a precise and reliable simulation platform to predict the angular distribution of gas molecules injected in vacuum through a nozzle of a given geometry. Several calculation techniques have already been proposed for MBE [1-6] and CBE [7].

However, the validity of such models needs to be established through a systematic experimental study that clearly isolates the contributions of each parameter. For this purpose, a test platform dedicated to the measurement of molecular beam angular profiles produced by a nozzle in UHV was designed and built. Its main features are discussed, especially regarding its ability to produce precise and reproducible data. For profiles being measured far away from the injector, the unwanted contribution from the molecules that reach the sensor after being scattered by the chamber walls (i.e. background level) is fairly large. In order to reduce it, several design strategies are considered and evaluated on the basis of the theory of rarefied gas dynamics. In particular, an innovative approach based on an angular selection tube is presented with a quantitative evaluation of its effect on the signal to background ratio. 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 possible.