Differential Pump

About

Summary Flux instability of the evaporation sources is one of the biggest problems in the multi-elemental oxide Molecular Beam Epitaxy techniques. Source oxidation of easily oxidizing elements such as Ca, Sr, Ba, and Ti in an oxidizing ambient leads to their flux instability. Such unstable flux rate is detrimental to precise control of film thickness and composition. The only way to prevent this source reaction from happening is to suppress the pressure of the reactive gas near the evaporation source while maintaining high enough gas pressure near the substrate where the film grows with the re- active gas. In complex-oxide MBE process, multiple source elements with significantly different oxygen affinities are used together. If all elements used are easily oxidized, such as in the case of Sr(Ca, Ba) TiO3, a low background oxygen pressure, in 10-7 Torr range suffices and flux instability is not critical, however, this issue becomes prominent when an easily oxidized element is used together with a difficult to oxidize element. Minimizing oxygen partial pressure near the source surface even in a harsh oxygen environment is a fundamental solution for flux instability problem. Ideally, if the O2 partial pressure near the source surface is negligible, the flux will be completely stable throughout the entire growth, thus eliminating the need for real-time monitoring.  Researchers at Rutgers University have developed a new scheme that can completely eliminate the source oxidation problem: a self-gettering differential pump using the source itself as the pumping medium. The pump simply comprises a long collimator mounted in front of the source in extended port geometry. With this arrangement, the oxygen partial pressure near the source was easily maintained well below the source oxidation regime, resulting in a stabilized flux, comparable to that of an ultra-high-vacuum environment. Moreover, this pump has a self-feedback mechanism that allows a stronger pumping effectiveness for more easily oxidizing elements, which is a desired property for eliminating the source oxidation problem. The same pump design can be used for any other reactive gases in MBE condition. This self-gettering differential pump uses the source itself as a pumping medium to eliminate the source oxidation problem and stabilize the flux even when the flux is as low as ~ 0.01Å/sec, which is an order lower than the typical growth rate.   Market Application Complex oxide and multi element Molecular Beam Epitaxy   Advantages Simple design easily implemented onto existing MBE equipment.