RF Sputtering Coating of Electron Transparent Materials for Photocathode Encapsulation

Alkali antimonide photocathodes are commonly used in high-brightness photoinjectors because of their ultra- high quantum efficiency QE) and relatively low requirements for growth. The biggest challenge of using the photocathode is that it has an extremely stringent requirement on the vacuum environment and can be destroyed by even low doses of residual gases in the chamber. Consequently, the transport of alkali antimonide photocathodes is eminently difficult. Furthermore, the cathode lifetime is limited by the residual gas in the photoinjector. An efficient encapsulation is needed to avoid contamination during transportation of cathodes, preserve their QE, and increase their lifetime. Euclid Techlabs PI: Dr. Ao Liu) and BNL PI: Dr. Erdong Wang) together propose to develop a thin-film deposition on the alkali antimonide photocathodes as an encapsulation, via the well-established RF magnetron sputtering, with sputtering enhancement techniques. The proposed encapsulation studies include research on the sputtering configuration and recipe, sputtering enhancement schemes, tests of QE and cathode lifetime, and material characterization. A thin film coating is the only viable encapsulation method other than physical encapsulation using tube encapsulation via a base plate or a cartridge. In Phase I, we will start with building a new sputtering source with a vendor, e.g. KJ Lesker, taking into account overall geometry and the available port size and distance to the photocathode surface. We will modify the magnetron details to cater to the vacuum requirements of lower than 10-9 torr and high baking temperature. Customized targets such as a h-BN target will be acquired. The system will be tested at Euclid before being shipped to BNL for installation. Meanwhile, Euclid will test the sputtering enhancement methods proposed in the previous sections, using the existing RF sputtering chambers. The technique can be applied to most of the alkali antimonide photocathode growth chambers, considering the easiness to add a customized sputtering system in the multi-port chambers. Users will benefit from the encapsulated photocathodes and their unaltered QE. More robust cathodes with longer lifetime will become available for heavy-duty experiments.