It was initially decided that the new system should be based around an electrostatic beam. This has the advantages that the monoenergetic positrons can be easily selected in a small space, avoids large coils surrounding the system,and has proven performance in delivering sub-millimetre beams eg. the positron microscope and surface science beams previously developed by our group.
Simulation of Positron Trajectories calculated by Simion
Proposed arrangements and sizes of electrodes were then simulated with Simion 6.0 to show their viability and performance. Results show the best produces a sub-millimetre beam at all implantation energies from 0.5 to 30 keV if a flat rather than mesh moderator can be used, still with less than 2mm across the range for a mesh.
With the arrangement of electrodes known the design of vacuum vessel could be finalized. The size of the vessel meant that a load-lock system to change and also manipulate the 100mm wafers became sensible. To reduce the internal volume of the vessel the parts nearest to the high voltage electrodes will be floated to the voltage of the source. This means that at times these parts of the vacuum vessel will be at 30kV so they will need to be effectively insulated.
