Low-energy electron beam focusing in self-organized porous alumina vacuum windows

Micromachined, micron-thick porous alumina membranes with closed pore endin gs show high electron transparency above an energy of 5 keV. This is due to the channeling of electrons along the negatively charged insulating pores after surmounting the thin entrance layer. We also find a sharp hightranspa rency energy window at energies as low as 2 keV which may be the result of a local maximum of channeling, as predicted by simulations, and positive ch arge up of the entrance layer causing electron electrostatic focusing. Appl ications for these membranes range from atmospheric electron spectroscopy t o self-assembled, nanoscale, large-area electron collimators. (C) 2000 Amer ican Institute of Physics. [S0003-6951(00)04824-5].