Micromachined thermionic emitters

Arrays of sputtered tungsten thermionic emitters have been fabricated. The emitters are microbridges which were fabricated by removing a sacrificial layer of SiO2 below a patterned film, and have two thicknesses: 0.45 and 1.1 mu m. The emitting shape varies from a single straight 'wires'. The emitters initially have a high resistance which drops an order of magnitude with an annealing current. A thermionic electron current of over 10 nA for a duration of 5 to 55 min has been measured. The lifetime depends on the microbridge thickness, shape, and emitting current. Micrographs after emission indicate recrystallization and melting at the midpoint of the microbridges. The thermionic emitters will be used as sources for a miniature scanning electron microscope (MSEM) with a column length of 1-4 mm.