Destruction and microcratering of field electron microscope tips by microparticles

Field emission tips of Mo and W, destroyed at emission current densities of i ≈ 104 A/cm2 (F ≈ 4 × 107 V/cm) and residual gas pressures of p ≈ 10−10 Torr, were studied using field emission, scanning, and transmission electron microscopic methods. Upon destruction, tip diameters increased to 15 to 20 μm as a result of the formation of a melting cap. Microcraters with diameters from 1 to 10 μm were arranged in clusters and decreased in density with decreasing local field strength. Electron microprobe analyses did not indicate the presence of any non tip material, only small concentrations of surface impurities could be detected by field emission from protruding spikes. A summary of relevant high vacuum electrical breakdown mechanisms is given. A microparticle initiated field emission mechanism, which includes phosphorescent screen particle participation, appears to dominate the tip destruction.