STABILITY OF THE EMISSION OF A MICROTIP

In order to evaluate the potential of microtips in novel electron opti cs applications, the emission stability of a single tip was studied. I t was found that after a few hours of ''self-shaping,'' a tip may emit a very stable spot (for 700 nA of global emission, 7 min is reported without any change over 1 nA, and a couple of hours with less than +/- 1% change). Unfortunately, a period of instability may follow a period of stability. It was observed that the spot is composed of several su bspots, each one having a much greater stability than the whole spot. These subspots are very likely to be emitted from independent nanoprot rusions or atoms, and changes in the emission occur because of motions of atoms and/or adsorption-desorption processes. It was concluded tha t, though the stability of an electron beam cannot be ''infinite'' as a laser beam is. very long periods of stability (hours) and high coher ence of wave packets can be obtained, if work was done under very high vacuum (several 10(-11) mbar) and with a unique beam emitted from a s ingle atomic site.