ELECTROSTATIC REMOVAL OF LITHIUM-FLUORIDE FROM FIELD-EMITTER TIPS AT ELEVATED-TEMPERATURES

The electrostatic removal of lithium fluoride (LiF) from field-emitter tips has been visualized at elevated temperatures in the transmission electron microscope (TEM). The apex of a field-emitter tip coated wit h similar to 1500 Angstrom of LiF provides a unique substrate for obse rving the removal process in the TEM in real time, and its curvature g enerates the required electrostatic field strength. The influence of t he imaging electron beam on coating morphology has been visually asses sed. A LiF coating can tolerate an electron dose of similar to 2000 e( -)/Angstrom(2) at room temperature without visible damage (at a resolu tion of 5 Angstrom). At elevated temperatures a higher dose can be tol erated before visible damage is observed. Removal of LiF coatings at r oom temperature occurs at 18 MV/cm. At 800 degrees C piecewise removal of the coating occurs at 9 MV/cm. Synergistic effects of the electron beam and the electrostatic field on the removal of the coating were n ot observed. The removal of LiF at any temperature is attributed to fi eld-induced fatigue stress of the coating. Field desorption does not a ppear to play a significant role in the removal process. Implications for the production of ions from lithium fluoride thin films exposed to high electric fields (in laboratory experiments and in particle beam fusion accelerators such as PBFA II) are discussed.