FIELD-EMISSION PROPERTIES OF SELF-SHIELDED TUNGSTEN SOURCES

The electron emission properties of self-shield W tips were studied by field emission microscopy and spectroscopy. Self-shielded tips were f ormed by focused ion beam milling an annular depression on the apex of an electrochemically etched W[111] tip. The resultant tip structure c onsists of a flat bottomed crater (the outer rim of which forms the sh ielding electrode) with a central protrusion (the emitter). In situ pr ocessing consisted of moderate thermal flashing and partial buildup. S ignificantly greater angular emission current densities were measured, as compared to both unshielded W[111] built-up tips and standard W[31 0] tips. Energy analysis of the emitted electrons showed no deviations from normal W[111] emission. Self-shielding may offer a generally app licable method for increasing the brightness of field emission sources in a variety of electron optical instruments. (C) 1995 American Vacuu m Society.