Relaxation of electric field by covering cathode-edge with vanadate glass

In scanning electron microscopy (SEM), restricting surface flashover is necessary to achieve stable insulation properties with a compact structure at high acceleration DC voltages. The flashover discharge model is regarded as an avalanche of field-emitted electrons caused by the concentration of electric fields and the high secondary electron yield on the insulating surface of alumina ceramics. An effective way to improve this phenomenon is by suppressing field emission electrons at the initial stage of the flashover phenomena. Herein, the electric field concentration was relaxed by covering the cathode edge with a vanadate glass to obtain favorable results. Vanadate glass has a conductive dielectric property and can be sintered to an amorphous and smooth surface that can be used to cover the cathode terminal edge to hide the cathode edge and smooth any microprojections. A vanadium-based vanadate glass was used as this has a glass transition temperature of $350^{\circ}C$, crystallization temperature of $450^{\circ}C$, resistivity of 10$^{7}-10^{8} \Omega$ -cm, and relative permittivity of 15. The voltage-holding capability of the cathode edge of a sample with a cathode metallized on an alumina surface was evaluated with and without a vanadate glass coating. The wettability of the vanadate glass to the metallized and alumina surfaces was excellent, and smooth surfaces were formed without defects such as peeling and cracking. Theoretical calculations showed that the effect of the field concentration mitigation was approximately three-fold greater than that of vanadate glass. The field emission electron emitter is used in the electron gun of the SEM, and the vanadate glass must be stable in an ultra-high vacuum environment without gas desorption. Therefore, the voltage-holding capability was evaluated using voltage-holding experimental tools incorporated in an ultra-high vacuum (around 10$^{-7}$ Pa) chamber. Consequently, a voltage-holding property of approximately 3-fold the electric field mitigation effect without outgassing was obtained, and a favorable field relaxation effect was confirmed.