Role of thermal properties of cathode materials for the nanoscale breakdown in vacuum

Nowadays, electrical insulation issue has attracted increasing concerns as the physical dimension of electrical devices and equipment shrinks to micrometer and even nanometer. Since almost all the nanostructures and nanodevices are driven by electrical bias, it is very vital to make sure a good robustness while operating under the high electric field. Hence, to understand the breakdown behaviors across a vacuum nanogap is of great interest for the insulation reliability evaluation of naonstructures and nanodevices. In the present work, cathode materials with various thermal properties were employed and the breakdown measurement was carried out across a 100-nm vacuum gap by utilizing the in-situ nanoscale discharge measurement system based on the scanning electron microscope and piezoelectric nano-manipulator. Results show that the ${I-V}$ curves for both Mo and W cathodes demonstrate the similar trend, which can be generally regarded to be three stages: leakage dark current stage (several pA), electron emission stage $( \sim 2.0 nA)$ and breakdown development stage (several mA). Moreover, W cathode with a higher melting point shows a larger average breakdown threshold $( \sim 241.2\mathrm {V})$ than Mo cathode $( \sim 199.4\mathrm {V})$, which indicates the metal atomic vapor evaporated from the cathode plays a key role on the electron avalanche and breakdown. As a consequence, the hypothesis for cathode process is put forward, regarding that the field emission dominates the initial stage and then thermionic emission takes over until the gap is filled with massive ions and electrons and the breakdown occurs. This would be of great benefit for better understanding of vacuum breakdown behavior at nanoscale.