Effect of Cathode Radius on the Pre-breakdown Characteristics in Vacuum Nanogaps

Vacuum microelectronic and nanoelectronic devices have drawn great attentions in recent years due to their high integration density, ultrafast electron transportation and low power consumption, etc. However, the shrinkage in physical size causes an extreme high electric field within a nanogap, therefore, to understand the failure mechanism and evaluate the electrical reliability in a vacuum nanogap become a critical concern. Here, we reported the experimental investigation on the influence of the radius ($R =$2 nm, 24 nm, 220 nm) of tungsten nanotip (cathode) on the pre-breakdown characteristics in vacuum nanogaps ($d =$2 nm-25 nm) based on the in-situ TEM electrical measurement system. Results show that the field emission characteristics of nanotips in nanogaps show the exponential increase, which is consistent with FN law. However, the field enhancement factor derived from the FN law is less than 1 at the turn-on stage of field emission for $R\gt \gt d$, and it could be attributed to the increase of the potential barrier due to the field penetration effects, which turn to be a significant role in specific conditions (in nanogaps while $R\gt \gt d$). Additionally, the cathode with a smaller radius has a smaller turn-on voltage because of a higher field enhancement factor, and the cathode with a bigger radius has a larger average turn-on electric field due to a higher field penetration degree, for example, the average turn-on electric fields in 5 nm gap for $R =$2 nm, 24 nm, 220 nm nanotips are 4 V/nm, 5 V/nm, 8 V/nm respectively, which are much higher than 0.325 V/nm in 400 nm gap in other study. The proposed in-situ transmission electron microscopy (TEM) method paves the way for investigation on field emission and breakdown in vacuum nanogaps, which would be of great interest to the device design and reliability evaluation for nanodevices.