Microscale electrical breakdown: a review of recent advances*

Microfabrication technologies have been largely advanced during the past decades, enabling the miniaturization of low-temperature plasma devices in practical applications, such as micro-switches, microsensors, detectors, and electron and ion sources. Understanding the electrical breakdown at microscales is essential for the downscaling and reliability of plasma sources, which on the other hand also aids the insulation design of microelectronic devices. It has been recognized that the dominant breakdown mechanisms can vary when the gap distance reduces from the macro to micro dimensional scales. The secondary electron emission and electron avalanche dominate the breakdown for the gap larger than ten-micron meters. The field emission becomes dominant when the gap distance shrinks to several micron meters; the thermal field emission comes into play if the cathode is under high field and temperature conditions. This talk summarizes and discusses state-of-the-art experimental diagnostics (e.g., the breakdown path) and theoretical models (e.g., the breakdown threshold) for microscale electrical breakdown. Recent studies on the space charge effects in the early stage of the field emission-driven microgap breakdown are introduced, which demonstrate a double-layer sheath and ultrafast oscillations. The dimensional and nondimensional models for predicting gap breakdown voltage from field emission to secondary electron emission-dominated regimes are discussed. A unification of the breakdown criterion is proposed which considers both the secondary electron emission, thermionic emission, and field emission under high-temperature, high-field conditions. The effects of cathode electrode surface morphology (e.g., hemispherical surface protrusion) are examined for the gap of several hundred-micron meters, showing a combined Paschen's curve with an extended range of the breakdown minimum. Other simulation results considering multiple cathode protrusions and floating metal perturbations are also included. The most recent advances in the modeling of electrostatic-induced microgap breakdown with moving electrodes are reported. This talk aims to provide a comprehensive overview of microgap breakdown characteristics and illustrate the underlying mechanisms of breakdown physics, highlighting the recent progress and ongoing challenges, which may promote more in-depth understanding, physical discoveries, and optimal design of miniaturized electronic devices.