Field emission characteristics of a tungsten microelectromechanical system device

We have investigated the field emission properties of free-hanging tungsten microelectromechanical system structures. These tungsten structures are designed to serve as electrodes in a Paul ion trap. Since the outer edges of the trap end cap electrodes are adjacent to the inner edges of the trap ring electrode and approximately 0.5μm apart, field emission may occur between these two edges when hundreds of volts are applied, thereby defining an edge field emitter. The arrays were tested under vacuum (10−5Torr) and at atmospheric pressure (625 Torr) to understand the field emission behavior. Vacuum tests show turn-on voltages of about 200 V for the 1 and 1.5μm radius traps and currents of ∼80nA for both sizes of trap with the largest array (106) at 6 MV/cm. The atmospheric tests showed lower turn-on voltages of approximately 150 V for both the 1 and 1.5μm radius traps. Currents up to a few μA were achieved at 6 MV/cm for smaller trap size (1μm) in the largest array indicating a gas ionization contribution. The measured current-voltage responses fitted the Fowler–Nordheim characteristics well, confirming that the current increase in vacuum was due to field emission. A stable emission current of 2.03 nA was obtained at 10 MV/cm for 11 min.