Measurements and Analysis of Advanced Field Emission Cold Cathodes

We report measurements and analysis of metal cold field emission cathodes utilizing an advanced cathode test facility. The facility is designed to measure the field emission current densities from cathodes on the Micro-and Macro-scale. Measurements are obtained under UHV vacuum (10~10 Torr) conditions. The vacuum chamber is a stainless steel six-way cross. We use a scroll pump, turbo pump and a Vacion pump to achieve UHV. We bake the system at 450 C for several days to eliminate residual water vapor and other possible contaminants. An emitting tungsten filament is used to clean the anode surface. The current vs. electric field (I-E) characteristics incorporating the effects of cathode uniformity, edge effects, space charge, thermal effects and the Fowler-Nordheim coefficients are examined. With a Glassman high DC voltage supply and a DEI high voltage pulse supply, 0~20 kV, 1~5000 mus duration negative pulses with rise times as fast as 60 ns are applied between the cathode and anode to obtain current-voltage characteristics. A Tektronics high voltage probe is used to monitor the cathode-anode voltage. A Lesker manual linear shifter is used to adjust the gap between the cathode and anode from 0~2.5 cm with a resolution less than 0.1 mm. Another MDC XYZ micro-positioner moves a second small anode (with a diameter between 100 mum and 1 mm) located behind a Mo anode screen along the whole cathode surface to obtain the emission current distribution. This is used to examine emission current density uniformity and edge effects. Carefully designed shielded circuits and signal averaging techniques are used to measure the emission current down to the nano-ampere range. Measurements have been made on ALF (Ablation-Line-Focus) aluminum cathodes and CKE (Copper-Knife-Edge) cathodes. The I-E curve and extracted beta are obtained for these cathodes. Simulations of temperature effects on quantum electron tunneling that affects field emission have been made. It demonstrates that for - low electric fields, the emission current density is much higher at room temperature than that of the 0 K Fowler-Nordheim equation. A break point divides the expected linear relationship between ln(J/E") and 1/E into two regions with different slopes. This prediction will be compared with experimental measurements. Both single sharp tip and multi-tip cathodes made of different materials (Cu, Al and W) are being tested, in order to determine the interaction between local neighboring tips during the electron emission process.