Measurements and analysis of advanced field emission cold cathodes

Abstract

Summary form only given. We report the measurements and analysis of metal cold field emission cathodes utilizing an advanced cathode test facility and a TMM (transfer matrix method) model. The facility is designed to measure the field emission currents from cathodes on the micro- and macroscale. Measurements are obtained under UHV (10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-10</sup> torr) conditions. The vacuum chamber is a stainless steel six-way cross. We use a scroll pump, turbo pump, and ion pump to achieve UHV. We bake the system at 450degC for several days to eliminate residual water vapor and other possible contaminants inside. The current vs. electric field characteristics incorporating the effects of uniformity, edge effects, space charge, and thermal effects are examined. With two sets of high-voltage pulse supplies, 0~20 kV, 1 mus~5000 ms duration negative pulses with rise times as fast as 60 ns are applied between the cathode and anode to obtain current-voltage characteristics. A high voltage probe is used to monitor the cathode-anode voltage. An XYZ micro-positioner is used to adjust the gap between the cathode and anode from 0-1 cm with a resolution less than 0.1 mm. It also moves the cathode along an anode with a hole of diameter between 100 mum and 1 mm located in front of another stainless steel anode to obtain the emission current distribution. This diagnostic is for examination of the local emission current variation and edge effects. Carefully designed shielded amplification circuits and signal averaging techniques are used to measure the emission current from the nA to A range. Measurements have been made for CKE (Copper Knife Edge) cathodes fabricated by electrode discharge machining. The I-E curve and extracted effective betas, effective emission area ratios, and effective work functions have been obtained for these cathodes. A TMM simulation which can precisely estimate emission current density with finite temperature effects has been developed. It shows that for low electric fields, the emission current density is much higher at room and elevated temperatures (277-350degC) than for the 0 K Fowler-Nordheim equation. A break point will divide the expected linear relationship between ln(J/E <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) and 1/E into two regions with different slopes. Using a fitting procedure in the low-E-field (thermionic) and high-E-field regimes, CKE cathode work functions of 1.7-1.8 eV and field enhancement betas of 8-9 are obtained. Both single sharp tip and multi-tip cathodes made of different materials (Cu and W) are being measured to determine the local J(r) current density character, in order to determine the interaction between neighboring tips during electron emission.