Robust high current field emitter tips and arrays for vacuum microelectronics devices

We seek to identify the most promising materials and geometries for field emitter tips and arrays (FEA) in applications requiring stable high current and conductance in a moderate vacuum. Mo FEAs have been used successfully in relatively high current (50–100 mA) TWTs. But molybdenum has some undesirable characteristics. For several years we have studied field emission from refractory carbides, particularly ZrC, and found that ZrC surfaces have very desirable characteristics for stable high current field emission. We calculate thermal effects and instabilities for five emitter materials and conclude that W or Mo emitters coated with a thin coating of ZrC have the highest current capability, particularly for dc emission. We then calculate β factors for various field emission microdiodes and triodes. We conclude that broad cone angle ZrC/Mo emitters have the highest conductance capability. We then discuss several factors which cause FEA performance to fall short of theoretical calculations in practice. We then review experiments with ZrC field emitters. We discuss individual single crystal ZrC field emitters made from zone refined ZrC single crystal wires, by the standard electrolytic etch technique or by a temperature enhanced field desorption technique which produces moderately sharp tips (50–100 nm) with broad cone half-angles (0.5–0.6 radian). Hundreds of tips have been tested. Noisy but fairly stable emission of 200–300 μA has been achieved at up to 3×10 −5 Torr in air. At high vacuum (10 −9 Torr ) dc currents up to 25 mA and pulsed currents up to 100 mA have been achieved with fairly blunt (r≅100 nm) , broad cone angle single tips, in fair agreement with calculations. We have fabricated small 10×10 FEAs made of bulk ZrC cones formed by physical vapor deposition, and show typical I(V) characteristics. Finally, we have coated Mo FEAs with thin ZrC coatings and achieved a 30% reduction in turn on voltage due to the reduced work function of ZrC. Much remains to be done, particularly on the noise, stability, and life of high current ZrC or ZrC/Mo FEAs at various pressures.