The Preparation Technology And Simulation Research On High-density Molybdenum Cone Spindt Cathodes

With the advantages of instant turn-on,low power dissipation,and room-temperature operation,field emission arrays(FEAs)have potential for using as electron sources in microwave vacuum electronic devices to decrease devices' size and improve devices' performance.Spindt cathode,as the typical integrated grid-controlled FEA,is especially suitable for microwave devices.At present,the C,X and Ku band traveling wave tubes(TWTs)with Spindt cathodes have been fabricated in Stanford Research Institute(SRI).However,the Spindt cathodes can not be applied in microwave devices since the total current is not high enough in China.To improve the total current,there are mainly two ways:(1)improving the current of individual emitter,and(2)increasing the number of the effective emitters that emit electrons.Increasing packing-density of emitters can increase the number of the emitters in certain area.In this work,we try to improve emission current of Spindt cathodes by increasing packing-density of emitters.Photolithography has been using to fabricate Spindt cathodes in our country.However,in this work,the preparation technology research on high-density molybdenum cone Spindt cathodes is studied based on electron beam lithography(EBL)and nanosphere lithography(NSL)respectively.Firstly,field emission theory is summarized.The factors have effects on field emission characteristics are studied.The independences of geometrical parameters of individual emitter on field emission characteristics and effective field emission areas are studied.Field screening effects between emitters are analyzed.These studies provide theoretical supports for the design of individual emitter and packing-density of FEA.This work is mainly focused on the preparation of high-density molybdenum cone Spindt cathodes based on EBL and NSL techniques respectively.Though EBL has been widely used for fabricating high-density Spindt cathodes in international,there are few corresponding systematic works in China.In this work,the fabrication process of high-density Spindt cathode with EBL technique is divided into several key steps,such as film deposition,mask pattern formation,pattern transfer and two electron beam sources deposition.The preparation process is then studied step by step in details.Spindt cathodes with packing-density of 2.5?107 tips/cm~2 have been successfully obtained.The area,arrangement and packing-density of Spindt FEAs can be adjusted based on actual needs.Fabicating high-density molybdenum cone Spindt cathodes by using EBL technique features good repeatability and high reliability,but the efficiency is low and the cost is very high.To reduce cost and improve efficiency,a new method for fabricating high-density molybdenum cone Spindt cathode by using NSL technique is proposed.The technological route is challenging since there are few reports to be referenced.In this work,the key techniques are studied.The preparation of sacrificial layer window,polystyrene nanosphere monolayers,gate aperture arrays and cone emitter arrays are studied respectively.Finally,Spindt cathodes with density of 1.8?108 tips/cm~2 have been obtained.The packing-density is determined by the initial size of nanospheres.The method provides possibility for fabricating high-density Spindt cathodes with low-cost and high efficiency.Field emission characteristics and failure mechanism of high-density molybdenum cone Spindt cathodes are explored.Test system and circuits are built.Different sizes of high-density Spindt FEAs are tested.The turn-on gate voltage is in the range of 30-40 V.The maximum emission current density is about 196 A/cm~2.The maximum emission current is about 55 ?A.The low emission current should be mainly attributed to the small area of FEA and the ununiformity of the emitters.Field emission characteristics are analyzed.The effects that pulse width of driving voltage,parameters of Spindt FEA and test environment has on the field emission characteristics are analyzed.The failure mechanisms caused by short between base and gate,structure disruption and flashover across oxide walls are discussed.Then,the reliability of Spindt cathode is discussed.The methods for improving field emission properties are proposed.Finally,application of high-density Spindt cathode in THz folded waveguide traveling wave tube(FWTWT)is explored.The synthetic methods for designing Pierce electron gun are summarized and analyzed.A general synthetic method is modified.The initial parameters of the electron guns of 0.14,0.22 and 0.34 THz FWTWTs are calculated by using the modified general synthetic method.Finally,an electron gun based on high-density Spindt cathode is designed for 0.14 THz FWTWT by simulation.