Research On Field Emission Array Cathode And Properties Based On Single Crystal Lanthanum Hexaboride

In contrast with conventional thermoionic cathode,field emitter array (FEA)cathode has many special advantages,such as room-temperature operation without acathode heater,high current density,low power dissipation,excellent ON/OFF isolationcharacteristics and instant turn-on characteristics.At present,FEA has potential for useas an electron source in a wide variety of applications,including microwave poweramplifiers,fiat panel displays,electron microscopy,and electron beam lithography.Themost famous and important cold cathodes are inevitably the silicon and molybdenum tipfield emitter arrays.However,both of them have some disadvantages,such as relativelyhigh work function of more than 4 eV and low stability to the sputtering of ions.Thus itis important to seek new materials of cold cathodes,besides improve the existingtechnology.It has been proved that,single crystal lanthanum hexaboride (LaB₆),withhigh conductivity,good heat stability and chemical stability,low work function,andactive cathode appearance,is an ideal material for field emission array cathodes.Stimulated by the unique properties of LaB₆,the interest in LaB₆ based on cold cathodeis increasing in recent years,mainly concentrated on the following aspects:LaB₆coating on conventional single tip or field emitters,LaB₆ nanowire and LaB6 film FEA.However,to our knowledge,no results on the preparation and properties of singlecrystal LaB6 field emitters have been reported as yet.Aiming at those problems,somebasic and systematic works have been performed in this paper to focus on thefabrication process to obtain single crystal LaB₆-FEA with good emission performance.1.The orientation and structure of LaB₆ emitters are the most important factorsdetermining the emission performance of LaB₆-FEA,which were analyzed andstimulated by EBS software.The results suggested that,the optimum orientation forLaB₆-FEA was [111],which offered adequate crystal faces with low work function,andthe optimum cone angle was 60°,which sastified the demands of low work function andhigh electric field for LaB6-FEA.The optimized structure for LaB₆-FEA was introducedin this paper,which improved the cathode performance effectively according to the EBSresults.In addition,the fabrication process of [111] LaB₆-FEA was also designed, including mask layer deposition,mask pattern fabrication and LaB6 emitters etching.2.Silicon nitride (SiNx) thin film used as mask layer for LaB₆-FEA was depositedon the smooth and clean surface of LaB6 substrate by plasma enhanced chemical vapordeposition (PECVD) growth mechanism.The influencing factors were discussed andthe optimum parameters for deposited SiNx were:deposition temperature 270℃,radio-frequency power 20W,flow ratio of ammonia to silane 40:5,reaction pressure60Pa.Electrical resistivity,mol ratio of Si to N of the film prepared with aboveparameter were 6.5×10¹³Î©.cm and 3:4.3,respectively.3.The prepared SiNx film was patterned into a dot array with the diameter of 4μmand 10μm spacing center to center by conventional UV photolithography and reactiveion etching (RIE) method.The optimum photolithographic parameters were:coatingrotation speed 3000rpm,rotation time 30s,soft-baking 100℃/1 min,exposure time 90s,developing time 30s,and hard-baking 100℃/10min.The optimum RIE parameters were:etching gas SF₆/30SCCM,etching power 50W and etching time 8min.4.LaB₆ emitter etching is the most difficult and important during the LaB₆-FEAfabrication process.Five programmes were discussed in this paper,includingwet-etching method with nitric acid solution,dry-etching method with argon plasma,oxygen plasma oxidization combing oxide etching with argon plasma,oxygen plasmaoxidization combing oxide etching with hydrochloric acid,and electro-chemical etchingmethod.In the first method,the SiNx mask was damaged,resulting in unsatisfiedappearance of field emitters.In the second method,argon plasma took no effect on LaB₆.In the third method,due to the high energy of Ar+,the mask layer was etched and brokeoff in advance.It concluded that the three methods were unsuitable for the fabrication ofLaB₆-FEA.The last two programmes were the feasible methods of fabricatingLaB₆-FEA.The influencing factors in the forth method were discussed and a LaB6-FEAwas achieved,having a tip height of more than 2μm.The substrate and tip surface ofLaB₆ sample were smooth with undamaged mask layer.In addition,the influencingfactors in the fifth method were also discussed and the results showed that a pattern ofLaB₆ field emitters having a height of more than 3μm could be clearly seen by SEM,suggesting excellent tip uniformity with smooth surface.5.Field emission measurements of fabricated LaB6 field emitters were carried outin diode geometry devised and fabricated by ourselves.It exhibited high emission stability at high vacuum of 6×10^-5 Pa with low turn-on field and threshold field of 3.2V/μm and 8 V/μm,respectively.The emission current reached the maximum of 32mAat 1500V,equal to 0.1μA for single tip,which higher than reported LaB₆-film FEA.Theemission properties of LaB₆-FEA at low vacuum of 2×10^-3 Pa signified the strongability of antipoisoning,which confirmed that FEA with LaB₆ emitter tips was an idealcandidate as the cold cathode for high current density devices.