【作者】 王小菊；

【导师】 蒋亚东；

【作者基本信息】 电子科技大学 ， 光学工程， 2009， 博士

【摘要】 场发射阵列阴极与热阴极相比，具有许多独特的优点，如无需加热，可以在室温下工作；电流密度比热阴极高几个数量级，并可工作在低电压调制下；功耗低；极好的开关特性；可瞬时启动等。目前，场致发射阴极的应用领域主要包括微波器件、平板显示器、电子显微镜及电子束刻蚀系统等。在各种结构和材料的场发射阵列阴极中，较为常用的是钼微尖场发射阵列和硅微尖场发射阵列。然而，这两种阴极材料逸出功偏高（硅：4．14eV，钼：4．4eV），抗离子轰击溅射的能力较差，且化学性质不够稳定。因此，在改善制作工艺的基础上，寻找新型的阴极材料变得非常重要。经试验证明，单晶六硼化镧（LaB₆）具有高导电率和良好的热稳定性、化学稳定性、低功函数以及活性阴极表面，是制备场发射阴极的理想材料。然而，目前国际上关于LaB₆场发射阴极的报道仅限于单尖、薄膜及纳米线领域，而对于单晶LaB₆场发射阵列（field emission array，FEA）的制备工艺，还未见相关文献报道。针对上述问题，本论文在单晶LaB₆场发射阵列阴极的制备及特性方面进行了一系列探索性和创新性的工作，具体包括：1．LaB₆晶向和发射体结构是影响单晶LaB₆-FEA发射性能的重要因素。论文对这两项参数进行了理论分析，并采用EBS软件模拟了发射体结构对尖端电场的影响，得到如下结论：制作LaB₆-FEA的最优晶向是[111]向，它可以长时间地保证有低逸出功发射面；LaB₆发射体的最佳锥角约60°，这是综合考虑阴极功函数和尖端场强的结果。同时论文提出了一种优化的场发射体结构，EBS模拟结果表明，该结构能有效提高阴极的场发射电流密度。在理论分析的基础上，论文还设计了LaB₆-FEA的制备工艺流程，包括：掩膜沉积、掩膜图案化及LaB₆场发射体刻蚀。2．采用等离子体增强化学气相沉积法制备单晶LaB₆-FEA所需的氮化硅（SiNx）掩膜，考察沉积温度、射频功率、反应源气体（NH₃和SiH₄）的流量比、反应压强四个因素对SiNx薄膜性能的影响。得出的优化工艺参数为：沉积温度270℃，射频功率20W，NH₃和SiH₄的流量比40：5，反应压强60Pa。按优化工艺参数制备出的SiNx薄膜致密度较好且颗粒细小，电阻率约6．5×10¹³Ω·cm，Si、N的摩尔比约为3：4．3，接近于Si₃N₄的标准化学配比。3．采用传统的光刻工艺和反应离子刻蚀法对上述SiNx薄膜进行图案化。光刻工艺的具体参数为：匀胶转速3000rpm，匀胶时间30s，软烘温度100℃，软烘时间1min，曝光时间90s，显影时间30s，坚膜温度100℃，坚膜时间10min。反应离子刻蚀的具体参数为：刻蚀气体SF₆，流量30SCCM，功率50W，刻蚀时间8min。最终形成的SiNx掩膜图案为圆形点阵，点阵直径4μm，点阵间距10μm。4．LaB₆场发射体刻蚀是制备单晶LaB₆-FEA流程中最难、最关键的步骤，论文考虑了以下五种方案：硝酸湿法腐蚀、氩等离子体干法刻蚀、氧等离子体氧化与氩等离子体干法刻蚀氧化层相结合、氧等离子体氧化与盐酸湿法腐蚀氧化层相结合、电化学腐蚀。结果发现，在硝酸湿法腐蚀过程中，随着刻蚀时间的增加，掩膜出现破损，导致尖锥形貌发生改变；氩等离子体对LaB₆基片没有任何的刻蚀效果；采用氧等离子体氧化与氩等离子体干法刻蚀氧化层工艺时，由于Ar离子轰击能量太强，在刻蚀LaB₆氧化物的同时，也对掩膜材料形成了很强的刻蚀作用，导致掩膜提前脱落。因此，前三种方案均不适合制备单晶LaB₆场发射阵列。在第四种方案中，论文考察了O₂流量、极间电压、基片加热温度对发射体形貌的影响，最终成功制备出单晶LaB₆场发射阵列，尖锥高度超过2μm，基片底部和发射体侧面较光滑，且掩膜保护完好，表明该方案具有较强的可行性。而在电化学腐蚀方案中，论文考察了电解液成分、阴阳极间电压、电解液温度、电解液浓度、缓蚀剂及水溶剂对LaB₆发射体形貌的影响，最终也成功制备出LaB₆-FEA，尖锥阵列具有较好的均匀性，发射体高度超过3μm，底面较平整。5．自行设计和制作LaB₆场发射阵列二极管的场发射性能测试系统，对上述获得的LaB₆场发射阴极进行性能测试。结果表明，在真空度6×10^-5pa下，单晶LaB₆场发射阵列阴极表现出优良的发射稳定性，开启电场和阈值电场分别为3．2V／μm和8V／μm，可同其它新型场发射阵列阴极（如硅纳米尖、碳纳米管）相比拟。在阳极电压为1500V时，获得最大发射电流32mA，折合单个尖锥0．1μA，超过文献报道的LaB₆薄膜场发射阵列。阴极在低真空下(2×10^-3Pa)的发射性能测试表明，LaB₆场发射阵列阴极具有较佳的抗中毒能力，从而证实了将单晶LaB₆作为场致发射阴极材料的优越性。更多还原

【Abstract】 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 6.5×10¹³Ω.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.更多还原