Research On Field Emission Characteristics Of Lanthanum Hexaboride

Vacuum microelectronic devices (VMDs), which is based on the theory of fieldemission, have received extensive attentions due to their mini size, low powerconsumption, low operating voltage, fast response, anti-radiation and wide operatingtemperature range. The performance of the device as a whole is essentially depends onthe performance of the so called field emission cathodes, which is considered as thekernel of VMDs and always be referred as cold cathodes. In the past decades,significant progresses have been achieved in the techniques of cold cathodes, includingcathode structure, material, and fabrication processes. The most famous and importantcold cathode structure is inevitably the so called Spindt-type cathode, which wasproposed by C.A. Spindt of Stanford Research Institute (SRI) in 1968. This cathode iscomposed of arrays of sharp tips and every tip has its own gate electrode to control theemission of electrons. Field emission cathodes with arrays of tips are usually calledfield emission arrays (FEAs). Spindt-type FEAs with Mo emitter tips have alreadyfound their application in power vacuum microelectronics devices, for example,traveling wave tubes (TWTs).It is recommended that the tips of FEAs be fabricated from materials with low workfunction value as well as high thermal and electrical conductivity, high melting point,and high chemical stability. Mo and Si are materials that are most frequently used as theemitter material, presently. Unfortunately, these materials suffer from many shortages,such as relatively high work function of greater than 4 eV and low stability to thesputtering of ions.On the contrary, Lanthanum hexaboride (LaB₆), which is known to be a low workfunction material (2.3～2.8 eV), possesses some unparalleled properties, such as highmelting point, high chemical stability, high thermal and electrical conductivity, andresistance to ion bombardment. These excellent characteristics make it an idealcandidate for FEAs.In this paper research works on fabrication and investigation of Spindt-type FEAswith LaB₆ emitter tips are reported. Emphasises are given to the field emissioncharacteristics of LaB₆, electron beam evaporation, fabrication processes of fieldemission array, and measurements as well.By investigation the field emission properties of single crystalline LaB₆ single tip emitter, work function of LaB₆ under different vacuum levels was investigated. It iscertain that the work function remaines unchanged under different operating vacuumlevels. Adsorption and desorption of gaseous molecules to and from the tip's surface arethe main causes of emission current instability. It is also found that operating under lowvacuum in order to put the LaB₆ tip under ion bombardment for a short time, such asseveral minutes, can clean the tip surface instead of demage it. This can be consideredas an activation process for LaB₆ field emitters.It is the unique structure of LaB₆ crystal that makes it possesses high melting point,high thermal and electrical conductivity, high chemical stability, and high resistance toion sputtering. However, this structure also makes evaporation of LaB₆ difficult. Thefact that vapor of LaB₆ is composed of La and B atoms when the material is evaporatedmakes it difficult to obtain films without oxidation. In order to e-beam evaporate LaB₆tablet, self-made e-beam evaporation configurations were designed, fabricated andmodified. Special attentions were given to minimize the oxide content in the films.Relatively high purity LaB₆ films were deposited with high rate as a result. Its workfunction under thermal emission and field emission applications was also measured,which is proved to be the same value of bulk LaB₆ crystal.Gate-hole arrays were fabricated using standard Spindt process as well as throughlocal oxidation of silicon (LOCOS) technique. Gate holes with different depth werefabricated. Issues concern with short circuit of LOCOS structure between the metallicgate and silicon substrate was also discussed, and solutions proposed.Parting layer is another special issue in the fabrication process of LaB₆ FEAs, dueto the lower work function of LaB₆ than that of silicon. By investigating severalmaterials for the parting layer, erode of LaB₆ tips during lift off process wassuccessfully overcome.Finally, FEAs with array area of 1 mm² and number of LaB₆ tips of 25000 weresuccessfully fabricated. Emission current density of 0.6 A/cm² and emission current pertip of 0.24μA/tip, which is almost 3 orders of magnitude higher than ever reported,were achieved, suggesting that FEAs with LaB₆ emitter are promising candidate forapplications of some power vacuum electronic devices.