UNIVERSITÀ DEGLI STUDI DI ROMA
"TOR VERGATA"
FACOLTA' DI INGEGNERIA
DOTTORATO DI RICERCA IN INGEGNERIA DEI
SISTEMI SENSORIALI E DI APPRENDIMENTO
CICLO DEL CORSO DI DOTTORATO XXI
Fabrication and characterization of carbon nanotube-based
vacuum triode
RICCARDO RICCITELLI
A.A. 2007/2008
Tutor: Prof. A. Di Carlo
Dr. A. Reale
“Mille anni al mondo mille ancora che bell'inganno sei anima mia e che bello il mio tempo che bella compagnia sono giorni di finestre adornate canti di stagione anime salve in terra e in mare […] ore infinite come costellazioni e onde spietate come gli occhi della memoria altra memoria e non basta ancora cose svanite facce e poi il futuro …” Anime Salve – Fabrizio De André ‐ Anime Salve (1996)
Abstract
This three years work dealt with the fabrication and characterization of carbon nanotube‐based vacuum triode. By reporting the manufacturing aspects and the related problems of the most widely investigated field emission devices, Spindt‐type arrays, innovative materials like Carbon Nanotubes (CNTs), Silicon Nanowires (SiNWs) and some promising Metal Oxide Nanostructures (namely ZnO, CuO, WO3, SnO2) have been shown and
described as potential materials for the realization of field emission cathodes. As a result, their relative figures of merit in diode characterization in terms of turn‐on electric field, threshold electric field, current densities, emission stability and field enhancement factor have been carried out. Technological processes for the design and the fabrication of carbon nanotube‐based nanotriode with high field enhancement factor have been investigated.
Problems correlated to the behavior of the device in high frequency characterization have been evaluated and possible solutions devised to overcome them have been analyzed and proposed.
Results obtained in this work contributed to two publications, a review chapter, six proceedings and two patents and it has permitted at our group to participate as coordinator at the project OPTHER (Optically Driven TeraHertz Amplifiers), financed by FP7 in 2008.
Abstract
Il lavoro svolto in questi anni di dottorato si è focalizzato sulla realizzazione e caratterizzazione di un triodo su scala micro/nanometrica. Il dispositivo in questione, in cui il fascio elettronico è ottenuto a partire da un catodo freddo realizzato con nanotubi, fonde in se le caratteristiche e le proprietà sia dei dispositivi a stato solido, sia dei dispositivi valvolari, offrendo da un lato un’elevata resistenza fisico‐termica, la capacità di miniaturizzazione, gli elevati tempi di vita e il peso ridotto, dall’altro alte frequenze e potenze di uscita. In questa attività sono stati messi a punto i passi tecnologici e di processo relativi alla realizzazione del dispositivo, è stato studiato ed analizzato il comportamento fisico dei catodi freddi realizzati con nanotubi di carbonio (CNTs), nanowires di silicio (SiNWs), o nanorods di Ossido di Zinco (ZnO). Sono state inoltre valutate le problematiche relative al funzionamento del dispositivo in frequenza e sono state proposte soluzioni a tal proposito. I risultati ottenuti in questo lavoro si sono concretizzati in 2 pubblicazioni su rivista, un articolo di review, 6 proceedings e 2 brevetti. Le competenze acquisite durante questo lavoro di dottorato hanno inoltre consentito di implementare il Progetto Europeo FP7 OPTHER, (Optically Driven TeraHertz Amplifiers), finanziato dalla comunità Europea nel periodo 2008‐2011 di cui il nodo di Roma risulta essere coordinatore.
Table of contents
Abstract i Table of contents iii Acknowledgements vii Background and Motivation ix List of publications and activities xv1
Field emission vacuum devices for electronic
application
1 1.1 Field emission and Fowler‐Nordheim equation 2 1.2 Field Emitter Arrays 12 1.2.1 Field emission from microtips 12 1.2.2 Spindt‐type cathodes 17 1.3 Novel Cold Cathode Materials 24 1.3.1 Carbon Nanotubes 25 1.3.2 Application: Carbon nanotubes for vacuum electronics 29 1.4 Other alternative emitters 35 1.4.1 Silicon Nanowires (SiNWs) 35 1.4.2 Metal Oxide Nanostructures 37 References 41
2
Field Emission from nanostructured materials
472.1 CNTs synthesis process 48 2.1.1 HF‐CVD – Hot Filament CVD 51 2.1.2 PE‐CVD – Plasma Enhanced CVD 54 2.2 Carbon Nanotube‐based nanotriode characterization 62 2.2.1 Field emission set‐up 62 2.2.2 Carbon nanotubes characterization 64 2.2.3 Measurement procedure 74 2.3 Silicon Nanowires 75 2.3.1 Synthesis process 75 2.3.2 SiNWs characterization 76 2.4 ZnO Nanorods 81 2.4.1 Synthesis process 81 2.4.2 ZnONRs characterization 82 References 86
3
Carbon nanotube‐based vacuum triode
893.1 Fabrication process of the device in Spindt‐type configuration 90 3.2 Fabrication process of the optimized device 93 3.2.1 Technological phase 1 94 3.2.2 Technological phase 2 97 3.2.3 Technological phase 3 100
3.3 Flip‐Cathode Configuration 105 3.3.1 Overview of the patent 106 3.3.2 Technological process 108 3.4 Characterization of the Flip‐Cathode Vacuum Triode 114 References 118
4
Design and Simulation of CNT based
vacuum triodes for high‐frequency regime
1194.1 Frequency limit of conventional Spindt‐type triodes 120 4.2 The Cross‐bar concept 123 4.3 Microwave characterization of the device 127 4.3.1 Simulation results in CST Microwave suite 127 4.3.2 Coplanar Waveguide Vs. Microstrip Solutions 130 4.3.3 Array structure 138 4.4 Other novel vacuum tubes for THz frequency range 142 4.4.1 THz sources and the problem of THz gap 144 4.4.2 THz vacuum electronic devices 147 4.4.3 Carbon nanotube‐based THz vacuum electronic devices 149 4.4.4 OPTHER project 151 References 156
5
Identification of the technological steps and
the measurement system for high‐
frequency triodes
1595.1 Lithographic Mask 160 5.1.1 Bonding techniques 163 5.1.2 Getters 165 5.2 Design of new high frequency measurement vacuum system 168 References 175
Conclusions
177
Acknowledgments
“… E adesso aspetterò domani per avere nostalgiasignora libertà signorina fantasia così preziosa come il vino così gratis come la tristezza con la tua nuvola di dubbi e di bellezza …” Se ti tagliassero a pezzetti – Fabrizio De André – Indiano (1981) E’ giunto il momento dei ringraziamenti e almeno questi, dove posso, li faccio in italiano. Sono molte le persone che direttamente o indirettamente hanno contribuito, anche a loro insaputa, affinché questo lavoro venisse pensato, affrontato e realizzato. Il primo ringraziamento va inevitabilmente al Prof. Aldo Di Carlo, “The Boss”. Molteplici sono le cose, le occasioni e gli avvenimenti che mi vengono in mente in questo momento, che hanno contraddistinto questi anni di dottorato e che spero di racchiudere in un GRAZIE di cuore, soprattutto per essere stato artefice e complice di questo mio desiderio di rendere reale una “febbre” esplosa durante il periodo di tesi. Al Dr. Andrea Reale, con il desiderio e la speranza di far mie, l’imbarazzante disponibilità e l’abnegazione incondizionata in tutto quello che fa, a tutti i livelli, in questo mondo, un vero esempio. Se poi tutto questo va a discapito dell’organizzazione e della puntualità, pazienza. A Francesca Brunetti, che prima di me ne ha seguito l’esempio e se ne vedono in maniera evidente i frutti. Non basterebbe una tesi intera per esprimerti i ringraziamenti per tutto quello che hai fatto in questi anni e che continui a fare. Ti basti sapere che tutto quel poco che so e che ho realizzato in questi anni lo devo a te. Un grazie particolare va a Giacomo Ulisse, per i continui confronti e per tutto l’aiuto dimostrato, con la consapevolezza che non poteva esserci persona migliore in grado di continuare questo lavoro. Un sentito ringraziamento va a i coordinatori di dottorato di questi tre anni Prof. Riccardo Marino e Prof. Corrado Di Natale.
Un doveroso ringraziamento va anche a tutti i ragazzi che, anche per il solo periodo di tesi, hanno dato e continuano a dare un contributo in questo lavoro, come Luca, Fabrizio, Damiano, Antonino, Mauro, Remo, Andrea, Francesco e tutte le straordinarie persone incontrate in questi anni, che fanno dell’Optolab (e non solo), un posto veramente familiare, come Giuseppe Romano, Gabriele Penazzi, Fabio Sacconi, Alessandro Pecchia, Matthias Auf der Maur, Stefano Bellocchio, Giorgia Di Lorenzo, Francesca Buccarello, Monica Coppola, Mauro Mineo, Alessio Gagliardi,
Desiree Gentilini, Giuseppe Latessa, Lyuba Prokopova, Stefano Russo, Mario Arcari, il mio vecchio maestro Pietro Regoliosi, Claudio Paoloni e la “socia” Claudia Bettiol. Grazie a questo lavoro ho avuto la possibilità di interagire con diversi gruppi di ricerca e soprattutto conoscere persone eccezionali da ogni punto di vista. Un ringraziamento speciale al gruppo della Prof.ssa Terranova del Dipartimento di Chimica, ed in particolare a Silvia Orlanducci, Angelamaria Fiori, Emanuela Tamburri, Francesco Toschi, Valeria Guglielmotti per tutto l’affetto e la disponibilità dimostrata in questi anni. Un grazie di cuore anche al gruppo del Prof. Cirillo del Dipartimento di Fisica, per la continua stima dimostrata ed in particolare a Massimiliano Lucci, Ivano Ottaviani e Federica Stella.
Many thanks go to Prof. Viktor Krozer and his group, for his fundamental help during my experience in Denmark, in particular his support was determinant for my approach to simulation studies and so important I remember are those long chats, discussing about microwave structures, S parameters and Danish life style.
I would like also to thanks Dr. Rajendra Kumar for his help in clean room processes and carbon nanotubes’ growth, and to have given me the opportunity to offer my contribution and to be his co‐author of the review chapter, really thanks you so much.
Un grazie di cuore va necessariamente a quella che è diventato da qualche tempo la mia nuova “casa”, a tutto il gruppo dei ragazzi che lavora o ha lavorato sul fotovoltaico organico ed in particolare, il Prof. Thomas Brown punto di riferimento dal punto di vista scientifico e persona straordinaria, Massimiliano Liberatore, Adalberto Brunetti, Simone Mastroianni, Fabrizio Giordano, Enrico Leonardi, Luigi Salamandra, Daniele Colonna, Giordana Roma, Daniele D’Ercole, Valerio Zardetto, Massimo Cecchetti, e quello che è diventato ufficialmente il mio gruppo, il “Gruppo dei Record”, Luigi Vesce, Girolamo Mincuzzi, e Francesco Felici.
In ultimo, mi tocca ringraziare anche loro. Più passano i giorni, più li guardo e penso ad una massima sull’amicizia, descritta come una presenza che non ti evita di sentirti solo, ma rende il viaggio, qualunque esso sia, piacevole e di sicuro più leggero. E’ bellissimo trovare il tempo di voltarsi indietro e accorgersi che senza di loro i percorsi fatti e i problemi superati non sarebbero stati tali. E’ ancora più bello non avere la minima certezza di quello che sarà il futuro, avere il coraggio di scommettere su traversate ancora da fare e farlo con le persone giuste. Semplicemente loro, Eleonora Stefano. Grazie.
E’ soprattutto per tutte queste persone che ho ringraziato in queste pagine che in questi anni non c’è mai stata una mattina in cui mi sia svegliato con il fastidio e la pesantezza di andare a lavoro. Grazie di cuore anche per questo, il viaggio in questo senso è stato indimenticabile.
Background and
Motivation
Field emission was described for the first time at the end of the 19th century. Many experiments in the 1920s showed clearly that this emission occurred under high electric field and was not related to thermoelectronic emission.
In 1928, Fowler and Nordheim used the concepts of quantum mechanics to estimate the current drawn from a metal surface by elastic tunneling under high electric fields: their model is still widely used today, and its derivation will be considered in detail in section 1.
Following the development by Müller of the field emission microscopy in 1938 and the field ion microscopy in 1956, field emission was extensively employed as one of the most powerful techniques in surface physics. Field emission was also used by Young in his “topografiner”, the forerunner of the scanning tunneling microscope. However electron sources based on field emission did not find their use in scanning probe microscopes but in conventional electron microscopy. Most high‐end electron microscopes take part of the low energy dispersion and high brightness of cold or Schottky field emission sources to reach high resolutions.
In contrast to the commonly used thermionic emission based on a hot filament, field emission from an unheated “cold” cathode is activated by electron tunneling effect through a field‐thinned barrier at room temperature. The effectiveness of field emission phenomenon is revealed by the lowering of power efficiency consumption respect to thermionic emitters which requires heating. In addition, field emission sources also offer several attractive characteristics such as the instantaneous response to field variation, the resistance to temperature fluctuations and radiation, the high collimation of emitted beam, a good on/off ratio, the ballistic transport and a nonlinear current‐voltage relationship in which a small change in voltage results in a large change in emission current. Field emission, however, requires a very large local field of a few V/nm to obtain useful currents for applications, so that practical cold cathodes utilize the local field enhancement at the apex of a tip or protrusion and/or a low‐work function material to lower the threshold voltage enabling emission and opening the way of so called field emission vacuum microelectronics.
Serious investigation took off in the late 1960s, when Spindt‐type cathodes also known as Spindt‐type field‐emitter arrays (Spindt FEAs) were
developed. These are basically microfabricated Molybdenum tips in gated configuration. Subsequently, Silicon microtips arrays were fabricated, and silicon vacuum microtriodes were introduced. The Mo and Si microtip FEAs were then developed for large‐area addressable electron emitters, for prototype field emission displays (FEDs). High resolution displays based on this novel technology were being produced by various commercial organizations and demonstrated since the beginning of the 1990s. Although they have not been successfully made to become popular household, the research on cold cathode has become a main stream activity in solid state chemistry and physics.
Initially vacuum devices were developed in the form of diode, triode, tetrode, pentode, that are being used as high power amplifiers and oscillators at low RF frequency. Subsequently ultra‐high vacuum devices were developed in the form of magnetrons, klystrons, travelling wave tubes (TWTs), backward‐wave oscillators (BWOs), for their high power applications at microwave frequencies up to 100GHz. These microwave vacuum devices called slow‐wave devices, have the particularity that the RF wave is slowed down to nearly 1/3rd to 1/10th of light velocity for its synchronism with the accelerated electron beam in an ultra‐high vacuum enclosure. During this synchronized interaction of accelerated electron beam with the slowed down RF wave, part of the dc energy of the accelerated electron beam is transferred to the RF wave leading to amplification/generation of high power RF wave. Dimensions of these vacuum devices reduce with increasing RF wave frequency. Therefore, the fabrication of these vacuum devices at high frequencies beyond 100GHz is hampered by the ability of the present manufacturing technology because of the reduced dimensions of critical components, such as RF structures and electron guns. Alternatively, fast
wave vacuum devices such as Gyro devices, FEL (Free Electron Laser) were developed for the generation and the amplification of very high power RF waves even up to 200GHz and beyond, but these devices are very large in size and complex.
This led to the urgency to develop compact vacuum devices at terahertz (THz) frequencies, above 100GHz, for their enormous applications for wide band communication, imaging radars, spectroscopy, and in many more unexplored areas of scientific, industrial and medical applications. THz devices provide added diagnostic tool for surface or small depth imaging that is very useful for security purpose, medical and agriculture field. At present, semiconductor devices cannot replace the vacuum devices at THz frequencies for power level even at few mW. Fundamental drawback of semiconductor devices at THz frequency is that electron transport is impeded by the silicon crystal lattice, which places a limit on both the miniaturization and the switching speed of such devices. A possible solution to this is therefore to create an active electronic device which relies on electron transport through vacuum.
Although recent developments in microelectronics and micro‐electro‐ mechanical systems (MEMS) technology are being explored for fabrication of small‐size RF structure and FEA cold cathode, a lot of work still has to be done. The importance to develop small size vacuum structures bring to consider new appropriate materials as possible candidates to be used as cold cathodes in the fabrication of the device. In this direction, by exploiting their unrivaled aspect ratio and high robustness, 1Dimensional nanostructures like carbon nanotubes, semiconductors nanowires and metal‐oxide nanostructures have been widely investigated as promising field emitters. In the last 15 years in fact, research on field emissions has been
mainly focused on such structures: hundreds papers on the argument testify the worldwide effort in measuring, understanding field emission from such nanostructures, as well as in the realization of field emission devices.
This dissertation contains the results of a three year investigation in the direction of the fabrication and characterization of carbon nanotube‐based vacuum triodes. It begins with the analysis of the Fowler‐Nordheim equation and an overview on the field emission devices is reviewed. By reporting the manufacturing aspects and the related problems of the most widely investigated field emission devices, Spindt‐type array, innovative materials like Carbon Nanotubes (CNTs), Silicon Nanowires (SiNWs) and some promising Metal Oxide Nanostructures (namely ZnO, CuO, WO3, SnO2) will
be shown and described as potential materials for the realization of field emission cathodes.
Subsequently, starting from the analysis of the synthesis process, field emission behavior of these high field enhancement factor materials as CNTs are investigated. In particular the characteristics and the physics behind the synthesis techniques used in this work to perform the CNTs growth, namely Hot Filament Chemical Vapour Deposition (HF‐CVD) and Plasma Enhancement Chemical Vapour Deposition (PE‐CVD) are described. Characterization in diode configuration is then evaluated in terms of field emission behavior.
Thereafter technological processes for the design and the fabrication of carbon nanotube based nanotriode with high field enhancement factor are investigated. Afterward the devices manufactured are measured in terms of field emission behavior in triode configuration by evaluating the modulation behavior and the extrapolation of the characteristic parameters.
The optimized flip‐chip structure is then studied using the CST simulator in order to obtain a complete microwave characterization of the small‐signal properties in terms of S‐parameters of the device.
Finally, the aspects of this work related to world of high frequency are discussed. In particular the attention is focused on the design of a new lithographical mask where the realization of a packaged and integrated device is considered a milestone in the fabrication of a high frequency device. The other important design activity regards the project of the new high frequency measurement vacuum system in order to avoid parasitic elements that typical limit the performances of the device in a frequency characterization. In this direction a new patented device design with an innovative geometry for high frequency applications is proposed and detailed and the state of art related to other vacuum tubes operating in THz frequency range are overviewed and described.
List of publications and
activities
List of Publications
‐ R. Riccitelli, A. Di Carlo, A. Fiori, S. Orlanducci, M.L. Terranova, A. Santoni, R. Fantoni, A. Rufoloni, F.J. Villacorta “FIELD EMISSION FROM SILICON NANOWIRES: CONDITIONING AND STABILITY”, Journal Applied Physics. 102, 5 (2007)
‐ F. Brunetti, R. Riccitelli, A. Di Carlo, A. Fiori, S. Orlanducci, V. Sessa, M.L. Terranova, M. Lucci, V. Merlo, M. Cirillo ‐ “FLIP‐CATHODE DESIGN FOR CARBON NANOTUBE BASED VACUUM TRIODES” ‐ IEEE Electron Device Letters, Vol. 29, No. 1, January 2008
‐ Review chapter entitled “Metal oxide nanostructures for field emission application” in the book series on Metal Oxide Nanostructures and Their
Applications, R.T. Rajendra Kumar, R. Riccitelli, K. Senthil, American Scientific
Publishers, 2009 (In Press)
International Conferences
‐ FIELD EMISSION PROPERTIES OF SELECTED SINGLE WALL CARBON NANOTUBE SAMPLES – A. Fiori, S. Orlanducci, V. Sessa, E. Tamburri, M.L. Terranova, A. Di Carlo, A. Reale, F. Brunetti, P. Regoliosi, R. Riccitelli, A. Ciorba, M. Rossi – Applied Diamond Conference Nanocarbon 2005 ‐ Argonne May 2005
‐ TOWARDS THE REALIZATION OF A MULTIELECTRODE FIELD EMISSION DEVICE: CONTROLLED GROWTH OF SINGLE WALL CARBON NANOTUBE ARRAYS – F. Brunetti, A. Di Carlo, R. Riccitelli, A. Reale, P. Regoliosi, M. Lucci, A. Fiori, M.L.
Terranova, S. Orlanducci, V. Sessa, A. Ciorba, M. Rossi, M. Cirillo, V. Merlo, P. Lugli, C. Falessi – Microtechnologies for the new Millenium (SPIE) 2005 Sevilla Spain, May 2005
‐ REALIZATION OF CARBON NANOTUBE‐BASED TRIODE – F. Brunetti, P. Lugli, A. Fiori, S. Orlanducci, V. Sessa, E. Tamburri, F. Toschi, M.L. Terranova, R. Riccitelli, E. Petrolati, L. Von Neumann, C. Paoloni, A. Reale, A. Di Carlo, A. Ciorba, M. Cirillo, V. Merlo ‐ 6th IEEE Conference on Nanotechnology 2006, Cincinnati(OHIO), July 2006
‐ INNOVATIVE DESIGN OF NANO‐VACUUM TRIODE – R. Riccitelli, F. Brunetti, E. Petrolati, C. Paoloni, A. Di Carlo, F. Toschi, M. L. Terranova – IVEC 2007 – 8° IEEE International Vacuum Electronics Conference (IVEC), Kitakyushu Japan, May 2007
‐ THZ VACUUM TRIODE BASED ON CARBON NANOTUBE – R. Riccitelli, F. Brunetti, C. Paoloni, A. Di Carlo, V. Krozer, M.L. Terranova, A. Ciorba – ISMOT 2007 ‐ 11° International Symposium on Microwave and Optical Technology (ISMOT), Monte Porzio Catone, December 2007
‐ COLD CATHODES ASSEMBLED WITH CNT AS ELECTRON SOURCES FOR MINIATURIZED ELECTRONIC DEVICES –M.L. Terranova, M. Lucci, S. Orlanducci, V. Sessa, F. Toschi, A. Di Carlo, F. Brunetti, A. Reale, R. Riccitelli, A. Ciorba, M. Rossi, D. Hampai, Nanotech 2008, Venezia, March 2008
‐ FIELD EMISSION VACUUM TRIODE: THZ WAVEGUIDE SOLUTIONS FOR THE TRANSMISSION LINES – R. Riccitelli, F. Brunetti, C. Paoloni, G. Ulisse, A. Di Carlo, V. Krozer – IVEC 2008 – 9° IEEE International Vacuum Electronics Conference (IVEC), Monterey California, April 2008
‐ INNOVATIVE VACUUM DEVICE FLIP‐CATHODE BASED ON CARBON NANOTUBES FIELD EMITTERS – R. Riccitelli, F. Brunetti, G. Ulisse, C. Paoloni, A. Di Carlo, S. Orlanducci, V. Sessa, M.L. Terranova, M. Cirillo – First Mediterranean Photonics Conference, Ischia June 2008
‐ SIMULATION OF FIELD EMISSION BEHAVIOUR IN CARBON NANOTUBE BASED VACUUM TRIODE – R. Riccitelli, C. Paoloni, F. Brunetti, G. Ulisse, A. Di Carlo –
IVNC 2008 – 21° International Vacuum Nanoelectronics Conference (IVNC), Wroclaw Poland, July 2008
National Conferences
‐ TOWARDS THE REALIZATION OF A NANO‐VACUUM TUBE – F. Brunetti, A. Di Carlo, R. Riccitelli, A. Reale, E. Petrolati, C. Paoloni, A. Fiori, S. Orlanducci, E. Tamburri, M.L. Terranova, A. Ciorba, M. Cirillo – Congresso annuale Gruppo Elettronica, Naxos June 2005 (PRIMO PREMIO ST MICROELETTRONICS)
‐ FIELD EMISSION PROPERTIES OF DIFFERENT KINDS OF SINGLE WALL CARBON NANOTUBE SAMPLES – M.L. Terranova, A. Fiori, S. Orlanducci, V. Sessa, E. Tamburri, A. Ciorba, R. Riccitelli, L. Von Neumann – Congresso Nazionale ISTM – Cagliari, September 2005
‐ CATHODE OPTIMIZATION FOR FIELD EMISSION NANOTRIODES – G. Ulisse, R. Riccitelli, F. Brunetti, C. Paoloni, A. Di Carlo, Congresso Annuale Gruppo Elettronica, Otranto (LE) June 2008
‐ OPTHER‐OPTICALLY DRIVEN TERAHERTZ AMPLIFIERS – A. Di Carlo, F. Brunetti, C. Paoloni, R. Riccitelli, G. Ulisse, Riunione Annuale del Gruppo Elettronica, Otranto (LE), Italy, June 2008
Workshops
‐ INNOVATIVE DESIGN FOR CARBON NANOTUBE BASED VACUUM TRIODE – ICNTE ‐ 1° Italian Workshop on Carbon Nanotubes for Electronic Applications, Bologna, May 2007
‐ NANOVALVOLA: CONFIGURAZIONE ALTERNATIVA SEGREDIFESA – IV Simposio sulle Tecnologie Avanzate “Nuovi Orizzonti Tecnologici Applicativi”, Roma, June 2007
‐ WORKSHOP ON THz AND mmW TECHNOLOGIES IN ITALY – An Advanced Materials & Enabling Technologies Community – Finmeccanica, Roma 2007
‐ FLIP‐CATHODE FIELD EMISSION VACUUM TRIODE FOR THZ APPLICATIONS – Microwave Technology and Techniques Workshop 2008 ‐ Innovation and Challenges – European Space Research and Technology Centre (ESTEC) in Noordwijk, Netherlands, May 2008
Patents
‐ Innovative structure for a triode type field emission based vacuum tube (based on CNTs) (PCT/IT2006/000883)
‐ High frequency triode‐type field emission device and process for manufacturing the same (PCT/IT2007/000931) (Patent pending)
Schools
‐ ITSS2005 (International Traveling Summer School of Microwaves and Lightwaves) L’Aquila, Italy
‐ ITSS2006 (International Traveling Summer School of Microwaves and Lightwaves) Warsaw, Poland
‐ Scuola Dottorato Gruppo Elettronica GE08 (Nanophotonics and Nanoelectronics: technologies, devices and applications), Otranto (LE), Italy
Other Activities
‐ Member of organization committee of ISOPHOS2007 e ISOPHOS2008 (International School on Organic Photovoltaics), Ventotene (LT), Italy
‐ Foreign experience to DTU (Danmarks Tekniske Universitet) in Copenhagen.
1. Field emission
vacuum devices for
electronic applications
In this chapter field emission concepts and theoretical derivation of the Fowler‐Nordheim law are investigated. The description of the developments that have taken place in the past couple of decades in microfabricated field‐ emitters have been reported and figures of merit like turn on‐field, threshold field and field enhancement factor (β) that characterize the field emission performance of an emitter have been proposed and illustrated. Those different approaches are presented focusing the attention on the manufacturing aspects and the related problems. Spindt‐type arrays of innovative materials like Carbon Nanotubes (CNTs), Silicon Nanowires (SiNWs) and some promising Metal Oxide Nanostructures (namely ZnO, CuO, WO3, SnO2) have been shown and described as potential materials for
1.1 Field emission and Fowler‐Nordheim
equation
The emission of electrons from a solid can be achieved by two means: either by heating to a temperature that is sufficiently high for electrons to reach over the potential barrier (thermoelectronic emission, Fig. 1.1(a)), or by applying an electric field that is sufficiently high for electrons at or near the Fermi level to tunnel through the potential barrier (field emission, Fig. 1.1(b)). Furthermore, several transition regimes appear in between these two extremes, depending on field and temperature (Fig. 1.1(c)) [1].
Figure 1.1 (a) Electron emission at high temperature and low applied field; (b)
Electron emission at low temperature and high applied field; (c) Emission regimes as a function of temperature and field for an emitter with = 5 eV [1].
At high temperature and zero applied electric field, the thermoelectronic current density is given by:
4
Where J is the emitted current density, the work function, T the temperature, k the Boltzmann constant, h the Plank constant, m and e the mass and the charge of the electron respectively. Typically, the emission probability becomes significant above 2500 K for most metals, but this value can be significantly reduced with a low work function.
An increase of the applied electric field will induce a decrease of the effective work function: this first transition regime is called Schottky emission, and is described by:
4 ⁄
(2) As the field is further increased, electron emission by tunneling becomes also significant: this is the extended‐Schottky regime, where sin (3) √ 4 (4) As the field is further increased and/or the temperature reduced, electron emission occurs with a tunneling through the potential barrier. Following the thermal field emission model, the current is given by:
sin
where ⁄ , ⁄ 2 2 , and is the Fowler‐ Nordheim equation (29) that will be derived in the following. Finally, the last regime is the cold field emission, which corresponds to T = 0 K. Setting T = 0 K induces typically an error of 6 % at 300 K respect to the behavior at room temperature (300K).
As outlined above, field emission is the extraction of electrons from a solid by tunneling through the surface potential barrier under a strong electric field. Figure 1(a) shows that the potential barrier is square when no electric field is present. Its shape becomes triangular when a negative potential is applied to the solid, with a slope that depends on the amplitude of the local electric field F just above the surface. Although the Fowler‐Nordheim (F‐N) model has been originally developed to describe field emission from flat metallic surfaces at 0 K [2], it has proven adapted to describe field emission from a wide range of materials, among these carbon‐based electron emitters [3].
To derive the current density emitted from a flat unit surface, one starts from the equation:
2 , ,
2
(6) In the frame of the free electron model, which is in first approximation valid for most solids around the Fermi level, one has the electron wavevector , , , the velocity ⁄ , and the occupation probability described by Fermi‐Dirac statistics:
, , 1
(7) To evaluate the emitted current for the free‐electron model, one has therefore to determine the transmission probability D(E) for an electron through a barrier described by (see Fig. 1(b)):
(8) One takes often into consideration the contribution of the image charge, which induces a decrease of the effective work function:
1 4 4
(9) The transmission factor can be determined analytically in the case of the triangular barrier without the image charge. Schrödinger’s equation is solved in two separate domains (the solid, and the barrier and vacuum, respectively) with simple wave matching in between the domains. The solution involves Airy functions Ai, Bi, Ai’ and Bi’ and amounts to: κ⁄ (10) κ 2 ⁄ (11) η κ η (12)
η κ η (13) η κ ⁄ (14) The sub‐ and superscript of indicate that D has been obtained through matching and is an exact solution. One can simplify the exact expression for by setting 1. In that case, the Airy functions Ai and Ai’ are negligible and the Bi and Bi’ functions can be approximated with exponentials. One obtains that: 4 ⁄ ⁄ 4 3 2 ⁄ ⁄ (15) The expression can be further simplified by restricting the energies involved to . This allows one to perform a series expansion of the energy‐ dependant term in the exponential: ⁄ ⁄ 3 2 ⁄ (16) With , one obtains the MWFM, or Matching Wave Function Method, approximation: 4Φ ⁄ 3 2 ⁄ 2Φ ⁄ 2 ⁄ (17) The matching wave function approach does not make possible to take into account the decrease of the effective work function due to the image charge.
One uses to this end the expression developed by Wentzel, Kramers and Brillouin (WKB) that gives the probability for an electron to tunnel through a potential barrier of arbitrary shape V (x):
2 2 ⁄
(18) where is the kinetic energy perpendicular to the surface. The integration limits are given by , . With the triangular potential
barrier , we get: 2 2 ⁄ (19) which, in turn, yields: 2 2 ⁄ 4 3 ⁄ (20) Using again the approximation of Equ. 16, one finally obtains: 4 ⁄ 3 2 ⁄ 2 ⁄ 2 ⁄ (21) This is in fact the same result as the MWFM approximation.
To take the image charge potential (9) into account is not easy, and was done by Nordheim in 1928 for the first time. He obtained the following expression:
2 2 ⁄ 4 3 ⁄ (22) which, with (16), is approximated by: 4 ⁄ 3 2 ⁄ 2 ⁄ t y 2 ⁄ (23) The functions v(y) and t(y) are related to elliptical functions, where y corresponds to the relative decrease of the work function due to the image charge potential: 4 1 (24) The functions can be approximated by 1.049 or 1.1 and
0.96 .
To finally derive the Fowler‐Nordheim equation, one inserts the WKB transmission factors (21) or (23) into Equ. (6), with , , described by Fermi‐Dirac statistics (7) at T = 0 K. Without the image charge, the current J [A] per unit area varies with the local field at the emitter surface F [V/m] as: 2 2 | | (25) and, once the integration is performed,
4 2
4√2 ⁄
3
(26) Inserting numerical values with F in V/m, in eV, and in A/m2 yields:
1.56 · 10 6.83 · 10 ⁄ (27) With image charge, the integration yields: 4 2 4√2 ⁄ 3 (28) so that one obtains, with numerical values, 1.56 · 10 exp 1 6.83 · 10 ⁄ (29) The values of the constants have been known to vary depending on the approximation used for the Nordheim elliptical functions. Also, the physical quantity that is usually measured is a current, whereas the F‐N equation gives a current density. One writes therefore that:
(30) where A has the dimension of an area and represents in first approximation the emitting area. It is useful at this point to remind the assumptions that lead to the model: the tunneling electrons are taken as a free‐electron gas described by Fermi‐Dirac statistics, with an energy comparable to the Fermi energy. The temperature is taken equal to 0 K (inducing an error of 6 % at
300 K). The tunneling is elastic, and occurs from a flat surface, yielding a current density in the F‐N equation. Several models based on the Fowler‐ Nordheim approach have been developed, especially for semi‐conductor structures. Formula (29) refers strictly to the limit T=0, but is a valid approximation so long as 1/ is very large. Now is of the order of 8.5X10‐5T volts. This is sufficient to guarantee the observed independence of T for all ordinary temperature.
According to FN model and considering the expression , the expression for the current density (J) obtained for an applied electric field (E) can be rewrote as: J ⁄ (31) where A and B are constants, (A = 1.56 x 10‐6 AeV‐2, B = 6.83 x 103 eV‐3/2 V µm‐1), β is the field enhancement factor, Φ is the work function of the material and E is the electric field given by V/d. V is the macroscopic applied voltage and d is the distance between the anode and the field emitter. Dividing the equation (1) by E2 and taking ln on both sides yields: ln ln ⁄ (32) A plot of ln(J/E2) versus 1/E is called Fowler‐Nordheim or FN plot. Generally, the field emission data are presented by FN plot in which the FN region is represented as a straight line with negative slope, from the slope value, the field enhancement factor ‘β’ can be calculated if the work function of Φ the material is known as it is shown in the next chapter. Turn on‐field, threshold field and field enhancement factor (β) are the figures of merit that
characterizes the field emission performances of an emitter. Turn‐on field is the field required to achieve a level of current density of 10 µA/cm2 from the emitter higher than the background noise (varies from 10 nA/cm2 to 0.01 mA/cm2 depends on the system). Threshold field is the field required to achieve a significant current density. In this work, it has been assumed the threshold field is the field required to achieve current density of 1mA/cm2 typical value required for flat panel display applications (the minimum to produce the luminance of 300 cdm–2 for a video graphics array field‐emission display with a typical high‐voltage phosphor screen efficacy of 9 lmW–1). Field enhancement factor β is the ratio of microscopic field at the apex of the emitter to the macroscopic applied field and it can be considered as the attitude of the emitter to collect field lines depending only by its geometry. In first approximation an infinitely long emitter with a radius at the apex r is approximated with a sphere with the same radius and:
/ ; /
(33) The meaning that this approximation is too drastic is shown by the models for field enhancement factor of protrusions on flat surface recently reviewed [4]. This studies based on numerical simulations have proposed the most accurate expression to date rely to β valid for 4 / 3000 without make any prediction on the influence of the shape or position of the counter‐electrode:
1.2 2.15 / .
The above models assume that the counter electrode is flat. This is not the case in most experimental set‐ups, and the measured value is therefore not only due to the emitter, but also to the anode where the field will be greater than V/d.
In spite of it, it is reasonable to believe that the shape of the anode does therefore not have a significant influence on the obtained value of β, which comes probably from the fact that the radius of curvature of the nanotube is much smaller than that of the anode.
1.2 Field Emitter Arrays
Active investigation on field‐emitter arrays began in the 1970s with a view to developing vacuum microelectronic devices including flat panel displays. Historically, microstructured tips have been at the heart of the field emission technology, produced from molybdenum and subsequently other materials. This section provides a brief description of the developments that have taken place in the past couple of decades in microfabricated field‐ emitters.
1.2.1 Field emission from microtips
As explained above, the principle of field emission is based on the application of a very high electric field to extract electrons from a metal or a highly doped semiconducting surface. An ideal electron source for micron‐ sized devices would be characterized by the following properties:
1. Being fabricated to submicron tolerances in order to insure that the emitting area in precisely defined and it does not change during operation;
2. The emission has to be voltage driven;
3. The source has to be capable to emit a very high current density level (~A/cm2);
4. The energy density supplied has to be manageable;
5. The energy spread of the emitter charges should be lower than conventional thermoionic cathodes (≤0.5 eV);
6. The emission should be reproducible among different sources and it should be stable over long lifetime (tens of thousands of hours); 7. The fluctuations must to be small enough to not limit the
performance device;
8. Cathodes must to be resistant to ion bombardment, reaction with residual gases, arcing and temperature effects;
9. Manufacturing should be inexpensive, without critical processes and possibly adaptable to wide variety of applications.
For a parallel flat electrode configuration the field is of the order of 109 V/m and this means approximately 1000 kV for an anode–cathode separation (vacuum gap) of 1 mm. However, if the cathode surface has a high point or a protrusion as shown in Fig. 1.2, electrons may be extracted at a considerably lower applied field. This is because the lines of force converge at the sharp point and the physical geometry of the tip provides an enhancement of the applied electric field [5].
Another important aspect to take in consideration in field emission analysis is the density of the emitters and in particular how the distance between the emitters plays an important role for the field emission properties.
Figure 1.2 Illustration of field electron emission from a tip [5].
In fact, emitters are typical assembled to array structures and their emission properties is influenced to the electrostatic interaction between the emitters and a so called screening effect become significant even for large distance between the emitters . For high density emitters, screening effects reduce the field enhancement factor end thus the emitted current. For emitters of a medium density, there is an ideal compromise between the emitter density and the emitters distance which is sufficiently large to avoid screening effect. Several investigation both theoretical and experimental, shown that the electric field at the apex of the emitters will decrease with decreasing spacing. In fact, the effective field amplification falls rapidly for spacings of 2 [6]. This effect influences critically the field emission properties of both individual and array emitters. As shown in the figure below, when the distance is less than the height of the emitters, the field enhancement factor decreases rapidly with distance. On the other hand when distance is larger than 1.5 times of the height of emitters, the enhancement factor hardly changes and closes to the enhancement factor
of individual emitter, but the current density decreases with the increasing the spacing. When the distance is much larger than the height of array, the enhancement factor is the same as the case of individual emitter. Figure 1.3 Simulation of the equipotential lines of the electrostatic field for emitters of 1 μm height and 2 nm radius, for distances between emitters of 4, 1, and 0.5 μm (a); along with the corresponding changes of the field enhancement factor β and emitter density (b), and current density (c) as a function of the distance [6].
According to the above equations, the emission current is strongly dependent on the following three factors: (i) the work function of the emitter surface, (ii) the radius of curvature of the emitter apex and (iii) the emission area. It is clear that at a specific field, lower work function materials can produce higher electron emission current. However, not all low work function materials are ideal for constructing field emission cathodes and this is because their other material properties may not be
suitable for field emission applications. For instance, the work function of cesium Φ = 1.8 eV, is one of the lowest, however, stable emission and long lifetime (from cesium or cesiated cesium coated cathodes) it can be very difficult to obtain. Materials commonly used for making microtips, such as illustrated in Fig. 1.3 include molybdenum and silicon, which have a work function of more than 4.5 eV. Therefore, it is important to make the microtips as sharp as possible in order to reduce the field required for emission. Microfabrication is commonly used to produce sharp microtips. In addition, since the current generated from a single microtip is quite small (~μA), arrays of emitters are produced for application in large‐area electronics; for instance, pixels in a display application and field emission gun in microelectronics applications. Such arrays are called field emitter arrays
(FEA). In order to enhance the lifetime of microfabricated emitters, it is
prudent to operate them at low current levels (fraction of a μA/microtip). Some redundancies are also built into the array to take in account of the fact that not all the microtips will be producing equal amount of current.
Essentially, microfabrication involves a combination of lithography, deposition and etching of thin films of a number of materials as it will be discussed in the next chapter, to create structures which are of the order of a few microns at the most. Within these techniques there are many variations, depending on the specific application. From the material point of view, microfabricated emitters may be broadly divided into four types including old Spindt‐type emitters like molybdenum and silicon, carbon‐ based compounds, silicon and metal‐oxide nanostructures.
One major consideration in large volume manufacture of field‐emitters is the cost of production. Most of the microfabrication techniques for
producing FEAs are well developed and routinely used in the semiconductor industry. However, there are a few techniques that are not so common for the semiconductor industry. They include for instance large‐area high‐ resolution laser interference lithography to define sub‐micron patterns for high‐definition displays. Also electron beam evaporation, which it is seldom used in the manufacture of microelectronic devices, is fundamental for certain type of FEA fabrication technology. Another technique, quite unique to realize silicon microtips, is the oxidation sharpening used to obtain uniform ultra‐sharp silicon microtips with an oxidation sharpening step to overcome the not simple problem of tips’ oxidation in order to enhance the tip effect of the emitters.
1.2.2 Spindt‐type cathodes
Most widely investigated FEAs are the Spindt‐type. They are relatively easy to manufacture and quite robust for applications such as field emission displays (FEDs). This type of cathodes was invented in the late 1960s and developed in the early 1970s by Spindt and coworkers [7‐10]. Over the years, there have been very considerable improvements in the quality of Spindt FEAs, owing largely to the advancements in fabrication technology. Spindt FEAs consists of molybdenum cones few micron height, where the microtip radius is in the region of a few nanometers. A concentric aperture hole in the metal gate electrode surrounds the apex of the cone and is separated from the cathode structure by a thin insulating layer, usually silicon dioxide (Fig. 1.4). The small aperture allows operation of device at low voltages.
Figure 1.4 Basic structure of a Spindt cathode—a molybdenum emitter is located at the center of an extraction gate electrode and an insulator layer separates the gate and the emitter [5].
In the early versions, heavily doped silicon wafers were mostly used. Currently, glass substrates are considered more suitable for large‐area FED applications. The substrate is coated with a thin layer of silicon dioxide (~1 μm) to act as the gate insulation. This is followed by a sputtered or evaporated (using electron beam) layer of molybdenum, which is typically 0.5 μm thick to act as the gate metal. Using optical lithography in conjunction with a suitable mask the gate metal is patterned to define a hole, 1 μm in diameter. Recently, laser‐interference lithography has been employed to define sub‐micron holes to reduce the operating voltage of the device [5].
Following resist patterning, the molybdenum layer is selectively etched using either a wet chemical or dry plasma process. The photoresist is then removed by dissolving it in a suitable solvent. Using wet chemical etching (buffered hydrofluoric acid) the silicon dioxide layer is subsequently etched down to the substrate creating an undercut in the top metal layer (Fig. 1.5a). A clever innovation in the fabrication process is the deposition of an aluminum partition layer, at grazing incidence. The wafer or the substrate is
mounted in a vacuum deposition system and rotated about an axis perpendicular to its surface. This allows the definition of the hole to any desired diameter (Fig. 1.5b). Molybdenum (cathode material) is then deposited through the defined hole at normal incidence using electron beam evaporation. As molybdenum is deposited on the substrate, the simultaneous condensation of the material on the aluminum parting layer decreases the hole until it is completely closed. This results in the formation of a sharp cone on the substrate as shown in Fig. 1.5c. Finally, the aluminum parting layer is dissolved in a solvent and this lifts off the top molybdenum layer, leaving the cone behind (Fig. 1.5d).
Figure 1.5 Fabrication steps used in the manufacture of Spindt cathodes. An
aperture (a) is defined by patterning a multilayer of thin films on a substrate. This is followed by the deposition of a parting layer (b) using electron beam evaporation at aglancing angle and subsequent deposition of a molybdenum layer to produce a self‐aligned tip (c). Finally, the excess metal is dissolved in a suitable chemical (d) [5].
The cone height, angle and the microtip radius can be controlled by the initial aperture size, thickness of the oxide layer and the source–substrate distance in the vacuum coater. It is usual in Spindt cathodes to have 1:1 aspect ratio for the microtip height and the thickness of the insulating layer.
Figure 1.6 SEM image of a molybdenum field‐emitter [5].
A SEM image of Spindt FEAs is provided in Fig. 1.6. Spindt FEAs can be fabricated over large areas. It is possible to produce very high densities of emitters using high‐resolution lithography. Figure 1.7 SEM images of a fabricated Spindt cathode array (a) and an example of high‐density cathodes (b) an array of 550,000 tips with a packing density of 6.4x107 tips/cm2 [5].
Using focused ion beam lithography, densities in the region of 6.4 107 tips/cm2 have been demonstrated (Fig. 1.7). In conjunction with holographic lithography techniques, Spindt FEAs with gate apertures as small as 70 nm have been also fabricated [11].
One of the key advantages of using low operating voltage FEAs in display applications is the possibility to use low voltage complementary metal oxide semiconductor (CMOS) drivers.
Operating FEAs to obtain stable emission currents is not as straight forward as fabricating them. There are a number of factors that contribute to the ‘‘randomness’’ or instability of the emission current. From the emission mechanism, it is clear that the emission is strongly dependent on a number of factors including the applied potential, microtip geometry and the cathode material work function.
Variation in any of these factors is reflected in the emission current. It is believed that the emission from these microtips originates from one or two atoms or clusters of atoms. From the fabrication point of view, it is difficult if not impossible to ensure that each emitter has identical (atomic scale) spatial resolution across the entire wafer. According to the simplified expression F ∝ V/r (F: electric field, V: applied voltage and r: microtip radius), variations in the microtip sharpness can results in non‐uniform field at the microtips leading to non‐uniform emission current.
Dyke and Dolan [12] reported that a variation of F of only 20% increase or decrease the emission of an individual microtip by a factor of more than 10 (assuming a work function of 4.5 eV and vacuum gap of field of about 5 × 107 V/cm). Similarly the adsorption of contaminants from the environment leads to changes in the emitter’s surface work function causing instability in the emission. Under normal mode of operation it is common to see the
emission current steadily decrease with time and the applied voltage is required to be raised in order to maintain the same level of emission current.
Similarly, the adsorption of contamination may decrease the emission by increasing the work function and this requires the increase of the applied voltage to obtain a constant current [13]. The overall effect is that the current density is not proportional to the array size.
In order to overcome some of the above problems, the procedure of emitter conditioning is employed. Usually, this involves heating the microtip at high temperatures. This has two effects: (a) desorption of adsorbed impurities and (b) re‐crystallization of the microtip apex due to localized melting. Essentially, two types of self‐heating behaviors are involved in the emission of a field‐emitter, i.e. Joule heating and Nottingham heating [14]. What concerns Joule heating, in the usual case where resistivity increase rapidly with temperature, resistive heating by itself leads to inherent unstable situation at high emission densities. Since stable high‐density emission is observed, there must exist another factor having a strong and stabilizing influence on cathode‐tip temperature. Such stabilizing factor is provided by the energy exchange resulting from the difference between the average of the emitted electrons, and that of the replacement electrons supplied by the external circuit. In the case of thermoionic emission this phenomenon, discussed by Richardson [15] and later by Nottingham [16], is well known and produces the effect of cooling the cathode. In pure field emission (T=0°K), energy levels above EF are empty, all emitted electrons have less
than Fermi energy, and the Nottingham effect necessarily produces heating of the cathode. However if the cathode temperature T is increased (T‐F emission) energy levels above EF become populated and contribute
preferentially to the emission, causing a decrease in the average heat transfer per emitted electron. Thus, in contrast to Joule heating, Nottingham heating increases less rapidly with the current and decreases with the temperature, becoming negative (cooling) at sufficiently high temperature. Clearly the relative magnitude of the two factors varies markedly with operating conditions. For an initially cold field emitter, Nottingham heating is the predominant effect at low emission densities. At high emission densities it is the triggering mechanism which raises the cathode tip temperature to where resistive heating becomes predominant. The Nottingham effect then changes to cooling and exerts a stabilizing influence on tip temperature.
As it is clear, in Spindt FEAs, Nottingham heating is the dominant mechanism where the temperature at the microtip apex can be controlled by the applied field.
A self‐annealing process was investigated by Spindt et al. [13], where the emission current itself is used to heat the microtip. Significantly, higher currents (several hundred microamperes) were obtained from a single microtip when activated by pulsed fields. Spindt et al. concluded that there are two distinct regimes of operation. When the emission current is in the region of 200 μA, a reversible change takes place due to desorption of weakly coupled species such as hydrogen, and when the emission exceeds 400 μA, an irreversible change in the I–V characteristics is observed which is consistent with the irreparable smoothing of the microtip surface due to thermally activated field‐assisted surface self‐diffusion.
1.3 Novel Cold Cathode Materials
The reported magnitude of emission current obtained from Spindt FEAs by various investigators varies significantly. The fabrication process used and the post‐fabrication conditioning seem to play a major role in the performance of Spindt FEAs. Most reliable data is obtained from emission characteristics of single microtips. There are many reports of emission current in the region of several hundred μA/microtip. To date Spindt et al. have demonstrated emission current of several mA/microtip. However, the microtip life‐time at such current levels is still low and in order of ~10000h [17].
As already remarked FEAs have a number of problems, including microtip sharpness variations, microtip height non uniformity and also surface work function variations. The cumulative effect of these is that the emission current can vary from microtip to microtip at a fixed applied voltage. Sharper microtips and microtips with lower work functions generate more currents and in extreme cases can be damaged due to excessive heating. For instance in display application, even moderate variations can result in spatial variation in brightness and also colour non‐uniformity. The problem is overcame to a large extent by placing a resistor between the emitter and the cathode electrode under the emitter. The function of this resistor is to limit the current emitted by the microtip and to normalize the emission current for all the microtips in a pixel to an average level. One other strategy for improving the uniformity is to introduce redundancy by increasing the number of microtips per area. The current contribution per microtip is reduced and the operating voltage can be lowered.