Special Issue solely devoted to Invited Papers on “Microfabricated THz Vacuum Electron Devices”
The Special Issue is motivated by the fact that while the THz region (100 GHz–1 THz) of the electromagnetic spectrum has attracted increased attention in the last few years for a variety of unique applications including all-weather navigational aids, millimeter wave imaging, and advanced communication systems with high Shannon capacity and data conveyance rate, the success of these applications depends critically on the availability of practical coherent sources which are currently not available. This need has motivated extensive investigations into extending vacuum electron device technology into this regime resulting in a number of novel VED approaches. However, terahertz vacuum electronic circuits require submillimeter features of a few microns to a few hundred microns, which pose significant challenges for conventional machining techniques due to tool size and fabrication tolerance requirements. Recently, the MEMS techniques of lithography, etching, and deposition developed by the semiconductor chip industry have been applied to microfabrication and integration of vacuum electronic devices. In addition, the use of multiple beam and sheet beam approaches together with new cathode technologies have made possible a new generation of vacuum electron devices and the intention of this Special Issue is to broadly disseminate the results.
This special issue contains six invited papers which provide a broad view of the frontiers of micro-fabricated THz VEDs and cover the actual devices as well as key enabling technologies. The lead article by Paoloni et al. describes a major effort aimed at the development of a 1 THz backward wave amplifier featuring a carbon nanotube cold cathode for electron generation and a double corrugated rectangular waveguide slow-wave structure (SWS) with fabrication using both LIGA and UV SU-8 photolithography. The second article by Feng et al. describes studies of backward wave oscillators with folded waveguide circuits at frequencies at W-band and Y-band where both UV-LIGA and DRIE microfabrication techniques are investigated. The following paper by Baig et al. describes an ultra-wideband near-THz sheet beam TWT utilizing a staggered vane slow wave circuit. A variety of fabrication techniques are described including the newly developed nano-CNC. The paper by Chua et al. is concerned with two planar helical slow-wave structures that offer both wide bandwidth and a good potential for microfabrication. Proof-of-principle studies involving a W-band structure are described. The sine quo non of vacuum electronics is the electron gun and this becomes increasingly challenging as one moves into the THz regime. The paper by Ives et al. discusses both the challenges and recent advances. Finally, the article by Zhao et al., discusses advanced high current density, long life tungsten-scandate nanocomposite dispenser cathodes which provide the high current densities required by THz VEDS.
Guest Editor
N. C. Luhmann, Jr. (ncluhmann@ucdavis.edu)
University of California, Davis, CA
December 29, 2011
TST, Vol. 4, No. 4, PP. 149-163
(Invited Paper) Design and Fabrication of a 1 THz Backward Wave Amplifier
Claudio Paoloni 1*, Aldo Di Carlo 1°, Francesca Brunetti 1, Mauro Mineo 1, Giacomo Ulisse 1, Alain Durand 2, Viktor Krozer 3, Mikko Kotiranta 3, Anna Maria Fiorello 4, Massimiliano Dispenza 4, Alberto Secchi 4, Vitaly Zhurbenko 5, Faycal Bouamrane 6, Thomas Bouvet 6, Stephan Megtert 6, Emanuela Tamburri 7, Costel-Sorin Cojocaru 8, Aurelien Gohier 8
1 Dept. of Electronic Engineering, University of Rome Tor Vergata, Via del Politecnico 1, 00133, Rome, Italy
* Email: claudio.paoloni@uniroma2.it
°OPTHER project coordinator: aldo.dicarlo@uniroma2.it
2 Thales Electron Devices, Vélizy, France
3 Physikalisches Institut, Goethe-Universität Frankfurt am Main, Frankfurt am Main, Germany
4 Selex-SI, Rome, Italy
5 Technical University of Denmark, Kgs. Lyngby, Denmark
6 Unité Mixte de Physique CNRS-THALES et Université Paris sud 11, Palaiseau, France
7 Dept. of Chemical Science and Technology, University of Rome Tor Vergata, Rome, Italy
8 LPICM – École Polytechnique, (UMR 7647) CNRS, Palaiseau, France
(Received September 16, 2011)
Abstract: The THz frequency range represents a true challenge for designers, fabrication technologies and characterization systems. So far, huge technological obstacles have prohibited any system realization different from laboratory one. Furthermore, most of the applications in the THz frequency range require a level of power not achievable by optoelectronic devices at room temperature or by solid-state technology. The recent availability of three-dimensional simulators and high aspect ratio micro-fabrication techniques has stimulated a class of vacuum electron devices operating in the THz regime, to get a level of output power to enable applications at these frequencies. The OPTHER (Optically driven THz amplifier) project, funded by the European Community, is on the road to realize the first 1 THz vacuum tube amplifier. Technology at the state of the art has been used for the realization of the parts with dimensions supporting THz frequencies. A backward wave amplifier configuration is chosen to make the parts realizable. A carbon nanotube cold cathode has been considered for electron generation. A thermionic micro electron gun is designed to test the tube. A novel slow-wave structure (SWS), the double corrugated rectangular waveguide, is devised to support a cylindrical electron beam and to guarantee high interaction impedance with limited losses. Both LIGA and UV SU-8 photolithography have been tested to realize the SWS.
Keywords: Terahertz, Carbon nanotube, Micromachining, LIGA, Vacuum electron device, Backward wave amplifier
doi: 10.11906/TST.149-163.2011.12.22
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TST, Vol. 4, No. 4, PP. 164-180
(Invited Paper) Study of High Frequency Folded Waveguide BWO with MEMS Technology
Jinjun Feng *, Dapeng Ren, Hanyan Li,Ye Tang and Junyi Xing
National Key Laboratory of Science and Technology on Vacuum Electronics，Beijing Vacuum Electronics Research Institute, P. O. Box749-41, 100015
* Email: fengjj@ieee.org
(Received September 14, 2011)
Abstract: The study of backward wave oscillators with folded waveguide circuits at frequencies at W band and Y band have been carried out in this paper. W band TWT with folded waveguide manufactured with EDM has reached continuous wave 10W and pulsed 100W at 20% duty in the authors’ research group, while for higher frequency devices, we start from W band folded waveguide pencil beam BWO of 100mW power level, for which the entire design and simulation have been finished and the permanent magnet system has been assembled and tested. Y band 220GHz folded waveguide BWO with pencil beam for practical tubes and sheet beam for transmission experiment are also designed and simulated. The impedance and field flatness across the beam were monitored in the simulation. UV-LIGA and DRIE techniques are investigated for the research of micro-fabrication process for W band folded waveguide slow wave structures.
Keywords: BWO, MEMS, Terahertz
doi: 10.11906/TST.164-180.2011.12.23
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TST, Vol. 4, No. 4, PP. 181-207
(Invited Paper) Design, Fabrication and RF Testing of Near-THz Sheet Beam TWTA
Anisullah Baig Student Member, IEEE *, Young-Min Shin, Larry R. Barnett, Diana Gamzina, Robert Barchfeld, Calvin W. Domier, Jianxun Wang, and Neville C. Luhmann Jr., Fellow IEEE
Department of Electrical and Computer Engineering, University of California – Davis, CA 95616, USA
* Email: abaig@ucdavis.edu, anis.ucd@gmail.com
(Received September 16, 2011)
Abstract: There is increasing realization that the region of electromagnetic spectrum between 100 GHz-1 THz has a myriad of potential applications that could directly impact society. These include security applications like non-invasive detection of concealed weapons, explosives and contraband items; imaging of thermonuclear fusion plasmas, medical imaging or detection of cancer, industrial quality control, and future ultra-wide band/high data rate communication systems. For the quest for high frequency and high power sources required to realize these applications and close the so-called THz gap, microwave vacuum integrated technology is an extremely attractive choice for their ability to handle high power in a relatively compact volume. However, the beam-wave interaction physics imposes constraints on the fabrication tolerances and surface roughness that an RF structure can possess. This directly impacts the cold (RF transmission only) and hot (beam and RF interaction) characteristics/performance of the tube. Thus, as we proceed to frequencies in the THz region, conventional machining is unable to handle the required structure fidelity and surface quality.
In this paper, our efforts involving the design, fabrication and RF measurements of an 0.22 THz ultra wide band sheet beam travelling wave tube amplifier are described. Eigen mode dispersion curve analysis and particle-in-cell analysis of the UC Davis designed TWT demonstrated wide band width (> 50 GHz; i.e., ~ 30% instantaneous BW). The output power was calculated to be > 50 W in the pass band for an input drive of 1 W. A PPM based sheet beam transport focusing structure employing SmCo6 magnets and an existing sheet electron gun developed by CPI [1]for use in a proof-of-principle experiment is also described that showed a beam transmission of 80 % that corresponds to a transmitted current of ~ 207 mA for a 20 kV electron beam.
We also describe MEMS fabrication technology to make micro-metallic structures/waveguides possessing the requisite high dimensional definition and low surface roughness (< skin depth). Our efforts in MEMS precision fabrication have primarily focused on the following areas: (a) LIGA technique for high aspect ratio structures in a single process employing KMPR[2, 3] and SU-8[2, 3]; (b) Si-DRIE process[4]; and (C) Nano-machining / nano-CNC milling[5]. We were successful in fabricating completely metalized 0.22 THz TWTA circuits within 3-5 μm tolerance and a surface roughness ranging from 30-80 nm. An extensive SEM and 3D microscope analysis was also conducted and described in detail. A scalar network analyzer system was configured for RF measurements employing a BWO in the frequency range 180-265 GHz2. Both KMPR LIGA and nano- machined circuits showed an excellent agreement with the simulations with S21 ~ -5 dB in the passband and also matching well the predicted 1 dB bandwidth of ~ 65 GHz predicted from 3D FDTD and FEM electromagnetic solvers. S11 remained a little high for the case of LIGA circuits as compared to the simulated value of ~ -10 dB, but for the nano machined circuits S11 gave an excellent agreement with the simulation.
We also describe in this paper our idea/preparation for an exploratory proof-of-principle hot test employing MEMS fabricated TWTA circuits. The PIC analysis for MEMS fabricated circuits placed in a holder assembly that connects an existing sheet beam electron gun, PPM structure, vacuum ports and input/output couplers suggested an output power of ~ 70 W for an input drive of ~ 1 W at 0.22 THz.
It is hoped that MEMS fabricated micro-scale vacuum electron devices will pave the way for the elimination of the so-called “THz gap” by scaling for high frequency operation. This is also important for many applications in the THz region that demands compact and mobile device with reasonable power and bandwidth.
Keywords: MEMS Fabrication, Vacuum Electron Devices, THz Gap, LIGA Fabrication, Nano-Milling/Machining, Vacuum integrated Power Amplifiers, mm-wave/THz technology
doi: 10.11906/TST.181-207.2011.12.24
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TST, Vol. 4, No. 4, PP. 208-229
(Invited Paper) Microfabricated Planar Helical Slow-Wave Structures Based on Straight-Edge Connections for THz Vacuum Electron Devices
Ciersiang Chua 1, 2, Sheel Aditya 1*, Julius M. Tsai 2, Min Tang 2, and Zhongxiang Shen 1
1 School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798.
2 Institute of Microelectronics, A*STAR (Agency for Science, Technology and Research), Singapore
* Email: ESAditya@ntu.edu.sg
(Received September 14, 2011)
Abstract: This paper reports on two planar helical slow-wave structures that offer wide bandwidth and a good potential for microfabrication. The paper begins with a review of progress made in recent years in the development of a planar helix slow-wave structure with straight-edge connections (PH-SEC). This is followed by new results that compare the performance of the PH-SEC with that of the circular and rectangular tape helices; specially considered is the interaction impedance for the forward- and backward-wave. The focus of the rest and the major part of the paper is a new slow-wave structure, namely, the rectangular ring-bar with straight-edge connections (RRB-SEC). It is shown that, similar to the case of the circular ring-bar structure, the RRB-SEC enhances the interaction impedance for the fundamental forward-wave while reducing the interaction impedance for the backward-wave. Detailed results for phase velocity and interaction impedance of the RRB-SEC are presented to show the effect of structure dimensions. Two configurations which are suitable for microfabrication of the RRB-SEC on a silicon wafer are also presented. As a proof-of-concept, one of these configurations, designed for operation at W-band, is microfabricated. The fabricated structures include a coplanar waveguide feed. On-wafer cold-test S-parameter measurements are reported for frequencies from 80 to 110 GHz. The measured results match well with the simulation results when the effect of surface roughness of the different parts of the fabricated structures is accounted for in the simulations. The RRB-SEC, with rectangular or square cross section, has potential application in high frequency travelling-wave tubes that aim to achieve high power operation.
Keywords: Electron devices, Interaction impedance, Phase velocity, Planar helix, Ring-bar, Slow-wave structure, Traveling-wave tube.
doi: 10.11906/TST.208-229.2011.12.25
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TST, Vol. 4, No. 4, PP. 230-239
(Invited Paper) Electron Guns for Terahertz Vacuum Electron Sources
R. Lawrence Ives *, George Collins, Michael Read, George Miram, David Marsden
Calabazas Creek Research Inc., 690 Port Drive, San Mateo, CA 94404 USA
* Email: RLI@Calcreek.com
(Received September 22, 2011)
Abstract: Advanced techniques are allowing fabrication of high frequency RF circuits to high precision. Dimensional tolerances of a few microns are routinely achieved, allowing precise fabrication at frequencies approaching, and sometimes exceeding, 1 THz. Fabrication is usually performed by computer-controlled machines, often without human intervention. A more challenging task, however, is fabrication and assembly of electron guns operating at thousands of volts and temperatures often exceeding 1000℃. Electron gun assembly, especially for thermionic guns, is still primarily a manual process. Because the electron gun operates at a high negative potential, it is necessary to electrically isolate the gun from the RF circuit, requiring one or more bonding processes, which usually include brazing or welding. A further complication is integrating the electron gun and circuit with a magnetic field providing beam confinement. Precise alignment is required to achieve adequate beam transmission. Again, this alignment is typically a manual procedure. This publication identifies the issues associated with the design, fabrication, assembly, and integration of high voltage thermionic electron guns with Terahertz RF circuit and describes simplification provided by the latest generation of high current density cathodes.
Keywords: Cathodes, Reservoir cathodes, Electron guns, Electron beams, RF sources, Terahertz
doi: 10.11906/TST.230-239.2011.12.26
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TST, Vol. 4, No. 4, PP. 240-252
(Invited Paper) Scandia-added Tungsten Dispenser Cathode Fabrication for THz Vacuum Integrated Power Amplifiers
Jinfeng Zhao 1*, Na Li 2, Ji Li 2, Diana Gamzina 1, Anisullah Baig 1, Robert Barchfeld 1, Larry Barnett 1, Subhash Risbud 3, and Neville C. Luhmann Jr., Fellow, IEEE 1
1 Dept. of Electrical and Computer Engineering, University of California, Davis, CA 95616, USA
2 Beijing Vacuum Electronics Research Institute, Beijing 100015, China
3 Dept. of Chemical Engineering and Materials Science, University of California, Davis, CA 95616, USA
* Email: jfzhao@ucdavis.edu, jfzhao2008@gmail.com
(Received December 24, 2011 )
Abstract: The sine quo non of terahertz (THz) vacuum electron beam devices is a high current density thermionic cathode, the development of which has been a major focus of our efforts. As a specific application, we have fabricated a high current density, long life scandate dispenser cathode to develop a 220 GHz sheet beam electron gun for a high power traveling wave tube (TWT) amplifier for the DARPA High Frequency Integrated Vacuum Electronics (HiFIVE) program. Using the solution-gellation (sol-gel) method, Sc2O3-added tungsten powders were made for use in high current density thermionic cathodes. The particle size of the powders was uniform, spanning the range from nanometers to micrometers, and was controllable by adjusting the sol-gel processing parameters. The densified cathode matrix fabricated from the powders has high porosity, uniform grain size and scandia dispersion, and open pore distribution.
By using the Mori Seiki NN1000 nano-CNC (developed by DTL, Davis CA), high current density nano- and micro-composite scandate dispenser cathodes were machined with high precision resulting in good surface smoothness, tight tolerance, and sharp edges. Sc2O3-added tungsten dispenser cathodes were tested in both UHV cubes employing a closely-spaced diode (CSD) configuration under pulse mode and in Cathode Life Test Vehicles (CLTV) with a Pierce gun configuration under CW mode. Space charge limited current densities of 38 A/cm2 at 915oCbr, and 80 A/cm2 at 1050oCbr were obtained by using Sc2O3-added (3.56 wt.%) tungsten powders. The cathode was sintered at 1700oC by using a batch of Sc2O3-W powder with an initial particle size of 700 nm yielding cathode pellets strong enough for machining. In CLTV #1, owing to a perveance issue (there is a 100 micron gap between the focus electrode and cathode emission surface), 10 A/cm2 dc current density can be achieved at practical temperature of 1120oCbr for more than 2000 hours. In CLTV #2 with a reduced focus electrode gap (30 µm), 45 A/cm2 dc current density has been obtained. The collector pulse current density with 56 A/cm2 at 960 oCbr at 4 kV, and up to 104 A/cm2 at 1040oCbr was obtained in the CLTV #3 gun with a cathode out of 70 microns beyond electron focus. This CLTV will be under CW life testing with 40 A/cm2 current density which is the design value for the 220 GHz sheet beam TWT.
Keywords: Scandate cathodes, High current density, CLTV (Cathode Life Test Vehicle), Nano CNC machining.
doi: 10.11906/TST.240-252.2011.12.27
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