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IEEE TRANSACTIONS ON ELECTRON DEVICES 1
Demonstration of a High Power, Wideband220-GHz Traveling Wave Amplifier
Fabricated by UV-LIGAColin D. Joye, Senior Member, IEEE, Alan M. Cook, Member, IEEE, Jeffrey P. Calame, Senior Member, IEEE,
David K. Abe, Senior Member, IEEE, Alexander N. Vlasov, Senior Member, IEEE,Igor A. Chernyavskiy, Member, IEEE, Khanh T. Nguyen, Member, IEEE, Edward L. Wright,
Dean E. Pershing, Takuji Kimura, Mark Hyttinen, and Baruch Levush, Fellow, IEEE
Abstract— We present the first vacuum electronic travelingwave amplifier to incorporate an interaction circuit fabricatedby ultraviolet (UV) photolithography and electroforming, demon-strating over 60 W of output power at 214.5 GHz from a 12.1 kV,118 mA electron beam. The tube also achieved an instantaneousbandwidth of ∼15 GHz in G-band in the small signal regime.The all-copper circuit was fabricated in two layers using aUV-transparent polymer monofilament embedded in the photore-sist to form the beam tunnel prior to electroforming. Effects aris-ing from fabrication errors and target tolerances are discussed.This microfabrication technique and demonstration paves theway for a new era of vacuum electron devices that could extendinto the 1–2 THz range with advances in high-current-densityelectron guns.
Index Terms— Electron tubes, lithography, millimeter waveamplifiers, millimeter wave tubes, traveling wave tubes (TWTs).
I. INTRODUCTION
THERE continues to be a lack of convenient high-powercoherent sources and devices in the 100 GHz–10 THz
frequency range [1], the well-known terahertz gap thatpromises abundant scientific and commercial applications.Solid-state technologies continue to advance, but the outputpower achievable at 220 GHz, a key atmospheric transmissionwindow, is still limited to several hundred milliwatts [2].Vacuum electron devices (VEDs) dominate in high-powerapplications, but a major challenge for scaling toward theterahertz mark is that the frequency-determining components
Manuscript received October 30, 2013; revised January 9, 2014; acceptedJanuary 10, 2014. This work was supported in part by the U.S. Office of NavalResearch and in part by DARPA. The review of this paper was arranged byEditor C. Paoloni.
T. Kimura is with Communications and Power Industries, Inc., Palo Alto,CA 94304 USA (e-mail: [email protected]).
M. Hyttinen is with Communications and Power Industries, Inc., George-town, ON L7G 2J4, Canada (e-mail: [email protected]).
Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TED.2014.2300014
shrink with increasing frequency, becoming problematic tofabricate and intolerant of high temperature during opera-tion [3]. Here, we present the first vacuum electronic amplifiermicrofabricated in copper by ultraviolet (UV) photolithogra-phy and electroforming, demonstrating over 60 W of outputpower at 214.5 GHz and ∼15 GHz of instantaneous amplifi-cation bandwidth in G-band. This paper is based on a 2013International Vacuum Electronics Conference presentation [4].
Traveling-wave tube (TWT) amplifiers [5] are the mostwidely used VEDs in which an input signal is amplified due tothe synchronous interaction between the electromagnetic (EM)slow wave of a periodic structure and an electron beam trans-ported inside a narrow vacuum channel. Advantages of thistype of device include moderate beam voltage, moderate ratioof slow-wave structure period to operating wavelength, highefficiency of transformation of beam power to radiated power(up to 40%), demonstrated lifetimes of over 30 000 h [6], andintrinsic radiation hardness. The most attractive characteristicof the TWT is its extremely wide instantaneous bandwidth,which can be comparable with the band center frequencyin magnitude. Therefore, complex waveform signals neededfor many advanced applications can be linearly amplified inthe TWT up to the kilowatt level even in the millimeter-wavelength (mmW) bands. Expansion of these workhorseTWTs into the upper-mmW and sub-mmW bands will createoutstanding opportunities for numerous applications, includingultrahigh data rate communications [7], all-weather imaging[8], biology and medicine [9], satellite telecommunicationsnetworks [10], and nuclear magnetic resonance [11]. Untilrecently, the only device capable of providing even modestpower above 200 GHz was the gyrotron oscillator or amplifier,relying on the cyclotron motion of the electrons at ∼28 GHzT−1 in a strong magnetic field to produce an efficient fast-wave interaction [12], [13]. Compact slow-wave devices, suchas the TWT amplifier presented here, were once ruled out dueto insufficient output power [11], [14].
A key barrier to realizing mmW and sub-mmW VEDsis accurate microfabrication of the slow-wave circuit [3].Photolithographic methods using photomasks are accurateto well below the micrometer-scale, and precisions of bet-ter than 100 nm are not uncommon. The UV photolithog-raphy and electroforming technique (collectively known asUV-LIGA, following the German acronym) described here
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2 IEEE TRANSACTIONS ON ELECTRON DEVICES
Fig. 1. (a) View of the vacuum portion of a 205-μm half-period of the circuitwith the location of the electron beam (EB), dimensions in micrometers.(b)–(g) Two-layer UV-LIGA microfabrication process using a polymermonofilament embedded in the PR to hold the size, shape, and location ofthe electron beam tunnel. APR: activated photoresist. F: filament. BTH: beamtunnel hole. (h)–(i) Photomicrographs of the completed all-copper serpentinewaveguide TWC with a 183-μm-diameter steel gauge pin inserted into thebeam tunnel. (j) Photomask pattern. (k) Photo of completed body withmicrofabricated circuit brazed in place.
uses embedded polymer filaments to achieve simultaneousmonolithic growth of highly accurate, solid copper circuits,and integrated electron beam tunnels [15]. Copper is anexcellent material for high-power VED circuits because ithas the high electrical and thermal conductivities needed forhigh mmW power generation, and is vacuum compatible andnonmagnetic [3]. Copper circuits had been fabricated usingUV-LIGA with KMPR and SU-8 photoresists (PR) in the past[16], but complete devices had not materialized. X-ray LIGA[17], an alternative process, is capable of superior aspect ratioand resolution, but the need for a costly X-ray source and masklimits its use. Deep-reactive ion etching was used successfullyin 220 GHz [18], 670 GHz [19], and 850 GHz [20] VEDdemonstrations, but the technique produces circuits in platedsilicon that lack the ideal thermal and electrical properties ofsolid copper. To date, we have fabricated copper circuits from95 GHz [21] to 670 GHz [22] and circuit mold structuresup to 1.35 THz using UV-LIGA techniques [22], spanning anorder of magnitude in frequency. This new microfabricationmethod and device demonstration paves the way for a newera of high-power VED amplifier sources in the mmW andsub-mmW bands.
This compact VED uses a circuit microfabricated byUV-LIGA [15], [23], the first such device. The entire TWT,including the circuit, solenoidal focusing magnets, electron
gun, vacuum windows, and electron beam collector, is con-tained in a compact volume of ∼1 dm3 (= 0.001 m3) andweighs ∼3 kg. The amplifier circuit is a rectangular waveguidefolded back and forth on itself in a serpentine fashion with atunnel for the electron beam passing through it [24], [25].Fig. 1(a) shows the dimensions of the vacuum region of ahalf-period of this serpentine waveguide traveling-wave circuit(TWC), including the beam tunnel. The required verticalaspect ratio of this waveguide is nearly 8:1, and the beamtunnel aspect ratio through the 16-mm-long circuit block is88:1. The TWC was designed around off-the-shelf parts froma G-band extended interaction klystron from CPI, Inc., modelVKY2444T, which limited the gain-bandwidth product thatcould be achieved. In addition to compatibility with planarfabrication techniques, this serpentine waveguide circuit offersbroad bandwidth, high power capability, and moderate gainper unit length [25], in contrast to lower-power but extremelybroadband helix circuits [26], difficult-to-fabricate coupled-cavity circuits [26], [27], and narrow-band extended interac-tion klystrons [26]. The electron beam powering the deviceoperates at an accelerating voltage of 11.5–12.1 kV. This roundbeam is emitted by a thermionic cathode and guided by a0.66 T axial magnetic field through the circuit at a highlyfocused current density of ∼500 A cm−2. The spent electronbeam is then dissipated as heat in the collector, though energyrecovery techniques are possible [26]. In experiment, over86% of the beam power is completely transported throughthe circuit at full mmW output power, with over 91% beamtransmission in the small signal regime.
II. UV-LIGA MICROFABRICATION
Referring to Fig. 1 for the UV-LIGA microfabricationprocess for the TWC [15]: (b) SU-8 PR [28] is applied toa copper substrate and (c) after 365 nm UV pattern exposurethrough a photomask and a developing step, the SU-8 activatedphotoresist (APR) acts like a mold for the desired circuitshape. (d) Copper is electroformed around the SU-8 mold andlapped to thickness. (e) A polymer filament (F) is aligned tothe circuit as a mold for the beam tunnel, and more PR isapplied to embed the filament and create the second layerof the circuit. The filament is UV-transparent and has anindex of refraction similar to SU-8, preventing distortion ofthe UV rays as they pass through. (f) Steps (c) and (d) arerepeated. (g) The PR and filament are removed, and a flatcover is brazed on to complete the TWC. This two-layerprocess produces superior tolerance control for structures over500-μm deep. The filament holds a tolerance of about ±1 μmon the diameter.
Fig. 2(a) shows the dispersion relation for guided waves inthe circuit and for the electron beam; the slope of the latter isproportional to the electron velocity. Wideband synchronousinteraction is achieved using the fundamental forward wavemode, which is dominant for this structure, between the π and2π rad phase advance per period. The required beam voltageis lower relative to possible operating points in the backwardwave regime between 0 and π for the same structure. Based onthe optically measured final TWC dimensions, the disper-sion relation of the as-built circuit was calculated using the
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JOYE et al.: DEMONSTRATION OF A HIGH POWER, WIDEBAND 220-GHz TRAVELING WAVE AMPLIFIER 3
Fig. 2. (a) Comparison of ideal circuit dispersion relation (solid red line)with the as-built circuit (green circles) along with electron beam resonanceline at 11.7 kV (solid blue line). (b) Cold mmW transmission measurementof the final circuit including all tapers and waveguides shown with (dottedred line) and without (solid blue line) the vacuum windows.
3-D finite-element EM solver ANALYST (AWR, Corp.) andcompares very well to the ideal dispersion relation [29].
After the circuit was brazed into the vacuum body, thefrequency response was electromagnetically characterized withand without the broadband beryllia (BeO) mmW vacuumwindows, as shown in Fig. 2(b). This test provides the cutofffrequency, fc, of the circuit, verifying the optical measure-ments of the dimensions. A narrow vacuum window resonancecan be seen at 208.8 GHz, but the windows still exhibit over25 GHz of transmission bandwidth [30]. The insertion lossmeasured matches well to simulations using half the dc valueof conductivity for copper. The passband is very clean exceptfor a dip of ∼3 dB at 223.6 GHz, which corresponds to3π /2 rad phase advance per period and is caused by smallasymmetries in the circuit fabrication [31]. Extraneous nascentstopbands are a well-known property of periodic systems withslight errors in symmetry and have persisted in the VEDindustry for decades. They are a potential cause of oscillationsif sufficiently deep. We show that, by rigorous control oftolerances, these kinds of oscillations can be prevented evenat these frequencies.
Fig. 3. (a) Circuits with poor beam tunnel alignment produce deep stopbandsat the 3π /2 phase advance point near 220 GHz. Beam tunnel tilt amountsare shown in legend with beam tunnel position relative to circuit indicatedby arrow in inset (*Layers also slightly misaligned). (b) Batch of threecircuits from the same wafer with negligible beam tunnel tilt showing veryhighly repeatable transfer characteristics. Beam tunnel offset amounts shownin legend.
A. Beam Tunnel Alignment
While the microfabrication process is rather straightforwardconceptually, small deviations from the ideal periodic structurelead to significant problems. A beam tunnel hole is itself asmall perturbation to the local EM fields in the waveguide gap(the gap is the interaction volume where the beam crosses thewaveguide), but over many periods, it represents a significantcollective effect. In particular, if the electron beam tunnel ispurely offset in the x-direction [see Fig. 1(a) for coordinates],a biperiodicity is created because the EM path length betweengaps alternates between longer and shorter than the ideallength. This biperiodicity results in a stopband appearing at the3π /2 phase advance per period, which, for this circuit design,falls at ∼220 GHz. In contrast to the offset is a small tilt in thebeam tunnel along the length of the circuit. For a slight tilt inthe x-direction, the gap-to-gap phase seen by the EM wavesdiverges from the ideal path length as the waves travel downthe length of the circuit, and the resulting perturbation is nolonger truly periodic. Fig. 3(a) shows early circuits exhibitingvarying degrees of stopband depth due to tilt-type beamtunnel misalignments and some layer-to-layer misalignment,
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4 IEEE TRANSACTIONS ON ELECTRON DEVICES
the worst case approaching 0.05° between layers. Once thismisalignment had been identified, care was taken to ensuremore accurate alignment. A possible rule of thumb is to keepthe beam tunnel misalignment below 10% of the beam tunnelradius for circuits with fewer than ∼100 gaps. In Fig. 3(b),the transmission response is plotted from three circuits withpure offsets (no tilt) below 10% of the beam tunnel radius.These circuits, taken from the same wafer, nearly match intheir characteristics, resulting in a 50% yield. Disqualifyingdefects include excessive beam tunnel misalignment and voidsin the electroformed circuit around the finest features.
Beam tunnel misalignments in the y-direction do not affectthe gap-to-gap phase and hence do not contribute to stopbandsin the circuit. Such misalignments can, however, lead to aslight reduction in gain [23].
III. EXPERIMENTAL RESULT
Fig. 4 shows the experimental setup of the mmW inputdriver sources, TWT configuration and power connections, anddiagnostics. For small-signal gain measurements, an AgilentE8257D analog signal generator drove a solid-state amplifier-multiplier chain capable of ∼3 mW maximum over the fre-quency range 180–260 GHz (Virginia Diodes, Inc., modelWR9.0AMC and WR4.3×2 broadband doubler). The highpower drivers used in the characterization were VEDs fromCommunications and Power Industries. An extended interac-tion klystron (EIK) amplifier (model VKY2444T) provided upto 3.5 W at the input flange of the TWT at 218.4 GHz, andan extended interaction oscillator (EIO, model VKY2441T)provided 13.3 W at 214.5 GHz.
Fig. 5(a)–(c) show small-signal circuit gain curves for theTWT measured at three beam voltages, with the gain adjustedto remove the effect of input and output waveguide losses,which are listed in Table I. Changing the beam voltage hasa large effect on the frequency response of the amplifier,in accordance with the dispersion relation. At 11.5 kV, thepeak gain is >10 dB and amplification can be obtained over∼206–244 GHz. As the beam voltage increases to 11.7 kV,the amplifying bandwidth pushes slightly lower in frequencyand higher in gain to 14 dB. The 3-dB bandwidth is ∼15 GHz.At 11.9 kV, the gain of the circuit approaches 15 dB andgain can be obtained as low as 201 GHz. As the voltage isvaried over this 3.4% range, gain is observed over a range of∼201–244 GHz. In each case, simulations with 3-D particle-in-cell code NEPTUNE [32] using the as-built and as-testedparameters are in a good agreement with these measurements.The gain ripple observed in the TWT response is attributedto spurious reflections from imperfect braze joints in theinput and output waveguides, and was not modeled in thesimulation. A voltage standing wave ratio of no more than∼1.5 is estimated at these joints [29].
High-power amplification results are presented inFig. 5(d)–(f). All high-power measurements were takendirectly at the output flange, including waveguide losses. Dueto the lack of wideband, high-power drivers in this range,high-power operation could only be demonstrated at twofrequencies: 1) 218.4 GHz and 2) 214.5 GHz. At 218.4 GHz,
Fig. 4. (a) Three different driver source configurations were used to supplyinput power to the TWT: 1) An amplifier-multiplier chain AMC provides∼3 mW to the input waveguide flange IWF; 2) the AMC powers an EIK forup to 3.5 W; or 3) an EIO provides over 13 W. The input window (IW) andoutput window (OW) contain the vacuum. The electron beam is acceleratedby voltage VK applied to cathode K. After being guided through the TWCvia magnetic field, electron beam terminates on the electron beam collectorEC producing current ic . Stray electrons striking the TWC become bodycurrent ib . Contents under vacuum are inside the vacuum envelope (VE), anddiagnostics D measure the signal at the output waveguide flange (OWF).
TABLE I
LOSS BUDGET AT 220 GHz
drive power from the EIK was insufficient to saturate theTWT, as shown in Fig. 5(d). Up to 33 W was measureddirectly at the output flange at 218.4 GHz. The simulationpredicts over 40-W output power at saturation. At 11.7 kV,greater output power is obtained with the limited input power,but at 11.9 kV, we obtain better linearity, a key metric forapplications such as radar and telecommunications.
In Fig. 5(e) and (f), the EIO was employed to providemore drive power to the TWT. Fig. 5(e) shows a voltagesweep at a constant 10-W input drive power to explorethe output power variation with beam voltage. The varia-tions in the TWT collected beam current plotted reflect theChild–Langmuir current emission law and the electrostaticfocusing characteristic of the electron gun. Fig. 5(f) showsthe power drive curve of the TWT along with a NEPTUNEsimulation. The beam current of the EIO tube was varied tochange the drive power, which also affects the frequency ofthe drive output, hence the drive frequency is also plotted.At 12.1 kV and 13.3 W at the input flange, 63 W was
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JOYE et al.: DEMONSTRATION OF A HIGH POWER, WIDEBAND 220-GHz TRAVELING WAVE AMPLIFIER 5
Fig. 5. Measurements of small signal gain with IW and OW and taper losses removed (dotted circles), compared with simulations in the NEPTUNE codeof the as-built circuits (solid line) at (a) 11.5 kV and 103 mA collected, (b) 11.7 kV and 103 mA, and (c) 11.9 kV and 104 mA. (d) Measured high-powerresults using the 218.4-GHz EIK as a driver for the TWT amplifier at two voltages (circles) compared with simulations of the as-built circuit in NEPTUNE(solid lines). (e) Using the EIO at 214.5 GHz as a driver for the TWT with constant 10-W input power at various beam voltages, and the collected current(dashed line). The error bars for the cathode voltage reflect quantization error in the measurement, and do not include a systematic error of ±60 V due tocalibration uncertainty. (f) Using the EIO as a driver for the TWT at 12.1 kV and 118 mA showing an output power of over 60 W achieved (circles) atfrequency shown (squares, dotted curve), and with NEPTUNE simulation (solid curve). Error bars for power indicate calibration uncertainty.
Fig. 6. Zero-drive stability test from conditions in Fig. 5(f) showing that,for zero input power, there is no output power detected.
obtained at the output flange at 214.45 GHz. Due to ohmiclosses between the circuit and output flange of ∼1 dB, thepower generated in the circuit is estimated to be 79 W;the electron beam power is ∼1.4 kW, for an interactionefficiency of approximately 5.5%. The amplifier was operatedin pulses as long as 0.5 ms at maximum output power at 2-Hzrepetition rate. Due to concerns of potential circuit damage
from the intercepted beam current, longer pulse lengths werenot attempted.
In Fig. 6, the TWT demonstrates zero-drive stability underthe conditions above for full output power, showing that thereis no output power measured when no input drive power isapplied, confirming the absence of oscillations.
IV. CONCLUSION
We have demonstrated the first VED employing a UV-LIGAmicrofabricated interaction circuit, an amplifier producingover 60 W output power at G-band with wide instantaneousbandwidth. The inherent interaction efficiency is over 5%,and the overall device efficiency could increase dramaticallyusing voltage-depressed electron collector techniques [26].These results show that microfabricated VEDs can signifi-cantly advance the available coherent source power in themmW to terahertz frequency range, making possible manyapplications requiring wide bandwidth and high power wellabove 100 GHz.
ACKNOWLEDGMENT
The authors would like to thank Dr. T. M. Antonsen fordiscussions on biperiodicity, Dr. L. D. Ludeking of ATK, Inc.,for modeling support of the circuit design, and R. E. Myers,F. N. Wood, B. S. Albright, and L. N. Blankenship fortechnical assistance.
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6 IEEE TRANSACTIONS ON ELECTRON DEVICES
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Colin D. Joye (M’03–SM’13) received the Ph.D.degree from the Massachusetts Institute of Technol-ogy, Cambridge, MA, USA, in 2008.
He joined the Vacuum Electronics Branch, U.S.Naval Research Laboratory, Washington, DC, USA,in 2008.
Alan M. Cook (M’07) received the Ph.D. degreein physics from the University of California, LosAngeles, CA, USA, in 2009.
He joined the Naval Research Laboratory, Wash-ington, DC, USA, in 2011.
Jeffrey P. Calame (M’96–SM’11) received thePh.D. degree in electrical engineering from the Uni-versity of Maryland, College Park, MD, USA, in1991.
He has been with the Naval Research Laboratory,Washington, DC, USA, since 1997.
David K. Abe (M’92–SM’12) received the Ph.D.degree in electrophysics from the University ofMaryland, College Park, MD, USA, in 1992.
He has been with the Vacuum Electronics Branch,U.S. Naval Research Laboratory, Washington, DC,USA, since 1997, where he is currently the Head ofthe Electromagnetic Technology Branch.
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JOYE et al.: DEMONSTRATION OF A HIGH POWER, WIDEBAND 220-GHz TRAVELING WAVE AMPLIFIER 7
Alexander N. Vlasov (M’95–SM’03) received thePh.D. degrees in physics from Moscow State Univer-sity, named after M.V. Lomonosov, Moscow, Russia,in 1987.
He has been with Vacuum Electronics Branch,Naval Research Laboratory, Washington, DC, USA,since 2008.
Igor A. Chernyavskiy (M’05) received the Ph.D.degree in physical electronics from the Institute ofHigh Current Electronics, Tomsk, Russia, in 1996.
He has been with the Naval Research Laboratory,Washington, DC, USA, since 2011.
Khanh T. Nguyen (M’07) received the Ph.D. degreein nuclear science from the University of Michiganat Ann Arbor, Ann Arbor, MI, USA, in 1983.
He became the Founder and President of Beam-Wave Research, Inc., Bethesda, MD, USA, in 1994.
Edward L. Wright received the B.S. degree in electrical engineering fromthe University of Alaska, Fairbanks, AK, USA, in 1988.
He has been with Beam-Wave Research, Inc., Bethesda, MD, USA, since2006, where he is a Senior Member of a research team developing state-of-the-art vacuum electronic devices that span the microwave-to-submillimeter-wavefrequency bands.
Dean E. Pershing received the Ph.D. degreein physics from North Carolina State University,Raleigh, NC, USA, in 1980.
He is currently with Beam-Wave Research, Inc.,Bethesda, MD, USA.
Takuji Kimura received the Ph.D. degree in physicsfrom the Massachusetts Institute of Technology,Cambridge, MA, USA, in 1997.
He joined the Microwave Power Products Divisionof Communications Power Industries, Palo Alto,CA, USA, in 2007. He is involved in research ondevelopment projects, including multibeam inductiveoutput tube and sheet beam traveling wave tubedevices.
Mark Hyttinen received the B.A.Sc. (Hons.) degreein electrical engineering from the University ofToronto, Toronto, ON, Canada, in 1980.
He is currently with Communications and PowerIndustries Canada, Inc., Georgetown, ON, as anEngineer of millimeter wave products.
Baruch Levush (F’01) received the Ph.D. degree inplasma physics from Tel-Aviv University, Tel Aviv,Israel, in 1981.
He joined the Naval Research Laboratory (NRL),Washington, DC, USA, in 1995, and became theHead of the Vacuum Electronics Branch, ElectronicsScience and Technology Division (ES&TD), NRL,in 2003, and was the Superintendent of the ES&TD,NRL, in 2012.
