Design and fabrication of low-cost

ferrite circulators

Denis C Webb

Naval Research LaboratoryWashington, DC USA 20375-5320

AbstractAdvances in microwave monolithic integrated circuit

(MMIC) technology have resulted in a dramaticreduction in the size and cost of active microwavecircuits over the past several years. There have beenseveral factors which have contributed to this success,some of the principal ones being more accurate deviceand circuit models, efficient user-friendly design tools,improved manufacturing processes, cost-effectivepackaging and high-volume testing enhancements.Efforts have been recently initiated to address manyof these same factors to reduce the cost and size ofMMIC-compatible circulators. This paper will reviewrecent related technical activities in the United States,much of it being conducted by members of the FerriteDevelopment Consortium which is sponsored in partby the Advanced Research Project Agency (ARPA).

IntroductionThere are a growing number of applications which

place a premium on small size and low cost. Amongthese are military systems such as missiles, decoys,multi-function active arrays and commercialapplications such as wireless communications andautomobile radar. The circulator is commonly used toseparate transmit and receive functions in suchsystems, and microstrip implementations offer thelowest overall size and cost. As the size and cost ofmicrowave semiconductor circuits has declined, theneed to address these issues for circulators hasbecome even greater since their relative importancein achieving overall system cost and size goals hasincreased. TheARPA Ferrite Development Consortiumwas formed with the goal of achieving order-of-magnitude reduction in the size and cost of MMICcompatible ferrite circulators. The principal elementsof this program are improved CAD tools, novel hybridconfigurations, low-cost material preparation (bulk andfilm) techniques and integration of magnetic andelectronic functions. This paper is based primarily onrecent work carried out by Consortium members.

Computer Aided DesignAlthough an approach for analyzing microstrip

circulators first appeared in the technical literature morethan thirty years ago there has been a dearth of user-frendly, fast, accurate circulator CAD tools availableto the microwave designer. Circulator development hastherefore relied heavily on experimental data and the

experience and skill of the designer. Multiple design-build iterations have been common in orderto convergeupon an acceptable geometry - a costly, time-consuming process. Fortunately, considerable progresshas been made in the past few years in improving thefidelity and utility of circulator CAD tools.The most widely used design methodology for

microstrip (and stripline) circulators (1 ,2,3) is based ona Green's function approach to relate the axialcomponent of the electric field to the circumferentialcomponent of the magnetic field at the circulatorperimeter. The original analysis entailed severalsimplifying assumptions, in particular, twodimensionality (no fringing fields), lossless media,uniform port excitation and uniform permeability andpermittivity (including, by implication, constant internalbias field) which limited its accuracy. A further restrictionis that the approach can only analyze a circulargeometry, a common but not exclusively-usedconfiguration.Several features have been recently incorporated into

the Green's function analysis to increase its accuracyand utility. Firstly, all dominant loss mechanisms -dielectrc, magnetic and conductor- have been included(4). Dissipation loss contributions for a typical C-bandmicrostrip circulator are shown in Figure 1 as a functionof thickness. Note that if the height exceeds a certaincritical thickness, approximately 0.25 mm for thisexample, the overall loss is relatively height-independent and is determined by magnetic anddielectric losses. As the thickness is progressivelyreduced below this critical height, overall dissipationloss becomes conductor-loss dominated and increaseddissipation loss (and accompanying mismatch loss dueto low impedance) sets limits on the minimumacceptable microstrip thickness. This is an importantconsideration in the realization of high performancefilm-based circulators to be discussed in a later section.A second important recent modification to the initial

Green's function formulation of the microstrip circulatorproblem has been the use of a recursive Green'sfunction technique to remove of the restriction ofhomogeneous material properties (5). This solutiontechnique breaks down the inhomogenoeous circulatorregion into a homogeneous center disk and a series ofhomogeneous concentric rings. All nonuniformities canbe put into the frequency dependent elements of thepermeabi-lity tensor. Not only does this approach allowanalysis of geometries consisting of annuli of differentdielectric and magnetic materals but can be used toaccount for bias-field variations arising fromdemagnetization effects (6).The original circulator analysis approach did not

attempt to account for fringing fields. Accurately treatingthese fields is especially important at the excitationports since the frequency response is very sensitive tothe value of coupling width. The effective coupling widthis in reality considerably larger than the physical width

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of the exciting microstrip line as assumed in the originalanalysis. A better approximation in determining thiseffective width is to base it upon the points where thevertical component of the electric field at thegroundplane has dropped to one-half of its value atthe center of the microstrip. This effective width thus-defined is typically -40% greater than the physicalwidth. Calculated loss and isolation of an X-bandcirculator are compared to measured values inFigure 2. Calculations employ the Green's functionapproach with all modifications described above, alldissipation and mismatch loss contributions,demagnetization effects, and effective excitation widthcorrection. It can-be seen that the principal features ofthe observed circulator behavior are captured by theanalysis. Without the demagnetization and widthcorrections the calculated center frequency is typically-1 0% too low and the observed low-frequency rolloffis not predicted. The latter arises from the intemal fieldbeing nearly constant in the interior but rising sharplyat the edges. The outer portion is thus biased neargyromagnetic resonance at the low end of thefrequency band when the center portion is just broughtinto saturation, resulting in increased loss. The fact thatthe calculated and observed minima in isolation do notoccur at the same frequency resufts from this parameterbeing highly sensitive to small phase and amplitudeerrors in the high isolation region.One of the important limitations of the above

approach is that it is restricted to circular geometries.Several techniques for analyzing non-circulargeometries have been reported in the literature (7,8).A finite-element approach was adopted by The NavalResearch Laboratory (NRL) (9) for the ARPAConsortium, using Maxwell's equations for 2D fields ina magnetic medium with Bosma-type boundaryconditions. This technique employs an establishedfinite-element engine (10). The user must define thegeometry and initial triangulation, the constants andfunctions of the partial differential equations, and theboundary conditions. The fields must must then be post-processed to obtain the S-parameters. This finite-element code is considerably slower than the analyticalcode, typically requiring one to two minutes perfrequency, but is valuable not only in solving arbitrarynon-circular 2D problems but provides insight and avalidity check for the 2D analytic code. Resultsobtained from applying the analytic and codes to anidentical circular geometry are indistinguishable.A useful feature available with finite-element code is

the capability of plotting the field contours. An exampleis shown in Figure 3. A qualitative understanding ofperformance versus frequency can be obtained byexamining the frequency dependence of the electricfield contours. Note, in particular, the presence of thefield null. Ideally it should remain at the isolated port atall frequencies of interest; departures represent adegradation in isolation.For highest fidelity a 3D code solving the full set of

Maxwell's equations in an arbitrary geometry must beemployed. Ansoft Corporation is adding gyrotropicmedia to its commercial 3D electromagnetic simulator,Eminence, as part of theARPA Consortium effort. Thefirst phase of this effort has been completed and betatesting is underway by Consortium members. Initial

simulations agree well with experimental results.The utility of a simulation tool depends heavily on

establishing an effective user interface. For maximumbenefit to the microwave designer, the code forcirculator analysis should be coupled to a commercialcircuit design environment to take advantage of theenvironments capability for graphics, layout, impedancematching, and integration with other components.Versatility is greatest for fast-running codes sinceoptimization in a circuit environment can be done inreal time. The Green's function analytic code has beenshown to perform well when employed as a user-defined circuit element in Hewlett Packard's MDS andTouchstone simulators. As noted earlier the finite-element code is relatively fast (1 to 2 minutes/frequency) on a workstation but not sufficiently fast toperform real-time ierative analysis and optimization.However, by using the code to generate S-parametersin a look-up table format, it can be used to efficientlyoptimize the embedding circuitry for the chosencirculator geometry. The 3D ferrite element tool willoperate in this same manner.

MaterialsGeneral ConsiderationsThere are several different ferrite microstrip circulator

topologies which have been found useful; each hassomewhat different material and device fabricationimplications. The simplest, shown in Figure 4, employsa metal shield on a ferrite substrate with a metal groundplane on the opposite side. A permanent magnetprovides the magnetic field bias of sufficient magnitudeto just bring the entire junction area into saturation.Reasonable performance can also be achieved withlayered ferrite-on-dielectric structures as long as thedielectric consumes no more than about half of the totalvolume. The principle drawback to this approach is thatthe matching sections use a ferrite medium whichincreases loss and complicates design.This problem is commonly avoided by embedding a

ferrite puck in a low-loss dielectric material such asalumina. Although the matching transformer problemsare minimized, fabrication is more complex since theassembly must be rigid and air gaps between puckand substrate must be minimized. A third topologyrecently described (11) employs two different ferritesin the junction area to produce a flatter bias-field profileand achieve circulation over a wider bandwidth. Thiscan be done, for example, by surrounding a highsaturation magnetization puck with a concentric ringof a lower saturation magnetization ferrite.The diameter of the distributed circulator discussed

above is constrained to be approximately one half-wavelength in the ferrite medium to achieve goodperformance, thus the lateral dimensions are set bythe frequency. An X-band device is typically 4-5 mm indiameter. Although, unlike the transverse dimension,there are no fundamental modal-imposed limitsrestricting the height, high conductor and mismatchlosses set a practical lower limit. This is illustrated inFigure 5 which summarizes results of a thicknessversus performance study done by ElectromagneticSciences (EMS) for the ARPA Ferrie Consortium. Asingle matching section was employed and the design

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was optimized in terms of material, shield diameter,and coupling aperture for each frequency. Note thatthe ferrite thickness should be 250 g±m or greater forX-band operation, but 100 gm is acceptable atmillimeter wavelengths. As will be discussed below, theoptimum material technology approach dependsstrongly on the needed ferrite thickness.Only distributed circulator configurations have been

considered in the discussion to this point. Lumped-element techniques hold considerable promise forreducing size and cost (12,13) but the majority of efforthas been concentrated on relatively narrow bands andlower microwave frequencies. Rapidly improvinganalysis tools and better fabrication techniques shouldsignificantly extend their range of applications.

Tape-CastingTape-casting is well suited to making sheets of

ceramic materials with thicknesses in the 300 gm to600 pn range as required for microstrp circulators atmicrowave frequencies. It has proven to be anexcellent high-volume, low-cost technique for producingdielectrc substrates but little has been done in adaptingtape-casting techniques to the production of ferritematerials. The general tape-casting method entails firstforming a slurry of the desired material with an organicbinder and organic solvent. Because magneticmaterials tend to flocculate, a dispersant must be usedto produce uniform ferrite sheets. A doctor blade is usedto spread the materal to a uniform thickness to producegreen sheets. The sheets are heated to evaporate theorganic materials and sintered at temperatures of1 200°C to 1 500°C. During the initial phase of the ARPAConsortium effort, Trans-Tech Inc. developed atechnique for tape-casting yttrium-iron-garnet (YIG),one of the most useful ferrite materials for mid-microwave frequency (C-, X-, Ku-band) applications.Tapes were fully dense and exhibited magnetic andelectric properties comparable to conventional bulkcounterparts. Following the trend to developenvironmentally benign manufacturing processes,Trans-Tech has recently developed an aqueous-basedapproach to supplant the organic-based sy-stem,successfully overcoming problems in foaming and lowviscosity which are common for this approach. Theresultant YIG material has excellent properties formicrowave applications as can be seen from Table 1.In subsequent work, post tape-cast rolling (calendarng)was used to increase the fired density to 99.7% withequivalent microwave properties. The tape-cast sheetshave been rolled up for storage for four months withoutevidence of cracking, sticking or loss of strength.Tapes of a single-component ferrite material are

suitable for low-cost fabrication for some circulatorconfigurations as noted above; others require multiple-ferrite compositions. Trans-Tech and Raytheon aredeveloping geometries consisting of a high saturationmagnetization (M8) circular plug in a low M, hostmaterial for very broadband microwave applications.Only a minimal gap between the materials can betolerated in order that subsequent metallization stepscan be reliably carried out. Raytheon demonstrated thatthis geometry could be successfully produced by co-firing two different members of the lithium manganese-titanium ferrite family with 4icMs = 2750 gauss and

4KM62 = 1300 gauss. However, tape-casting of thiscomposition would be a less expensive process. Thegeneral approach being used to tape-cast thisconfiguration is to cast thick (75 mil) tapes of theconstituent materials and reduce their thickness bycalendaring to avoid firing shrinkage anisotropy.Different amounts of bismuth oxide are added to thetwo compositions to control shrinkage and produce azero gap between the two materials after firng. Theinner and outer portions of the substrate are thenstamped out of the green tape, pressed together andfired to the desired density. To date a zero gap sizehas been achieved and material composition is beingadjusted to achieve the desired magnetic properties.

Hexagonal FerritesOne of the principal contributors to the present size

of the circulator is the bias magnet structure. At presentthis consists of a permanent magnet and a lowpermeability magnetic circuit to confine the flux. Thisstructure is commonly mounted rigidly above thecirculator substrate. Highly anisotropic alignedhexagonal ferrites, similar in structure to thosefabrcated for permanent ceramic magnets, can alsoperform this role and since they are insulators can beincorporated into the microwave circuit to realizecompact internally-biased configurations. The materialsthen must not only exhibit good permanent magnetcharacteristics but also exhibit good microwavecharacteristics.EMS has been focusing on growth of BaM and SrM

compounds [Ba, Sr) Fe12O19 or (Ba,Sr) 0 6 Fe2O3].forthis application. These compounds have hexagonalplatelet grains with a large uniaxial anisotropy fieldalong the c-axis normal to the platelet. Grain growthoccurs primarily in the a-axis direction resulting in largerarea platelets. To produce a good permanent magnetthe material must have a high coercive field andremanent flux. The coercive field is promoted by smallparticle size while remanent flux relies upon a highdegree of grain alignment.A thermomechanical process (press forging) is used

by EMS in the sintering of these compounds to achievegrain-oriented structures. This has resulted in highdensity with very small grain size. Sintering aids suchas CaCo3 and Bi2O3 allow nearly full densification attemperatures as low 9500C. In some compositions,A103 has been added as grain growth inhibitoryieldingstructures with very high coercive fields (up to 6000oersteds). Typical properties achieved are summarzedin Table 2.

Film TechnologyEstablishment of a viable ferrite film technology would

enable monolithic integration of circulators with activedevices on a common semiconductor substrate withresulting advantages in size, cost, manufacturabilityand reliability. Advantages of film technology withdielectric substrates can also be anticipated atmillimeter wave frequencies where substrate and circuitsize become small and difficult to fabricate byconventional approaches.Producing ferrite films which are useful for fabricating

circulators on semiconductor substrates represents amajor technical challenge. Not only must they have

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good physical, magnetic and electrical properties butthey must be manufacturable. Goals and acceptablefilm properties for a circulator operating at lowermillimeter wave frequencies are summarized in Table3. The thickness (Figure 5) should approach 100 gmto realize low loss. Studies conducted by EMS revealedthat a surface finish rougher than 5 micro inches causedincreased insertion loss at frequencies of 30 GHz andhigher. Crack-free, rugged films are essential not onlyfor good performance but are needed for high yieid inmetallization and high reliability. High deposition ratesover large areas are required for low cost and highthrough put. Gallium arsenide, the most widely usedmillimeter wave semiconductor degrades attemperatures > 300°C if unprotected. With use ofappropriate protective layers it can withstandtemperatures approaching 6000C. This can still be amajor problem since ferrites commonly require hightemperature formation and anneal to achieve goodcharacteristics. Low temperature ferrite depositiontechniques have been reported (temperature -1 00°C)(14), but scaling to the thick films required for circulatorswithout encountering high dielectric loss due to excessFe+2 has proven difficult. Low linewidth and losstangent are required to for good insertion loss. Asaturation magnetization of -2000 gauss is acceptablefor narrowband millimeter wave applications but largervalues are needed for many others.Pulsed laser deposition (PLD) has been the most

successful approach to date in producing ferrites forcirculators. It is a conceptually simple flash evaporationtechnique which transfers the stoichiometry of the targetmateral to the film. It has been successfully applied toa variety of metal oxides including high-temperaturesuperconductors, ferroelectrics and dielectrics and sowas a logical choice for ferrite growth. Growth ratesare typically in the 1 gm to 30 gm per hour rangedepending upon substrate size and growth parameters.PLD has been prirnarly a laboratory tool which allowsresearchers to deposit films of complex materials thatare difficult to produce by other physical depositionprocesses. Recently, however, it has been scaled upto 150 mm diameter wafers fora variety of metal-oxides(15).The greatest success in producing ferrite films for

circulators has been produced by PLD of YIG. YIG ismechanically stable and exhibits low dielectric andmagnetic loss. Good quality films of thicknesses up to100 gm have been produced by Westinghouse for theARPA Ferrite Consortium on both Si and GaAssubstrates (16), i.e. a saturation magnetization of 1800gauss, a magnetic linewidth of 50 Oe. and a losstangent of less than .001. The films are grown attemperatures of 5500C and then undergo a rapid-thermal-anneal (RTA) at 850°C for 20 sec. The hightemperature processing and thermal mismatchbetween film and substrate have presented numerousthermomechanical problems including wafer breakage,ferrite film cracking and delamination, metal peelingand arsenic contamination. These were overcome withcarefully selected processing temperatures and useof special layers to protect the substrate. The goldground plane also helps relieve the stress and reducemechanical problems.The relatively low saturation magnetization of YIG

makes it useful for only narrowband millimeter waveapplications. The ARPA Consortium is investigatingvarious spinel compositions to achieve a highermagnetization the most promising PLD film being NiZnferrite. NRL has produced 30 micron thick NiZn filmson -alumina substrates with excellent properties,namely, loss tangent <.001, 4 itM >4000 gauss and aperpendicular FMR linewidth of 100 oersteds (17).Deposition was performed at 5500C at rates exceeding15 gm/hour; no subsequent anneal was required.Efforts are now underway to grow thicker films in acirculator configuration on a semiconductor substrate.The Jet Vapor DepositionTM (JVDTM ) process a

proprietary method developed by Jet ProcessCorporation (JPC) is a production oriented techniquefor depositing thin and thick films. It employs sonic jetsof inert carrier gas in "low vacuum" to convect vaporzedfilm constituents to a substrate at high speed (18). Theprocess provides high rates over large areas and ishighly versatile, capable of producing films of metal,alloys, multilayers, and multi-components such asoxides and nitrides. JPC has been depositing nickelferrites for circulator applications. They employ anelectron jet source which deposits nickel and otherferrites at rates between 50 and 300 gm/hour. The jetalso provides an intense plasma for RF ionbombardment control of film crystallinity.The deposition process has produced nickel ferrites

with thickness from 25 gm to 125 gm and 4nMexceeding 2500 Gauss. Excellent results have beenachieved at deposition temperatures less than 550 OCfollowed by RTA at 850 OC for 20 sec. Aluminum andcopper have also been added to adjust magnetizationand improve microwave properties. Current workfocuses on balancing composition, deposition rate,temperature and RF ion bombardment to optimizemagnetic properties and achieve required surfacemorphology. Results to date are very encouraging, theJVDTM process shows high promise of both generatingthe high quality magnetic materials, and providing ahigh rate, manufacturing method for commerciallyuseful devices.For all film deposition processes investigated to date

thermomechanical and morphology problems place apractical limit of about 100 gm on thickness. Fromearlier discussion this means that the prmary utility offilm technology will be at millimeter wave frequencies.It thus forms a natural complement to tape-casting,which is best suited to microwave frequencies as alow-cost production approach for ferrite circulatorsubstrates.

DevicesSelf-Biased CirculatorsAs noted above, hexagonal ferrites can be used to

realize self-biased (or intemally biased) circulators andthus avoid the size and weight penalty imposed byconventional permanent magnet biasing circuitry. lnitialproof-of-concept demonstrations were conducted in themillimeter wave region using a BaM ferrite post in ametal waveguide configuration (19). The reportedinsertion loss was less than 1 dB over a 2 GHzbandwidth at 35 GHz.The M-type hexagonals operate most naturally at

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millimeter wave frequencies since self-biasedcirculators require high coercivity and hence a largeanisotropy field HA For example, HA for BaM is 17 kOewhich results in a natural gyromagnetic frequency of50 GHz. This is unsuitable for low microwave frequencyoperation since the off-diagonal tensor component Kresponsible for circulation is too small for efficientoperation when the device is biased so far fromresonance.A configuration to circumvent this limitation was

proposed and demonstrated by Raytheon (20). Itemploys a two material configuration consisting of alow coercivity ferrite plug within a high coercivity BaMsubstrate (Figure 6). The low-coercivity insertestablishes the active circulator junction while the BaMferrite provides the bias field. High permeabilitypermalloy sheets are used to confine the magnetic flux.Excellent performance was demonstrated over a 35%bandwidth centered at 9 GHz with less than 0.4 dBloss and more than 20 dB isolation. Total thicknesswas less than 0.9 mm, more than five times thinnerthan models which use an external magnet.

Film CirculatorsAs noted earlier matching and insertion loss

considerations inherent in thickness limitations imposedby current film-growth technology favor millimeter waveapplications of film-based circulators. The smallercirculator disk at the higher frequencies also mitigatesmechanical problems arising from thermal expansionmismatch between film and substrate. By consumingless of the semiconductor substrate area, it alsoenhances the economic viability of monolithic circuit/circulator configurations. The ARPA Ferrite Consortiumis investigating prototypes in the 20 to 77 GHz regionwhich are based on film technology.Westinghouse has successfully fabricated YIG

circulators on both Si and GaAs substrates (16).Fabrication on GaAs presents more technicalchallenges but is of greatest practical significance sowill be discussed in some detail here. Because of thehigh processing temperature required for YIG, thesurface is first covered by an oxy-nitrde layer to preventGaAs degradation and arsenic contamination of othermaterials. A high temperature T/Pt/Au groundplane isthen defined using liftoff. This is followed by a 80 pm to100 pm thick YIG film deposited through a siliconshadow mask by PLD at a temperature of 5500C andannealed with an 8500C/20 sec RTA. The metalcirculator shield and input/output circuitry is thendefined. A coplanar configuration is employed tosimplify fabrication by eliminating the need for waferthinning and via formation as well as providing aconvenient means for probing the devices. The shadowmask deposition technique for the YIG film results in asmooth taper at the edges of the YIG film allowing themetal to be brought from the top surface of the film tothe substrate level.Performance of the first successful YIG-on-GaAs

circulator is given in Figure 7. The minimum insertionloss is 2 dB and isolation exceeds 18 dB over a 1 GHzbandwidth. The explanation for the higher thanexpected midband loss and rapid degradation in lossand isolation at the high end of the band is still underinvestigation but high groundplane-resistance and a

too-thin YIG film are known to be convtbuting factors.A low value of 4nM, and the high processing

temperature make YIG a non-optimal material formillimeter wave applications. NiZn and/or NiCu ferritesare being developed by NRL and JPC with the intentof applying them to millimeter-wave circulators to befabrcated by Westinghouse and Alpha in the next year.Their higher saturation magnetization should increasethe bandwidth by a factor of three. Furthermore, lowerdeposition temperatures and improved coefficient-of-thermal-expansion (CTE) match should reduce film/substrate stress. A viable film technology will beespecially important in the 77 GHz version underdevelopment by Alpha. This device requires a 50 pmthickness and the junction area is only 0.6 mm indiameter, a difficult geometry to realize usingconventional bulk approaches.

Summary and ConclusionsA number of technology advances have been

described in this paper which promise to significantlyreduce the size and cost of ferrite microstrip circulators.These can be broadly characterized as better CADtools, improved material fabrication and processing,and new compact geometries. Both fast analytic 2Dcodes and high fidelity 3D codes for circulator designhave been demonstrated which should prove useful inreducing circulator design-cycle time, lowerng NREcosts and permitting rapid analysis of non-standardconfigurations. The 2D codes have been incorporatedinto commercial circuit simulation tools to permit realtime optimization of circulators in active circuits.Tape-casting and film-deposition of ferrites have been

pursued as a means of low-cost production of microstripcirculators at microwave and millimeter wavefrequencies, respectively. Excellent results have beenachieved with tape-casting of materials for single-ferrite-components structures and promising initialresults obtained for dual-ferrite structures. Good qualityferrite films up to 100 microns thick have beendemonstrated on semiconductor substrates.An approach to thin, high-performance circulators

based on hexagonal ferrites has been identified whichis applicable to all microwave and millimeter wavefrequencies. Excellent results were obtained for awideband X-band unit. Finally, the first practical filmcirculator on GaAs was demonstrated and exhibitedlow-loss and good isolation at 20 GHz albeit for anarrow bandwidth.Although additional refinement of the above

techniques is required, especially in extending the filmand tape-casting to other material systems, theirfeasibility has been established and a pathway to low-cost production of compact circulators identified. TheARPA Consortium has focused primarily on thedistributed circulator; a concentrated effort on designand fabrication of lumped element circulators shouldalso produce a large payoff. By employing acombination of the above techniques not only shouldsize and cost of circulators be reduced for conventionalapplications but new applications could be enabled,e.g. interstage amplifier coupling and multi-portcirculator configurations in cost/volume constrainedconfigurations.

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AcknowledgementsThe author is indebted to the many participants in

the ARPA Ferrite Development Consortium for theircontributions to this paper. Particular thanks go to DougAdam and Harry Buhay of Westinghouse, John Ingsof Trans-Tech, Ernst Schloemann and Ron Blight ofRaytheon, Gordon Harrison and Dave Popelka of EMS,Dan Donoghue of Alpha, Bret Halpern of JPC and mycolleagues at NRL - Robert Neidert, Harvey Newmanand Cliff Krowne. Support and encouragement of FrankPatten, Stu Wolf and Lee Buchanan of ARPA is alsogratefully acknowledged.

References[1] H. Bosma, "On stripline Y-circulation at UHF,"

1964; IEEE Trans. on MT, 12, 61-72.[2] C.E. Fay and R. L. Comstock, "Operation of the

ferrite junction circulator," 1965, IEEE Trans. on MTU,13, 15-27

[3] Y.S. Wu and F.J. Rosenbaum, "Wide-bandoperation of microstrip circulators," 1974, IEEE Transon MUT, 2, 849-856.

[4] R.E. Neidert and P.M. Philips, "Losses in Y-junction stripline and microstrip ferrite circulators," 1 993,IEEE Trans. on MUT, 41, 1081-1086.

[5] C.M. Krowne and R.E. Neidert, "Inhomogeneousferrite microstrip circulator: theory and numericalcalculations using a recursive Green'sfunction," 1995Proceedings of 25th European Microwave Conference.

[6] R.l. Joseph and E. Schloemann, "Demagnetizingfield in nonellipsoidal bodes," 1965, J. Appl. Phys., 36,1579-1593.

[7] T Miyoshi, S. Yamaguchi and S. Goto, "Ferriteplanar circuit in microwave integrated circuits," 1977,IEEE Trans. on MTT, 25, 593-600.

[81 R. W. Lyon and J. Helszain, "A finite elementanalysis of planar circulators using arbitrarily shapedresonators,:" 1982, IEEE Trans. on MUT, 30 1964-1974.

[91 H. Newman, Naval Research Laboratory, privatecommunication.

[10] The software used is PDE2D, available for leasefrom Granville Sewell, P.O. Box 12141, El Paso, TX79913, USA.

[11] R.E. Blight and E. Schloemann, "A compactbroadband circulator for phased array antennamodules," 1992, IEEE MTT-S int'l MicrowaveSymposium Digest, 1309-1392.

[12] Y. Ishikawa, T. Okada, T. Kawanami, K. Okamuraand T. Nishikawa, "A miniature isolator for 800 MHzband mobile communication systems," 1991, IECETransactions, 74, 1226-1232.

[13] C. Ngounou Kouam, J. P Coupez, S. Toutain,G. Forterre an P. Desmarest, " Frequency limits of theultra-miniature circulator," 1994, MTT-S SymposiumDigest, 929-932.

[14] M. Abe, T. Itoh, Y. Tamaura, Y. Gotoh and M.Gomi, " Ferrite plating on GaAs for microwavemonolithic integrated circuit," 1987, IEEE Trans. onMag., 23, pp 3736-3738.

[15] J. Greer and M. Tabat, "Large-area pulsed laserdeposition; techniques and applications," 1994,Proceedings of American Vacuum Society Meeting,Denver, CO.

[16.]J. D.Adam, H. Buhay, M.R. Daniel, M. C. Driver,G. W. Eldridge, M.H. Hanes and R. Messham,"Monolithic integration of an X-band circulator withGaAs MMICs," 1995, Proc. of MTT-S MicrowaveSymposium, 97-98.

[17] Paul Dorsey, Naval Research Laboratory, privatecommunication.

[18] B. L. Halpern and J.J. Schmitt Jet VaRorDeposition, Chapter 16, Deposition Technologies forThin Films and coatings, R.F. Bunshah, Editor, NoyesPublications, Park Ridge, NJ, 2nd edition, 1994.

19] J. A. Weiss, N. G. Watson and G. F. Dionne,"New uniaxial-ferrite millimeter wave junctioncirculators," 1989, MTT-S Symp. Digest, 145-148

[20] Raytheon Company, Ferrite DevelopmentCorporation 6th Quarterly Report.

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Table 1. Properties of Tape-Cast YIG Sheets

Fired densitySurface finishHc4 iMstan

98% T. D.20 inch

7 Oe.1730 gauss

.0002

DensityRemanent MagnetizationCoercive Field'Dielectric Contact *Loss Tangent*Anisotmpy Field

4.9 gm/cm33400 gauss3950 oersted21-2.00119,000 oersted

* Measured at 10 GHz

Table 3. Desired Film Characteristics for Millimeter Wave Circulators

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-----_-- Goal AcceptablePhyjical ,geRtiesThickness 100 gm 50 pmArea 50 cm2 5 cm2Surface Finish 5 p inch 20 p inchSurface Adhesion passes 100% passes 50%(Tape Test)Crack-Free Growth no cracks cracks reduce device yield to 20%

Proce prmetersDeposition Rate 2,500 gmucm2hr lOOpmcm2hr1Dep. & Extended 300°C 6000CAnneal Temperature RTA<8500C

Saturation Mag. 2,500-5,000 G 1,5000-5,000 GFMR Linewidth 200 Oe 500 OeLoss Tangent 0.001 0.01

Table 2. Properties of BaM Ferrites

1.0

m 0.8v G113 (YG)CD) R-.130'W 0.6 W =.130"0-j

z0

0.4 - TOTALLOSS(dB)IL --- METAL LOSS (dB)C5 0.2 0s _L DIEL and MAG LOSS (dB)

HEIGHT (mil)

Figux 1. Dissipation losses of a C-banld ncrostip crulator.

m0(00

oC0 .

Es

0

-5

-10

-15

-20

-25

-30

-35

-404 6 8 10 12

Frequency (GHz)14 16

Figure 2. Circulator insertion loss and return loss. Calculations performed using 2D analytical theory.

f =7GHz f GHz

Figur 3. Electric field contours calculated by 2DFE. Excitation 'is through port 1; port 3 'is the isolated port.

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Figure 4. Ferrite Y-junction microstip circulator.

0 50 100 150 200 250 300 350 400 450 500 550Substrate Thickness (microns)

- 30 GHz; BaM 17 GHz; NkFerrite --* X-band; YIG

--- C-band; Gamet - 77 GHz; BaM

Figure 5. Circulator insertion loss as a function of substrate. Thickness for several material-frequencycombinations

rostripult

\ MicrowaveFerrite

Permalloy Sheet

,,~

\'Hard- Ferrite(BaM)

Figure 6. Microwave self-biased circulator.

1199

3.5

3com 2.5(0

-J

o 1.5:E01C

0.5

0

Circ

Groundplarx

ie

Figure 7. YIG film/GaAs circulator performance.

1200