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8/6/2019 Space Applications

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Applications of MEMS in Comm unicationsSatellitesSlawomir Jerry FiedziuszkoSpace SystemsLORAL

3825 Fabian Way, Palo Alto, CA 94303

ABSTRACT: Micro-Electro-Mechanical-Systems (MEMS) are finding more and more applications inmicrowave systems. Communications satellites ,which typically have a large microwave hardwarecontent, will be discussed as an example of these potential applications. In the first part, general MEMStechnology and early examples will be described. Next, space applications of MEMS ( sensors, actuators,propulsion etc.) in communications satellite vehicle (bus) will be shown. Finally, microwave applicationsof MEMS in satellite payloads (switches, micromachined structures, tunable filters, and phased arrayantennas) will be presented.

I . Introduction to MEMSFor centuries people were fascinated by the prospect of making mechanical devices smaller and smaller,while maintaining their functionality. Limits of miniaturization were studied extensively in 1959 by suchrenown physicistsas Nobel laureate Richard P. Feynman who is considered the father of MEMStechnology. He explored basic technological limitations for making very small mechanical devices. Thisincluded: forces( weight and inertia insignificant- material strength important), materials(grain structureand inhomogenity), magnetic behavior ( magrietic domains) and friction ( superlubricity). RichardFeynman sponsored several prizes for very small manmade objects - claimed by a superminiature electricmotor, and very small prints. These objects are considered early MEMS(Figure 1).

Figure 1.Example of early MEMS(Piezoe1ectric micromotor on finger, Kyushu University, YaskawaElectric Co. Japan)

These devices were one of a kind, hand-made by highly skilled and inventive individuals, therefore theywere not suitable for insertion in any practical, mass produced system. The MEMS manufacturingbreakthrough came, when people realized the benefits of applying IC ( silicon) technology ( waferphotographic and chemical etching) to MEMS fabrication .This extended IC manufacturing tomicromechanical structures and the MEMS revolution was on its way.


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Presently, the following MEMS fabrication techniques (with IC fabrication origin) arebeing usedSurface MicromachiningBulk MiromachiningLIGAWafer-to-Wafer BondingThese techniques are illustkted %Figure 2f 11.

a. b.Common MEMS Fabrication Processes Bulk Micromachiningm

ning

C. d.

LIGA*, Deep UV Wafer-to-Wafer Bonding

Pmcessm nd etch m i

-in wer-acceleration

athographie, Galvanoforming, Abformung h p SEtch beam and bond pyrex da mping

Figure 2. Fabrication techniques forMEMS surface micromachining, bulk micromachining, LIGA andWafer-to-wafer bonding.The most critical feature of these techniques (modifications of basically planar IC technology) is the abilityto realize three-dimensional structures, which is essential in fabrication of complex mechanical devices. Inthe fabrication technique called surface micromachining( Figure 2a), a thin-filmof material to beprocessed is initially deposited on a wafer or substrate. Next, this material is coated by a photoresist, whichis subsequently exposed through a desired photomask. Final processing involves steps of wet etching ,dryetching and additional thin film deposition. Quite often in this technique a thin-film material is used as asacrificial material. Bulkmicromachining ( Figure 2b) allows for fabrication of well defined, threedimensionalMEMS. his technique is relatively mature; however its accuracy of fabrication is limiied bye.g. crystallographic properties of the wafer. An improvement over bulk micromachining is a fabrication


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process called LIGA, from German RoentgenLItographie, Galvanik, Abformung(Figure2).hichprovides increased resolution due to the X-ray lithography and improved depth resulting from an additionalmolding step. Several etched wafers can be bonded together in a MEMS rocess called Wafer-@Waferbonding (Figure 2d). The resulting structures have great verticality and quite complex , hree dimensionalMEMS were demonstrated using th is technique. Basic MEMS manufacturing process follows the stepslisted in Figure 3. This process can be described as producing mechanical chips.

Thicker films Multiple Processing Cycles Remav al o f uwderfymydeeper etches materials to releasefewer steps0I

spectar prczhmg. se%3mmngand haMfringprocedures Io protect released pans Encapsulate snme pansof device but expose ovlers Test more thanjustelectrtcal fUf"

Figure 3. Basic process flow for manufacturing of MEMS [ 11.MEMS production is increasing very rapidly worldwide .SomeMEMS re very successful commercially,particularly in automotive applications( Sensorsfor airbags) and computer accessories ( inkjet cartridges).MEMS development activity grows rapidly, and this is evidenced by MEMS patent activity shown inFigure4 [ 11.

USPatents a on* Patents

Year

Figure 4. MEMS patent activity


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Figure 5 shows the incredible miniaturization and dimensional resolution for mechanical devicesachievable using MEMS fabrication techniques described in this paragraph.

Figure5. Example of MEMS ( Sandia)

11. MEMS n Space: Satellite Platform (Bus)ApplicationsMEMS offer significant benefits for future satellite systems since they can realize various electrical andmechanical functions in a fraction of the size, weight, and power consumption of corresponding traditionalmacro systems. This makes these devices quite attractive in space applications, especially in commercialcommunications satellites, which are constantly driven by increased capabilities, high levels of integration,miniaturization and cost reductions. Figure6hows generic configurationof a communications satellite.Essentially, it can bedivided into two parts: the basic satellite vehicle or platform ( often called satellitebus), and the satellite payload ( in this casecommunications payload located on satellite platform panels).

a. b.--\

m.U I Y ~ U OUFigure 6. Generic communications satellite (a. satellite,b. exploded view)


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Several applications of MEMS in satellite platforms are presently under consideration. This includesmicrosensors, microactuators, microheatpipes for thermal management, propulsion, active conformablesurfaces, etc.Inertial sensors may include gyroscopesor accelerometers with integrated MEMS. An example of thesedevices is shown in Figure 7.

Figure 7. Multifunctional MEMS chip (accelerometer)- SandiaMEMS technology has led to a number of novel satellite propulsion concepts. MiniatureMEMSmicrorockets have been recently demonstrated. An example of a novel propulsion device is shown inFigure 8[ 2 I.

Figure 8. Prototype MEMS Thrusters developed at NASA Glenn.

In another implementation, MEMS actuator arrays can be u s e d to selectively deform relatively largesurfaces (Figure 9). This technology can be applied in the aerospace industry (deformable wing)or in th ecase of satellites for mechanically reconfigurable antennas.


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Figure 9. Intelligent ,servo actuated surfaces using MEMS [ 1 1.Miniaturization and increased functionality are integral to the future of satellite platforms.TheMEMScomponents described aboveare only a limited sample of vast possibilities which will revolutionizesatellite industry ( pic0 and nano-satellites, satellite clusters etc.).

111. MEMS n Space: Satellite Payload Microwave ApplicationsThe MEMS fabrication process and novel applicationsof various micromachining techniques provideaccurate control of three dimensional structuresat lmicron level resolution. Since the electricalperformance of passive microwave components is determined almost entirely by the mechanicaldimensions of these devices, th e precision of the manufacturing process is extremely important. Thisrequirement makes MEMS processes very attractive for fabrication of miniature passive microwavecomponents and integrated assemblies. For this reason, microwave applications ofMEMS echnology areone of the fastest growing area s in the microwave field. Since communications satellite payloads havevery large microwave content (Figure 10)and miniaturization is very important, microwave MEMSarebeing considered asreplacement for various traditional microwave components. This can provide apotential paradigm shift in microwave devices technology and enable dramatic size, weight, and costreductions.

i

!i-

I Potential I

I multiplexer I -1IpotentialI MEMS) outputI multiplexer

t'ii;A""

i!

Figure 10. Simplified communications satellite transponder.


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Figure 13. SEM Photograph of a micromachined6dB oupler ( fc=15GHz)Microwave MEMS SwitchedSwitchArraysSatellite payloads utilize a large number of electromechanical switches for routing and redundancy.These switches are quite large and heavy, and reliable actuation is always a problem. Solid state switcharrays including MMIC implementations were developed in the past, however they suffer from excessivepower consumption and limited isolation as well as reduced reliability. MEMS echnology is a naturalsolution for this problem since microwave switching and tuning applicationsare very attractive and one ofth e first applications to be explored . Many MEMS switch configurations are currently under development.Most of them use th e silicon based process. However, from the integration with MMICs ,GaAs substratesfor these devices are most attractive. Two basic configurations for MEMS microwave switches are shownin Figure 14 [ 3 1.

SWltCh "ClomOd

'Shmt'--a)i l k 10 Putl-Down RF CU IElectrode

Ib)

Switch ' C l o w l '

Pull-OawnElaclrado!dl

Figure 14. MEMS witch designs a. cantilever,c. air bridge


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These switches can be arranged in switch mays for further reduction of size and weight. Typicalperformance of MEMS based switches, especially when optimized for a narrower band of frequencies(allocated for satellite communications) includes . I dB insertion loss and 60 dB isolation. An example of aMEMS switch is shown in Figure I5 [ 4 1.

Ground

SignaIPaw

Gmund

Membrane

un~rcairHoles

ACCe66

Figure 15. SPST ir bridge MEMS switch developedby RaytheonRt.

MEMS Tunable Filters

High performance tunable filters are considered the Holy Grail f microwave technology. Thefrequency and bandwidth tunability is essential for the next generation of communication satellites sincethis enables user reconfigurable satellites. In addition, the custom tuned, fixed frequency filters can bereplaced by electrically tunable filters, hence standardizing the front end of the communications payload.This has th e potential to significantly reduce the satellite cost and fabrication span. MEMS tunable filtersare the primary candidates to achieve this ambitious goal. Two basic concepts are presently underconsideration: switchable filters or switchable element filters (using MEMS switches) and tunable filtersusing MEMS variable capacitors. In such lumped element filters both filter elements namely inductors andcapacitors can be realized using MEMS technology. This is shown in Figure 16[ 8 1.

Figure 16.MEMS unable filter elements- inductor and capacitor


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A MEMSbased tunable filter is shown in Figure 17 [ 8 1.

Figure 17.MEMS based, lumped-element tunable filter.

Applications ofMEMS in Phase Array AntennasReconfigurable antennas are of great interest to microwave industry due to their flexibility .MEMS technology is under consideration for these applications. Several different approaches are possiblenamely: using MEMS switches to alter the conductive surface of the antenna ( Figure 18).

t

Figure IS. Topological view of three configurationsof an array of planar elements reconfigurable byMEMS switches [ 4 1.


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utilizing MEMS basedphase shifters for electronically steered phased array antennas, and finally the us eof MEMS actuators ( in array form) to physically alter the shape of the antenna surface. Work is continuingto take full advantageof this technology ,especially for wireless applications.

Conclusions

Applications of MEMS technology in microwave components and subsystems ore growing very rapidly.This technology is very attractive for insertion in communications satellites where. size, weight, and costreductions are essential. Implications of this technology for these demanding systems were discussed andexamples of candidate satellite MEMS components were shown.REFERENCES:[ 1 ] A. Pisano " MEMS Overview" DARPA presentation, Summer 1997[ 21 J. Riordon " Dawn of the Microrocket" Ad Astra MarcMApril 1999[ 3 ] L.E. Larson "Microwave MEMS Technology for Next- Generation Wireless Communications"1999 IEEE MlT-S IMS Digest, Anaheim,Ca.[ 4 ] E. R. Brown " RF-MEMS Switches for Reconfigurable Integrated Circuits", IEEE Tran. onMicrowave Theory and Techniques, vo1.46, No. 1 I , November 1998.[ 51 V.M. Lubecke, K.Mizuno, G.M. Rebeiz ''Micromachining for Terahertz Applications", IEEE Tran.on Microwave Theory and Techniques, vo1.46, No. 11, November 1998

[ 6 ] T.L. Willke, E. Onggosanusi and S.S. Gearhart. "Micromachined thick-metal coplanar coupled-line filters and couplers." 1998 MTT-S International Microwave Symposium Digest 98. I (1998Vol. I [MWSYM]): 115-1 18.[ 7 ] K.J.Herrick, J-G.Yook, L.P.B.Katehi "Microtechnology in th e Development of Three-DimensionalCircuits", IEEE Tran. on Microwave Theory and Techniques, vo1.46, No .1 1. November 1998.[ 8 ] W. Tang " MEMS at DARPA' DARPA presentation, 1211999

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