rf mems based tunable filters rfmcae01

8/21/2019 RF MEMS Based Tunable Filters RFMCAE01

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RF MEMS-Based Tunable Filters

James Brank, Jamie Yao, Mike Eberly, Andrew Malczewski, Karl Varian,Charles Goldsmith

Raytheon Systems Company, P. O. Box 660246 MS 35, Dallas, Texas 75266

Accepted 17 May 2001

ABSTRACT: This paper overviews the application of RF MEMS switches in tunable filters

as well as circuit developments for bandpass filters covering 110 MHz to 2.8 GHz. RF MEMS

have several desirable features, including small size, low power requirements, and low loss.The basic operation of Raytheon’s RF MEMS capacitive membrane switch is described. An

overview of the technique used to integrate the switch into a variable capacitor structure with

sixteen capacitance states is provided. Variable capacitor structures are used to construct

multipole lumped bandpass filter designs, each with sixteen states. Finally, measured data

from two representative five- and six-pole bandpass filters are presented. Characterization

data demonstrates that the insertion loss for the five-pole filter using on-chip inductors was

between 6.6 and 7.3 dB, and between 3.7 and 4.2 dB for the six-pole filter using off-chip

inductors. © 2001 John Wiley & Sons, Inc. Int J RF and Microwave CAE 11: 276–284, 2001.

Keywords: MEMs; microelectromechanical system; tunable capacitors; varactors; membranecapacitor; tunable filter; tunable bandpass filter

I. INTRODUCTION

Filters are the basic building blocks within fre-quency converting systems such as receivers andtuners. At microwave frequencies (1 GHz andabove), filters are composed of high-Q resonatorssuch as printed transmission line, suspended rods,or dielectric pucks. Depending on the media

used to create these resonators, excellent perfor-mance can be achieved with Qs in the hundredsfor printed lines to tens of thousands for dielec-tric resonators. The need for frequency tunability within broadband receiving and transmitting sys-tems usually necessitates switching of multiplefixed-tuned circuits. The use of tunable filters andresonators can significantly simplify complexityand reduce losses within complex multiband sys-tems. Unfortunately, there is not yet a tunable

Correspondence to: James BrankContract grant sponsor: Raytheon.Contract grant number: DARPA F 30602-97-C-D1 8la,

resonator component that affords the high perfor-mance achieved by fixed resonators. YIG filterscome the closest to having very good filter selec-tivity, but at the expense of being bulky, requiringsignificant quiescent current, and being expensive.To date, diode varactor-tuned circuits, thoughsimple and requiring little bias current and size,have not met the expectations of most modern

receiver requirements in terms of loss. As such,inexpensive and high performance tunable res-onators have become one of the “holy grails” of receiver components.

The advent of microelectromechanical sys-tems (MEMS) for radio-frequency (RF) appli-cations provides new possibilities for achievingthe desired characteristics of a tunable resonator.RF MEMS devices, a new paradigm in the con-struction of electronic devices, created mechan-

ical structures on the microscale. Being con-structed entirely of low-loss metals and dielectrics,these mechanical structures inherently have lowloss.

276

© 2001 John Wiley & Sons, Inc.


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RF MEMS-Based Tunable Filters 277

Development of RF MEMS switches has beenunder way seriously since about 1995 from severalindustrial and university groups. These deviceshave distinguished themselves as having very lowloss, requiring practically no power consumption,and having very high linearity. The application

of RF MEMS has already proven to providerevolutionary (rather than evolutionary) improve-ments in electronic switching performance forphase shifters at microwave and millimeter-wavefrequencies.

This paper explores the use of RF MEMScapacitive switches in the application of tun-able filters. Since these devices are operated in abistable manner, with either a high or low capaci-tance, they are a natural device for accomplishing

digital frequency selection within a filter. Thecapacitive membrane switch is used to create amultibit variable capacitor which serves as a dig-ital varactor. This varactor is in turn used withinresonators and coupling circuits to create tunable,lumped-element LC filters for receiver front-endapplications.

II. MEMS VARIABLE CAPACITOR

CONSTRUCTION/ ELECTRICALPERFORMANCE

To create variable capacitors, fixed MIM capaci-tors are combined in series with RF MEMS capac-itive switches as shown in Figure 1. This creates atwo-state capacitor whose value is set by the seriescombination of the fixed cap and the capacitanceof the RF MEMS. The minimum value of the two-state capacitor is limited by the off-capacitance of

the MEMS, and the maximum value is limitedby the on-capacitance of the RF MEMS. Gen-erally, the value of the fixed cap is kept belowthe on-capacitance of the MEMS switch to mini-mize the effect of MEMS variation. Combinationsof these two-state capacitors with fixed capacitorsallow construction of variable capacitor structures,ref. [1].

Fixed

Cap

RF

MEMS

Figure 1. Schematic of a two-state variable capacitorusing RF MEMS.

Cross Section

High resistivity silicon

Buffer Layer

Post

Dielectric Electrode

Top View

Signal

Path

Membrane

Dielectric

Lower

Electrode

Undercut

Access

Holes

Figure 2. Views of the RF MEMS capacitive switch.[Color figure can be viewed in the online issue, which isavailable at www.interscience.wiley.com.]

The basic RF MEMS capacitive switch isshown in Figure 2. The structure is basically aparallel-plate capacitor with a movable top plate. Applying a voltage between the membrane (top

plate) and electrode (bottom plate) creates anelectric field. When the field is strong enough,the membrane will flex downward and contact thedielectric. A simple electrical model of the switchis shown in Figure 3. Typical on capacitance(membrane down) is 3 pF, and off capacitance(membrane up) is 30 fF, [2, 3].

The fixed capacitor used were metal-insulator-metal (MIM) capacitors, as shown in Figure 4.Both top and bottom plates of the capacitor are

gold, with a thin silicon nitride dielectric layer(r = 68). Average capacitance, resistance, andQ at 1 GHz for four MIM test structures areshown in Table I. It should be noted that whenmeasuring high-Q devices it can be difficult toextract accurate values for Q. The Q values are

R

S E

R

S E

R

S H

R

S H

C

O N / O F F

Figure 3. Simple schematic of the RF MEMS capac-itive switch. [Color figure can be viewed in the onlineissue, which is available at www.interscience.wiley.com.]


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278 Brank et al.

Figure 4. Layout of a MIM capacitor.

very sensitive to small errors in R and can be

difficult to extract. Other measurements of simi-lar capacitors yielded Qs that varied by as muchas ±100%. This large variation depends uponthe specific measurement conditions, such as thecalibration method used or the condition of theRF probes.

The schematic of a four-bit variable capaci-tor is shown in Figure 5. It consists of five fixedcapacitors, four of which are in series with an RFMEMS capacitive switch. A layout of a four-bit

variable cap is shown in Figure 6. Dependingon which combination of switches are actuated,the capacitance across the variable capacitor canbe set. In the design of a variable capacitor, thefixed capacitors are designed to give even stepsof capacitance between the minimum and maxi-mum required values of capacitance. The variablecapacitor layout shown in Figure 6 incorporatessome subtle improvements over the previous ver-sions of variable capacitors. The fixed capacitor values are shown in Table II. The variable capac-itor structure is “straightened out” in order toplace the larger capacitor states closer to theRF signal path. This was done to minimize theseries inductance. Reducing the series inductanceof the capacitors is especially important in the

TABLE I. Average Measured Resistance, Capacitance,

and Q for MIM Capacitor Test Structures

R, ohms QNominal CpF (@ 1 GHz) (@ 1GHz)

0.56 1.60 179.081.06 0.60 251.745.11 0.10 302.40

10.28 0.13 120.19

C0

C1

C2

C3

C4

Control

Lines

Figure 5. Schematic of a four-bit variable capacitorusing RF MEMS.

Figure 6. Photograph of a variable capacitor. [Color

figure can be viewed in the online issue, which is avail-able at www. interscience.wiley.com.]

larger capacitors, as unwanted self resonance candegrade the frequency response of a filter. A sidebenefit of this rearrangement is that the digi-tal control lines can easily be routed to the RFMEMS devices without having to cross over anyof the RF paths.

Measured data for a typical variable capacitor

is shown in Figure 7. The capacitance increasesin even steps except for the step between steps 7and 8. The C4 capacitor value was slightly toolarge, which caused the gap in capacitance val-ues. The capacitance is very flat versus frequency, which is due to the high self-resonant frequencyof the structure.

TABLE II. Fixed Capacitor Values for

the Circuit Shown in Figure 6.

C0 3.08 pFC1 0.231 pFC2 0.498 pFC3 1.18 pFC4 3.88 pF


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2.00

2.503.00

3.50

4.00

4.50

5.00

5.50

6.00

6.50

7.00

0.050 0.100 0.150 0.200 0.250

Frequency, GHz

C a p a c i t a n c e , p F

Figure 7. Capacitance versus frequency for the sixteenstates of the variable capacitor.

III. TUNABLE FILTER TOPOLOGY

Of the many filter topologies available, only a feware amenable to construction of a tunable filter.The capacitively coupled LC resonators bandpassfilter, shown in Fig. 8, was chosen as the base-line design [4 5]. The redundancy of the elementsallows an extra degree of freedom in the choiceof component values, which allowed the induc-tance to stay constant as the center frequency istuned.

Choosing the component values is challenging,

and several design and simulation iterations arenecessary. The designer is given electrical require-ments, such as frequency range over which thefilter must tune, bandwidth, insertion loss, andnumber of tunable states, that must be satisfied.Unfortunately, it may be impossible to simultane-ously satisfy all requirements. For example, if aconstant bandwidth is required, the insertion lossof the filter will vary across the tuning range. If aconstant insertion loss is required, the bandwidth

must vary across the tuning range [6].Choice of the inductor is determined primar-

ily by the Q of available devices, as in a fixedfilter design. An added constraint is that if atunable filter using fixed inductors has a tuningrange of

α =f max f min

Cp1 Cp2 Cp3 Cp4 Cp5

L1 L2 L3 L4 L5Cs01 Cs12 Cs23 Cs34 Cs45 Cs56

Figure 8. Schematic of a capacitively coupled five-pole bandpass filter.

then the variable capacitors must be roughly capa-ble of tuning

C max C min

= α2

The implications can be seen in the followingfigures. Two tunable filters were designed. Both were five-pole capacitively coupled 0.1 dB Cheby-shev bandpass filters with sixteen states and fixed180 MHz bandwidth but with different tuningranges. Both filters used a fixed 2 nH induc-tor. Using a spreadsheet, the series and shuntcapacitances of Figure 8 were calculated foreach frequency step. These values are shown inFigures 9 and 10. The starting point for thesecalculations is given in ref. [1].

The filter of Figure 9 had a tuning range of 885 to 986 MHz, or an 11% tuning range. Theseries and shunt capacitor values versus tun-ing state have very slight curvature, and can beapproximated quite well with a straight line. This works well with the variable capacitor structuredescribed in Section III. Step size for this fil-ter will be even, and bandwidth will be relativelyconstant across the tuning range.

The filter of Figure 10 had a tuning range of

996 to 2068 MHz, or a 108% tuning range. Theseries and shunt capacitor values versus tuningstate have a noticeable curvature, and a linearapproximation is not as good. In practice, the variable capacitors will be designed to have thecorrect value at the maximum and minimum tun-ing ranges, and vary linearly with state betweenthe extremes. Step size for this filter will beuneven, with larger steps at the high end of thetuning range. As the values of the coupling capac-itors are somewhat flat, the bandwidth will beroughly constant across the tuning range, witha slightly larger bandwidth in the center of theband.

IV. TUNABLE FILTER REALIZATIONS

The designs presented here incorporate theimprovements in variable capacitor design


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280 Brank et al.

0

2

4

6

8

10

12

14

885 935 985

Center Frequency, MHz

C a p a c i t a n c e , p F

Cp1, Cp5

Cs12,Cs45

Cp2, Cp4

Cs23, Cs34

Cp3

Cs01, Cs56

Figure 9. Variation of capacitance with tuning state for a five-pole filter, f c = 885–986 MHz. [Color figure can be viewed in the online issue, which is available at www. interscience.wiley.com.]

described earlier, as well as processing improve-ments related to the metalization. These designshave improved electrical performance, have abetter control structure, and demonstrate theability to incorporate these devices into higherlevel assemblies. Improved wafer processing alsoreduced the filter insertion loss and improveddevice quality.

To date, seventeen different tunable filters

using RF MEMS have been built. They range incomplexity from simple one-pole structures with

01

2

3

4

5

6

7

8

9

996 1496 1996

Center Frequency, MHz

C a p a c i t a n c e , p F Cp1, Cp5

Cs12, Cs45

Cs23, Cs34

Cp3

Cp2, Cp4

Cs01, Cs56

Figure 10. Variation of capacitance with tuning state for a five-pole filter, f c = 996–2068 MHz.

l=2057 umw=125 um

l=2057sw=125 um

l=2057 umw=125 um

l=2057 umw=125 um

l=2057 umw=125 um

8.55 pF

1.62 pF

0.65 pF

0.29 pF

5.32 pF

8.55 pF1.62 pF

0.65 pF

0.29 pF

5.32 pF

0.71 pF0.32 pF

0.15 pF

0.075 pF

2.13 pF

0.075 pF

0.15 pF

0.32 pF0.71 pF

2.13 pF

0.24 pF

0.12 pF

0.52 pF

0.06 pF

1.62 pF

0.06 pF

0.12 pF

0.24 pF0.52 pF

1.62 pF

3.43pF

1.22pF

0.54pF

0.22pF

7.21

pF

0.22p

F

0.54p

F

3.43

pF

1.22pF

7.21

pF

1.87pF

0.80pF

0.39pF

0.17pF

6.96pF

0.80pF

0.39pF

6.96pF

1.87pF

0.17pF

2.16pF

0.89pF

0.42pF

0.17pF

7.69pF

Figure 11. Schematic for the UHF filter.

twelve MEMS devices to six-pole filters with 139MEMS devices. Both on-chip and off-chip induc-tors have been studied. The filter frequenciescovered 70 MHz to 2.8 GHz. As space is limited,only two representative designs will be discussed.

UHF Tunable Filter

A five-pole 0.1 dB Chebyshev bandpass filterdesign, denoted as the UHF filter, is presented


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Figure 12. Layout of the UHF five-pole filter. Die size is 3.5 mm by 14 mm. [Color figure can be viewed in the

online issue, which is available at www. interscience.wiley.com.]

here. This filter had a center frequency tuningrange of 885 MHz to 986 MHz with a constantbandwidth of 180 MHz. As described earlier, thecapacitively coupled LC resonator design waschosen. A lumped element design was derivedfor the maximum and minimum tuning frequen-cies with constant resonator inductances of 2.9

nH. On-chip inductors were used to demonstratethe potential for integration of entire filters ona chip, with the path to ground provided by rib-bon bonds to the carrier plate. Four-bit variablecapacitors were designed to cover the requiredtuning ranges based on the lumped designs. Theschematic and layout are shown in Figure 11 andFigure 12.

The measured insertion loss and return lossof the five-pole filter is shown in Figure 13. The

filter tuned from 880 to 992 MHz, with the cen-ter frequency insertion loss across all tuningstates from 6.6 and 7.3 dB. Measured bandwidth varied between 168 and 174 MHz. Return loss was better than 10 dB for all tuning states. Thebandwidth and passband shape stayed relativelyconstant as the center frequency of the filter wastuned.

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-35

-30

-25

-20

-15

-10

-5

0

0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4

Frequency (GHz)

-30

-25

-20

-15

-10

-5

0

5

10

R e t u r n L o s s ( d B )

I n s e r t i o n L o s s ( d B )

Figure 13. UHF frequency response. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

VHF FILTER

A six-pole 0.1 dB Chebyshev bandpass filter,denoted as the VHF design, is presented here.This filter had a center frequency tuning range of 110 MHz to 160 MHz with a variable bandwidthfrom 37 MHz to 58 MHz. A lumped element

design was derived for the maximum and mini-mum tuning frequencies with constant resonatorinductances of 27 nH. Off-chip inductors wereused in this case because the required value of inductance was too large to incorporate on-chip. Also, the improved Q of the off-chip inductorsimproved the insertion loss compared to the on-chip designs. As with the previous design, four-bit variable capacitors were designed to cover therequired tuning ranges based on the lumped

designs. Some of the larger capacitor states usedmultiple MEMS devices to switch large capaci-tancies. The schematic and layout are shown inFigure 14 and Figure 15.

The measured insertion loss and return lossof the five-pole filter is shown in Figure 16.Center frequency insertion loss across all tun-ing states was from 3.7 to 4.2 dB. Return loss


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282 Brank et al.

90.0 pF

5.20 pF

14.620 pF

24.150 pF

43.290 pF

27

nH

127.45 pF

14.71

0

pF

0.620

pF

12.99

0

pF

1.560

pF

6.490

pF

10.78 pF

20.92 pF

4.15 pF

71.73 pF

28.34 pF

27

nH

12.

990

pF

1.560

pF

9.920

pF

0.620

pF

6.490

pF

5.20 pF

14.620 pF

24.150 pF

43.290 pF

27

nH

127.45 pF

12.99

0

pF

1.560

pF

9.920

pF

0.620

pF

6.4

90

pF

5.20 pF

14.620 pF

24.150 pF

43.290 pF

27

nH

127.45 pF

12.990

pF

1.5

60

pF

9.9

20

pF

0.6

20

pF

6.4

90

pF

10.78 pF

20.92 pF

4.15 pF

71.73 pF

28.34 pF

27

nH

14.71

0pF

0.620

pF

12.99

0pF

1.560

pF

6.490

pF

5.20 pF

14.620 pF

24.150 pF

43.290 pF

27

nH

127.45 pF

90.0 pF

Figure 14. VHF filter schematic.

was better than 15 dB for all tuning states. Thepassband shape stayed relatively constant as thecenter frequency of the filter was tuned, while thebandwidth increased as the center frequency wastuned higher. This resulted in the filter havingless insertion loss at the highest tuning state than

at the lowest tuning state.When compared to a conventional switched-

filter bank, the advantages of RF MEMS-basedfilters are remarkable. Analysis indicates that com-pared with a typical switched-filter bank, use of RF MEMS tunable filters allow a 60X reduction insize, 150X reduction in weight, and a 10X reduc-tion in the number of RF support switches. A single MEMS-based filter can have sixteen tun-able states, replacing sixteen fixed frequency fil-

ters. The low power requirements of RF MEMScan reduce filter assembly power requirements 8X.Such size, weight, power, and circuit complexityreductions are crucial in modern communicationsdesigns.

V. CONCLUSION

Two tunable bandpass filters designs using RFMEMS were demonstrated. Insertion loss for thefive-pole UHF filter with on-chip inductors was

measured to be between 6.6 and 7.3 dB, andbetween 3.7 and 4.2 dB for the six-pole VHF fil-ter with off-chip inductors. Both filters exhibitedgood return loss across the tuning range. Pass-band shape was also maintained across the tuningrange. With their high degree of integration, RFMEMS show great potential for weight, powerconsumption, and size reduction. Ongoing designand process improvements will reduce the inser-tion loss further, as well as extend the operating

frequency range.

ACKNOWLEDGMENTS

Raytheon, RF MEMS group, DARPA contract numberF30602-97-C-0186.

REFERENCES

1. Charles L. Goldsmith, Andrew Malczewski, ZhiminJamie Yao, Shea Chen, and David Hinzel, RF

MEMS variable capacitors for tunable filters, Inter-national Journal of RF and Microwave Computer-

Aided Engineering, 9 (1999), 362–374.2. Zhimin Jamie Yao, Shea Chen, Susan Eshel-

mann, David Denniston, and Charles Goldsmith,Micromachined low-loss microwave switches, IEEEmicroelectromechanical systems, 8 (1999), 129–134.


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Figure 15. Layout of the VHF five-pole filter. Die size is 4 mm by 16 mm. [Color figure can be viewed in the onlineissue, which is available at www. interscience.wiley.com.]

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-35

-30

-25

-20

-15

-10

-5

0

0.05 0.1 0.15 0.2 0.25 0.3

Fr

R e t u r n L o s s ( d B )

I n s e r t i o n

L o s s ( d B )

equency (GHz)

-30

-25

-20

-15

-10

-5

0

5

10

Figure 16. VHF frequency response. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

3. Charles L. Goldsmith, Zhimin Jamie Yao, SusanEshelmann, and David Denniston, Performanceof low-loss RF MEMS capacitive switches, IEEEmicrowave and guided waves letters, 8 (1998),269–271.

4. Anatol I, Zverev, Handbook of filter synthesis,Wiley, New York, 1967.

5. G.L. Matthaei, L. Young, and E.M.T. Jones,Microwave filters, impedance-matching networks,and coupling structures, McGraw-Hill, New York,1964.

6. Thomas R. Cuthbert, Broadband direct-coupledand matching RF networks, TRCPEP Publications,Greenwood, AR, 1999.

BIOGRAPHIES

James Brank received his Bachelor’sdegree in Electrical Engineering from

Texas A&M University in 1982. Hereceived his Master’s degree in Electri-cal Engineering from Southern MethodistUniversity in 1987. From 1983 to 1987he was employed at E-Systems, Garland

Division where he performed integrationand test of electronic warfare receivers.

From 1987 to 1988 he worked for Raytheon in Bristol, Ten-nessee where he designed components for the Standard Missile2 program. From 1989 to the present, he has been employed by

Raytheon Systems Company (formerly the Defense Electron-ics Group of Texas Instruments) where he has been involved ina wide variety of projects, ranging from the design of X-band

radar modules to the application of phased array antenna tech-

nology for cellular telephone Smart Antennas. Currently he is

designing low loss MEMS tunable bandpass filters.

Zhimin J. Yao received her Ph.D. from

the School of Materials Science and Engi-

neering at Georgia Institute of Technol-ogy in 1995. She then worked as a post

doctoral research associate at the School

of Electrical Engineering, Cornell Univer-

sity for one year. Her research emphasis

was on silicon bulk micromachining. Dr.

Yao is currently working at Rockwell Sci-

ence Center in Thousand Oaks, CA. Her research interests


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284 Brank et al.

include design, fabrication and characterization of microelec-tromechanical systems.

Michael Eberly has been a member of the IEEE since 1988 as a student mem-ber. He earned his BSEE from the Uni-

versity of South Florida in 1992. He was with the United States Navy, on activeduty, from 1992 to 1994, transferring thento the Naval Reserve. He has continued

with the Naval Reserve to the present andcurrently holds the rank of Lieutenant. In

1994, he began studying part time for his Masters degree, whileteaching basic courses in Electrical Engineering Technology atTampa Technical Institute. In 1996, he was selected to study asthe Texas Instruments Fellowship Student at the University of South Florida. He worked for Raytheon since the summer of 1997 as an engineering intern as part of the previously men-tioned fellowship and permanently since August 1998. He wasawarded his Master’s Degree in Electrical Engineering in the

fall of 2000. He is currently working in the Applied ResearchLaboratory at Raytheon on RF/MEMS.

Andrew Malczewski was born in War-saw, Poland in May, 1973. He earned aBachelor’s degree in Electrical Engineer-ing from the University of Texas at Arling-ton in 1996. Since 1996, he has beeninvolved in the design and developmentof microwave and millimeter-wave circuitsfor Raytheon Systems Company (formerlythe Defense Electronics Group of Texas

Instruments). He is also involved in the development of RFMEMS technology for receiver and antenna applications. Heis presently pursuing his Master’s degree in Electrical Engi-neering. Karl Varian photo and biography not available.

Karl Varian photo and biography not available.

rf mems based tunable filters rfmcae01

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