RF MEMS Filtersd2xunoxnk3vwmv.cloudfront.net/uploads/RF-MEMS-Filters.pdf · 2018. 6. 8. · EC462 :...
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EC462 : RF MEMSDr. S. Raghavan
NIT TRICHY
RF MEMS FiltersRF MEMS FiltersRF MEMS FiltersRF MEMS Filters
By By By By
Durai Praveen. D (108106024)Durai Praveen. D (108106024)Durai Praveen. D (108106024)Durai Praveen. D (108106024)
Gautham Muthukumar. S (108106026)Gautham Muthukumar. S (108106026)Gautham Muthukumar. S (108106026)Gautham Muthukumar. S (108106026)
B.Tech (ECE), Final Year, Batch 2006B.Tech (ECE), Final Year, Batch 2006B.Tech (ECE), Final Year, Batch 2006B.Tech (ECE), Final Year, Batch 2006----10 10 10 10
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INTRODUCTION TO RF Filters
An area that has got significant attention, and remains technically challenging, isminiaturization of the well-proven mechanical filters
These filters involve a form of mechanical wave propagation at some stagebetween their input and output terminals (often vibrations)
Most filters are only fabricated with micromachining techniques and do notinvolve mechanics for their operation.
Classification on basis of frequency bands
Low Pass
High Pass
Band Pass
Band Stop
Others ( Extreme Narrow pass band and rapid roll of characteristics)
Most important factors of filters are Insertion loss, Quality factor, Roll off andthe Stop band rejection.
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Parameters for characterizing BP Filters
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RF Filters in Communication systems
These systems are designed to handle many communication channelsoperating simultaneously, in which Band Pass Filters play an important role.
More channels have to be packed in the limited spectrum available.
So , BPF should have uniform pass band with very low insertion loss, rapidroll off and high out of band rejection ratio.
Simplest design of a filter involves usage of inductors and capacitors
The above approach has limitations of maximum sampling frequency in highspeed processors and modern digital signal processing algorithms
Various forms of electromechanical filters have been used to obtain desirablecharacteristics like high Q factor. These use electromechanical transducersand a transmission line connecting them.
Strong resonance properties have been observed which results in excellent Qof such filters.
Modeling of these systems are done by using their equivalent circuitstranslated into electrical domain by simple transformations for design andoptimization.
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RF Filters in Communication systems
High Q filters use mechanical components for communication systems andradars for frequencies in the KHz range.
As frequency increases the size of these devices become smaller, and arealmost infeasible to fabricate, therefore not amenable for mass production.
By RF MEMS, most of the VHF bands can be covered by a few small designmodifications and improvements in fabrication accuracies.
Current fabrication limits use of RF MEMS above ~100 MHz and planardistributed filters below gigahertz frequencies.
Devices such as the InterDigital Transducer launch surface acoustic waves(SAW) which provide high Q devices upto 2 GHz.
At microwave and millimeter wave frequencies distributed components areused extensively. Q factors obtained so are limited by parasitics. Severalmicromachining techniques are used to minimize these effects.
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Modeling of Mechanical Filters
Transducers behave as resonators and in terms of electrical and mechanicalproperties , performance can be improved by joining such resonatorstogether.
The number of resonators plays a key role in determining the shape factor offilter performance while their resonant frequency is the center frequency ofthe band pass filters. The filter B/W is reduces by increasing the equivalentmass o f the resonators, or by increasing the compliance of coupling wires.
A simplified analysis of the results include several assumptions :
vibrations are of small amplitude, and the stress–strain relationship is linear;
there are no internal losses and no external damping of vibrations by air resistance,
etc.;
effects of external gravity and magnetic forces can be neglected.
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Analysis of Resonators
Start with the differential equations of wave motion within the resonator. Theseare second or fourth order in space coordinates and second order in time.
To eliminate the time dependency, sinusoidal excitations are assumed, and phasornotation is used.
Solutions to these equations are expressed in terms of trigonometric, hyperbolicor Bessel functions.
The boundary conditions are mathematically represented. These are thensubstituted into the solutions for displacement to eliminate constants. Thefrequency equation is obtained for different modes.
Substituting these in the original differential equation, one can obtain arelationship between the wave number and frequency.
Using this relation, and the frequency equation, the resonant frequencies fordifferent modes can be obtained.
The equivalent mass is defined as an equivalent lumped mass placed at a specifiedlocation on the resonator that matches with the kinetic energy of the distributedparameterelement vibrating at a given mode and resonant frequency.
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Types of resonators
Longitudinal mode rod resonators
Torsional mode rod resonator
Flexural mode bar resonator
Flexural disk resonators
Thick disks and plates
Circular and rectangular membranes
Mechanical coupling components
Electrical transmission lines
Longitudinal mode in a solid bar
Stretched string transmission line
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Assumptions & Theorems for modeling
A simple straight forward analysis has been done for homogenous, isotropic,continuous, elastic, lossless solids.
Valid for grain size of crystalline materials much smaller than the wavelength.
Assumed that the disturbances that travel along these solids are continuousmotions around their rest positions with a relatively small magnitude ofvariation.
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General considerations for mechanical filters
Consist of series of resonators coupled together with some form of couplingelements
These elements critically affect the performance of the filter.
NumberNumberNumberNumber ofofofof resonatorsresonatorsresonatorsresonators decide the shape factor for filter response and centerfrequency is decided by the operational frequencies of these resonators.
Compliance of coupling wires as well as the equivalent mass of the resonatordetermine the B/W of the filter
Accuracy of the formulations is plausible at the micro scale for reasons suchas the structural dimensions being not large enough compared with thewavelength, nonidealities of boundary conditions.
Goal is to fabricate devices such as filters so small that they can beintegrated nto rest of circuit in a single chip leading to a SoC.
Performance of RF Filters is enhanced by presence of coupling networks.
Number of tanks used is equal to order of its polynomial transfer function
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Micromechanical Filters
A parallel plate capacitor configuration is common for such largeelectromechanical filters
ElectrostaticElectrostaticElectrostaticElectrostatic combcombcombcomb drivedrivedrivedrive
An electrostatically driven parallel plate actuator has a clamped –clampedbeam configuration. Non linearity can cause frequency instability in the filteroperation.
Two resonator configurations are possible.
First is a 2 port configuration driven on one of the comb structures
Second configuration, both comb structures are used to drive differentially
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Comb drive filter calculations
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Static Displacement at drive portSpring constant
Electromagnetic transfer function relates phasor disp X to phasor drive voltage Vd
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Comb drive filter calculations
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Quality factor of circuit
Sensed current is
Magnitude of transconductance
Resonant frequency of the structure determined by Rayleigh method is :
•The fabrication uses a single mask for most of the critical features. This eases the process design and can potentially reduce cost.•A grounded planar electrode which also helps suppress excitation of undesired modes.
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Micromechanical filters using comb drives
A number of resonant structures can coupled together in either series orparallel configuration to obtain high quality filter characteristics.
Series Configuration Parallel configuration
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Filters using electrostatic coupled beam
structures
Lateral drive actuators have a linear transfer function between displacementand voltage and hence have a significant advantage on filter performance.
However, these are relatively large structures. Resonant freq. of Spring masssystem is :
F is proportional to k and inversely to m
The perspective view and equivalent
circuit of a resonator with 2 beams.
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Equivalent circuits for filters
Filter analysis and synthesis is significantly simplified using an electricalequivalent circuit
The corresponding mechanical model is shown below
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Surface Acoustic Wave Filters
Maximum operational frequency is limited to tens of MHz.
Compared to resonant vibrations, acoustic waves can be used to extend theupper limit of frequency
They have a monopolistic market share for HF applications.
These filters require special piezoelectric substrates that prevent theirintegration with the circuits in a single chip.
Design aspects :
Basic principle of operation is described in order to provide a preliminaryunderstanding.
Surface wave excitations on these solids are compared
Design if IDT helps in reduce loss.
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Surface Acoustic Wave Filters
IDTs are reciprocal devices and can be used as input and output transducers.
Consists of pair of metallic IDTs patterned on a piezoelectric substrate.
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Surface Acoustic Wave Filters
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Bulk Acoustic Wave Filters
Fabricated for higher frequencies that surface acoustic waves
Similar fabrication except that thin film of materials like ZnO, PZT are used
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Medium-Q Resonators
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Switchable LCBandpass Filter
Problem:Problem:Problem:Problem: Switch losscompromises filter loss
Medium-Q best achieved via tunable micromachined capacitors and inductors
Medium-Q Resonator Needs
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• Medium-Q best achieved via tunable micromachined capacitors and inductors
Tunable LCBandpass Filter
Mechanically tunable LC tank with higher Q than conventional on-chip tanks
Eliminates switch loss better insertion loss
Medium-Q Resonator Needs
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Micromachined, movable, aluminum plate-to-plate capacitors
Tuning range exceeding that of on-chip diode capacitors and on par with off-chip varactor diode capacitors
Challenges: microphonics, tuning range truncated by pull-in
Voltage-Tunable High-Q Capacitor
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Vtune
Larger Capacitive Tuning RangeLarger Capacitive Tuning RangeLarger Capacitive Tuning RangeLarger Capacitive Tuning Range
Use comb-transducers to actuate multiple plate capacitorsa
• Left: lateral comb-capacitor in deep RIE’ed silicon
• Nearly 250% tuning range with ~100V of actuation input
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Suspended, Stacked Spiral Inductor
Strategies for maximizing Q: 15µm-thick, electroplated Cu windings reduces series R suspended above the substrate reduces substrate loss
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Molybdenum-chromium metal solenoids perpendicular to the plane of the substrate
reduced substrate loss high Q
Assembled out-of-plane via curling stresses, then locked into place
Record Q’s: ~70 on glass, ~40 on 20Ω-cm silicon (85 w/ Cu underside)
LockingMechanism
SolenoidInductor
Stress CurledMetal
Design/Performance:D=600µm, t=1µm
On Glass Substrate:L = 8nH, Q = 70 @ 1GHz
On 20Ω-cm Silicon:L = 6 nH, Q = 40 @ 1GHz(Q ~ 85 w/ Cu underside) D
Out-of-Plane Micromachined Inductor
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High-Q Resonators
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Best if Q >300
Would likeQ’s >2,000 Would like
Q’s >5,000
Best when highest Qused
Would likeQ’s >10,000
• High-Q best achieved via vibrating micromechanical resonators
High-Q Resonator Needs
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Thin-Film Bulk Acoustic Resonator (FBAR)
Piezoelectric membrane sandwiched by metal electrodes
extensional mode vibration: 1.8 to 7 GHz, Q ~500-1,500
dimensions on the order of 200µm for 1.6 GHz
link individual FBAR’s together in ladders to make filters
Agilent FBAR
• Limitations: Q ~ 500-1,500, TCf ~ 18-35 ppm/oC difficult to achieve several different freqs. on a single-chip
h
freq ~ thickness
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Basic Concept: Scaling Guitar Strings
Guitar String
Guitar
Vibrating “A”String (110 Hz)
High Q
110 Hz Freq.
Vib
. Am
plit
ude
Low Q
r
ro
m
kf
ππππ2
1====
Freq. Equation:
Freq.
Stiffness
Mass
fo=8.5MHzQvac =8,000Qair ~50
µMechanical Resonator
Performance:Lr=40.8µmmr ~ 10-13 kgWr=8µm, hr=2µmd=1000Å, VP=5VPress.=70mTorr
[Bannon 1996]
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-60
-50
-40
-30
-20
-10
0
8.7 8.9 9.1 9.3Frequency [MHz]
Tra
nsm
issio
n [dB
]
Pin=-40dBm
In Out
VP
Sharper roll-off
Loss Pole
Performance:fo=9MHz, BW=20kHz, PBW=0.2%I.L.=2.79dB, Stop. Rej.=51dB20dB S.F.=1.95, 40dB S.F.=6.45
Design:
Lr=40µm
Wr=6.5µm
hr=2µm
Lc=3.5µm
Lb=1.6µm
VP=10.47V
P=-25dBmRQi=RQo=12kΩ
3CC 3λ/4 Bridged µMechanical Filter
RF MEMS Filters, Dr. S. Raghavan, EC462, NIT Trichy
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Radial-Contour Mode Disk Resonator
VP
vi
Input ElectrodeOutput Electrode
io ωωο
ivoi
Q ~10,000Disk
Supporting Stem
Smaller mass higher freq. range and lower series Rx
(e.g., mr = 10-13 kg)
Young’s Modulus
DensityMass
Stiffness
R
E
m
kf
r
ro
1
2
1⋅⋅⋅⋅∝∝∝∝====
ρρρρππππ
Frequency:
R
VP
C(t)
dt
dCVi Po ====
device offNote: If VP = 0V device off
RF MEMS Filters, Dr. S. Raghavan, EC462, NIT Trichy
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-100
-98
-96
-94
-92
-90
-88
-86
-84
1507.4 1507.6 1507.8 1508 1508.2
1.51-GHz, Q=11,555 Nanocrystalline Diamond Disk
µMechanical Resonator
Impedance-mismatched stem for reduced anchor dissipation
Operated in the 2nd radial-contour mode
Q ~11,555 (vacuum); Q ~10,100 (air)
Below: 20 mm diameter disk
PolysiliconElectrode
R
Polysilicon Stem(Impedance Mismatchedto Diamond Disk)
GroundPlane
CVD Diamond µMechanical DiskResonator
Frequency [MHz]
Transm
ission
[dB
]
Design/Performance:R=10µm, t=2.2µm, d=800Å, VP=7Vfo=1.51 GHz (2nd mode), Q=11,555
fo = 1.51 GHzQ = 11,555 (vac)Q = 10,100 (air)
Q = 10,100 (air)
RF MEMS Filters, Dr. S. Raghavan, EC462, NIT Trichy
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Need for Q’s > 10,000
Antenna
Demodulation Electronics
The higher the Q of the Pre-Select Filter the simpler the demodulation electronics
Pre-SelectFilter in the GHz Range
Presently use resonators with Q’s ~ 400
If can have resonator Q’s > 10,000
1.5-GHz Polydiamond Disk
Wireless Phone
Non-Coherent FSK Detector?(Simple, Low Frequency, Low Power)
Substantial Savings in Cost and Battery PowerFront-End RF Channel Selection
RF MEMS Filters, Dr. S. Raghavan, EC462, NIT Trichy
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Need for Q’s > 10,000
Antenna
Demodulation Electronics
The higher the Q of the Pre-Select Filter the simpler the demodulation electronics
Pre-SelectFilter in the GHz Range
Presently use resonators with Q’s ~ 400
If can have resonator Q’s > 10,000
1.5-GHz Polydiamond Disk
Wireless Phone
Non-Coherent FSK Detector?(Simple, Low Frequency, Low Power)
Substantial Savings in Cost and Battery PowerFront-End RF Channel Selection
RF MEMS Filters, Dr. S. Raghavan, EC462, NIT Trichy
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RF Channel-Select Filter Bank
Bank of UHF µmechanicalfilters
Switch filters on/off via application and removal of dc-bias VP, controlled by a decoder
Tr an s mi
ssi
on
Freq.
Transmission
Freq.
Tr an s mi
ssi
on
Freq.
1 2 n3 4 5 6 7RF Channels
RF MEMS Filters, Dr. S. Raghavan, EC462, NIT Trichy
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Thank You
We express our deepest gratitude to Dr. S. Raghavan, Professor of
ECE Dept, NIT Trichy who has been more than just a guide in
helping us with both academics and organizational activities. He
has shown us that with utmost passion, even rigorous work is fun !
We would always be your humble students Sir.