RF MEMS Filters - Microwave and RF Information for … MEMS Filters.pdf · At microwave and millimeter wave frequencies distributed components are ... RF Channels RF MEMS Filters,

EC462 : RF MEMSDr. S. Raghavan

NIT TRICHY

RF MEMS FiltersRF MEMS FiltersRF MEMS FiltersRF MEMS Filters

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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

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.

RF MEMS Filters, Dr. S. Raghavan, EC462, NIT Trichy2

Parameters for characterizing BP Filters


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.


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.


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.


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.


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


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.


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


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


Comb drive filter calculations


Static Displacement at drive portSpring constant

Electromagnetic transfer function relates phasor disp X to phasor drive voltage Vd

Comb drive filter calculations


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.

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


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.


Equivalent circuits for filters

Filter analysis and synthesis is significantly simplified using an electricalequivalent circuit

The corresponding mechanical model is shown below


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.



IDTs are reciprocal devices and can be used as input and output transducers.

Consists of pair of metallic IDTs patterned on a piezoelectric substrate.




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


Medium-Q Resonators

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

RF MEMS Filters, Dr. S. Raghavan, EC462, NIT Trichy

• 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


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


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


Suspended, Stacked Spiral Inductor

Strategies for maximizing Q: 15µm-thick, electroplated Cu windings reduces series R suspended above the substrate reduces substrate loss


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


High-Q Resonators

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


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


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]


-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


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


-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)


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


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 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


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.

Documents

RF MEMS Filters - Microwave and RF Information for … MEMS Filters.pdf · At microwave and millimeter wave frequencies distributed components are ... RF Channels RF MEMS Filters,