Bulk Acoustic Wave Devices - VGTU slides/Bulk AW devices.… · Bulk Acoustic Wave Devices In electronic devices, electrical signals are used for the excitation of acoustic waves

ELEKTRONIKOS ĮTAISAI 2009

VGTU EF ESK [email protected]

1

Bulk Acoustic Wave Devices

In electronic devices, electrical signals are used for the excitation of acoustic

waves. Mechanical vibrations and acoustic waves are excited due to

piezoelectric effect or magnetostriction.

The piezoelectric effect, discovered by Pierre Curie and Jacques Curie in

1880, is exhibited by certain crystals, e.g., quartz and ceramic materials.

The production of a voltage between opposite sides of a piezoelectric crystal

as a result of pressure or twisting. Also the reverse effect which the

application of a voltage to opposite sides causes a deformation to occur at

the frequency of the applied voltage. (Converts mechanical energy into

electrical energy and electrical energy into mechanical energy.)

Magnetostriction is a property of ferromagnetic materials that causes them

to change their shape when subjected to a magnetic field. The effect was

first identified in 1842 by James Joule when observing a sample of nickel.

The essence of the effect is in the mechanical deformation of a feromagnetic

material when it is subjected to a magnetic field. Mechanical deformations

depend on magnetic induction (magnetic flux density). Conversely, when the

material is subjected to mechanical stress, changes in its magnetisation and

magnetic permeability are observed.



2

Electromechanical resonators and transducers

The structure of an electro-

mechanical resonator or transducer

The disc is a mechanical resonator. Due to the

piezoelectric and reverse piezoelectric effects the

device has properties of an electrical resonator.

SE

lL

2π

2=

2

ρlSC =

LCf

π2

1r1 =

0r1

0

0

2r 1

π2

1

C

Cf

CC

CCL

f +=

+

=



3

0

jX

-jX

Reacta

nce

Fundamental mode

3rd overtone

5th overtone

Frequency

Spurious

responsesSpurious

responses

Spurious

responses

Overtone Response of a Quartz Crystal



4

Electromechanical resonators and transducers

The resonant frequencies and their electrical quality depend on the crystal

dimensions, how the surfaces are oriented with respect to its axes and how the

device is mounted. Frequencies ranging from a few kilohertz to a few hundred

megahertz and quality values in the range from several thousand to several

hundred thousand are commercially available. Quartz resonators are stable, but

at the same time expensive. Therefore piezoelectric ceramics are used for

piezoelectric resonators. Piezoelectric ceramics are often used for piezoelectric

transducers that convert electric oscillations to mechanical vibrations and

mechanical vibrations to electric oscillations in filters, delay lines and other

devices.



5

3-3

In quartz, the five strain components shown may be generated by an electric field.

The modes shown on the next page may be excited by suitably placed and shaped

electrodes. The shear strain about the Z-axis produced by the Y-component of the

field is used in the rotated Y-cut family, including the AT, BT, and ST-cuts.

STRAIN

EXTENSIONAL

along:

SHEAR

about:

FIELD along:

X

Y

Z

X

Y

Z

X Y Z

√√√√

√√√√

√√√√

√√√√

√√√√

X

Y

Z

The Piezoelectric Effect in Quartz



6

Flexure Mode Extensional Mode Face Shear Mode

Thickness Shear

Mode

Fundamental Mode

Thickness Shear

Third Overtone

Thickness Shear

Modes of Motion



7

Quartz resonators: modes of vibrations

The Flexure Mode is used in the low frequency range.

The Thickness Shear Mode is used

in the high frequency range.

sf

8,2...6,1r =

Third Overtone

Thickness Shear



8

x xl

y

φ

z

θ

θ

The AT, FC, IT, SC, BT, and SBTC-cuts are some

of the cuts on the locus of zero temperature

coefficient cuts. The LC is a “linear coefficient”

cut that has been used in a quartz thermometer.

Y-cut: ≈ +90 ppm/0C

(thickness-shear mode)

X-cut: ≈ -20 ppm/0C

(extensional mode)

90o

60o

30o

0

-30o

-60o

-90o0o 10o 20o 30o

AT FC IT

LC SC

SBTCBT

θ

φ

Singly

Rotated

Cut

Doubly Rotated Cut

Zero Temperature Coefficient Quartz Cuts



9

Singly

Rotated

Cut

Doubly Rotated Cut

X

X’

Y

θ

θ

ϕ

Z

3-18

Singly Rotated and Doubly Rotated Cuts’

Vibrational Displacements

Singly rotated resonator

Doubly rotated resonator



10

Base

Mounting

clips

Bonding

area

ElectrodesQuartz

blank

Cover

Seal

Pins

Quartz

blank

Bonding

area

Cover

Mounting

clips

SealBase

Pins

Two-point Mount Package Three- and Four-point Mount Package

Top view of cover

Resonator Packaging



11

Piezoelectric resonators and transducers

Ceramic resonators are made of high-stability piezoelectric ceramics,

generally lead zirconium titanate (PZT) which functions as a mechanical

resonator. A ceramic resonator is often used in place of quartz crystals in

electronic circuitry because of its lower cost and smaller size. It is used on

circuits where frequency specifications aren't highly critical (quartz has a

0.001% frequency tolerance, while PZT has a 0.5% tolerance).

Piezoelectric ceramics are often used for piezoelectric transducers that

convert electric oscillations to mechanical vibrations and mechanical

vibrations to electric oscillations in filters, delay lines and other devices.

Metal

electrodes

Acoustic

waveguide

Piezoelectric

plateAcoustic

load



12

Magnetostriction is also used for transducers.

Magnetostrictive resonators and transducers



13

Mechanical

resonator

The alternating current flowing in the coil creates magnetic flux in the

resonator and vibration of the resonator arises due to the magnetostrictive

force. Because the characteristic δl(B) is nonlinear at the origin, a permanent magnet is used for bias. So the device may be used to convert

electric energy to mechanical.

If mechanical vibrations of the resonator are excited, its permeability

changes periodically. Due to the change of permeability, magnetic flux

created in the resonator by the permanent magnet varies periodically.

Then the electromotive force is induced in the coil due to the

electromagnetic induction effect. Thus, the device may be also used for the

conversion of mechanical vibrations to electric oscillations.

Magnetostrictive resonators and transducers



14

Piezoelectric and magnetostrictive resonators

Piezoelectric resonators are used in crystal oscillators and piezoelectric filters.

Piezoelectric and magnetostrictive transducers are used in electromechanical

filters and bulk acoustic wave delay lines.

Electromechanical and piezoelectric filters are used as band (usually – band-

pass) filters having high selectivity.



15

Electromechanical filters

Excitement of torsion

vibrations

Electromechanical filters:

100 Hz – 1 MHz



16

Piezoelectric filters

Piezoelectric filters are used in the frequency range to 100 MHz.

Quartz or ceramic resonators may be used in these filters. Quartz

resonators have high quality and are very stable. Due to the high quality a

few resonators can ensure high selectivity.

The prices of ceramic resonators are lower. But the quality of these

resonators is also lower and more resonators are needed to obtain high

selectivity. Application of ceramic resonators is also limited by requirements

of high stability.

Electrical coupling of resonators…



17

Piezomechanical and monolithic filters

In piezomechanical filters

piezoelectric resonators are

coupled mechanically

Mechanical coupling of piezoelectric

resonators is also used in monolithic

piezoelectric filters. Metal electrodes

with piezoelectric material between

them function as piezoelectric

resonators. The space between the

adjacent metal electrodes functions as

a coupling element.

Monolithic filters are applied in the

frequency rage from 2 to 200 MHz.

They are small in size like monolithic

ICs.



18

Ultrasonic Delay Lines

td = L / va

Ultrasonic delay lines are used in the frequency range to 100 MHz. Their

frequency response is like that of the band-pass filters. For this reason

ultrasonic delay lines may be used for delay of radio wave pulses having

band spectrum.

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Bulk Acoustic Wave Devices - VGTU slides/Bulk AW devices.… · Bulk Acoustic Wave Devices In electronic devices, electrical signals are used for the excitation of acoustic waves