Lecture 8-7 acoustic.pdf

8/17/2019 Lecture 8-7 acoustic.pdf

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Acoustic Wave Devices

Lecture 8-7ME2082


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Acoustic wave devices

Surface acoustic wave sensors

– biological and chemical sensor

– viscosity sensor

– force sensors

– stress/strain sensors Bulk Sensors

– acoustic pressure sensor

– pressure sensors

– accelerometers

Plate wave ((lamb wave) – Piezo-motors

Monolithic SAW device

– SAW on piezoelectric substrate

– SAW on non-piezoelectric

substrate (e.g.)

» Bi-layer or multi-layer

» Polycrystalline structure

» roughness larger

– Common configuration to excite

SAW wave

» IDT--interdigital transducer

» frequency determined by thephotolithographic process


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Acoustic wave devices Acoustic waves can be excited by various means such

as mechanical impact, pulsed thermal energy, or the

inverse piezoelectric effect. The last is by far the most

important for acoustic devices.

There are two groups of acoustic devices which make

use of two distinctive transducer types: surface acousticwave (SAW) and bulk acoustic wave (BAW) devices.

– The SAW employs interdigital transducers (IDTs), i.e.,

thin-film metal comb (‘finger’) structures as depicted in

the Figure. The principle of IDTs: if an alternating

electric field is applied at the comb structure, a time-

harmonic periodic deformation is induced in the

piezoelectric substrate underneath. Thus, an acoustic

wave perpendicular to the finger direction is excited by

each finger pair.

– The BAW is generally excited by means of thin-film

transducers covering the excited volume of a

piezoelectric material.

Interdigital transducer (IDT)

for the excitation of surface

acoustic waves (SAW)


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Top view of a SAW delay line consisting of

a quartz plate and interdigital metal

transducers. The lower figure indicates the

wave motion and direction of surface

particle displacement.

Top view of a TSM consisting of a

quartz disc, metal electrodes and

electric contacts. The smaller figure

illustrates the wave motion and the

direction of particle displacement.

BAW SAW


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Example: BAW-- Quartz thickness shear mode (TSM) resonators

A

m f f

QQ

∆×−=∆

2/1

2

0

)(

2

µ ρ

For the case of thin, rigid and

uniform mass layer on the surface of

the device, there is a relationship between the mass increase (∆m) at

the quartz surface and the resonant

frequency shift (∆ f )

where f 0 is the resonant frequency ofthe quartz crystal, A is the active area

of the coated crystal, µ Q is the shear

modulus of the quartz crystal and ρ Q is

the density.


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C0 C

1

R1

L1

liquid loading

C0

L2

C1

R1

L1

R2

unperturbed

TSM resonator

L3

mass loading

liquid loading

C0

L2

C1

R1

L1

R2

unperturbed

TSM resonator

L3

mass loading


For the liquid loading, Kanazawa and Gordon found that

for the Newtonian liquid contacting with one side of

resonator sensor, the resonant frequency shift depends upon

the viscosity (µ L) and density ( ρ L) of the liquid

2/1

2/12/3

0

)(

)(

QQ

L L f f µ πρ

η ρ −=∆


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The electrical admittance spectra Y(w) generated

from BVD model is given by:

( )

0

111 1

1 C j

C j L j R

jBGY

ω ω ω

ω

+

++

=

+=

2/1

11 )(

1

2

1

C L f s

π =

For unperturbed TSM resonator, the series resonance

frequency f s at which G is maximum (Gmax), is given by:


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Acoustic wave devicesQuartz TSM resonator impedance


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Quartz thickness shear mode BAWFrequency

phase


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Acoustic wave devicesExample: BAW-- Quartz crystal microbalance (QCM) gas sensor

Typical response to 50 ppm of NO2 of the device coated with

NO2-absorbing polymer


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Acoustic wave devicesExample: BAW-- Quartz crystal microbalance (QCM) gas sensor forammonia detection at ppb level

QCM is coated with fibrous

polyacrylic acid (PAA)

membrane


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For SAW device – If multiple finger pairs exist, interference occurs

yielding to a maximum SAW magnitude if

,.....3,2,1,0,

12=⋅

+=

nv

n

f n λ

with the excitation frequency f , the acoustic wavevelocity v of the particular mode, and the transducer

period λ=2(a+b).

– Hence an IDT is a frequency-selective element.The frequency of the lowest order mode f o iscalled the characteristic frequency.


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SAW: Basic Acoustic Structures

– One of the simplest and best known acoustic structures is the delay line

shown in Figure. The transducer IDT1 serves as the input for an electric

radio-frequency (RF) signal which is transformed by means of the inverse

piezoelectric effect into an acoustic wave traveling to the outputtransducer IDT2. Here, a re-transformation takes place back into the

electrical domain. Hence the electrical signal experiences a delay τ

because the acoustic wave is five orders of magnitude slower than the

velocity of light:

⋅=

ν τ CC


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If no transmission loss occurs, the delay linetransfer function H DL(f ) can be written as

with the transfer functions H 1(f) and H 2(f) of IDT1

and IDT2, respectively.

( ) ( ) ( ) ( )⋅⋅−⋅= τ π f j f H f H f H DL 2exp21

The magnitude of the transfer function depends on the IDTcharacteristics, whereas CC determines the phase ϕ

( ) ( )o f f vCC f −⋅⋅−= π ϕ 2

Hence, amplitude and phase characteristics can be chosenindependently of each other. Changes of both quantities due toexternal influences may serve as measuring effects.


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By Materials

– Bulk wave

– Surface wave

– Flexural plate wave

By wave forms

– Longitudinal wave

– Transverse wave

– Raleigh wave (mixed long. and trans. wave)

Bulk Materials – Quartz

– Piezo-ceramic materials

Thin films materials

– ZnO

– PZT

– organic polymer

– GaAs

Surface acoustic wave transducers

Electrical -Mechanical

Transducer

Mechanical - Electrical

Transducer Electrical

input

Electrical

output

Mechanical

output

Frequency

Amp.

Phase

Delay line

(temperature, mass,

stress, morphology)Oscillator


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Frequency: f

Pitch

Phase velocity: v p

Important parameters:


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λ=4d


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SAW chemical sensors


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Acoustic wave sensors

Dual delay line configuration.


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SAW Devices Measurement Methods: Oscillator vs. delay line approach

– Oscillator: frequency f/f = Sm m

Sm: mass sensitivity, ∆m: mass/unit area

Delay line: phase shift (time delay)

amplitude(dispersion)quality factor (Q)

Love Mode SAW

– High sensitivity

Flexural Wave (Lamb) Plate

High sensitivity than SAW

Using a low velocity lamb wave allows the device to operate at low frequency

The velocity can be made lower than the velocity of compression wave in

common liquid, permitting low loss operation

Heat capacity is low due to the reduced thickness


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Love Mode SAW Devices as gas sensormolecularly imprinted materials (MIM).


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SEM of liquid traps at the border between an IDT

and the propagation path of a Love mode sensor


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Love Mode SAW Devices


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Love Mode SAW Device as gas sensor

Relative frequency change

∆ f/ ∆ f c due to MMP exposure

for imprinted and non-imprinted (control)device.

2-methoxy 3-methyl pyrazine,

MMP, is a substance used in

perfume industry.,


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Love Mode SAW Devices as gas sensor


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Lamb Wave sensor

Effective mass


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Consideration when designing anacoustic wave devices


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Measurement Methods: Oscilaltor vs. delay line approach – Oscillator: frequency f/f = Sm m

Sm: mass sensitivity, ∆m: mass/unit area

Delay line: phase shift (timr delay)

amplitude(dispersion)

quality factor (Q)


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Acoustic Wave

Devices

Love mode


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Influence of Newtonian

liquid viscosity on Love

wave phase velocity

Acoustic Love wave devices


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Cantilever beam accelerometer


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Accelerometer


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Vibration and acceleration sensors


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Vibration and acceleration sensors


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Torque Sensors


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Gyro Sensor


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Humidity Sensors


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Dew point sensor


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Voltage sensor


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Flow Sensor


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Others


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Electrostatic ink jet printer


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Lecture 8-7 acoustic.pdf