Lallart M. Ferroelectrics: Applications

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Periodically Poled Acoustic Wave-Guide and Transducers for Radio-Frequency Applications

Fig. 11. Theory/experiment comparison for a 20µm period PPT on Silicon (LiNbO

thickness =

26µm)

Fig. 12. Theory/experiment assessment for a 40 µm period PPT (LiNbO

thickness = 50 µm)

5. Si/PPT/SI-based waveguide, resonator and oscillator

Finally, we have developed an isolated wave guide allowing for the propagation of acoustic

waves within a PPT plate in between two silicon substrates, yielding advanced packaging

opportunities. Fig. 13 illustrates this configuration and Fig.14 shows the kind of theoretical

prediction one can obtained using FEM/BEM harmonic computations to demonstrate the

targeted guiding effect.

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Electrodes

p = λ

Exponential

vanishing of the

waves

Periodically poled transducer

(PPT)

Silicon

Fig. 13. Principle of the PPT isolated wave transducer

Fig. 14. Example of harmonic admittance computed for a Si/LiNbO

/Si

transducer/waveguide, the period of the PPT (wavelength) is 50µm, the niobate layer is

30µm thick (the pole is the signature of a guided mode).

The fabrication of acoustic waveguides based on PPTs consists in bonding a silicon wafer on

each side of the periodically poled wafer, as described in fig. 15. In that purpose, the 500 µm

thick Z cut lithium niobate wafer is poled and bonded on a (100) 3" doped silicon wafer

using a wafer bonding technique developed in our group based on a metal-metal adhesion

at room temperature promoted by a high pressure applied to the material stack (Fig.15). The

study of the dispersion properties enables to define a specific configuration using a thinned

PPT layer of about 30 µm. The LiNbO

wafer thinning is achieved by home-made lapping

and polishing techniques. After this step, the stack of Si(380 µm)/LiNbO

(20 µm) is bonded

again on a doped silicon wafer with the same properties that the first one (Fig.16). Several

devices have been built along this approach but we mainly have focused our attention on

thicker structures (using 500µm thick lithium niobate wafers) for characterization and

application purposes.

Periodically Poled Acoustic Wave-Guide and Transducers for Radio-Frequency Applications

Fig. 15. Flowchart which summarizes the different steps of fabrication

Fig. 16. SEM view of a Si/PPT/SI transducer, clearly showing the periodic poling of the

transducer

Operational test vehicles have been achieved using doped silicon wafers to ease the

electrical contact. The transducer was built in lithium niobate with a 50 µm period and a

thickness equal to 500 µm. Theoretical and measured electrical admittances agree well and

allow for identifying a main contribution corresponding to a guided longitudinal mode at

131 MHz (fig. 17). The corresponding phase velocity is very close to the one of the PPT alone

(i.e. 6500 m.s

-1

). The elliptically polarized mode excited using the PPT alone and exhibiting a

phase velocity of about 3800 m.s

-1

is not excited nor guided in this configuration. This mode

actually needs a free surface to satisfy its boundary conditions (similarly to a Rayleigh

wave) and therefore, the existence of rigid boundary conditions on each side of the PPT

prevents its excitation and propagation.

This resonator operating near 131 MHz exhibits a quality factor of 13000 and an

electromechanical coupling k

equal to 0.25 % (twice higher than the one of a SAW resonator

on Quartz). The corresponding phase rotation (320°) and the dynamic of its electrical reflection

coefficient (S

=-8 dB) are suitable for oscillator applications. Such a device therefore has been

built using a negative resistance scheme (the so-called Colpitts circuit [Colpitts]).

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Fig. 17. Theoretical and experimental admittances of a Si(380µm)/LiNbO

(500µm)

PPT/Si(380µm) sandwich

A specific printed circuit has been built in that matter (Fig.18). Note that thanks to the

isolation of the mode, one could glue the resonator directly on the board allowing for easily

grounding the device. A single gold wire then is used to connect the resonator to the

oscillator (such a connection yields a notable sensitivity to RF parasites and hence will be

improved in the next future). The phase noise of the oscillator at 100 kHz from the carrier

shows a value less than -160 dBc/Hz, which can be honestly compared with other acoustic

wave oscillators at such frequency, accounting for the fact that the device was excited with a

quite low signal level (-6dBm). Therefore, increasing the excitation should allow for a

significant reduction of the noise floor and then advantageously compete with standard

solutions. Moreover, as the wave guide appears really robust concerning packaging and

back end conditioning, it can be integrated more easily than any other acoustic wave based

solutions and benefit from a clear applicative potential.

Fig. 18. The oscillator board implemented for phase noise tests

Periodically Poled Acoustic Wave-Guide and Transducers for Radio-Frequency Applications

Fig. 19. Phase noise of the 131 MHz oscillator stabilized with a Si/PPt/Si resonator. The

noise floor is better than -160 dBc/Hz.

6. Conclusion

In this chapter, we have discussed the standard techniques implemented for optimizing

PPTs for fabricating test vehicles and we have proposed a detailed analysis of the

experimental tests. We propose some guidelines for future developments and

implementation of these new waveguide principles to answer the requirements for the next

generation of passive signal processing components, and more particularly resonators and

filters. However, because of its very particular configuration, the Si/PPT/Si structure is

considered as a potential candidate for sensor applications, particularly when the sensor is

expected to be inserted in hosting bodies submitted to parametric perturbations such as

stress, vibration or pressure. In that case, the device can be connected directly to the proof

body without the need to protect any surface, providing therefore more robustness than

SAWs or even bulk-wave-based sensors.

7. Acknowledgment

This work has been achieved in the Dominos program framework, funded by the European

Community as the InterReg project DOMINOS and was also funded by the french DGA

(Délégation Générale pour l’Armement) under grant #07-34-020.

8. References

S. Ballandras et al, (2003); A novel surface wave transducer based on periodically poled

piezoelectric domain, Frequency Control Symposium and PDA Exhibition Jointly

with the 17th European Frequency and Time Forum, 2003. Proceedings of the 2003

IEEE International, vol., no., pp. 893- 896

Ferroelectrics - Applications

S. Ballandras et al., (2009) A mixed finite element/boundary element approach to simulate

complex guided elastic wave periodic transducers. Journal of Applied Physics 105 1

014911

F. Bassignot et al, (2011) A New Acoustic Resonator Concept Based on Acoustic Waveguides

Using Silicon/Periodically Poled Transducer/Silicon Structures for RF

Applications, IEEE IFCS, San Francisco

P. Bradley et al, (2000), “Film bulk acoustic resonator (FBAR) duplexer”-- U.S. Patent (USP)

#6262637; Agilent inventors Paul Bradley, John Larson, Richard Ruby).

E. Courjon et al, (2007) Lamb wave transducers built on periodically poled Z-cut LiNbO3

wafers, Journal of Applied Physics, 102, 114107

E. Courjon et al, (2008) Characterization of guided modes excited by periodically poled

transducers on Si," Frequency Control Symposium, 2008 IEEE International , vol.,

no., pp.604-608

O. Elmazria et al, (2009), AlN/ZnO/diamond structure combining isolated and surface

acoustic waves, Applied Physics Letters, vol.95, no.23, pp.233503-233503-3

D. Gachon et al, (2008) Fabrication of High, Frequency Bulk Acoustic Wave Resonator Using

Thinned Single−Crystal Lithium Niobate Layers, Ferroelectrics, Vol.362, n°1, pp.

30−40

D. Gachon et al. (2010) Prediction and Measurement of Boundary Waves at the Interface

Between LiNbO3 and Silicon. IEEE Transactions on Ultrasonics Ferroelectrics and

Frequency Control57 7 1655−1663

J.A. Greer et al, (1990) Metalllisations for Surface Acoustic Wave Resonators: film properties

and device characteristics, Proc. of the IEEE Ultrasonics Symposium, Vol.1, pp.483-

491

G.F. Iriarte et al, (2003) SAW COM parameter extraction in AlN/diamond layered

structures, IEEE Trans. on UFFC, Vol. 50, N°11, pp. 1142–1547

K. Higaki et al, (1997) High power durability of diamond surface acoustic wave filter,

Ultrasonics, Ferroelectrics and Frequency Control, IEEE Transactions on , vol.44,

no.6, pp.1395-1400

M. Kadota et al, (2009) Acoustic Wave Devices using Periodical Poled Z-cut LiTaO

Plate,

Proc. of the joint IEEE IFCS-EFTF, Besançon, pp919-922, 2009

H. Kando et al (2006) 6B-4 RF Filter using Boundary Acoustic Wave, Ultrasonics

Symposium, IEEE, vol., no., pp.188-191

R. Lanz, P. Muralt, (2005) Band pass filters for 8 GHz using solidly mounted Bulk Acoustic

Wave Resonators, IEEE Trans. on UFFC, Vol. 52, N°6, pp. 946–946

K.M. Lakin, (2003) “Thin film resonator technology”, Proc. of the joint IEEE IFCS-EFTF, pp.

1-14

D.P. Morgan, (1985) Surface Wave Devices for Signal processing, Studies in Electrical Eng.

19, Elsevier Ed.

L. E. Myers et al, (1995) Quasi-phase-matched optical parametric oscillator”, JOSA B, Vol. 12,

Issue 11, pp. 2102-2116

R. Salut & al, (2010) Fabrication of GHz Range Oscillators Stabilized by Nano-Carbon-

Diamond-Based Surfcae Acoustic Resonators, Proc of the IEEE International

Ultrasonics Symposium, San Diego

A. K. Sarin Kumar et al (2004), High-frequency surface acoustic wave device based on thin-

film piezoelectric interdigital transducers, Appl. Phys. Lett. 85, 1757

doi:10.1063/1.1787897 (3 pages)

M. Wilm et al, (2002) A full 3D plane-wave-expansion model of 1-3 piezoelectric composite

structures, J. Acoust. Soc. Am., vol. 112, n°3, pp. 943-952

Ferroelectric Polymer for Bio-Sonar Replica

Antonino S. Fiorillo and Salvatore A. Pullano

School of Biomedical Engineering, University of Magna Græcia,

Italy

1. Introduction

The sensorial knowledge paradigm has captured the interest of many eminent scholars in past

centuries (the philosophical trend of “Sensism” was developed around the “Gnoseologic

Paradigm”, which has found its highest expression in Étienne Bonnot de Condillac, 1930) as

well as in the modern era, particularly in the attempt to interface the external environment to

humans through artificial systems. Of the five human senses, which have been investigated by

scientists involved in artificial perception studies, vision, touch and hearing have received the

most attention, each one for different reasons. When referring to hearing as the sense which

perceives sound (the mechanical perturbation induced in a medium by a travelling wave at

suitable frequency), a distinction should be made. Indeed, sound between 100 Hz and 18 kHz

refers mainly to the range of human perception, while infrasound (up to 20 or 30 Hz) and low

frequency ultrasound (from 20 to 120 kHz) refer to animal (mammalian) perception.

Low frequency ultrasounds have been amply investigated in the last century and the

resulting applications have been made in both military and civil fields. In any case, it

appears relevant and necessary to improve the performance of the ultrasonic system (more

properly named sonar) for use in a variety of industrial, robotic, and medical applications

where ranging plays a basilar role. Nevertheless, other important information can be

extrapolated through proper use of the ultrasonic signal as is evident from the study of the

biology and mammalian behaviour (Altringham, 1996). Up to now, attempts have been

made to try to emulate animal auditory systems by using both commercial or custom

piezoelectric transducers. In this context, the latest investigation in artificial perception was

mainly inspired by bat bio-sonar, which has been extensively studied and described by

biologists.

As a result of the damping exerted by the propagation medium, which increases as the

ultrasound frequency increases, conventional transducers normally function at relatively

low frequencies (40 ÷ 50 kHz) in air. Sometimes this restricts choices of piezoelectric

materials, besides transducer shape and dimensions. In order to increase the frequency and

hence to improve the performances of ultrasonic transducers, flexible plastic materials, such

as the ferroelectric polymer polyvinylidene fluoride (PVDF) were investigated and

assembled in different geometries. It was discovered that, when properly shaped, PVDF

films can resonate at frequencies superior to 100 kHz, covering the full range frequency of

the majority of bat bio-sonars (20 ÷ 120 kHz).

The first part of a work aimed at emulating the auditory system of Pteronotus Parnellii, (also

known as the moustached bat) is described in this chapter. We have simulated some of this

Ferroelectrics - Applications

bat’s most important strategies, based on the techniques used by its sophisticated echo-

location system in gathering ultrasonic information. Specifically, in this phase of the study,

the piezoelectric transducer is used to emulate the function of the bat cochlea in a real

distance measurement, although bio-sonar capabilities are far superior.

The chapter is organized as follows: first the design and characterization of the ferroelectric

polymer ultrasonic transducer is discussed. Based on the piezoelectric equilibrium rules, a

suitable transducer geometry has been designed in order to improve the device’s

performance in air.

Then the transducer impedance has been characterized from 30 to 40 kHz up to 120 kHz,

exactly in the same range frequency in which most bat bio-sonars operate. The design of the

electronic circuits and the matching of the electric impedance with the ultrasonic transducer

received particular attention because of the inherent noise of the PVDF. The sensory unit

operates at distances of 10 ÷ 2500 mm with an axial resolution of about 2 mm down to 500

µm. A critical comparison between the custom transducer (based on ferroelectric polymer

technology) and similar devices (based on different, but standard, technologies) is also

carried out.

These and other problems are considered in this work, including a neural network approach

carried out in order to verify the potentiality of the PVDF electronic sonar and the

analogism with the bat bio-sonar, in all its complexity, with the aim of exploring how

artificial perception in the acoustic field can support human sensorial perception.

Nomenclature

c Stiffness coefficient [

−

⋅ ]

d Piezoelectric constant [

−

⋅ ]

C Capacitance [

F ]

Normal electric displacement [

−

⋅ ]

e Piezoelectric constant [

−

⋅ ]

E Electric field [

−

⋅ ]

Frequency [Hz ]

Piezoelectric constant [

VmN

−

⋅⋅ ]

h Piezoelectric constant [

−

⋅ ]

Electric current [

]

k Piezoelectric coupling factor

L Inductance [ H ]

Figure of merit

P Polarization [

−

⋅ ]

Q Quality factor

Resistance [

]

r Bending radius [ m ]

S Strain

s Elastic compliance [

−

⋅ ]

T Tangential stress [

−

⋅ ]

Dielectric impermeabilities [

−

⋅ ]

Permittivity [

−

⋅ ]

Relative permittivity

Vacuum permittivity [

−

⋅ ]

Ferroelectric Polymer for Bio-Sonar Replica

Angle [de

]

SNR Signal-to-noise ratio

2. PVDF transducer analogy to bat cochleas

The auditory system of mammals is characterized by a common basic layout in which one

can identify three anatomical regions—the external and middle ears (air filled) and the inner

ear (filled with biological fluid). The acoustic waves are received, conveyed, and amplified

by the external and middle ears, while the vibrational energy related to sound pressure is

converted into bio-electric energy by the inner ear. Sound amplification is carried out

mechanically by a system which includes the ossicles: the malleus, the incus, and the stapes;

and the eardrum.

In the system we propose for bio-sonar replication, the amplification of the signal and all

other steps included in acoustical signal conditioning are carried out electronically, while

the piezoelectric transducer is concerned with the conversion of mechanical energy. Our

attention was focalized on the third anatomic region, particularly the cochlea, which

transduces mechanical energy into bio-electrical energy, and the acoustic nerve, which

carries the bio-electrical signal to the cerebral cortex.

In this section the anatomic structure of the cochlea and its working is reviewed in order to

clarify and justify the choice concerning materials, particularly the ferroelectric polymer,

and methods used to emulate the bio-sonar of bats.

The cochlea of a bat, similar to that of other mammals, boasts a hollow spiral geometry

divided into three channels; the vestibular and tympanic channels are filled with

endolymph, which is very similar to intracellular liquid, and the middle channel is filled

with perilymph, similar to extracellular liquid, each one separated by an endothelial

membrane (see Figure 1). As far as the electronic system is concerned, the most important

component is the basilar membrane which, through vibration, activates the receptors of the

organ of Corti, that lie over it (the organ of Corti includes the tectorial membrane, two

different systems of hair cells, and nervous fibres; these generate bio-pulses as a

consequence of the vibration).

Fig. 1. Main part of the inner ear and arrangement of cochlear component.

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When the acoustic wave reaches the eardrum after passing through the outer ear canal, it

has already been primarily amplified in this resonant cavity of the external ear.

Successively it is mechanically amplified in the middle ear by the chain of ossicles acting

as levers on the oval window, located in the inner ear. The travelling wave, once in the

cochlea, propagates along the basilar membrane (see Figure 2), which acts as a mechanical

filter with respect to the frequency spectrum. Different frequency components of the

signal cause the motion of different parts of the membrane in a tonotopic organization.

The behaviour of the basilar membrane is related to its geometry, because moving from

the base toward the apex the membrane increases its width and thickness, while the

resonance frequency decreases.

Fig. 2. Basilar membrane winding inside the cochlea

The receptors located in the organ of Corti are divided into outer and inner hair cells, and

perform two different functions. The motion of the basilar membrane intimately connected

to the organ of Corti is first amplified by the outer hair cells and then transmitted through

the liquid to the tectorial membrane that induces the deflection of the hair in the inner cells.

The inner hair cells transduce the mechanical signal, after amplification, into an

electrochemical signal through the activation of ionic channels and the release of a

neurotransmitter (glutamate) to the acoustic nerve, as a consequence of the polarization

level of the cells itself. The neurotransmitter reaches the central nervous system through the

afferent fibres, which account for 90 ÷ 95 % of the total connections, while the rest are

connected to the inner hair cells. From the bio-electrical point of view, this process is

concerned with the generation of an action potential which drives ionic currents along the

axons of the afferent fibres (see Figure 3); conversely, the majority of the efferent fibres connect

the central nervous system to the outer hair cells.

Referring to the analogism with the artificial system, the PVDF ultrasonic transducer acts in

a way similar to the basilar membrane, except for the absence of the outer hair cells. In effect

though, the acoustic pressure is not mechanically amplified, but merely converted, by the

direct piezoelectric effect , into an electrical signal and, hence, only

afferent electronic pathways

are considered.

PVDF is a light and flexible plastic material, of several tens of μm in

thickness, which can be shaped into hemicylindrical geometries in order to fabricate the

ultrasonic transducer which resonates in the frequency range of bat biosonar.

The resonance

frequency is inversely proportional to the bending radius and can be easily tuned to the

suitable values necessary for the specific tasks accomplished by the bat. In fact the curved

PVDF ultrasonic transducer works in a way similar to the basilar membrane.