This work relates to a novel piezoelectric transformer to be used in an
autonomous sensor unit, possibly in conjunction with a RF-MEMS retro-modulator.Comment: Submitted on behalf of EDA Publishing Association
  (http://irevues.inist.fr/handle/2042/16838

Bourouina, T.

Lissorgues, G.

Sangouard, Patrick

English

arXiv

Submitted on behalf of EDA Publishing Association (http://irevues.inist.fr/handle/2042/16838)International audienceThis work relates to a novel piezoelectric transformer to be used in an autonomous sensor unit, possibly in conjunction with a RF-MEMS retro-modulator

Lissorgues, Gaelle,

Bourouina, Tarik

HAL - UPEC / UPEM

A Novel Piezoelectric Microtransformer for Autonmous Sensors Applications

This work relates to a novel piezoelectric transformer to be used in an autonomous sensor unit, possibly in conjunction with a RF-MEMS retro-modulator

I-Revues

9-11 April 2008 
©EDA Publishing/DTIP 2008  ISBN: 978-2-35500-006-5 
A Novel Piezoelectric Microtransformer for 
Autonmous Sensors Applications 
 
Patrick SANGOUARD, Gaëlle LISSORGUES and Tarik BOUROUINA, Senior Member, IEEE 
 
Université Paris-Est, ESIEE, Laboratoire ESYCOM, EA 2552 
2 Bd Blaise Pascal 93162, Noisy-le-Grand, France 
 
 
 
Abstract- This work relates to a novel piezoelectric transformer 
to be used in an autonomous sensor unit, possibly in conjunction 
with a RF-MEMS retro-modulator. 
  
I.  INTRODUCTION 
This works relates to autonomous devices like sensors. 
The concept refers to sensors or actuators without any 
embedded power source. These are positioned for instance 
inside a wall, a human body or in space. Such devices are 
passive most of the time, but must provide, on request, 
information to the outside world or perform actions. 
Wireless and with no internal energy source, the energy 
needed for the operation of these autonomous devices must 
therefore be provided by the extern world. One can perform 
a transformation of this external energy, which can be 
originally ultrasonic or in the form of electromagnetic 
waves, to an electrical energy.  
 
External  Energy Source Electromagnetic or ultrasonic
Integrated Micro transformer
Obtaining an AC voltage with a  few 
tens  mV and a few hundred Khz
Autonomous Electronics of 
Amplification and storage 
Without power  
SENSOR 2
Electromagnetic Emission
of the  Sensor information
Frequency Modulator
Reception of the 
sensor Information
SENSOR n 
SENSOR 1
AUTONOMOUS DEVICE
 
Fig. 1. Illustration of the proposed concept of autonomous device 
. 
II. OPERATION PRINCIPLE 
The micro transformers have to transform an 
electromagnetic (or an ultrasonic) energy into an electrical 
energy. Their operation is based on some properties of 
piezoelectric thin films. They also must comply with the 
constraints associated with the ultimate goal of integration 
on the same silicon wafer of the sensors (or actuators), the 
frequency modulator and the electronics circuit. The shape 
of the micro-transformers is necessarily planar as they are 
created by micro-machining of thin- piezoelectric film of a 
few microns in thickness. The exploitation of thickness 
distortion is very difficult because in this case the resonance 
frequency of the piezoelectric structures quickly becomes 
excessive (several tens of Megahertz). On the other hand, the 
planar structures, the corresponding deposition processes and 
the layers polarization do not allow directly to use the 
transverse and shearing piezoelectric effects (d13 and 
d15.piezolectric coefficients, respectively)  
Taking into account these constraints, we are developing 
micro transformers using some resonance modes of bending 
deformation. These are better suited to micro systems. The 
main interest of this solution is that we can control the 
resonance frequency by modifying the geometrical 
dimensions of the structure.  
 
III. FIRST GENERATION OF PIEZOELECTRIC 
TRANSFORMERS 
In a previous work [1-3] we made a first generation of 
micro transformers, which take advantage of the direct and 
reverse piezoelectric effect and a mechanical amplification, 
taking place between the center and the boundary of a 
vibrating electromechanical structure (Fig. 2).   
 
 
(a) 
 
(b) 
  
(c) (d) 
Fig. 2.Photographs of fabricated micro transformers. Direct and reverse 
piezoelectric effects take place at the center and at the boundaries of 
mechanical structures, where sets of electrodes are deposited. The different 
strain level in these regions is used to obtain the required voltage 
amplification in the transformers 
9-11 April 2008 
©EDA Publishing/DTIP 2008  ISBN: 978-2-35500-006-5 
 
These first versions of micro transformers have been done 
with simple geometric configurations and took the form of 
beams, bridges and circular or square discs. 
A modelling of these devices has been established [4] by 
splitting these structures into simple elements, one formed 
by the primary (input) of the micro transformer, the other by 
his secondary (output). These elements are connected by 
boundary conditions involving efforts (equal bending 
moments, and cutting efforts), as well as speed (equal linear 
and angular velocity). Calculation details are given in the 
Appendix. 
The equivalent diagram (Fig. 4) is deduced from this 
analytical model of the beam micro transformer, which 
consists here of a single film piezoelectric, which is common 
to the primary and the secondary electrodes. 
 
 
 
Fig. 4: Equivalent electric circuit and gain a beam micro transformer 
consists of one piezoelectric film 
 
The previous model was validated on a macro transformer 
using a single film piezoelectric (shown in Fig.5), whose 
geometrical details and physical properties are given in 
Table 1 below.  
 (a) 
 
 
(b) 
10 0 150  20 0 25 0 3 00  350  4 00 4 50 5 000 .2 
0 .4 
0 .6 
0 .8 
1  
1 .2 
1 .4 
1 .6 
1 .8 
2  
2 .2 x  10 
-5 
F ré q u e n c e  e n  H z  
M
o
d
u
le
 d
e
 l
’a
d
m
it
ta
n
c
e
 d
’e
n
tr
é
e
 M o d è le  
M e s u re  
M o d e  1  (m 1)  
 
 
Fig. 5: Macroscopic piezoelectric transformer (15mm* 6.3 mm) 
 (a) Picture (b) Comparison between measurements and theoretical model  
 
 
 
 
Table 1: Characteristics of the macroscopic piezoelectric transformer 
 
The characteristic of the first micro transformers of simple 
geometric shape, made on a silicon wafer with a single 
piezoelectric film of either PZT or AlN (Fig. 6) were 
consistent with the theory developed in thesis [4], but the 
gain measured electrical thereof were quite weak. 
 
 
Fig. 6: Gain (0.093) and phase of a beam micro transformer using an uniform 
AlN film on the primary (input) and secondary (output) electrodes 
 
IV. SECOND GENERATION OF MICROTRANSFORMERS WITH 
TWO PIEZOELECTRIC MATERIALS 
Taking into account these first results, and in order to 
improve the voltage gain, we explored a new idea, which 
consists of using two different piezoelectric films, called 
"complementary". Indeed, the first piezoelectric material is 
chosen with a high piezoelectric coefficient - like PZT (Lead 
Zirconate Titanium), whose d31 is typically equal to 1.8 10
-10
 
m/V. This first material is used in the primary (input) 
electrode). The second piezoelectric coefficient is chosen for 
that it has a much lower piezoelectric coefficient - such as 
AlN (Aluminium Nitride), whose d31 is typically equal to 
2.65 10
-12
 m/V. This second material is used on the 
secondary (output) electrode 
The comparative gains simulations with Matlab software 
for identical micro transformers structures are shown in Fig. 
7, for micro-transformers based on a bridge-type resonator 
and in Fig. 8 for micro-transformers based on disc-type 
resonators, respectively. These two kinds of micro-
transformers are compared in two configurations, where we 
used either the same piezoelectric film (AlN) on the primary 
and secondary electrodes (Figs. 7.a and 8.a) or two 
"complementary" piezoelectric films : PZT and AlN (Figs. 
7.b and 8.b) on the primary and secondary electrodes, 
respectively.  
 
 
 
9-11 April 2008 
©EDA Publishing/DTIP 2008  ISBN: 978-2-35500-006-5 
 
 
Fig. 7: Matlab simulation of bridge-type micro transformers showing the 
gain response around the 2nd bending mode. (a) Low gain of 0.43 is obtained 
when using only AlN films : (b) Higher gain of 29 is reached when using 
PZT at the input (primary) and AlN at the output (secondary) 
 
 
 
 
 
Fig 8 :  Matlab simulation of disc-type micro transformers  showing the gain 
response around the 3rd resonance mode. (a) Moderate gain of 17  is 
obtained when using only AlN films : (b) Higher gain of 300 is reached 
when using PZT at the input (primary) and AlN at the output (secondary) 
Load resistance is 10 MΩ 
 
 
One can note on Figs. 7 and 8 the improvement, by a 
factor more than 20x of the gain for the devices with two 
complementary in nature piezoelectric films. The deposited 
piezoelectric film at the vicinity of the primary electrode 
must have a piezoelectric coefficient d31 much greater than 
the one placed on the secondary electrode. The use of 
different piezoelectric films at the vicinity of the primary and 
secondary electrodes looks promising but of course only a 
realization in clean room will confirm this theoretical 
prediction. 
New geometries (shown in Fig. 9) for the micro 
transformer were also designed. They combine 
"complementary" piezoelectric layers, a geometrical shape 
that is aimed to increase further the strain on the secondary 
electrode. As these structures are open, they are also 
expected to decrease the effect of air damping. 
 
 
Fig.9: New resonator shapes for a  Micro  transformer  
 
 
V. DESIGN OF AN ELECTRONIC CIRCUIT FOR THE MICRO-
TRANSFORMER 
For micro transformers made with a single piezoelectric 
material, common to the primary and secondary and 
submitted to ultrasonic waves of variable frequency, the 
measured secondary (output) voltage V1 is in the order of 5 
mV (for the resonant frequency of mode 3). However, in the 
case of micro transformer involving two complementary 
piezoelectric materials, the performed simulations predict a 
voltage V1 exceeding 50 mV at the secondary (output) 
electrode. This alternating voltage output is in any case 
insufficient to feed electrical power to a sensor. The use of 
an electronic amplification without power is therefore 
necessary. 
The proposed electronic system (schematized in Fig. 10) 
was simulated with SPICE software. Its function is to 
amplify and generate a continuous voltage from an 
alternative voltage of about 50mV. This circuit consists of a 
SCHENKEL voltage doubler type, where diodes are 
replaced by N and P type MOS transistors with very low 
threshold voltage and also with the substrate and grid 
connected together.  
As shown in Fig. 10, these transistors can be made on the 
same SOI wafer used to manufacture other 
electromechanical structures. They are all isolated each other 
by engravings of their local layers of silicon to the buried 
oxide layer of the SOI wafer. 
 
 
 
(b) PZT / AlN 
 
Gain = 29,   
f=105985 Hz 
 
(a) AlN / AlN  
 
Gain = 0.43 ,  
f=238270 Hz 
 
 
(a) AlN / AlN  
 
Gain = 17  
f=70715 Hz 
DISK Resonator 
beam 
Resonator 
beam 
Resonator 
(b) PZT / AlN 
 
Gain = 300 
f=75585 Hz 
 
DISK Resonator 
9-11 April 2008 
©EDA Publishing/DTIP 2008  ISBN: 978-2-35500-006-5 
 
 
P
Drain
Grid
PBulk
Source
N
Drain
NBulk
C
a
p
a
c
it
y 
C
ap
a
ci
ty
 
Grid
Source
 
Fig 10: isolation of MOS transistors on SOI wafer 
 
This amplifier (as shown in Figs. 11-12) operates without 
any power supply and allows sufficient amplification to 
generate up to 5.4 V DC starting from a voltage of only 50 
mV AC (available at the output of the micro transformer). 
Also, the with power consumption is very low : 60 nW in the 
transient (of 2 ms) and 2.97 pW in the steady state, with the 
corresponding current of 60 pA at the input. 
 
 
Fig. 11: The principle of electronic amplification of an AC voltage 
of a few tens of mV into a continuous voltage of several volts 
 
 
  
 
PROSPECTIVE WORK 
The realization of prototypes and their characterization is 
still required in order to confirm the theoretical simulation 
studies on the second proposed version of micro-
transformers involving two “complementary” piezoelectric 
materials. The same applies for the corresponding electrical 
circuit. Then we will be able to check the effectiveness of 
the proposed system consisting in a micro transformer, an 
electronic amplification circuit without power supply, a set 
of sensors or actuators, and a frequency modulator [5,6]. 
Target application of such autonomous device is based on its 
ability  to transmit to the outside world, information from an 
abandoned sensor. 
 
ACKNOWLEDGEMENTS 
The authors would like to thank Dejan Vasic, Emmanuel 
Sarraute and François Costa (ENS-Cachan), Eric Cattan 
(IEMN), Philippe Bois (CEA) for their valuable contribution 
to this work. 
Microfabrication of the prototypes was done in the clean-
room facilities of ESIEE.  
This work has the support of the French National Research 
Agency (ANR)in the frame of the project Ref. JC05/ 54551. 
 
 
 
REFERENCES 
[1] D. Vasic ,E.Sarraute,F.Costa,P.Sangouard,E.Cattan , « Piezoelectric 
micro-transformer based on SOI structure » Sensors and Actuatirs 
A-physical , Volume 117 ,Issue 2,pages 317-324, 14 january 2005  
[2] D. Vasic ,E.Sarraute,F.Costa,P.Sangouard,E.Cattan  ,  “Piezoelectric 
micro-transformer based on PZT unimorph membrane” Journal of 
Micromech. Microeng. 14 S90-S96, September 2004.  
[3] D. Vasic ,E.Sarraute,F.Costa,P.Sangouard,E.Cattan, «  Piezoelectric 
Micro-Transformer Based on SOI structure » , MME 2003 14 st 
MicroMechanics Europe Worksop , 2-4 Novembre 2003 Delf, 
Netherlands  
[4] D. Vasic , « Modélisation des micro-transformateurs » Ph.D Thesis 
ENS-Cachan September 2003  
[5] P. Sangouard, P.Bilstein ,G.Bazin ,P .Nicole . Patents EP0P21633 
,US6137323, FR2772213, DE69814803T, JP112309025 
[6] G. Bazin-Lissorgues, P. Sangouard, P. Bildstein, "Design of a 
Micro-mechanical capacitor with microwave functions", EPJ 
Applied Physics, N° 1, Vol. 9, pp. 75-80, January 2000. 
Fig 12:  Spice Simulation of: 1/ power charged by the micro transformer 
Producing a signal (50mV, 150 kHz), number of level= 30 
2/ the amplifier with a final voltage of 5.4 V, low power  
and current consumed by the micro transformer 


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A Novel Piezoelectric Microtransformer for Autonmous Sensors Applications

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