# Fabrication of periodically polled domains transducers on liNbO3

E. Courjon, N. Bodin, G. Lengaigne, L. Gauthier-Manuel, W. Daniau, S.

Ballandras, P. Paruch, J.M. Triscone, J. Hauden

► To cite this version:

E. Courjon, N. Bodin, G. Lengaigne, L. Gauthier-Manuel, W. Daniau, et al., Fabrication of periodically polled domains transducers on liNbO3. International Frequency Control Symposium and Exposition, Jun 2006, Miami, United States. pp.831-834, 10.1109/FREQ.2006.275497. hal-00194288

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# Fabrication of periodically poled domains transducers on LiNbO₃

E. Courjon, N. Bodin,

G. Lengaigne, L. Gauthier-

Manuel, W. Daniau, S. Ballandras

FEMTO-ST, UMR CNRS 6174

Besançon, France

ballandr@femto-st.fr

P. Paruch, J.M. Triscone

University of Geneva, DPMC

Geneva, Switzerland

jean-marc.triscone@unige.dpmc.ch

J. Hauden

Photline Technologies

Besançon, France

jerome.hauden@photlinetechnologies.com

**Abstract**— The development of piezoelectric transducers based on periodically poled ferroelectrics domains is investigated. Optical quality Z-cut LiNbO₃ wafers have been used for the fabrication of test devices operating in the range 7–70 MHz. The fabrication process is detailed and characterization results are reported. A very good agreement between theoretical predictions achieved by periodic finite element analysis and experiments is observed, allowing for a precise identification of the excited modes.

## I. INTRODUCTION

The possibility to manufacture elastic wave-guide based on a thin ferroelectrics film deposited on a single crystal substrate and periodically poled to enable the excitation of surface acoustic wave has been demonstrated recently [1,2]. The so-called piezoelectric inter-digital transducer (PIT) simply consists in two electrically conductive media embracing a poled ferroelectrics layer. It has been successfully implemented and tested for the excitation of elliptically polarized waves in the frequency range 1-3 GHz [2].

In this work, the possibility to develop PITs on LiNbO₃ is investigated, since this material is classically identified as well-adapted for very high frequency applications. The first experiments have been performed on 500µm thick 3" wafers of optical quality Z-cut answering severe specification on total thickness variation and side parallelism. The fabrication of PITs on such wafers allows for the excitation of symmetrical Lamb modes with an operating frequency twice higher than those obtained using standard inter-digital transducers. A photo-resist pattern is defined on one side of the wafer, enabling the implementation of a poling process using a liquid electrode. A poling sequence [3] (max electric field of about 22 kV.mm⁻¹) was applied to successfully fabricate PITs of period 100 and 200µm (50 and 100µm line-width respectively). The corresponding devices exhibiting respectively 500 and 200 periods have been tested and the measured admittance are compared to theoretical predictions. The latter were obtained using our

periodic finite element code [4] providing an harmonic admittance considering a synchronous excitation of all the PIT cells comparable to the admittance of our very long implemented PITs. An excellent agreement is found between both results, enabling one to analyze almost all the peaks of the experimental setup response. Even low frequency parasitic signals were analyzed thanks to the theoretical analysis. The LiNbO₃ wafer then was lapped to check the robustness of the PIT to this kind of operation. The PIT still operates after reducing the wafer thickness by a factor of 2. As in the case of classical Lamb waves, the fundamental mode is almost insensitive to the wafer thickness. The frequency control is then totally achieved by the PIT period for that mode, offering attractive perspectives for the development of reliable high frequency devices.

In the first section of the paper, the fabrication process is reported, pointing out specific difficulties related to the use of LiNbO₃. The second part of the paper is dedicated to the characterization of the devices and to theory/experiment assessment. The evolution of the spectral behavior of the device versus its thickness is theoretically and experimentally investigated and the mode shape and polarization is deduced from theoretical calculation. As a conclusion, the possible applications of devices based on PITs on LiNbO₃ are listed and discussed.

## II. FABRICATION OF PIT ON Z-CUT LiNbO₃ PLATES

### A. Poling bench

We first recall here the overall principle of the piezoelectric inter-digital transducer (PIT). It is fundamentally based on a periodically poled piezoelectric medium. Each side of this medium is metallized, then providing a capacitive dipole in which elastic waves can be excited. Such a periodically poled structure can be advantageously achieved on a ferroelectrics material like PZT [1,2] thanks to the rather small value of its coercitive
electric field (the absolute value of the electric field above which the ferroelectrics poling can be controlled). The principal advantages of PITs compared to standard periodic inter-digital electrode transducer (IDTs) deposited atop piezoelectric materials is first the robustness of the excitation versus defects or surface pollution (no passivation required to avoid short circuit as for standard IDTs) and second the possibility to excite waves exhibiting a wavelength equal to the poling period, contrarily to standard IDTs for which the wavelength is equal to twice the mechanical period (Bragg condition). Figure 1 schematize the general outlook of such a device and points out the interest of the PIT compared to standard IDTs.

<figure><img src="image_2.png" /><figcaption>Figure 1. Typical geometry of piezoelectric interdigitated transducer (PIT) based on ferroelectric poling</figcaption></figure>

As mentioned in introduction, the poling process can be
rather easily applied to PZT for which the coercitive field is
small enough to allow for an efficient control of the domain
polarity. In the case of lithium niobate, this situation is quite
different because of the large value of its coercitive field (22
MV.m⁻¹ compared to 2.5 MV.m⁻¹ max. for PZT). As a
consequence, the development of a dedicated poling bench
was required to control the poling of thick (500µm) Z-cut
LiNbO₃ plates. This is detailed in ref.[3]. Consequently,
only a short recall of the bench principle is reported below.

The poling bench mainly consists in a high voltage
amplifier used to submit the LiNbO₃ wafer to an electric
field strong enough to invert the native poling of the plate.
To achieve such an operation, one needs the use of optical
grade Z-cut LiNbO3 plates. Wafers are cut in the same
boule to well control the poling condition. A photoresist
mask is achieved atop one wafer surface, which will control
the poling location. An lithium chloride electrolyte is used
to drive the poling high voltage atop the wafer surfaces. A
dynamic poling sequence is then imposed to the wafer,
progressively reaching the coercitive field of lithium
niobate. An evidence of succesfull poling is obtained by
measuring the current of the whole electrical system. Once
evidence of transient current measured, the device is
considered to be poled. Following this sequence, and
providing no short circuit occurs, an almost perfect poling
can be achieved. Figure 2 shows a principle scheme of the
poling bench.

<figure><img src="image_3.png" /><figcaption>Figure 2. Scheme of the ferroelectric plate poling bench</figcaption></figure>

B. *Design of the device*

The fabricated devices correspond to periodic transducers
exhibiting periods *p* equal to 400µm, 200µm and 100µm
respectively, with a volume fraction ideally equal to 50%.
The number of period of the corresponding transducers is
100, 200 and 500, yielding quite long transducers preventing
the fabrication of more than three devices on a 3" wafer. The
acoustic aperture was 10 mm for each device in order to
avoid any diffraction effect. This kind of structure is
assumed to only exhibit symmetrical modes, as there no
source of geometric nor material-related asymmetries. It is
expected to behave in a very close way to the one simulated
by our own periodic FEA code which is devoted to the
simulation of infinite periodic transducers [4]. We also
assume that edge effects should remain small enough to
prevent major signal pollution, as there is no in-phase
reflection effects on the wafer sides. Figure 3 shows the
implantation of the periodically poled sequence on the wafer
and of the top side electrodes (the wafer backside is totally
metallized). Note that the backside electrical contact is
reported on the top surface thanks to the wafer sides
metallization naturally generated by the sputtering process.

<figure><img src="image_4.png" /><figcaption>Figure 3. Implantation of the periodically poled sequences and of the top side electrode on the Z-cut LiNbO₃ wafer</figcaption></figure>

The dispersion curve of Lamb modes on a Z-cut LiNbO₃
plate has been computed first along the Green's function
approach and is plotted in fig.4, giving a clear idea of at least
the velocity of the first symmetrical mode (the so-called S₀
mode, assumed to be the most suited to be excited by our
transducer). This allows us to point out the operating
frequency of the first mode for each of our device
configuration.

<figure><img src="image_5.png" /><figcaption>Figure 4. Dispersion curve of Lamb waves on a Z-cut LiNbO₃ plate</figcaption></figure>

A simple geometric construction gives the operating
frequencies for each excited Lamb mode. For a plate
thickness *h*, the slope *m* of a straight line passing through the
origin is equal to the wavelength-plate thickness ratio
(*m*=*λ*/(*h*). In our case, the values of the slope *m* are 0.2, 0.4
and 0.8 respectively. Projecting the corresponding line on the
Lamb wave dispersion curves allows for the prediction of the
wave velocity and hence the operating frequency for each
mode. Since the values of *m* are always smaller than 1 (i.e.
the frequency-thickness products corresponding to the test
device are in the range 5-20 km.s⁻¹), we could not take
advantage of the high velocity of the S₀ mode. As a
consequence, the first coupled mode on our device should
always exhibit a wave velocity close to 3500 m.s⁻¹, yielding
operating frequencies respectively equal to about 8.5 MHz,
17 MHz and 35 MHz. As it will be shown further, this first
order prediction nicely meet the experimental values.

### III. EXPERIMENTAL RESULTS

In this section, we present the electrical measurements
obtained using a Süss Microtec RF probing bench using the
so-called Z-probes connected to a Rhode & Schwarz ZVRC
network analyzer. The electrical admittances of the test
devices have been measured and are compared to the
harmonic admittance of the corresponding configuration.
These results are reported in fig. 5(a,b,c) for the three
considered configurations. In fig.5c, we have reported the
response of an un-poled device, as only the bulk wave
signature and its odd harmonics appears on the experiments.
We have no explanation to propose for that result, since all
the transducers were processed at the same time.

<figure><img src="image_6.png" /><figcaption>Figure 5. Comparison between theory and experiment for the three test vehicles (a) period *p*=100µm (b) *p*=200µm (c) *p*=400µm (poling failed)</figcaption></figure>

We have related the contribution on the experimental
curves near 7 MHz of fig 5a&b to parasitic bulk waves due
to the actual volume fraction of positively and negatively
poled domains of the period. In the case the volume fraction
is not rigorously equal to 50% (for instance 40% Z+ / 60%
Z- within the period), the longitudinal bulk wave and its odd harmonics (see the contribution at 21 MHz on fig. 5a) can be weakly excited. This was simulated for the 100µm period device and is reported in fig. 6.

<figure><img src="image_7.png" /><figcaption>Figure 6. Influence of a volume fraction defect on the transducer response (40/60% instead of 50% as expected)</figcaption></figure>

Thanks to the very good agreement between theory and experiments on fig 5a&b, one can easily identify the shape of the mode of the different Lamb-wave contributions, allowing to select one of those for its particular properties and polarization. Fig.7 (a,b,c) shows some examples of modes excited in the plate for a 200µm period transducer. It can be seen on these graphs that elliptically polarized modes as well as almost purely longitudinal waves can be excited in the plate thanks to our PIT.

<figure><img src="image_8.png" /><figcaption>Figure 7. Shape of given modes of the 100µm period transducer (fig. 5b)
(a) fundamental at 19 MHz (b) mode#6 at 28 MHz (c) mode#8 at 34 MHz</figcaption></figure>

The last experimental work that has been achieved for this paper is to test the robustness of the polarization to lapping and polishing techniques. These operations are required in order to fabricate LiNbO3/Silicon wave-guides, as expected as a future issue of our work. We consequently have lapped down the plate to a thickness of 250µm and then achieved new admittance measurements. Again, the very nice theory/experiment agreement is met as shown in fig.8, proving that the poling is strong enough to persist even after such treatments.

<figure><img src="image_9.png" /><figcaption>Figure 8. Theory/experiment assessment after lapping the plate down to 250µm, 200 µm period PIT</figcaption></figure>

## IV. CONCLUSION

PIT have been successfully manufactured on LiNbO₃ Z-cut plate, allowing for a very nice demonstration of the piezoelectric inter-digital transducer's operation on such material. Almost all the contributions to the electrical admittance of different devices have been predicted by FEA, allowing for the mode shape identification. The poling is found very resistant to different technological treatment, allowing for any kind of process sequencing. This will be exploited for the fabrication of wave-guide consisting of Z-cut lithium niobate plates on silicon.

## ACKNOWLEDGMENT

This work has been achieved in the Dominos program framework, funded by the European Community as the InterReg project DOMINOS.

## REFERENCES

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[2] A. K. Sarin Kumar, P. Paruch, J.-M. Triscone, W. Daniau, S. Ballandras, L. Pellegrino, D. Marré, T. Tybell, A High Frequency Surface Acoustic Wave Device Based on Thin Film Piezoelectric Inter-digital Transducers, Appl. Phys. Let., Vol. 85, p.1717, 2004

[3] L. E. Myers, R. C. Eckardt, M. M. Fejer, R. L. Byer, W. R. Bosenberg, J. W. Pierce, "Quasi-phase-matched optical parametric oscillator JOSA B, Vol. 12, Issue 11, pp. 2102- (November 1995)

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