Fabrication of periodically poled domains transducers on LiNbO3

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


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 LiNb03 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]  In the first section of the paper, the fabrication process is LiNb03 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 LiNb03 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   [ 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.  III.

EXPERIMENT AL RESULTS
In this section, we present the electrical measurements obtained using a Suss 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.Sc, 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. 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. 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 LiNb03/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.