# An acoustic waveguide using doubly-bonded silicon/thinned PPT/silicon structures for RF applications

F. Bassignot, E. Courjon, G. Ulliac, Thierry Laroche, J. Garcia, S. Queste, J.-P. Romand, S. Ballandras, R. Petit

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## To cite this version:

F. Bassignot, E. Courjon, G. Ulliac, Thierry Laroche, J. Garcia, et al., An acoustic waveguide using doubly-bonded silicon/thinned PPT/silicon structures for RF applications. The 22nd European Frequency and Time IEEE International forum, Sep 2009, Rome, Italy. pp.923-926, 10.1109/ULT-SYM.2009.5441895. [hal-00485700](https://hal.science/hal-00485700v1)

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https://hal.science/hal-00485700v1

Submitted on 25 May 2021

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# An acoustic waveguide using doubly-bonded silicon/thinned PPT/silicon structures for RF applications

F. Bassignot, E. Courjon, G. Ulliac, T. Laroche, J.

Garcia, S. Queste, J-P. Romand, S. Ballandras

Institut FEMTO-ST

UMR 6174 CNRS-UFC-ENSMM-UTBM

26 che. de l'épitaphe, 25000 Besançon, France

sylvain.ballandras@femto-st.fr

R. Petit

CELAR

DGA

Rennes, France

**Abstract**—In this paper, we present new results on the development of piezoelectric transducers based on periodically poled ferroelectric domains in a lithium niobate plate bonded between two silicon wafers. The fabrication of the periodically poled transducers operating in the range 50–500 MHz has been achieved on a 3 inches 500 µm thick wafer. These devices then have been bonded on silicon wafers to fabricate a waveguide. Guided elliptic as well as partially guided longitudinal modes are excited. The experimental responses of the tested devices are compared to predicted harmonic admittances, showing a good agreement between both results and allowing for a reliable analysis of the nature of the excited modes. We also show interesting studies of material combinations used to guide ultrasonic waves. Dispersion properties have also been studied for a structure Si/PPT/Si. Operating points corresponding to a specific thickness/period ratio are found. Therefore a new conception with a Si/thinned PPT/Si structure is fabricated.

**Keywords-component:** Periodically poled Lithium Niobate; FEA-BEAM; waveguide; dispersion behaviour; isolated wave

## I. INTRODUCTION

Surface Acoustic Waves resonators may be used for the frequency stabilization of electrical oscillators until a few Gigahertzes (GHz). In some cases, these resonators are limited by technical and compactness specifications imposed by radar applications. Consequently, passive components using guided acoustic waves are well-suited for such an application. Periodically poled transducers (PPTs) have been investigated recently as an alternative to classical inter-digital transducers for the excitation and detection of guided acoustic waves. We propose here a new compact structure allowing high frequency operation with a simplified package as the wave is trapped in a sandwich structure based on Si/LiNbO3/Si material combination. We investigate the capability of such devices to answer the above specifications.

We explain the PPTs fabrication principle and present experimental results on 3" and 4" LiNbO3 and LiTaO3 wafers. We then report theoretical analysis results of the PPT operation obtained, using our finite and boundary element simulation tool. Thanks to the analysis of the transducer dispersion properties providing the optimum operating points and the temperature coefficient of frequency (TCF) analysis, we have

chosen the best combination Si/PPT/Si to fabricate test vehicles. Poled LiNbO3 wafers are first bonded on silicon (Si) using Au/Au compression, then thinned down to a few dozen microns. The resulting stack is metalized to process a second wafer bonding on Si. These devices exhibit a 50 µm period, yielding operating frequencies near 80 MHz. We have pointed out the existence of elliptically polarized guided modes for various material combinations and technological parameters. We also have identified the possibility to excite partially guided longitudinally polarized waves. The control of the ferroelectric material thickness has been achieved (lapping and polishing). The comparison between theoretical and experimental TCF shows that we are able to accurately predict the thermo-elastic behavior of the material stack.

## II. A NEW WAVEGUIDE CONCEPT: SUBSTRATE/PPT/SUBSTRATE:

In this part, we will first present a new waveguide concept. We present then the theoretical results achieved using our finite element/boundary element simulation tool [1], allowing the simulation of such material combinations. After that, the combination of material Si/PPT/Si is chosen (the easiest and the cheapest structure to fabricate) and the dispersion properties of this transducer are studied providing the best operating points.

### A. New waveguide concept

An innovative solution has been retained in order to answer to the need of high frequencies sources (RF until X band domain). These sources indeed need spectral purity, short-term stability and phase noise around the carrier wave. The purpose is to fabricate resonators based on the ultrasonic waves propagation through a waveguide. This resonator operates with a PPT structure inserted between two guiding substrates. A wave propagates without any acoustic losses and decreases exponentially in the structure (fig. 1). This definition is similar in these interface waves [2]. Nevertheless, the principle is substantially different for various reasons. First, the transduction structure corresponds to a layer in which ferroelectric domains have been artificially structured as
developed in [3-4-5]. Classical interdigital transducers have been given up in this kind of structure because the bonding is difficult due to the electrode pattern. In this case, a polymer film is deposited between both substrates to be bonded in order to create an elastic link introducing important acoustic losses.

<figure><img src="image_3.png" /><figcaption>Figure 1. Scheme of the new concept waveguide based on PPT</figcaption></figure>

## B. Analysis of several material combinations

The purpose of this analysis is to study influences on wave guidance and velocity. For that, we study two principal cases using LiNbO₃ and LiTaO₃ as ferroelectric material and we modify the guidance material (silicon, sapphire or diamond). Theoretical analysis results of these structures are obtained by means of our finite and boundary element simulation tool. Figures 2 and 3 show the theoretical admittance of LiNbO₃ and LiTaO₃ PPT respectively (p=50 µm).

<figure><img src="image_4.png" /><figcaption>Figure 2. (a) Scheme of the PPT structure (b) Theoretical admittance of a 50 µm period LiNbO₃ PPT for 30 or 60 µm thick for different guiding substrates</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Figure 3. Theoretical admittance of a 50 µm period LiTaO₃ PPT for 30 or 60 µm thick for different guiding substrates</figcaption></figure>

There are two main differences between LiNbO₃ and LiTaO₃ transducers. The first one is a better spectrum purity for the LiTaO₃ configuration in comparison with the LiNbO₃ configuration. The second one concerns the Rayleigh wave velocity of LiTaO₃ (3500 m.s⁻¹) which is lower than the one of LiNbO₃ (3800 m.s⁻¹), that is why we observe the same acoustic waves for both materials and for each guiding substrates but at a lower frequency for the LiTaO₃. The guidance conditions also depend on the ratio between the ferroelectric material thickness and the transducer wavelength (h/p). The elliptic mode actually is guided from a ratio h/p equal to 1. The structure diamond/PPT/diamond is the most guiding configuration thanks to the high propagation velocities in the diamond (12500 m.s⁻¹ for the Surface Skimming Bulk Wave (SSBW)). In fact, the quasi-longitudinal mode is excited and guided at a velocity around 6500 m.s⁻¹. The last structure consisting of a silicon/PPT/silicon stuck presents interesting guidance properties for a ratio (h/p) smaller than 1, this parameter is treated in the next part. In this structure, the elliptic mode is mainly excited at about 5100 m.s⁻¹ whereas the longitudinal mode at about 7300 m.s⁻¹ is not guided, as the SSBW velocity in the silicon is near 5800 m.s⁻¹. This configuration is retained for the rest of the study because the elliptic mode is well guided, the fabrication process is quite simple and cheaper than structures based on other guiding materials.

## C. Analysis of dispersion properties

We propose to closely investigate how to efficiently derive the dispersion curves and then to show at least for the elliptic mode of the waveguide the influence of the PPT geometry and the operating frequency on the mode properties. In that matter, we have considered the general shape of the transducer symbolized in fig.4. We assume here a propagation between two silicon layers but the approach can be generalized to any guiding substrate. The period is arbitrarily fixed to 50 µm (i.e. width of each LiNbO₃ domain set to 25 µm, poling ratio=0.5).

<figure><img src="image_6.png" /><figcaption>Figure 4. Scheme of PPT principle and generic shape for dispersion curve derivation</figcaption></figure>

In Figure 5, we have plotted the theoretical admittance (conductance and susceptance) versus frequency for a thickness/period (h/p) ratio equal to 0.1, in order to identify the main contribution corresponding to an elliptically shaped mode.
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Harmonic admittance of the structure plotted in fig.4 for a thickness/period ratio equal to 0.1</figcaption></figure>

Once the mode identified, we follow the evolution of its
resonance frequency and electromagnetic coupling coefficient
for h/p ratios ranging from 0.1 to 10, is plotted on fig.6.

<figure><img src="image_8.png" /><figcaption>**Figure 6.** Evolution of the elliptic mode synchronism frequency (red curve) and electromagnetic coupling (green curve) for small values of h/p ratio (ranging from 0.1 to 10 in our calculation)</figcaption></figure>

There is no ambiguity concerning the elliptic polarization of
the studied mode because there is no possible interaction with
other modes (as often observed for Lamb waves for instance).
This operating point is of very particular importance as it
conditions the possibility to improve the design reliability by
choosing operation condition poorly affected by technology,
which is a key point for any practical exploitation of the device.
The next section shows the different technological steps of a
device Si/PPT/Si fabrication.

### III. EXPERIMENTAL VALIDATION OF THE CONCEPT SI/PPT/SI AND CHARACTERIZATION

The concept has been experimentally checked first using a
thick structure consisting of a Silicon(380 µm thick)/LiNbO₃
PPT (500 µm)/Silicon (380 µm) material stack.

#### A. Fabrication

The fabrication of PPT-based waveguides consists in
bonding a silicon wafer on each side of the periodically poled
wafer. In that purpose, the lithium niobate wafer is bonded on a
(100) 3" 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.7b). Connection via are etched in each
silicon substrate by Deep Reactive Ion Etching (DRIE) in order
to reach the buried metallic electrodes. After that, the structure
is diced to obtain 5 separated devices. The process flowchart is
reported in fig.7a, showing the different implemented coatings
and the technological sequence.

<figure><img src="image_9.png" /><figcaption>**Figure 7.**
(a) Flowchart which summarizes the different steps of fabrication (b) Double bonding of a lithium niobate wafer with two silicon wafers using room temperature Au/Au bonding process</figcaption></figure>

A test vehicle is mounted on a metallic surface (printed
circuit) to connect electrically the structure with the support
and a SMA port is added to connect the support to a RF
spectrum analyzer (fig.8).

<figure><img src="image_10.png" /><figcaption>**Figure 8.** Test vehicle of a Si/LiNbO₃ PPT/Si device</figcaption></figure>

#### B. Characterization and temperature sensibility

Figure 9 presents the comparison between the experimentally measured responses of the implemented devices and the theoretical harmonic admittances obtained with our periodic finite element code. Measurements have been achieved successfully for the 50 µm period devices, as explained above. Since the implemented single-port test devices are quite long and almost behave as single port resonators, the comparison between measurement and harmonic admittance results makes sense.

<figure><img src="image_11.png" /><figcaption>**Figure 9.** Comparison between the test vehicle admittance and the theoretical admittance</figcaption></figure>
This plot points out the good agreement between both results. We find two main contributions corresponding to the elliptical mode and the longitudinal mode. We can notice that the experimental admittance increases gradually. The main cause seems to be the wire length of the connection between the device and the support. This proves the feasibility of LiNbO₃-based Silicon/PPT/Silicon devices.

We also have measured the experimental admittance of the longitudinally polarized wave at different temperatures (between 278 and 323 K) of the test vehicle. The theoretical and the experimental Temperature Coefficient of Frequency (TCF) are in the same order of magnitude at around -80 ppm/K. We can notice that the TCF is sensibly the same than the one of isolated LiNbO₃ devices. So the waveguide concept based on a double silicon bonding on a thick PPT substrate (about 500 µm) has been validated. Now, the purpose is to use the ultrasonic wave excited to do a resonator for instance. Thus, following the dispersion properties modelling for a thin PPT layer, the wave is well coupled and the Q factor is high. So that, the next part presents the fabrication of a Si/thinned PPT/Si waveguide.

## IV. DOUBLE BONDING SI/THINNED PPT/SI: FABRICATION AND CHARACTERIZATION

The analysis of the dispersion properties of a device constituted by Si/PPT ($\lambda$=50µm)/Si has shown a specific operating point for which a single wave is excited with a maximum electromagnetic coupling.

### A. Fabrication

Technological steps have already been experimentally checked using a thick structure consisting of a Silicon(380 µm thick)/LiNbO₃ PPT (500 µm)/Silicon (380 µm) material stack. The study of the dispersion properties enables to define a specific configuration using a thinned PPT layer of about 30 µm. The thinning down is achieved by home-made lapping and polishing techniques. However, the layer obtained can have surface defaults due to dust presence at the bonding interface. Consequently, the structure can only be partly bonded.

### B. Characterization and comparison with theory

Test vehicles have been achieved and characterized with a network analyzer. Figure 10 presents the comparison between the measured responses of the implemented devices and the theoretical harmonic admittances obtained with our periodic finite element code. Measurements have been achieved successfully for the 50 µm period devices, as explained above. Since the implemented single-port test devices are quite long and almost behave as single port resonators, the comparison between measurement and harmonic admittance results makes sense.

<figure><img src="image_12.png" /><figcaption>Figure 10. Comparison between the test vehicle admittance and the theoretical admittance</figcaption></figure>

This plot points out the good agreement between both results. We find only one contribution (except bulk waves) corresponding to the isolated wave. We can notice that the experimental admittance decreases gradually as explained above for a thick structure. The parasitic bulk waves seem to come from the electrode shape which is bigger than the transducer.

## V. CONCLUSION

We have pointed out the existence of elliptically polarized guided modes for various material combinations and technological parameters. We also have identified the possibility to excite partially guided longitudinally polarized waves. The control of the ferroelectric material thickness has been achieved (lapping and polishing). Several devices have been successfully fabricated with different thicknesses and tested. As predicted, we have measured both contributions when the PPT is thick and only one contribution when the PPT is thinned. Besides, we prove that the final device can be packaged easily in a low cost housing.

## ACKNOWLEDGMENT

This work was supported by the french DGA (Délégation Générale pour l'Armement) under grant #07-34-020.

## REFERENCES

[1] S. Ballandras, R. Lardat, M. Wilm, Th. Pastureaud, A. Reinhardt, N. Champavert, W. Steichen, W. Daniau, V. Laude, R. Armati, G. Martin, "A mixed finite element/boundary element approach to simulate complex guided elastic wave periodic transducers", accepted for publication in Journal of Applied Physics, 2008

[2] H. Majjad, D. Gachon, V. Laude, S. Ballandras, "Interface acoustic wave devices made by indirect bonding of lithium niobate on silicon", Proc. of the IEEE US, 2006

[3] E. Courjon, J. Hauden, J. Masson, D. Gachon, L. Gauthier Manuel, W. Daniau, N. Bodin, S. Ballandras, "Pure longitudinal plate mode excited by poled domains transducers on LiNbO₃", Proc. of the IEEE IFCS, 2007

[4] E. Courjon, J. Hauden, P. Parruch, J. Masson, D. Gachon, L. Gauthier Manuel, W. Daniau, N. Bodin, J.M. Triscone, S. Ballandras, "Fabrication of periodically poled domains transducers on LiNbO₃", Proc. of the IEEE IFCS, Miami, 2006

[5] S. Ballandras, W. Daniau, B. Gautier, D. Hauden, M. Wilm, V. Laude, V. Ruch, S.Flambart, "A novel surface wave transducer based on periodically poled piezoelectric domain", Proc of the IEEE EFTF-IFCS, 2003