Mode conversion in high overtone bulk acoustic wave resonators

M. Pijolat,A. Reinhardt,B. Ghyselen,M. Aid,C. Deguet,S. Ballandras,D. Mercier,J.S. Moulet,E. Defay

doi:10.1109/freq.2009.5168188

Abstract

High overtone bulk acoustic resonators (HBAR) have been realized using the Smart Cuttrade technology to transfer a thin X-cut LiNbO 3 layer onto an X-cut LiNbO 3 substrate. When the bonding of the two wafers is performed, an additional rotation along the normal axis is set to generate mode conversion between the two acoustic shear waves electromechanically coupled in X-cut LiNbO 3 . This enables excitation of only one of the two acoustic shear waves.

Highlights

As the demand for frequency sources or filters for embedded communication systems is still increasing, High Overtone Bulk Acoustic Resonators (HBAR) are considered as solutions for applications requiring very high quality factors - tens of thousands - at GigaHertz frequencies
On a similar structure we present in this paper - with a different purpose - we obtained 7.9.1013 on High overtone Bulk Acoustic Resonators (HBAR) realized thanks to the Smart CutTM technology applied to piezoelectric layers. 3 inches sub-micron thick X-cut lithium niobate layers were successfully transferred onto lithium niobate wafers [3,4]
We describe a method to convert the two shear waves piezoelectrically coupled in X-cut lithium niobate into a single one

Summary

INTRODUCTION

As the demand for frequency sources or filters for embedded communication systems is still increasing, High Overtone Bulk Acoustic Resonators (HBAR) are considered as solutions for applications requiring very high quality factors - tens of thousands - at GigaHertz frequencies. For Bulk Acoustic Wave resonators for example, sputtered AlN layers are widely known to be non single crystal Layer transfer techniques such as grinding polishing or Smart CutTM technology preserve the integrity of the single crystal materials [5,6,7]. As those layers are both used as transducers and propagating medium, the quality of the thin film has a major impact on performances. AlN grows along the c-axis, extreme difficulty appears when one wants growth along a more interesting orientation [8,9] This extra degree of freedom allows temperature compensated orientations, improved electromechanical coupling factor or the choice for shear or longitudinal waves propagating in the device. Sponsors: * Substrates were developed in the frame of the CEA-SOITEC joined program. * This work has been performed with the help of the “Plateforme technologique amont” de Grenoble, with the financial support of the “Nanosciences aux limites de la Nanoélectronique” Foundation

HBAR devices

X-cut LiNbO3

Smart CutTM process applied to piezoelectric media

Additional rotation for mode conversion

Principal

Simulations approach

Characterization

CONCLUSION