Acoustic resonator based on periodically poled transducers: Concept and analysis

Abstract

International audienceThe demand for highly coupled high quality acoustic wave devices for radio-frequency (RF) signal processing based on passive devices has generated a strong innovative activity, yielding the investigation of new excitation principles and waveguide structures. Periodically poled transducers (PPTs) have been recently investigated [E. Courjon et al., J. Appl. Phys. 102, 114107 (2007)], as an alternative to classical interdigital transducers (IDTs) for the excitation and detection of guided acoustic waves. PPTs have two principal advantages compared to IDTs: the robustness of the excitation versus defects or surface contamination and the possibility to excite waves exhibiting a wavelength equal to the poling period. Here a new acoustic resonator concept is suggested, allowing high frequency operation with a simplified package. The idea consists of using a waveguide based on a PPT fabricated on a ferroelectric single-crystal substrate such as lithium niobate or tantalate inserted between two single-crystal substrates allowing the guidance of elastic waves without losses. The concept analysis points out optimum configurations of the structure allowing the excitation of elastic waves compatible with RF applications

Highly coupled resonator based on ridge-shaped periodically poled materials for radio-Frequency applications

“…The device was indeed not poled nor excited on its stuck extremity, yielding it very close to a "freefree model". Experimental coupling coefficient of these devices is not as high as predicted but it is found 20 times larger than former LiNbO 3 PPTs [4]. Moreover, the experimental electromechanical coupling evolution follows the theoretical prediction, as shown in Table 1.…”

“…This kind of PPT was successfully manufactured as shown in [3] and [4], but the best experimental electromechanical coupling remained smaller than 1%. More specifically in [4], a Periodically Poled Lithium Niobate (PPLN) wafer was stacked between two silicon wafers to guide acoustic waves and to reduce the temperature coefficient of frequency [4], allowing for transducer working from 65MHz to 1.3GHz with a TCF of -50 ppm.K -1 , i.e. twice smaller than the TCF of PPT on pure LiNbO 3 plates.…”

“…However, these two materials are known to allow for much more coupled mode excitation, particularly concerning bulk waves. Even if advanced transducer combinations provide advantageous conditions for wave trapping [4], PPTs based on this basic architecture are not capable to overcome such coupling values (PZT -Lead zirconate titanate -was found theoretically capable of larger mode coupling, but it poorly suits RF requirements due to excessive intrinsic acoustic losses [5]). To overcome this limitation, various electrode configurations and excitation conditions have been investigated, resulting in a new device architecture based on a high aspect ratio structure (ridge) which allows for coupling higher than 20%.…”

New acoustic resonator based on a ridge built in a periodically poled ferroelectric material for RF applications

“…However, such a structure does not allow for reaching coupling factors larger than 1%, which is far to comply with RF filter requirements. The leading idea of the present work consists in testing a new configuration in which the excitation is achieved on the (YX) surface instead of (ZX) as performed in previous work [2]. Figure 1 illustrates both configurations.…”

“…For the same period on a device, the frequency of PPT is twice higher than IDT's one, that's why it could reach frequencies higher than Gigahertz. This new device electromechanical coupling is about 1% but it depends on the design of the PPT [2]. The main technological progress is a new design with high aspect ratio structure which allows for coupling coefficient higher than 20%.…”