William Steichen

Design of coupled resonator filters using admittance and scattering matrices

The longitudinal coupling of thin film bulk acoustic resonator (FBAR) is investigated in order to design coupled resonator filter (CRF). Such devices are dedicated to high-frequency filter applications. A scattering matrix algorithm that was already used to simulate simple FBARs devices is used to study the behavior of these structures. The principle of the algorithm is recalled and is explained how it can be adapted to the simulation of multiport devices. Then an example of filter design is given.

Simulations of surface acoustic wave devices built on stratified media using a mixed finite element/boundary integral formulation

The demand for high frequency surface acoustic wave devices for modern telecommunication applications imposes the development of devices able to answer the manufacturer requirements. The use of high velocity substrates for which a piezoelectric layer is required to excite and detect surface waves has been widely investigated and requires the implementation of accurate theoretical tools to identify the best combinations of material. The present paper proposes a mixed formulation combining finite element analysis with a boundary integral method to accurately simulate the capability of massive periodic interdigital transducers to excite and detect guided acoustic waves in layered media. The proposed model is exploited for different typical configurations.

Finite-element analysis of periodic piezoelectric transducers

The need for optimized acoustic transducers for the development of high-quality imaging probes requires efficient simulation tools providing reliable descriptions of the behavior of real devices. The purpose of this work is the implementation of a finite-element model for the simulation of periodic transducer arrays. By using the assumption of harmonic excitation, the harmonic admittance of the studied structure can be derived. It is then shown how the mutual admittance is deduced from this feature, allowing one to estimate the amount of cross-talk effects for a given periodic transducer. Computation results are reported for standard linear acoustic probes, 2-2 (one-dimensional periodic) and 1-3 (two-dimensional periodic) piezocomposite materials. In the case of 2-2 connectivity composites, a comparison between nonperiodic and periodic computations of the mutual admittance is conducted, from which the minimum number of periods for which periodic computations can be trustfully considered can be estimated.

A FEA/BEM approach to simulate complex electrode structures devoted to guided elastic wave periodic transducers

A modelling approach able to address complicated SAW periodic structures with non homogeneous geometry has been developed and implemented. It is based on the combination of finite element analysis and a boundary element method. Validation of the computation is reported. An example of simulation of a passivated STW resonator is used for theory/experiment assessment.

High-frequency surface acoustic waves excited on thin-oriented LiNbO/sub 3/ single-crystal layers transferred onto silicon

The need for high-frequency, wide-band filters has instigated many developments based on combining thin piezoelectric films and high acoustic velocity materials (sapphire, diamond-like carbon, silicon, etc.) to ease the manufacture of devices operating above 2 GHz. In the present work, a technological process has been developed to achieve thin-oriented, single-crystal lithium niobate (LiNbO3) layers deposited on (100) silicon wafers for the fabrication of radio-frequency (RF) surface acoustic wave (SAW) devices. The use of such oriented thin films is expected to favor large coupling coefficients together with a good control of the layer properties, enabling one to chose the best combination of layer orientation to optimize the device. A theoretical analysis of the elastic wave assumed to propagate on such a combination of material is first exposed. Technological aspects then are described briefly. Experimental results are presented and compared to the state of art

Side radiation of Rayleigh waves from synchronous SAW resonators

Surface acoustic wave (SAW) resonators on lithium tantalate (LiTaOa) and lithium niobate (LiNbO3) are investigated. The amplitude of the acoustic fields in the resonators are measured using a scanning laser interferometer. The amplitude profiles of the surface vibrations reveal the presence of distinct acoustic beams radiated from the transducer region of the SAW resonators and propagating with low attenuation. We suggest that this radiation is generated by the charges accumulating at the tips of the finger electrodes. The periodic system of sources, namely oscillating charges at the fingertips, generates Rayleigh-wave beams in the perpendicular and oblique directions. Greens function theory is used to calculate the coupling strength and slowness of the Rayleigh waves on 42degY-cut LiTaO3 and Y-cut LiNbO 3 substrates as a function of the propagation direction. Furthermore, the propagation angles of the Rayleigh-wave beams as a function of frequency are calculated. The computed angles are compared with the measured ones for both the LiTaO3 and LiNbO3 substrates

FEM/BEM simulation and experimental study of LLSAW resonator characteristics on YZ-LiNbO/sub 3/

The high velocity (above 6000 m/s in LiNbO/sub 3/) of the longitudinal leaky SAW (LLSAW) mode makes it attractive for use in high frequency SAW devices in the 2-3 GHz range. We have investigated, both experimentally and theoretically, the dependence of one-port LLSAW resonator performance on YZ-LiNbO/sub 3/ on the metallization thickness and metallization ratio. The results indicate a strong dependence of the Q factor and resonance frequency on the aluminum thickness. The observed behaviour is addressed with the help of simulations using a combined FEM-Greens function method. Based on the simulated and experimental data, the optimal aluminum thickness is found to be approximately 8%.

Longitudinal leaky SAW resonators and filters on YZ-LiNbO/sub 3/

The high-phase velocity (above 6100 m/s in and aluminum (Al) grating on lithium niobate (LiNbO/sub 3/)) of the longitudinal leaky surface acoustic wave (SAW) (LLSAW) mode makes it attractive for application in high-frequency SAW ladder filters in the 2-5 GHz range. We investigate the dependence of one-port synchronous LLSAW resonator performance or YZ-LiNbO/sub 3/ on the metallization thickness and metallization ratio, both experimentally and theoretically. Our results indicate a strong dependence of the Q factor and resonance frequency on the aluminum thickness, with the optimal thickness that produces the highest Q values being about 8%. The optimal thickness increases with the metallization ratio. The observed behavior is interpreted with the help of simulations using a combined finite element method (FEM)/boundary element method (BEM) technique. As an application, bandpass filters have been fabricated in the 2.8 GHz frequency regime, based on LL-SAWs. The synchronous resonators constituting the ladder filters operate in the fundamental mode. The filters feature low insertion losses below 3 dB and wide relative passbands of 4.5-5%.

Investigation of spurious resonances in thin film bulk acoustic wave resonators

A finite element analysis is performed to simulate thin film bulk acoustic wave (FBAR) structures, and a coupled finite element analysis/boundary integral method (FEA/BIM) is used to simulate accurately a radiation medium, like a Bragg mirror underneath a resonator, without having to mesh it entirely. Displacement fields are extracted from these calculations and analysed. In particular, dispersion curves are obtained by a Fourier analysis of these fields and are used to explain phenomena arising at electrode edges, like standing wave resonances under the electrodes or mode conversions.

A new triply rotated quartz cut for the fabrication of low loss IF SAW filters

Analysis of the quartz properties shows the existence of unexplored angular domains for which Rayleigh waves can be efficiently excited, exhibiting physical characteristics better than the ones of the (ST,X) cut. This paper presents a family of quartz cuts allowing significant improvements of surface acoustic wave (SAW) devices on quartz. A first set of experiments has been performed, confirming the theoretical predictions of the basic properties of SAW on these cuts. A second set of measurements then was achieved to refine the identification of coefficients needed to perform industrial SAW design. A demonstration of the improvements accessible using this new cut is presented. A low loss SAW filter working at 71 MHz has been fabricated using smaller aluminum thickness than that for standard quartz cuts, and exhibiting all the properties required for its industrial implementation.