Benfeng Zhang

Thin plate model for transverse mode analysis of surface acoustic wave devices

In this paper, we propose a physical model for the analysis of transverse modes in surface acoustic wave (SAW) devices. It is mostly equivalent to the scalar potential (SP) theory, but sufficiently flexible to include various effects such as anisotropy, coupling between multiple modes, etc. First, fundamentals of the proposed model are established and procedures for determining the model parameters are given in detailed. Then the model is implemented in the partial differential equation mode of the commercial finite element analysis software COMSOL. The analysis is carried out for an infinitely long interdigital transducer on the 128°YX-LiNbO3 substrate. As a demonstration, it is shown how the energy leakage changes with the frequency and the device design.

Frequency domain FEM analysis of reflector scattering characteristics for SAW tags

This paper proposes a frequency domain analysis (FDA) based on the finite element method (FEM) for calculating reflector scattering characteristics of SAW tags. This method is applicable not only to devices employing conventional single-crystal substrates but also multi-layered ones. First, reflection characteristics are calculated for a short reflector on 128°YX-LiNbO3 and compared with the result previously reported. Then the method is applied to short reflectors on the AlN/Si layered structure, and their applicability to SAW tags is discussed.

Modeling of lateral SAW propagation including coupling between different SAW modes

This paper describes lateral propagation of surface acoustic waves (SAWs) when two types of SAWs are coupled. For the analysis, the thin plate model proposed by the authors is extended to include interaction between two SAW modes. Firstly, lateral wave propagation in an infinitely long periodic grating is modelled and discussed, it is found that SAW slowness curves can be changed by mode coupling. Then, the model is applied to the analysis of two-dimensional SAW propagation on the Al grating on the 42°YX-LiTaO3 (42-LT) substrate structure to investigate how the lateral SAW propagation changes with the Al grating thickness. Finally, SAW resonators on 42-LT substrate with Cu electrodes are fabricated and the results confirm the feasibility of the model.

Influence of coupling with shear horizontal surface acoustic wave on lateral propagation of Rayleigh surface acoustic wave on 128°YX-LiNbO3

In this paper, we investigate the impact of the coupling with shear horizontal (SH) surface acoustic wave (SAW) on the propagation of Rayleigh SAW in periodic grating structures on 128°YX-LiNbO3. First, the frequency dispersion behavior with longitudinal and lateral wavenumbers of Rayleigh SAW is calculated using the finite element method (FEM) software COMSOL. It is shown that the coupling causes (1) the satellite stopband and (2) variation of the anisotropy factor. It is also shown these phenomena remain even when the electromechanical coupling factor of SH SAW is zero. Then, the extended thin plate model which can take coupling between two SAWs into account, is applied to simulate the result of FEM. Good agreement between these results indicated that the mechanical coupling is responsible for these two phenomena. Finally, including electrical excitation and detection, the model is applied to the infinitely long interdigital transducer (IDT) structure and the calculated result is compared with that obtained by the three-dimensional FEM. The excellent agreement of both results confirms the effectiveness of the extended thin plate model.

Wavenumber Domain Analysis of Surface Acoustic Wave Scattering from Localized Gratings on Layered Piezoelectric Substrate

This paper proposes a general finite element method (FEM)-based wavenumber domain analysis (WDA) to calculate scattering characteristics of surface acoustic wave (SAW) on arbitrary piezoelectric substrates. We add a damping loss mechanism (DLM) to the SAW injection port to avoid interferences from the incident and backscattered modes. After checking the validity of the proposed method, we calculate and study Sezawa mode scattering using a small number of electrodes on the ScAlN/3CSiC structure for demonstration. The frequency dependences of reflection and transmission coefficients and that of the power dissipation ratio for different termination conditions and electrode thicknesses are calculated. Also, the influence of base substrate materials and that of gratings on scattering parameters are explored. Investigation results demonstrate that high reflectivity with suppressed mode conversion can be obtained for the ScAlN-based layer structure if a base substrate with an extremely large velocity is used and if proper grating design is applied.

A new threshold determination algorithm for SAW resonant sensors

Wireless SAW resonant sensors have been widely used in many applications, especially in harsh environment. The sensor is detected when the echo signal spectrum processed by Fourier transform exceeds a threshold. Because of complicated noise and electromagnetic interference in wireless channel, the threshold should be adaptive to interference in order to detect the sensor accurately. However, few researches have been reported on the threshold determination of SAW resonant sensors. The only reported minimum identification error probability method for SAW RFID-tags needs prior probability density functions of both the background noise and the signal. In addition, since the resonance frequency of sensor and the reading distance are unknown when the reading system interrogates a SAW resonant sensor, it is difficult to estimate the signal-to-noise ratio (SNR) in real-time before a threshold is determined. A false alarm means the detection of a sensor when it actually does not exist, and it will cause wrong frequency estimation in the reading system; so the false alarm is more unacceptable than the miss-detection. According to the constant false alarm rate (CFAR) method in radar signal processing, a threshold determination algorithm for SAW resonant sensors is proposed, which pursues the detection probability to reach a maximum in complicated noise, and only the probability density distribution of noise is needed to estimate in the algorithm.

Impact of coupling between multiple SAW modes on piston mode operation of SAW resonators

This paper investigates piston mode operation (PMO) when two kinds of surface acoustic wave (SAW) exist and are coupled to each other. The extended thin plate model is used for the analysis. Firstly, a PMO condition is derived in an analytical and closed form from the model. It is found that PMO is possible without adding phase shifters when two SAW modes are coupled. Then, parameters defined in the model are extracted for various SiO2-overlay/Al-gratmg/128°YX-LiNbO3 (128-LN) substrate structures. It is shown that the PMO condition can be satisfied when the structure is properly designed. Finally, resonance characteristics of the designed structure are calculated by the extended thin plate model and 3D finite element method (FEM). The results of both methods agree with each other fairly well.

Influence of coupling between Rayleigh and SH SAWs on rotated Y-cut LiNbO 3 to their electromechanical coupling factor

Suppression of the SH-SAW response is mandatory to design TC SAW devices on θ degree rotated Y-cut LiNbO<inf>3</inf>. However, it is not easy because K² for SH SAW changes rapidly with θ, and its optimal value in terms of the SH-SAW suppression is dependent on electrode and SiO<inf>2</inf> thicknesses. The authors found that the optimal θ always locates close to θ where difference in velocities of two SAWs is minimum. This suggested that their coupling is responsible for variation of their K².

Modeling and Analysis of Lateral Propagation of Surface Acoustic Waves Including Coupling Between Different Waves

This paper discusses lateral propagation of surface acoustic waves (SAWs) in periodic grating structures when two types of SAWs exist simultaneously and are coupled. The thin plate model proposed by the authors is extended to include the coupling between two different SAW modes. First, lateral SAW propagation in an infinitely long periodic grating is modeled and discussed. Then, the model is applied to the Al-grating/42° YX-LiTaO3 (42-LT) substrate structure, and it is shown that the slowness curve shape changes from concave to convex with the Al grating thickness. The transverse responses are also analyzed on an infinitely long interdigital transducer on the structure, and good agreement is achieved between the present and the finite-element method analyses. Finally, SAW resonators are fabricated on the Cu grating/42-LT substrate structure, and it is experimentally verified that the slowness curve shape of the shear horizontal SAW changes with the Cu thickness.