Effective electromechanical coupling coefficient of high-overtone bulk acoustic resonator

A high-overtone bulk acoustic resonator （HBAR） is composed of a substrate, a piezoelectric film and upper and lower electrodes, the influences of their structure parameter （thickness） and performance parameter （characteristic impedance） on effective electromechani- cal coupling coefficient K^2eff are investigated systematically. The relationship between K^2eff and these parameters is obtained by a lumped parameter equivalent circuit instead of distributed parameter equivalent circuit near the resonant frequency, and K^2eff at the resonance frequency closest to the given frequency is analyzed. The results show that K^2eff declines rapidly and oscillatorily with the continuous increase of the substrate thickness when the piezoelectric film thickness is fixed, and decreases inversely proportion to the thickness when the substrate thick-ness is greater than a certain value. With the ratio of the characteristic impedance of the substrate to the piezoelectric layer increasing, the maximum of K^2eff obtained from the vari- ation curve of K^2eff with the continuous increase of the piezoelectric film thickness decreases rapidly before reaching the minimum value, and later increases slowly. Fused silica with low impedance is appropriate as the substrate of HBAR to get a larger K^2eff. Compared with Al electrode, Au electrode can obtain larger K^2eff when the appropriate electrode thickness is selected. The revealed laws above mentioned provide the theoretical basis for optimizing parameters of HBAR.

PROPAGATION OF SURFACE ACOUSTIC WAVES IN PRESTRESSED ANISOTROPIC LAYERED PIEZOELECTRIC STRUCTURES

The propagation of surface acoustic waves in layered piezoelectric structureswith initial stresses is investigated. The phase velocity equations are obtained for electricallyfree and shorted cases, respectively. Effects of the initial stresses on the phase velocity and theelectromechanical coupling coefficient for the fundamental mode of the layered piezoelectricstructures are discussed. Numerical results for the c-axis oriented film of LiNbO_3 on a sapphiresubstrate are given. It is found that the fractional change in phase velocity is a linear functionwith the initial stresses, and the electromechanical coupling factor increases with an increase ofthe absolute values of the compressive initial stresses. The results are useful for the design ofsurface acoustic wave devices.

Propagations of Rayleigh and Love waves in ZnO films/glass substrates analyzed by three-dimensional finite element method

Propagation characteristics of surface acoustic waves（SAWs） in ZnO films/glass substrates are theoretically investigated by the three-dimensional（3D） finite element method. At first, for（11ˉ20） ZnO films/glass substrates, the simulation results confirm that the Rayleigh waves along the [0001] direction and Love waves along the [1ˉ100] direction are successfully excited in the multilayered structures. Next, the crystal orientations of the ZnO films are rotated, and the influences of ZnO films with different crystal orientations on SAW characterizations, including the phase velocity, electromechanical coupling coefficient, and temperature coefficient of frequency, are investigated. The results show that at appropriate h/λ, Rayleigh wave has a maximum k^2 of 2.4% in（90°, 56.5°, 0°） ZnO film/glass substrate structure; Love wave has a maximum k^2 of 3.81% in（56°, 90°, 0°） ZnO film/glass substrate structure. Meantime, for Rayleigh wave and Love wave devices, zero temperature coefficient of frequency（TCF） can be achieved at appropriate ratio of film thickness to SAW wavelength. These results show that SAW devices with higher k^2 or lower TCF can be fabricated by flexibly selecting the crystal orientations of ZnO films on glass substrates.

Stoney formula for piezoelectric film/elastic substrate system

With the trends in miniaturization, and particularly the introduction of micro- and nano-electro-mechanical system, piezoelectric materials used in microelectronic devices are deposited usually in the form of thin film on elastic substrates. In this work, the bending of a bilayer comprising a piezoelectric film deposited on an elastic substrate, due to the mismatch, is investigated. An analytic formula relating the curvature of the bilayer to the mismatch, the electroelastic constants and the film thickness is obtained, and from this formula, a transverse piezoelectric constant d31 can be estimated. Meanwhile the influence of electrornechanical coupling coefficient on the curvature is discussed.

Theoretical analysis and experimental validation of radial cascaded composite ultrasonic transducer

A radial cascaded composite ultrasonic transducer is analyzed.The transducer consists of three short metal tubes and two radially polarized piezoelectric ceramic short tubes arranged alternately along the radial direction.The short metal tubes and the piezoelectric ceramic short tubes are connected in parallel electrically and in series mechanically,which can multiply the input sound power and sound intensity.Based on the theory of plane stress,the electro-mechanical equivalent circuit of radial vibration of the transducer is derived firstly.The resonance/anti-resonance frequency equation and the expression of the effective electromechanical coupling coefficient are obtained.Excellent electromechanical characteristics are determined by changing the radial geometric dimensions.Two prototypes of the transducers are designed and manufactured to support the analytical theory.It is concluded that the theoretical resonance/anti-resonance frequencies are consistent with the numerical and experimental results.When R_(2) is at certain values,both the anti-resonance frequency and effective electromechanical coupling coefficient corresponding to the second mode have maximal values.The radial cascaded composite ultrasonic transducer is expected to be used in the fields of ultrasonic water treatment and underwater acoustics.

AlScN films prepared by alloy targets and SAW device characteristics

In this paper,we reported a surface acoustic wave(SAW)device prepared and optimized by piezoelectric films containing AIN,AIScN(Sc-20 at%)and AIScN(Sc-30 at%)by reactive magnetron sputtering using Al and AISc alloy targets.We calculated the material intrinsic electromechanical coupling coefficient k_(t)^(2) of AlScN(Sc-20 at%)and AlScN(Sc-30 at%)which are much better than AIN.It can be explained by the lattice softening.Furtherly,the results were confirmed by transmission electron microscopy(TEM)observation of the microstructure.Then the SAW devices based on three thin films were tested by vector network analysis obtaining the device equivalent electro mechanical coupling coefficient k_(eff)^(2).The value of AIScN(Sc-20 at%)k_(eff)^(2),which equals to 1.94%,is higher than that of AIN and AIScN(Sc-30 at%)while the value of AIScN(Sc-30 at%)k_(t)^(2) is higher than that of others.It is shown in our study that the crystallinity and orientation of the material still have a greater impact on k_(eff)^(2) but it does not have influence on k_(t)^(2) in the actual device preparation process.

Algorithm for the layout of a piezoelectric element in an elastic medium providing the maximal piezoelectric effect within a specified frequency range

An algorithm for the layout of a piezoelectric that provides the most efficient performance within a specified range of vibration frequencies is proposed in this paper.This algorithm is based on the consideration of a special parameter within the area of a piezoelectric element’s possible location.This parameter characterizes the superposition of electromechanical coupling coefficients’for all the natural vibration frequencies included in a specified frequency range.The condition for defining the best option for location of the piezoelectric element in the case of several equivalent positions is specified.The efficiency of the proposed algorithm is shown numerically.The electromechanical coupling coefficients were calculated numerically based on solution to the problem of natural vibrations for electroelastic bodies using a finite element method.The calculations were performed to define the best location for a single piezoelectric element at the surface of a thin-walled shell having a half-cylindrical shape.The results are presented for natural vibration frequencies within the frequency range from 0 up to 1100 Hz.The numerical results were obtained by solving the problem of natural vibrations with a finite element method using the ANSYS software package.