High Frequency SAW Devices Research Articles

Shear wave velocity in a functionally graded piezoelectric semiconductor plate clamped on a rigid base

Abstract This paper investigates the propagation of horizontally polarized shear waves in a Piezoelectric Semiconductor (PSC)
layered structure. The modal consists of a pre-stressed PSC thin plate atop an elastic dielectric half-space joined
perfectly at the interface. It is postulated that the material parameters and initial stress exhibit an exponential
variation exclusively along the depth. The velocity equation of the considered wave is analytically obtained based
on the traction-free boundary conditions. Numerical examples have been employed to examine the influences of
several parameters, including semiconducting properties, material gradient index, initial stresses, external biasing
electric field, and PSC film thickness, on the characteristics of the wave. Graphs have been generated to visualize
the dependency of wave velocity and attenuation on these factors. The wave’s velocity and damping properties are
significantly influenced by the thickness and steady state carrier density of the PSC plate. Besides yielding critical
results, current findings are instrumental in designing high-frequency SAW devices.

Electro-Acoustic Properties of Scandium-Doped Aluminum Nitride (ScxAl1-xN) Material and its Application to Phononic Crystal-Coupled SAW Devices

Within the framework of the Density Functional Theory, the elastic, dielectric, and piezoelectric coefficients of w-ScxAl1−xN material were investigated for scandium (Sc) concentrations x = 0 to 0.375. The electro-acoustic properties are used to investigate the frequency response of the SAW delay line, based on the tilt θ° of the normal c-axis of the w-ScxAl1−xN piezoelectric thin film. We found that the piezoelectric response is improved as the Sc concentration increases, which is consistent with existing works in the literature. A 2D-phononic crystal pillars was then grafted on top of the surface, and the dependence of the acoustic band gaps is investigated with the help of the finite element method as a function of the Sc concentration and the tilted angle of w-Sc0.375Al0.625N. It was found that the two first band gaps exhibit a shift toward low frequencies with increasing Sc concentration. Moreover, the second acoustic bandgap is more sensitive to the inclination angle than the first. Furthermore, the insertion loss (S21) of w-Sc0.375Al0.625N is improved by 22 dB at θ° = 60°. The c-axis tilted Sc0.375Al0.625N-SAW delay line coupled with 2D-phononic crystals is a promising structure for low-loss and high-frequency SAW devices.

Research of micro area piezoelectric properties of AlN films and fabrication of high frequency SAW devices

Aluminum nitride (AlN) thin films were optimized on Pt/Ti/SiO2/Si (100) substrates by magnetron sputtering at different substrate temperatures (room temperature, 400 °C, 500 °C, and 600 °C). The analysis of X-ray diffraction (XRD) and atomic force microscopy (AFM) indicated that the optimal deposition temperature for AlN films with high (002) orientation, smallest value of rocking curve FWHM and superior grain distribution is 500 °C. Micro area piezoresponse force microscopy (PFM) results show that AlN samples deposited under 500 °C have the highest relative piezoelectric coefficient (d⁎33(max) = 0.79) and uniform piezoelectric performance. Moreover, interdigital transducers (IDT) of submicron scale (finger width 300 nm) were designed and fabricated by electron beam lithography (EBL) and lift-off process. Finally, the Pt/AlN/Pt two-port SAW device cell was constructed, with a device frequency up to 4.47 GHz, and an acoustic velocity of deposited AlN of approximately 5300 m/s.

Microstructure and piezoelectric properties of reactively sputtered highly C-axis ScxAl1-xN thin films on diamond-like carbon/Si substrate

We report deposition of pure AlN and ScxAl1-xN thin films with different Sc concentration on DLC/Si substrate by RF and DC reactive magnetron co-sputtering method by using Al and Sc as targets. The X-ray diffraction (XRD) results showed that the films have high c-axis-orientation. Electron probe microanalyzer (EPMA) analysis reveals the presence of scandium atoms reduce the space occupied by aluminum atoms, leading to lattice distortion during the phase transition. The SEM cross-section results showed that the ScxAl1-xN films are highly aligned along c-axis and have columnar like morphology. The top view of SEM results indicated that the new phase formation above Sc 30.33%, however no new peak appeared in the XRD pattern. The piezoelectric coefficient (d33) of ScxAl1-xN thin films are measured and the highest value (11.54 pC/N) is achieved at x=30.33% increasing 14 times, as that with AlN/DLC/Si, 0.73 pC/N. ScxAl1-xN films on DLC/Si substrate have a great potential to be applied on high frequency SAW devices in the future.

FEM simulation of AlN thin layers on diamond substrates for high frequency SAW devices

AlN/Diamond structure is very promising for high frequency acoustic wave devices. In this paper, a comprehensive investigation of AlN/Diamond structure is presented using finite element method. The phase velocity and the coupling coefficient of the first two modes of SAWs for the AlN/Diamond structure are numerically analyzed and compared to experimental data. Close agreement between experimental and numerical results is obtained. Results show that the mode 1 (Sezawa mode) exhibits the largest coupling coefficient of 1.28% associated with a phase velocity of 9500m/s. Additionally, the temperature coefficient of frequency (TCF) and the reflection coefficient (r) are studied. The simulation results show that for a zero TCF, a high phase velocity of 10,800m/s associated with a coupling coefficient of 0.5% can be obtained. These results demonstrate that the AlN/Diamond structure can be used to design wide-band and temperature-compensated SAW devices. The dependence of SAW devices performance with the electrode height, metallization ratio and mass-loading is also investigated.

AlN/Sapphire: Promising Structure for High Temperature and High Frequency SAW Devices

In this paper, the performance of AlN/sapphire structure in high frequencies is investigated. Several SAW devices are fabricated with various designs (free-space delay, wavelength, metallization ratio, etc.) to study simultaneously different parameters (acoustic velocity, electromechanical coupling (K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ), acoustic propagation loss (α), and temperature coefficient of frequency) versus frequency and temperature. Experimental results show that, as expected, α increases with temperature while K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> is enhanced at high temperatures. Because of the antagonistic evolution of these two parameters, insertion loss decreases or increases as function of the free-space delay. We also demonstrate that this structure allows fabrication of devices operating up to 1.5 GHz and that the frequency varies linearly with temperature.

Loss Reduction of Longitudinal-Type Leaky Surface Acoustic Wave by Reverse Proton Exchange

The longitudinal-type leaky surface acoustic wave (LLSAW) is one of the SAW modes that is advantageous for application to high-frequency SAW devices. However, the LLSAW has huge inherent attenuation. In this study, to reduce the attenuation of the LLSAW, we proposed a layered structure of air, a bulk LiNbO3 (LN) layer, and an elastically softened LN substrate. When the layered structure was applied to X–36°Y LN with a large coupling factor K2 for the LLSAW, the calculated attenuation for the metallized surface decreased at a certain depth of the bulk layer. To realize such a layered structure, a reverse proton exchange (RPE) process was applied to X–36°Y LN, and the propagation and resonance properties of the LLSAW were evaluated. A “good region” existed on the RPE wafer, in which the value of K2 for the Rayleigh wave was recovered to approximately 80% of that of the virgin wafer. In comparison with the virgin wafer, the propagation loss for the free surface of the good region on the RPE wafer was decreased threefold, the insertion loss was decreased drastically, and the resonance properties were improved markedly.

Preparation of AlN/Diamond Film Structure for High Frequency SAW Devices

Freestanding diamond film was prepared by hot filament chemical vapor deposition (HFCVD) method. Inter-Digital Transducer (IDT) was formed on the nucleation side of freestanding diamond film by photolithography technique. Then piezoelectric AlN film was deposited by RF reactive magnetron sputtering to obtain the AlN/diamond film structure. Surface morphologies of the nucleation side and the IDT were characterized by atomic force microscopy (AFM) and optical microscopy. The results indicated that the surface of nucleation side was very smooth and the IDT was of high quality without discontinuity and short circuit phenomenon. Room temperature frequency dependence of dielectric constant and dielectric loss for freestanding diamond film suggest a good stability of dielectric properties in high frequency region for diamond film. X-ray diffraction (XRD) pattern of the structure of AlN /diamond film structure showed a strong diffraction peak of AlN (002), which indicated that as-sputtered AlN film was highly c-axis oriented.

High frequency SAW devices based on third harmonic generation

We demonstrate the third harmonic generation in a ZnO/Si layered structure to obtain high frequency SAW devices. This configuration eliminates the need of high lithography resolution and allows easy integration of such devices and electronics on the same wafer. A theoretical study was carried out for the determination of the phase velocity and the electromechanical coupling coefficient (K2) dispersion curves of the surface acoustic waves. These results are also in agreement with those measured on a SAW filter designed for the third harmonic generation and the operating frequency is up to 2468MHz.

Preparation of free-standing diamond films for high frequency SAW devices

Free-standing diamond films were prepared by hot filament chemical vapor deposition (HFCVD) method under different conditions. Inter-digital transducers (IDTs) were formed on the nucleation sides of free-standing diamond films by photolithography technique. Then piezoelectric ZnO films were deposited by radio-frequency(RF) reactive magnetron sputtering to obtain the ZnO/diamond film structures. Surface morphologies of the nucleation sides and the IDTs were characterized by means of scanning electron microscopy (SEM), atomic force microscope (AFM) and optical microscopy. The results indicate that the surfaces of nucleation sides are very smooth and the IDTs are of high quality without discontinuity and short circuit phenomenon. Raman spectra show the sharp diamond feature peak at about 1 334 cm −1 and the small amount of non-diamond carbon in the nucleation side. X-ray diffraction (XRD) patterns of the structure of ZnO/diamond films show a strong diffraction peak of ZnO (002), which indicates that as-sputtered ZnO films are highly c-axis oriented.

Q values of longitudinal leaky SAWs propagating on rotated Y-cut LN substrates along the perpendicular to the X axis

Longitudinal leaky SAWs (LLSAWs), which can be excited in a thick aluminum (A1) grating on a rotated Y-cut lithium-niobate substrate, were analyzed in a simulation that combined both FEM and BEM. The Q-value dependence of an LLSAW on the rotated angle and the Al-grating thickness was clear. The simulation results were confirmed experimentally. These SAWs have potential applications in high-frequency SAW devices because their velocities range from 6100 to 6500 m/s and their optimum Q values are theoretically larger than 4000.

Investigation of new‐mode‐type SAWs by procedures combined with finite‐element and analytical method, and their applications for devices used in mobile communications

AbstractSimulations using a combination of the finite‐element method and the analytical method as well as experiments are used to study new‐mode‐type SAWs. First, in 171°YZé‐LiNbO3, it is found that there exists a longitudinal leaky SAW with a high acoustic velocity and a high Q in a grating structure with an Al film thickness of 0.084 wavelength. The acoustic velocity is 6100 to 6500 m/s and the Q values are more than 4000 (computed) at both the resonance and antiresonance points. These excellent properties are suitable for high‐frequency SAW devices. Next, in 15°YX‐LiNbO3, it is found that there exists a grating mode SAW of a transverse wave type in a grating structure with an Al film thickness of 0.125 wavelength. In this case, k2 has an extremely large value of 28.5% and hence the mode is suitable for a SAW resonator for use in a voltage‐controlled oscillator. By means of this SAW, SAW‐VCOs at 400 and 170 MHz for mobile communication are fabricated. At 400 MHz, the frequency variable width is 54 MHz (3.2%/V) and the phase noise is less than –115 dBc/Hz at 25 kHz; the corresponding values at 170 MHz are 24 MHz (3.4%/V) and &lt; –12 dBc/Hz at 25 kHz. © 2001 Scripta Technica, Electron Comm Jpn Pt 2, 84(4): 49–58, 2001

Propagation of longitudinal leaky surface waves under periodic metal grating structure on lithium tetraborate

The dispersion properties of longitudinal leaky surface waves propagating under the periodic Al strip grating on lithium tetraborate (Li(2)B(4)O(7); LBO) are described theoretically and experimentally for applications of the mode to high frequency SAW devices. A theoretical method developed here is based on Floquet's theorem using space harmonics as an orthogonal function set and real boundary integral equations derived from the method of weighted residuals for a period of each region, i.e., substrate, metal, and free space. The boundary integral equations are solved by using the Galerkin procedure. The periodic strip gratings with both single-electrodes and double-electrodes are investigated, considering the convergency of the numerical computation for the number of the space harmonics. As a result, the propagation loss for shorted gratings was found to be relatively low in the thickness range of the Al strip below about 1% for the single-electrodes and 2% for the double-electrodes, although it greatly increases for a thickness over 2% for the single-electrodes and 3% for the double-electrodes.

Sputtered Al–Ti Electrodes for High Frequency and High Power SAW Devices

DC magnetron sputtering and Ti addition for Al electrodes are studied to achieve higher durability of SAW devices operating at high frequency and high power. Experiments on SAW resonators as a test medium show the following: the sputtering is better than conventional e-gun evaporation for both the reproducibility of film composition and for durability; 0.6 wt% to 0.9 wt% Ti addition to the sputtered Al films leads to durability for 3 times larger power than 0.7 wt% Cu addition to evaporated Al films, while less loss is experienced if resistivity is the same in each film.