Wideband Surface Acoustic Wave Research Articles

Enlarged phase velocities of ultra-wideband surface acoustic wave devices with relaxor based ferroelectric single crystal/diamond layered structure

The surface acoustic wave (SAW) characteristics of Y-cut X propagating Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (YX-PIMNT) single crystals on a diamond substrate have been theoretically calculated. The simulated results show that the phase velocity of the shear horizontal (SH) SAW may be greatly enhanced from the 1350m/s to 3350m/s by reducing the thickness of the PIMNT from 0.5λ to 0.05λ, with a corresponding decrease in the electromechanical coupling factor (K2) from 73.6% to 19.6%. The dispersion curves of phase velocity and K2 as a function of PIMNT thickness are given for the proposed layered structure. Besides the SH SAW, there are also higher order modes that would cause unwanted responses in the pass-band of wideband SAW filters. These were suppressed by properly controlling structural parameters including top electrode thickness, thickness and Euler angle (θ) of PIMNT substrate. The calculated results demonstrate the effectiveness of this approach to enlarge the phase velocity of the SH SAW without dramatically sacrificing its K2, which makes relaxor based ferroelectric single crystals promising for realizing ultra-wideband SAW devices working in ~GHz range.

Theoretical analysis of surface acoustic wave propagating properties of Y-cut nano lithium niobate film on silicon dioxide

The surface acoustic wave (SAW) propagating characteristics of Y-cut nano LiNbO3 (LN) film on SiO2/LN substrate have been theoretically calculated. The simulated results showed a shear horizontal (SH) SAW with enhanced electromechanical coupling factor K2 owing to a dimensional effect of the nanoscale LN film. However, a Rayleigh SAW and two other resonances related to thickness vibrations caused spurious responses for wideband SAW devices. These spurious waves could be fully suppressed by properly controlling structural parameters including the electrode layer height, thickness, and the Euler angle (θ) of the LN thin film. Finally, a pure SH SAW was obtained with a wide θ range, from 0° to 5° and 165° to 180°. The largest K2 achieved for the pure SH SAW was about 35.1%. The calculated results demonstrate the promising application of nano LN film to the realization of ultra-wideband SAW devices.

Enhanced coupling factor of surface acoustic wave devices employing ScAlN/diamond layered structure with embedded electrodes

A layered structure ScAlN/diamond configured with embedded electrodes is studied for high frequency and wide band SAW devices. Theoretical studies are performed to calculate the phase velocity and K2 dispersion curves of the Rayleigh mode and its higher modes. The influences of various ScAlN film thickness and electrode thickness as well as different electrode materials on K2 are also investigated. The maximum electromechanical coupling factor (K2) of surface acoustic wave (SAW) resonator based on such structure has been theoretically shown up to be 14.5%, which is about 2.5 times larger than the previously reported. Such an enhancement of K2 makes this layered structure great potential application to ultrawide band SAW filters operating in 2–3GHz range.

FEM simulation of Rayleigh waves for SAW devices based on ZnO/AlN/Si

In this study, the propagation characteristics of surface acoustic waves (SAWs) in ZnO/Si and ZnO/AlN/Si structures are investigated. The phase velocity, the coupling coefficient and the temperature coefficient of frequency (TCF) of the first two modes of SAWs for the ZnO/Si structure are numerically analyzed and compared to experimental data. The results show that a large coupling coefficient of 3.5% associated with a phase velocity of 4600m/s and a TCF of −42.8ppm/°C are obtained in mode 1. Close agreement is achieved between the numerical and experimental results.Inserting an AlN layer between the ZnO and Si layers is shown to increase the performance of the SAW resonator. The simulation results indicate that for mode 0, a temperature compensation of 0ppm/°C and high velocities of up to 4600m/s are obtained by optimizing the AlN/ZnO thickness ratio. These results demonstrate that the ZnO/AlN/Si structure can be used to design wide-band and temperature-compensated SAW devices.

Cost-effective and high frequency surface acoustic wave filters on ZnO:Fe/Si for low-loss and wideband application

ZnO:Fe films with various piezoelectric coefficient d33 were employed to fabricate high frequency surface acoustic wave (SAW) filters on Si by conventional photolithography technology. Comparing with the SAW filters on undoped ZnO films, the electromechanical coupling factor and bandwidth, respectively, increases 75.7% and 14.8%, while the insertion loss decreases 20.3% when using Zn0.988Fe0.012O films with a d33 of ∼127 pC/N. Through annealing demonstration, d33 is considered as the intrinsic factor determining the SAW properties of filters, and the properties are improved by O2 annealing. ZnO:Fe films with enhanced d33 is promising for high frequency, low-loss, and wideband SAW applications.

Surface acoustic wave characteristics of a (100) ZnO/(100) AlN/diamond structure

Propagation characteristics of surface acoustic waves (SAWs) in the (100) ZnO/interdigital transducer (IDT)/(100) AlN/diamond structure are investigated in this study. The temperature coefficient of delay (TCD), the phase velocity, and the coupling coefficient of the first three modes of SAWs for different thicknesses of (100) ZnO and (100) AlN are theoretically analyzed. Results show that the second mode (Sezawa mode) exhibits the largest coupling coefficient. In the second mode, the largest coupling coefficient of 6.05% associated with a phase velocity of 6234m/s and a TCD of 20.9ppm/°C can be found; for a zero TCD case, a large coupling coefficient of 5.35% associated with a phase velocity of 6460m/s can be obtained. Those coupling coefficients and phase velocities are better than the ones in the IDT/(100) ZnO/diamond structure that possesses a large positive TCD. Simulation results indicate that the (100) ZnO/IDT/(100) AlN/diamond structure exhibits a high phase velocity, a large coupling coefficient, and a small TCD, which can be used to design the wideband and temperature-compensated SAW devices for gigahertz-band applications.

Ultra-wideband surface acoustic wave resonator employing Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 crystals

Theoretical analysis showed that high performance surface acoustic wave (SAW) resonator with an extremely large electromechanical coupling factor K2 could be realized on Y cut X propagating Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (YX-PIN-PMN-PT) substrate. A one-port SAW resonator was designed, fabricated, and characterized on Cu-grating/YX-PIN-PMN-PT substrate structure. The experimental results demonstrate that very large K2 of 57.3% is realizable. Such an extremely large K2 makes PIN-PMN-PT single crystal great potential for realizing wideband SAW filters and tunable SAW filters. The influence of inhomogeneity and instability of the domain structure on the fabricated resonators has been discussed thoroughly for further improvement of the devices.

Influence of oxygen on the elastic properties of nanocrystalline diamond films studied by laser-induced surface acoustic waves

Laser-induced surface acoustic waves (SAWs) were used to study the influence of oxygen on the elastic properties of nanocrystalline diamond films. A series of samples was grown by chemical vapor deposition (CVD) on the (1 0 0) plane of p-type silicon for 3 h under different O 2/(CH 4 + H 2) flow ratios from 0% to 8%. The elastic properties of these nanocrystalline diamond films were determined from the SAW dispersion curves. The maximum frequency realized was about 310 MHz in wideband SAW experiments and about 700 MHz using a narrowband mask method with several higher harmonics. It was found that the densities of all samples were surprisingly high, approaching the ideal value of diamond, whereas the Young’s modulus increased from 700 to 950 GPa with the addition of oxygen. It is concluded that oxygen has a significant positive effect on the elastic properties of nanocrystalline diamond.

A novel matching network employing surface acoustic wave devices for W-CDMA power amplifiers

This paper describes a new approach of designing high Q surface acoustic wave (SAW) resonators as an inductive element in the matching network for W-CDMA power amplifiers (PAs). Spiral inductors based on CMOS/BiCMOS technologies presently possess relatively low Q (typically <10) and occupy a considerably large area. In order to break through the limitations of the spiral inductors, the authors attempt to apply higher Q and wideband SAW resonators employing Cu-grating/15°YX-LiNbO 3-substrate structure to the matching network for improved PA performance. An analysis was made on SAW resonators in detail, and an SAW resonator having a very small capacitance ratio of 3.28 and moderate Q of 147.8 was developed. After discussing the frequency dependence of the effective inductances, SAW resonators, which are used to be as inductive elements in the matching networks of PAs, were designed and fabricated. The PA including the matching circuit was simulated using the characteristics of the fabricated SAW resonators. The result showed that with better shape factor and good out-of-rejection, the SAW resonators definitely work as an inductive element and could replace widely used spiral inductors.

A novel weighted method for layered SAW filters using slanted finger interdigital transducers

In this paper, we propose a novel weighted method to improve the inclined pass-band problem that is intrinsic to surface acoustic wave (SAW) filters with slanted finger interdigital transducers (SFITs). By the dispersive characteristics of SAW in layered piezoelectric substrates, the present method could make the inclined pass-band flatter or the bandwidth wider. We derived a coupling-of-modes model for the calculation of the frequency responses of layered SAW devices with SFITs and calculated the dispersive characteristics of an AlN/silicon layered piezoelectric substrate, such as the phase velocity and the electromechanical coupling coefficient. Then we utilized the dispersive electromechanical coupling coefficient to flatten the inclined pass-band and the dispersive phase velocity to widen the bandwidth. Two designed examples of wide-band layered SAW filters using SFITs on AlN/silicon substrates are presented to prove the validity of the proposed method: one is designed to flatten the inclination of the pass-band; the other, to widen the bandwidth. Results show that the dispersions of layered piezoelectric media are favourable for improving the performance of SFIT-based SAW filters.

Evaluation of elastic properties of submicrometer thin films using slanted finger interdigital transducers

Accurate evaluation of film properties with thickness in the submicrometer range is very important throughout microelectronic industry as well as nano/microelectromechanical system industry. In this study, we developed a nondestructive evaluation technique, which is not only suitable for dielectric films but also valid for metallic films, to measure the elastic properties of submicrometer thin films. Firstly, we established the forward solution of the inverse evaluation and analyzed the dispersion of surface acoustic wave (SAW) in a SiO2∕YZ–LiNbO3 coated solid based on the effective permittivity approach. To measure the dispersion of SAW in the coated solid, a slanted finger interdigital transducer (SFIT) was employed to generate wide band SAW signals. The SFIT was designed by using the coupling of modes method to obtain the optimal frequency response. SiO2 films with submicrometer thickness were deposited on the piezoelectric YZ–LiNbO3 substrate via the plasma-enhanced chemical-vapor deposition process. Pairs of the SFITs were also fabricated on the substrate. The measured frequency responses were then processed using the spectral analysis of surface wave to obtain the SAW dispersion in the coated solid. Finally, based on the forward solution and measured dispersion, we determined inversely the elastic properties of the SiO2 film through the use of the Simplex algorithm. The inversion result shows that the elastic properties of the submicrometer SiO2 film were measured successfully. We note that the results of this study provide an important basis for developing a SAW sensor which can be adopted to measure in situ film properties.

New Low-Distortion Band-Switching Techniques for SAW Antenna Duplexers Used in Ultra-Wide-Band Cellular Phone

A new low-distortion band-switching technique for a surface-acoustic-wave (SAW) antenna duplexer used in ultra-wide-band cellular-phone systems has been developed. Both a transmitter (Tx) portion and a receiver (Rx) portion of the duplexer consist of not only wide-band SAW bandpass filters (BPFs), but also band-rejection SAW filters (BRFs) whose rejection bands can be shifted by switching circuits. The spurious-response sensitivity caused by the third-order nonlinear distortion of the switching circuits was dramatically improved by the new above configurations, i.e., the combination of BPFs and BRFs. The developed duplexer for the 900-MHz Japanese cdmaOne system has a size of 10 /spl times/ 8 /spl times/ 2 mm. It also has frequency characteristics of equivalent transmitted/received frequency bands as wide as 38 MHz and the guard band as narrow as 17 MHz. The insertion losses in both the low and high bands of the Tx portion are less than 2 dB, and those of the Rx portion are less than 3.3 dB. The other required specifications, e.g., attenuation at harmonic frequencies, attenuation at the image frequency, etc. are also satisfied.

Differential surface acoustic wave IDT gyroscope

This paper presents the design and experimental evaluation of a gyroscope based on differential design of a surface acoustic wave (SAW) resonator and a sensor on a piezoelectric substrate. The resonator is divided into two halves, one with metallic dot arrays and other without any metallic dots. Standing waves are formed inside both cavities. The particles at the antinodes of the standing wave pattern experience large amplitude of vibration that serves as the reference vibrating motion for this gyroscope. The metallic dots strategically positioned at the antinode locations experience Coriolis force due to rotation and acoustically amplify the magnitude in the orthogonal direction. However, the other half of the resonator cannot generate any secondary SAW. A wideband SAW sensor arranged orthogonal to the SAW resonator picks up these two secondary SAWs and is fed to a differential amplifier. Any drift in the gyroscope signal can be completely eliminated by this novel design. The performance of this 74.2-MHz gyroscope shows very high sensitivity and dynamic range, which is ideal for many commercial applications. Unlike other MEMS gyroscopes, this gyroscope has a planar configuration with no suspended resonating mechanical structures, thereby being inherently robust and shock resistant.

Research of Super-High Electromechanical Coupling Surface Acoustic Wave Substrates

The properties of surface acoustic wave (SAW) devices mainly depend on the choice of piezoelectric substrate materials. The important properties required for SAW substrates are a large electromechanical coupling coefficient (k2), small temperature coefficient of delay (TCD), and low propagation loss, etc. Especially, super-high coupling substrates are very important for obtaining wide-band SAW filters, wide-band voltage controll oscilater (VCO) and delay lines with small insertion loss. KNbO3 single crystal has an extremely large electromechanical coupling coefficient (k2=53%) and zero TCD at approximately room temperature. Large-scale single-crystal growth technologies are now under investigation. In this study, the propagation characteristics of SAW in PZNT and KTP substrates are investigated experimentally in order to obtain higher coupling substrates than those of KNbO3. The experimental results show that the substrates have large k2. Also, theoretical SAW propagation characteristics with a k2 of about 100% are analyzed and their physical phenomena are discussed.

Single crystal growth of KNbO3 and application to surface acoustic wave devices

Potassium niobate (KNbO 3 ) has large piezoelectric constants. The surface acoustic wave (SAW) substrates with high couplings are very important for wide-band SAW filters and SAW devices. Therefore, the large size KNbO 3 single crystals are required for SAW device applications. In this paper, the techniques of a large size of KNbO 3 using the top seeded solution growth techniques (TSSG) with large size of crucible of 60 mm φ platinum are described. The results show that 50×50×15 mm 3 single crystals are obtained. Also, the poling techniques of KNbO 3 crystal are investigated by using the nonlinear scanning dielectric microscope. The propagation characteristics of SAW and piezoelectric leaky surface waves in KNbO 3 single crystal were investigated theoretically. The results show that the electro-mechanical coupling coefficient of the surface wave propagating along the X -axis of the rotated Y -cut plane is extremely large: K 2 =0.53, compared to K 2 of 0.055 for LiNbO 3 . The experimental results of the above KNbO 3 substrates agree with the theoretical ones. Also the KNbO 3 substrates showed experimentally the zero temperature coefficients of frequency (TCF) around 20 °C.

Thin-Film Aerogel Porosity and Stiffness Characterized by Surface Acoustic Wave Spectroscopy

ABSTRACTDensity/porosity and stiffness are the most critical parameters determining properties of porous films, however they are difficult to measure. Here we report characterization of density/porosity and Young's modulus values of a range of nanoporous silica aerogel films via dispersion of laser-generated wideband surface acoustic waves. Results are compared to dielectric constant, Rutherford Backscattering Spectrometry (RBS), Ellipsometric Porosimetry (EP) and Nanoindentation (NI). RBS density correlates well to SAW. EP and NI also show a correlation, however the absolute values do not match. Low Young's modulus values show that the film stiffness is drastically reduced with increasing porosity. The technique is rapid, nondestructive and relatively inexpensive, and yields absolute values of nanoporous aerogel elastic properties which are useful for process control and are difficult to access with other techniques.

Surface Acoustic Wave Filter Using Floating Electrode Slanted Finger Unidirectional Transducers

This paper describes a wide-band surface acoustic wave (SAW) filter designed to achieve large stop-band attenuation. The SAW filters using slanted finger interdigital transducers are suitable for mid- or wide-band filters. To achieve a stop-band attenuation of more than 80 dB, two SAW filters using slanted finger unidirectional transducers (SFUDTs) were cascaded. Since these SFUDT filters have center-to-center distances that differ by one-fourth of the acoustic wavelength at the center frequency, the triple transit echo was effectively compensated. A cascaded-pair SFUDT SAW filter with a fractional bandwidth of 15 percent was designed and fabricated on a 128°Y-X LiNbO3 substrate. An insertion loss of 14 dB, amplitude ripples of 0.5 dB and stop-band attenuation of 80 dB or more were achieved.

Technique for low-loss and wideband surface acoustic wave filters

A ring surface acoustic wave filter on a high electromechanical coupling coefficient substrate is demonstrated. The insertion loss is 1.9 dB and the 3 dB fractional bandwidth is 30%. The filter also shows linear phase characteristics and low-passband ripples. The insertion loss is the lowest reported with such a wide bandwidth.

Super high electromechanical coupling and zero temperature coefficient surface acoustic wave substrates in KNbO/sub 3/ single crystal

The propagation characteristics of surface acoustic waves (SAW) and piezoelectric leaky surface waves in KNbO(3) single crystal have been investigated theoretically and experimentally. The results show that the electromechanical coupling coefficient k(2) of the surface wave propagating along the X-axis of the rotated Y-cut plate is very large with k(2)=0.53. This is about 10 times as large as that of LiNbO(3) in the surface wave branch. Experimental results for wideband SAW filters show low loss and temperature stable characteristics. The bandwidth is about 20%, and insertion losses are less than 2 to 6 dB. Simulation results for a ladder type filter using Y-cut KNbO(3) indicate that a bandwidth of 40% should be possible. The KNbO(3) substrates exhibit zero temperature coefficient of frequency (TCF) around 20 degrees C in rotated Y-cut substrates.

A Design of Surface Acoustic Wave Wideband Single-Phase-Unidirectional Transducers Using Coupling-of-Modes Theory

To realize wideband surface acoustic wave (SAW) filters, a weighted single-phase-unidirectional transducer (SPUDT) has been analyzed by direct numerical analysis of the coupling-of-modes equations. It was found that the directivity was obtained over a wide frequency range when the reflection coefficient of the SPUDT was weighted using a sinc function in order to distribute the positive and negative reflection elements into the SPUDT; this was called a wideband SPUDT. This paper also describes a high-performance wideband SPUDT, with both the reflection and transduction coefficients weighted on 128°Y-X LiNbO3 substrates, and a new wideband natural-SPUDT (NSPUDT) on 50°Y-23°X La3Ga5SiO14 substrates, which has recently attracted much attention for intermediate frequency (IF) SAW filters.