SAW Phase Research Articles

A finite state machine read-out chip for integrated surface acoustic wave sensors

A finite state machine based integrated sensor circuit suitable for the read-out module of a monolithically integrated SAW sensor on Si is reported. The primary sensor closed loop consists of a voltage controlled oscillator (VCO), a peak detecting comparator, a finite state machine (FSM), and a monolithically integrated SAW sensor device. The output of the system oscillates within a narrow voltage range that correlates with the SAW pass-band response. The period of oscillation is of the order of the SAW phase delay. We use timing information from the FSM to convert SAW phase delay to an on-chip 10bit digital output operating on the principle of time to digital conversion (TDC). The control inputs of this digital conversion block are generated by a second finite state machine operating under a divided system clock. The average output varies with changes in SAW center frequency, thus tracking mass sensing events in real time. Based on measured VCO gain of 16MHz/V our system will convert a 10kHz SAW frequency shift to a corresponding mean voltage shift of 0.7mV. A corresponding shift in phase delay is converted to a one or two bit shift in the TDC output code. The system can handle alternate SAW center frequencies and group delays simply by adjusting the VCO control and TDC delay control inputs. Because of frequency to voltage and phase to digital conversion, this topology does not require external frequency counter setups and is uniquely suitable for full monolithic integration of autonomous sensor systems and tags.

Theoretical Study of the Effect of Enamel Parameters on Laser-Induced Surface Acoustic Waves in Human Incisor

The laser ultrasound technique has great potential for clinical diagnosis of teeth because of its many advantages. To study laser surface acoustic wave (LSAW) propagation in human teeth, two theoretical methods, the finite element method (FEM) and Laguerre polynomial extension method (LPEM), are presented. The full field temperature values and SAW displacements in an incisor can be obtained by the FEM. The SAW phase velocity in a healthy incisor and dental caries is obtained by the LPEM. The methods and results of this work can provide a theoretical basis for nondestructive evaluation of human teeth with LSAWs.

A Novel Closed-Loop Readout Topology for Monolithically Integrated Surface Acoustic Wave Sensors

A new frequency to voltage closed-loop integrated sensor circuit suitable for the read-out module of a monolithically integrated SAW sensor on Si is proposed. This closed-loop system consists of a voltage controlled oscillator, a peak detecting comparator, a finite state machine, and the monolithically integrated SAW sensor device. The output of the system is forced to oscillate within a narrow voltage range that correlates with the SAW pass-band response. The period of oscillation is of the order of the SAW phase delay. The output voltage range varies with changes in SAW center frequency, thus tracking mass sensing events in real time. The analysis and simulation of the system are presented for two SAW devices operating at 374 and 140 MHz, respectively. Experimental results for a test chip fabricated with AMIC5N 0.5 μm process are presented for comparison with simulation data of the 140 MHz design. Because of this frequency to voltage conversion, this approach is uniquely suitable for full monolithic integration of autonomous sensor systems and tags.

Low propagation loss in a one-port SAW resonator fabricated on single-crystal diamond for super-high-frequency applications

Diamond has the highest known SAW phase velocity, sufficient for applications in the gigahertz range. However, although numerous studies have demonstrated SAW devices on polycrystalline diamond thin films, all have had much larger propagation loss than single-crystal materials such as LiNbO3. Hence, in this study, we fabricated and characterized one-port SAW resonators on single-crystal diamond substrates synthesized using a high-pressure and high-temperature method to identify and minimize sources of propagation loss. A series of one-port resonators were fabricated with the interdigital transducer/ AlN/diamond structure and their characteristics were measured. The device with the best performance exhibited a resonance frequency f of 5.3 GHz, and the equivalent circuit model gave a quality factor Q of 5509. Thus, a large fQ product of approximately 2.9 × 10(13) was obtained, and the propagation loss was found to be only 0.006 dB/wavelength. These excellent properties are attributed mainly to the reduction of scattering loss in a substrate using a single-crystal diamond, which originated from the grain boundary of diamond and the surface roughness of the AlN thin film and the diamond substrate. These results show that single-crystal diamond SAW resonators have great potential for use in low-noise super-high-frequency oscillators.

Theoretical approach on SAW characteristics of layered structures for gas sensing

Integrating the self-assemble sensing film with SAW devices has advantage for organic gas detection. It generally requires a thin metal layer like Au acting as a catalytic base for the sensing film formation. This leads to a double layer SAW device sensing area fabrication consisting of a metal layer deposited on the piezoelectric substrate. Therefore, the SAW characteristics will be affected when propagating through the sensing area. Various layer parameters such as stiffness constants and the thickness are needed for consideration. In this paper, the SAW phase velocities have been calculated theoretically for ST-X Quartz/Gold structure under different normalized thickness configurations.

Surface acoustic wave response to optical absorption by graphene composite film

Propagation of surface acoustic waves in YZ LiNbO3 overlaid with graphene flakes has been investigated and its optical response to illumination by 633-nm light from a He-Ne laser was studied. The heating of the sample surface caused by optical absorption by the graphene led to a downshift in the transmitted SAW phase caused by the wave velocity's dependence on temperature. The proposed simple model based on optothermal SAW phase modulation was found to be in good agreement with the experimental results.

A comparison of laser ultrasound measurements and finite element simulations for evaluating the elastic properties of tissue mimicking phantoms

Advances in the field of laser ultrasonics have opened up new possibilities in medical applications. This paper presents a finite element modelling technique, which studies laser generated surface acoustic waves in different concentration of soft tissue mimicking agar-agar phantoms. In addition, we propose a novel approach that utilises a low coherence interferometer to detect the laser-induced surface acoustic waves from the tissue mimicking phantoms. A Nd:YAG focused laser line-source is applied to the agar-agar phantoms, which as the same with the FE simulation. The generated SAW signals are detected by a time domain low coherence interferometry system. SAW phase velocity dispersion curves from both of the FE simulation and experiment are calculated. By comparison, we show that the experimental results agree well with those of the FE simulation and theoretical expectations. This study is the first report that a laser-generated SAW phase velocity dispersion technique is applied to soft materials. This technique may open a way for laser ultrasonics to detect the mechanical properties of soft tissues, such as skin.

High-temperature langatate elastic constants and experimental validation up to 900°C

This paper reports on a set of langatate (LGT) elastic constants extracted from room temperature to 1100 degrees C using resonant ultrasound spectroscopy techniques and an accompanying assessment of these constants at high temperature. The evaluation of the constants employed SAW device measurements from room temperature to 900 degrees C along 6 different LGT wafer orientations. Langatate parallelepipeds and wafers were aligned, cut, ground, and polished, and acoustic wave devices were fabricated at the University of Maine facilities along specific orientations for elastic constant extraction and validation. SAW delay lines were fabricated on LGT wafers prepared at the University of Maine using 100-nm platinumrhodium- zirconia electrodes capable of withstanding temperatures up to 1000 degrees C. The numerical predictions based on the resonant ultrasound spectroscopy high-temperature constants were compared with SAW phase velocity, fractional frequency variation, and temperature coefficients of delay extracted from SAW delay line frequency response measurements. In particular, the difference between measured and predicted fractional frequency variation is less than 2% over the 25 degrees C to 900 degrees C temperature range and within the calculated and measured discrepancies. Multiple temperature-compensated orientations at high temperature were predicted and verified in this paper: 4 of the measured orientations had turnover temperatures (temperature coefficient of delay = 0) between 200 and 420 degrees C, and 2 had turnover temperatures below 100 degrees C. In summary, this work reports on extracted high-temperature elastic constants for LGT up to 1100 degrees C, confirmed the validity of those constants by high-temperature SAW device measurements up to 900 degrees C, and predicted and identified temperature-compensated LGT orientations at high temperature.

Temperature coefficient of elastic constants of SiO2 over-layer on LiNbO3 for a temperature stable SAW device

The influence of sputtered SiO2 over-layer on the SAW propagation characteristics of a 128° rotated Y-cut X-propagating lithium niobate SAW filter has been studied. Experimentally measured SAW phase velocity and temperature coefficient of delay (TCD), with varying SiO2 over-layer thickness, show a significant deviation from the theoretically calculated values using the bulk material parameters of SiO2. The observed deviation is attributed to the differences in the material parameters (density, elastic and dielectric constants and their temperature coefficient) of the deposited SiO2 over-layer. The density and the dielectric constant of the deposited SiO2 layer were determined separately, and the elastic constants and their temperature coefficients were estimated by fitting the experimental velocity and TCD data, respectively. The deviation in the dielectric constant and the density in comparison to the bulk was insignificant, and the estimated values of the elastic constants (C11 = 0.75×1011 N m−2 and C44 = 0.225×1011 N m−2) were found to be lower, and the respective temperature coefficients (5.0×10−4 °C−1 and 2.0×10−4 °C−1) were high in comparison to the bulk material parameters.

GaAs surface modifications under light irradiation in vacuum

The interaction between metal atoms and near-surface semiconductor atoms from GaAs results in metal and Ga and As atoms intermixing, or solid solutions and new compounds formed during the metal deposition on the GaAs surface. Near-surface GaAs layer changes induced by non-loaded evaporator light emission in situ in vacuum were studied in this work. The GaAs near-surface layer state changes were followed by measurements of resistivity as a function of light intensity and duration of exposure, and of SAW amplitude and phase velocity variations. These measurements have been correlated with the structure and composition changes of the semiconductor native oxide layer. The GaAs surface changes were additionally studied with HEED and microscopy methods. They can be explained by diffusion of Ga and As atoms from the crystalline GaAs into the oxide layer, and back, caused by the plastic deformation, as well as the interaction between the Ga and the As atoms and residual gas in the vacuum chamber resulting in changing the thickness and composition of the oxide layer.

Conversion of 1/f fluctuations in crystal resonator within an inter resonance gap

This paper presents general relationships for transformation coefficients of BAW or SAW crystal resonator amplitude and phase fluctuations through the 1/f flicker noises of its motional and static equivalent parameters within the resonator inter resonance gap. Approximate functions of phase and amplitude power spectral densities are found based on Leeson's oscillator open loop model and are given with detailed consideration of Butler and Colpitts modes of operation with the assumption of full and zero inter noise correlation. It is also substantiated that a low-noise frequency region of crystal resonator operation exists in which the fluctuation influence of its motional inductance and capacity tend to zero in oscillators. Five examples are given as an illustration of a good agreement of the measured data with the prediction curves, giving a possibility of resonator power phase and amplitude spectral densities valuation at an arbitrary offset frequency from the carrier through the 1/f flicker noises of resonator parameters. Emphasis is laid in conclusion on the possible way of parameter spectral densities definition.

Coupling coefficient, temperature stability and mass sensitivity of the Rayleigh-type mode on (001) [110] AlAs/GaAs

The use of AlAs/GaAs layered structures for SAW sensor applications is discussed with the aim of exploring the potential of the material system for the integration of SAW and electronic devices. Based on the acoustic wave spectrum on the (001) cut of GaAs the development of the Rayleigh-type mode in the [110] direction of the AlAs/GaAs structure is described. Using a transfer matrix algorithm the phase velocity and the coupling coefficient of the dispersive structure are calculated as a function of the relative layer thickness , with k being the wave number and the layer thickness. Results of SAW phase velocity measurements carried out by a thermoelastic laser excitation method and a time delay technique are presented. The coupling coefficient has a maximum at kh = 1.8 being twice as high as the coefficient of bare GaAs. The temperature stabilization with and Au layers is calculated and the relation between and is determined for a vanishing temperature coefficient of delay (TCD). The mass sensitivity is increased by the application of the temperature stabilizing layers. It reaches a maximum value at and . Some aspects of optimization procedures including the AlAs layer thickness are discussed.

Diffraction of two optical beams by a SAW in an isotropic solid

This paper for the first time explores the diffraction of two spatially separated parallel light beams by reflection from a SAW, propagating in isotropic semi-space. In the analysis the attenuation and dispersion of SAW, the interference of light beams and scattering by different sites of acoustic wave are considered. The analytical expressions predict low-frequency oscillation of the resulting diffracted fields as observed in previous experiments. Using measurements of period oscillation of diffracted intensity vs. SAW frequency, the SAW dispersion curve and phase and group velocities were determined.

The characterization of sputtered polycrystalline aluminum nitride on silicon by surface acoustic wave measurements

Polycrystalline aluminum nitride films were deposited on Si/sub 3/N/sub 4/ coated [100] silicon substrates by the reactive sputtering method. We have carried out experiments to evaluate the effect of AlN material parameters on the SAW characteristics. The SAW transducers were fabricated by forming interdigitated Al electrodes on top of the AlN films and transmission measurements made over the frequency range from 50 MHz to 1.5 GHz. The SAW characteristics were correlated with material parameters of crystallite orientation, grain size, surface morphology and oxygen concentration. A key material parameter affecting the SAW characteristics was found to be the preferred degree of crystallite orientation with the c-axis normal to the plane of the substrate. The better oriented the AlN grains, the stronger the SAW response, the higher the SAW phase velocity, and the lower the insertion and propagation losses over the entire frequency range of measurement. Above 500 MHz the propagation losses of the well oriented films followed a frequency squared dependence only slightly higher than the reported values for the best epitaxial films. The coupling factors deduced from the transducer characteristics are in the upper range of values reported by Tsubouchi for epitaxial AlN films deposited on the basal plane of sapphire. There was a strong correlation between the X-ray diffraction intensity from the (002) planes and the oxygen content in the films. >

Scattering of SAW at discontinuities: some numerical experiments

In developing models to characterize the metallic-finger step discontinuity in SAW devices on piezoelectric substrates it is important to estimate the SAW reflection coefficients and the effect of other modes at step discontinuities. In the literature perturbation approximations, finite element methods and various phenomenological parameter-fitting using experimental data are among the techniques which have been exploited. In this paper, a nonperturbation approximation method, which attempts to parallel some well-established techniques used in solving electromagnetic waveguide-discontinuity problems, is explored as an alternate numerical procedure for calculating SAW reflections. Implementing such a method requires that the full solution to SAW propagation in the free and in the layered regions be obtained. Thus, for a free-to-metallized discontinuity, the two SAW phase velocities, the total SAW power now in the free region and in the layered region, and the electroacoustic fields, must be calculated. In this paper the reflection problem is formulated as an optimization problem subject to the necessary scattering parameter constraints. Numerical experiments are described, and the results of calculations for reflectivity are compared to perturbation approximations, nonperturbation approximations and the available experimental data for the effects of energy storage.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Performance of SAW transducers on submicrometre n-GaAs epitaxial layers

Performance of the SAW interdigital transducer with doubled Al electrodes placed on GaAs structures with submicrometre n-GaAs epitaxial layers was investigated. A novel mode of operation on even overtone frequencies of this tranducer was observed and the possibility of electronic controllable SAW phase shift of up to 180 degrees was demonstrated.

Transduction mechanisms in SAW gas sensors

Experimental results are presented for a surface-acoustic-wave gas sensor. At 150°C a pronounced selectivity towards NO2 was observed. The transduction of a chemical signal (concentration) into a physical signal (SAW phase velocity) is discussed in terms of the effect of mass and conductivity changes on sensor response.

SAW Phase Modulator Using Magnetostrictive Thin Films

The fabrication and testing of a SAW phase modulator based on the d effect in magnetostrictive thin films is described. 4n Fe-Si-B film on lithium niobate was used and peak phase deviations of about 5 degrees at nodulating frequencies of 100 KHz were observed at a center frequency of 115 MHz. A simple t heory for the phase modulator has been developed w hich is in reasonable agreement with experiment. The need for a dc bias field, in addition to the ac modulating field, for linear phase modulation is discussed.

Voltage Controlled Monolithic SAW Phase Shifter and Its Application to Frequency Variable Oscillator

Voltage Controlled Monolithic SAW Phase Shifter and Its Application to Frequency Variable Oscillator

Frequency Variable Oscillator Using a Monolithic SAW Phase Shifter

A new frequency variable oscillator with a monolithic SAW phase shifter is reported. A 214 MHz monolithic SAW phase shifter is fabricated on a silicon substrate with ZnO film. A voltage controlled phase shift is measured and discussed. An SAW oscillator is implemented with this phase shifter and its frequency is controlled by external voltage. A frequency shift of 120 kHz (a relative variation of 560 ppm) and excellent modulation characteristics are obtained without an external phase shifter.