Transverse Surface Acoustic Wave Research Articles

Numerical study on the vapor bubble removal by acoustic streaming in downward-facing pool boiling

The dynamic mechanism of vapor bubble removal by acoustic streaming on downward-facing heating surface during boiling process was numerically investigated. It is observed that under the excitation of longitudinal surface acoustic wave (SAW), vortices are formed at the root of bubble due to acoustic streaming. The vortices promote the bubble to detach from heating surface, while inhibiting the lateral growth and movement of the bubble. Thus, the bubble can be suspended in the liquid, almost statically. In addition, under the excitation of the transverse SAW, the generated vortices produce a translational thrust on the bubble, causing the bubble to move laterally and eventually leave the heating surface. For the case of superimposed SAWs excitation, the transverse SAW breaks the balance of vortical flow around the bubble which initially excited by the longitudinal SAW. Meanwhile, the unbalanced vortices destroy the boundary layer of heating surface, and weaken the influence of the viscous force as well. The results show that under the superimposed SAWs excitation, comparing with transverse SAW solely, the time for the bubble to detach laterally from the downward-facing heating surface is shortened by nearly 33%, and the removal speed of the bubble is also increased by nearly 45%. It indicates that SAW excitation can serve as an effective approach for active control and enhancement of boiling heat transfer on the downward-facing heating surface.

Elastic constants of epitaxial cubic MoNx(001) layers

The N-to-Mo ratio x in epitaxial cubic molybdenum nitride γ-MoNx(001) layers was varied from x=0.69–1.25 by adjusting the substrate temperature Ts=1000–600°C during reactive magnetron sputter deposition. X-ray diffraction, reflection, and reciprocal space mapping measurements in combination with Rutherford backscattering spectroscopy indicate that the layers exhibit negligible strain and that increasing x=0.69–1.25 leads to an increasing lattice constant a=4.16–4.26Å and a decreasing density ρ=8.7–6.8g/cm3 which is primarily attributed to a decrease in the cation site occupation from 0.89 to 0.70. Time resolved pump-probe reflectivity measurements of the sound velocity parallel to the [001] growth direction indicate that increasing the nitrogen concentration leads to a velocity reduction, corresponding to a decrease in C11 from 502±31 to 355±30GPa for x=0.69–1.25. Similarly, the transverse surface acoustic wave velocity measured by surface Brillouin scattering spectroscopy decreases from 4.00–3.52km/s, providing values for C44 from 100±7 to 73±3GPa. The softening with increasing N concentration is primarily attributed to the decreasing Mo site occupancy, which causes a reduction in the valence electron concentration leading to a weakening of NMo bonds associated with hybridized N 2p - Mo 4d eg orbitals.

Reduction of Transverse Surface Acoustic Wave Leakage in Resonator on Al/42° YX-LiTaO3 Substrate for High-Frequency Applications

In this paper, we propose a new structure for reducing the extent of transverse surface acoustic wave (SAW) leakage for the SAW resonator on a 42° YX-LiTaO3 substrate. Such leakage occurs from the interdigital region toward the busbar region in the SAW resonators. The new structure has a Ta2O5 film outside the interdigital region. This structure can make the SAW velocity in the busbar region lower than the velocity in the interdigital region. Therefore, the new structure could reduce the extent of leakage, and contribute to confine the SAW energy in the interdigital region. This structure was applied in SAW resonators and ladder-type SAW filters fabricated on a 42° YX-LiTaO3 substrate. The insertion loss could be improved by suppressing transverse SAW leakage. This technique could be applied to the fabrication of the filters and duplexers using leaky SAW on a 42° YX-LiTaO3 substrate, and the SAW devices could exhibit excellent performance.

High Quality

Ken Laker advanced surface acoustic wave (SAW) filter and resonator technology with significant decreases in the size, weight, and cost of oscillators, fast frequency-measuring filter banks, and compact fast-frequency- hopping synthesizers. His compact SAW resonator chip at 780 MHz had a Q factor of almost 5,000, five times that of larger electromagnetic equivalents. His technique of designing withdrawal-weighted SAW transversal filters increased out-of-band rejection from 30 dB to more than 60 dB.

Fabrication of a Slotted SiO2 Film/Embedded Grating Electrodes/128° YX-LiNbO3 Structure

In this paper, we describe the fabrication of Al grating electrodes embedded in a slotted SiO2 film on a 128° YX-LiNbO3 substrate in order to control both reflection characteristics and the temperature coefficient of frequency (TCF) of surface acoustic wave (SAW) devices. First, the fabrication process is discussed. It is shown that the combined usage of ion milling and reactive ion etching enables independent control of the thicknesses of the slotted SiO2 film and grating electrodes to be embedded in the slots. Next, the process is applied to the fabrication of an SAW transversal filter, showing the possibility of controlling the reflection characteristics and TCF independently.

Nonlinear effects in the time measurement device based on surface acoustic wave filter excitation

A transversal surface acoustic wave filter has been used as a time interpolator in a time interval measurement device. We are presenting the experiments and results of an analysis of the nonlinear effects in such a time interpolator. The analysis shows that the nonlinear distortion in the time interpolator circuits causes a deterministic measurement error which can be understood as the time interpolation nonlinearity. The dependence of this error on time of the measured events can be expressed as a sparse Fourier series thus it usually oscillates very quickly in comparison to the clock period. The theoretical model is in good agreement with experiments carried out on an experimental two-channel timing system. Using highly linear amplifiers in the time interpolator and adjusting the filter excitation level to the optimum, we have achieved the interpolation nonlinearity below 0.2 ps. The overall single-shot precision of the experimental timing device is 0.9 ps rms in each channel.

Time-Interval Measurement Based on SAW Filter Excitation

This paper deals with a novel time-interval measurement method that makes use of a surface acoustic wave (SAW) filter as a time interpolator. The method is based on the fact that a transversal SAW filter excited by a short pulse can generate a well-defined finite signal with highly suppressed spectra outside a narrow frequency band. If the responses to two excitations are sampled at clock ticks, they can be precisely reconstructed from a finite number of samples and then compared to determine the time interval between the two excitations. As a first step of a more comprehensive discussion of the method, the time-interpolation error caused by deterministic effects has been analyzed in this paper. It has been shown that the mean square of the interpolation error is limited by the upper bound, which is proportional to the relative energy of aliasing distortion and inversely proportional to the square of the filter center frequency. The upper bound has been compared to the results obtained from the simulation based on a linear-phase transversal filter. The RMS error resulting from the simulation exactly follows the slope of the theoretical upper bound, and it is approximately four times smaller than that. The described method excels in time interpolation efficiency since the time interpolation error relative to clock period is small; in other words, accurate measurement can be achieved even with a relatively low clock frequency. This has been practically demonstrated in a time-interval counter, which provides precision of 7-ps RMS using a clock frequency as low as 16.4 MHz. It results from the analysis that using state-of-the-art elements in the interpolator, the deterministic interpolation error on the order of 0.1 ps can be achieved.

Random Errors in Time Interval Measurement Based on SAW Filter Excitation

This paper deals with the time interval measurement method that makes use of a surface acoustic wave (SAW) filter as a time interpolator. The method is based on the fact that a transversal SAW filter excited by a short pulse can generate a finite signal with highly suppressed spectra outside a narrow frequency band. If the responses to two excitations are sampled at clock ticks, they can be precisely reconstructed from a finite number of samples and then compared to determine the time interval between the two excitations. We have analyzed noise contributions to the overall uncertainty of the interval measurement. The following four noise sources have been considered: (1) noise of excitation; (2) noise of amplifier; (3) quantization noise of the analog-to-digital converter; and (4) clock and sampling jitter. The main goal of the analysis was to determine the uncertainty of the measurement in dependence on the variances of contributing noises, and the magnitude of the frequency response of the filter. First, each uncertainty component has been analyzed under general assumptions. Second, a general narrow-band filter has been investigated. Finally, we have found the solution for an ideal narrow-band filter where we have obtained simple illustrative results. The results have been compared to Monte-Carlo simulations. Good agreement between theory and simulation has been demonstrated except for the outliers due to the false ambiguity solution that occurs with large measurement errors and small relative filter bandwidth.

Time interval measurement device based on surface acoustic wave filter excitation, providing 1ps precision and stability

This article deals with the time interval measurement device, which is based on a surface acoustic wave (SAW) filter as a time interpolator. The operating principle is based on the fact that a transversal SAW filter excited by a short pulse can generate a finite signal with highly suppressed spectra outside a narrow frequency band. If the responses to two excitations are sampled at clock ticks, they can be precisely reconstructed from a finite number of samples and then compared so as to determine the time interval between the two excitations. We have designed and constructed a two-channel time interval measurement device which allows independent timing of two events and evaluation of the time interval between them. The device has been constructed using commercially available components. The experimental results proved the concept. We have assessed the single-shot time interval measurement precision of 1.3 ps rms that corresponds to the time of arrival precision of 0.9 ps rms in each channel. The temperature drift of the measured time interval on temperature is lower than 0.5 ps/K, and the long term stability is better than +/-0.2 ps/h. These are to our knowledge the best values reported for the time interval measurement device. The results are in good agreement with the error budget based on the theoretical analysis.

High-Frequency Edge Reflection Type Resonators with Excellent Temperature Characteristics

Currently, some duplexers are required to have a good temperature coefficient of frequency (TCF). An Al electrodes/36–48° YX-LiTaO3 substrate, which is used for most surface acoustic wave (SAW) duplexers, does not have a good TCF. Its TCF can be improved by depositing a SiO2 film with a positive TCF on a transversal SAW filter consisting of a substrate with a negative TCF such as LiTaO3 and LiNbO3. However, resonator-type SAW devices combining a SiO2 film, Al electrodes, and a LiTaO3 substrate do not show good frequency characteristics because of their small reflection coefficient and so on. It is considered as a countermeasure that a resonator using the reflection of a shear horizontal (SH) wave at substrate edges will show good frequency characteristics because its reflection coefficient is very large regardless of the Al electrode and SiO2 films thicknesses. It has been difficult to obtain a high-frequency edge reflection resonator using ordinary machining techniques because it requires extremely fine substrate edges. However, a high-frequency edge reflection resonator with a good TCF and excellent frequency characteristics has been realized using our newly developed method of obtaining fine edges.

Analysis of laser damage threshold and morphological changes at the surface of a HgCdTe crystal

The laser damage threshold for the onset of surface melting is measured. The dynamics and evolution of surface morphology are observed for different pulse power densities and number of laser pulses. With the power density increase, a sputtering phenomenon in the radial direction towards the periphery of the ablated zone is observed. The accumulation effect of heat due to pulses successively bombarding the surface is also evident. The damage threshold value calculated by the thermal model is in good agreement with the measured data in our experiment. As for the morphological changes, the thermal pressure model and shock pressure model can give a reasonable explanation. The periodic structure of ripples on the surface can be explained well by transverse surface acoustic wave theory.

Shear horizontal BG surface acoustic waves on piezoelectrics: a historical note

In December of 1968 Jeffrey Bleustein of Yale University published an article predicting the existence of a new type of transverse surface acoustic wave (SAW) that could propagate on the surface of a piezoelectric crystal. This was followed within 20 days by the publication of an article by Yuri Gulyaev in January of 1969 predicting the same basic surface wave propagation. The wave took on the name Bleustein-Gulyaev or BG-wave, joining the names of Rayleigh, Love, Sezawa, and Stonely for distinct types of SAW. But is there more to the story than this? Were there works preceding those of Bleustein and Gulyaev which signaled an interest in exploring a new surface wave mode on piezoelectrics? What about the work of Shimizu, Nakamura, and Ohta, who in April of 1969 published both theoretical and experimental verification of the existence of such a wave independent of the knowledge of the Bleustein and Gulyaev papers? This paper explores the early roots and characteristics of the shear horizontal surface wave on a piezoelectric.

Development of an SH-SAW sensor for the detection of DNA hybridization

We have developed SH (shear horizontal) surface acoustic wave (SAW) sensors for the detection of DNA (deoxyribonucleic acid) hybridization on the gold-coated delay line of transverse SAW devices. DNA hybridization experiments were performed with 15-mer oligonucleotides (probe and complementary target DNA). The sensor consists of twin SAW delay line oscillators (sensing channel and reference channel) operating at 100 MHz fabricated on 36° rotated Y-cut X-propagation LiTaO 3 piezoelectric single crystals. The relative change in the frequency of the two oscillators was monitored to detect hybridization between the target DNA and probe DNA immobilized on the delay line of the SAW sensor. The measurement results showed a good response of the sensor to the mass loading effects of the DNA hybridization with a sensitivity level up to 1.55 ng/ml/Hz.

Development of SH-SAW sensors for underwater measurement

We developed SH (shear horizontal) surface acoustic wave (SAW) sensors to detect protein molecules in liquid solutions applying a particular antibody thin film on the delay line of transverse SAW devices. The antibody investigated was human-immuno-globulin G (HigG) to hold the antigens (anti-HigG) in the protein solution to be measured. The sensor showed stable response to the mass loading effects of the anti-HigG molecules with the sensitivity up to 10.8 ng/ml/Hz.

Analysis of surface damage and plasma properties of pulsed laser ablation of GaAs

The surface morphological changes produced by Nd:YAG pulsed laser ablation of GaAs are examined and analyzed with scanning electron microscope. The periodic structure on the irradiated surface is explained by the transverse surface acoustic wave theory. The dispersed emission was detected by a 10 ns gated, image-intensified optical multi-channel analyzer. The time-of-flight spectra exhibit the twin-peak structures of Ga atoms that can be explained by the recombination process. The experimental results show that the characteristics of the species in plasma are strongly influenced by the ambient pressure.

Design of withdrawal-weighted SAW filters.

This paper presents a new design algorithm for a withdrawal-weighted surface acoustic wave (SAW) transversal filter. The proposed algorithm is based on the effective transmission loss theory and a delta function model of a SAW transversal filter. The design process consists of three steps, which eventually determine eight geometrical design parameters for the filter in order to satisfy given performance specifications. First, the number of fingers in the input and output interdigital transducers (IDTs), plus their geometrical sizes is determined using the insertion loss specification. Second, the number and positions of the polarity reverses in the output IDT are determined using the bandwidth and ripple specifications. Third, the number and position for withdrawing and switching specific fingers in the output IDT and attached electrode area are determined to achieve the desired sidelobe level. The efficiency of the technique is illustrated using a sample design of an IF filter consisting of a uniform input IDT and withdrawal-weighted output IDT. The proposed algorithm is distinct from conventional techniques in that it can optimize the structural geometry of a withdrawal-weighted SAW filter in a direct manner by considering all the performance specifications simultaneously.

Structural and electroacoustic studies of AlN thin films during low temperature radio frequency sputter deposition

AlN is a material used in a wide variety of applications such as electroacoustic devices, blue diodes, IR windows, thermal conductors, metal–insulator–semiconductor structures, integrated circuit packaging, etc. In this work thin piezoelectric AlN polycrystalline films have been grown on Si and SiO2 using rf magnetron sputter deposition in an Ar/N2 gas mixture. The structural properties of the films have been optimized by varying the deposition parameters, such as process pressure, gas mixture, substrate temperature, discharge power, etc. [K. Tominaga et al., Jpn. J. Appl. Phys., Part 1 35, 4972 (1996); H. Okana et al., ibid. 31, 3446 (1992); K. Kazuya, T. Hanabusa, and K. Tominaga, Thin Solid Films 281-282, 340 (1996)]. It was found that the best film texture was obtained for a particular set of parameters, namely process pressure of 4 mTorr, substrate temperature 350 °C, discharge power 350 W, and a gas mixture of 25% Ar and 75% N2. The films as examined by x-ray diffraction exhibited a columnar structure with a strong (001) texture, and a full width at half maximum (FWHM) rocking curve of 1.6°. Atomic force microscopy measurements indicated a surface roughness with a rms value of 8 Å. Classical nonapodized transversal surface acoustic wave filters operating at a frequency of 534 MHz were fabricated to characterize the electroacoustic properties of the films. The measurements indicated a coupling coefficient of 0.37% and a phase velocity of 4900 m/s. Further, thin epitaxial films were grown on (001)α-Al2O3 (sapphire) under the same deposition conditions except the substrate temperature. The films exhibited a (001)AlN//(001)α-Al2O3 plane orientation with a (002) rocking curve FWHM value of about 0.4°, showing a relatively good alignment of the c axis. The in-plane orientation was [110]AlN//[120]α-Al2O3 corresponding to a rotation of the AlN film of 30° with respect to the (001)α-Al2O3 surface. Cross-sectional transmission electron microscopy studies indicated a population of both thread and edge dislocations with decreasing concentrations with film thickness.

Spatially resolved measurement of transverse surface acoustic waves for the investigation of elastic properties

This paper reports new developments in the field of spatially resolved surface acoustic wave (SAW) analysis. With scanning acoustic force microscopy (SAFM) the investigation of SAW phenomena with lateral resolution of the scanning force microscope became possible. This technique was limited to SAW modes with an out-of-plane oscillation component. Recently, we demonstrated that purely in-plane polarized SAWs can also be investigated by using a non-linear coupling to the cantilever's torsional movement. Now it is possible to measure the SAW phase velocity dispersion for any given SAW polarization. We used SAFM for investigation of the layered system SiO2 on ST-cut quartz. Copyright © 1999 John Wiley & Sons, Ltd.

Transverse surface acoustic wave detection by scanning acoustic force microscopy

We present a scanning acoustic force microscope (SAFM) for the study of surface acoustic wave (SAW) phenomena on the submicron lateral scale. Until now, SAWs with in-plane oscillation components could only be studied effectively via nonvanishing out-of-plane oscillation contributions. By operating the microscope in lateral force mode, where both bending and torsion of the cantilever are detected, additional amplitude-dependent signals are found, which are due to the interaction with purely in-plane polarized surface oscillations. To demonstrate the capabilities of this type of SAFM, Love waves were studied on the surface of layers deposited on ST-cut quartz with SAW propagation perpendicular to the crystal X-axis. The phase velocity of the wave as well as the amplitude of a standing wave field was measured and compared to calculated values.

Low-Loss Surface Acoustic Wave Filter on Natural-Single Phase Unidirectional Transducter Orientations of a Li 2B 4O 7 Substrate

Natural-single phase unidirectional transducers (NSPUDT) have attracted much attention for the implementation of low-loss surface acoustic wave (SAW) devices. In this paper, we describe a low-loss SAW filter using NSPUDT orientations of a Li2B4O7 substrate previously reported by the authors. Since the unidirectionality of NSPUDT is inherent to the substrate orientation, it is necessary to make the receiving transducer unidirectional in the opposite direction in order to obtain a low-loss filter using two transducers with reverse directivity. A new transducer configuration to reverse the directivity of an NSPUDT substrate is proposed, which utilizes reflector banks within the ladder transducer. The configuration has the advantage that it can be fabricated through a single level and a single metallization with minimum electrode width of λ/4. The principle of the operation of a reversal of directivity transducer is described. A 200-MHz-band low-loss SAW transversal filter with minimum insertion loss below 3 dB and low passband ripple are demonstrated with a theoretical filter response using coupling-of-modes theory.