Slanted Finger Interdigital Transducers Research Articles

Acoustofluidic Separation of Proteins Using Aptamer-Functionalized Microparticles.

We propose an acoustofluidic method for the triseparation of proteins conjugated with aptamer-coated microparticles inside a microchannel. Traveling surface acoustic waves (TSAWs) produced from a slanted-finger interdigital transducer (SFIT) are used to separate the protein-loaded microparticles of different sizes via the TSAW-driven acoustic radiation force (ARF). The acoustofluidic device consists of an SFIT deposited onto a piezoelectric lithium niobate substrate and a polydimethylsiloxane (PDMS) microfluidic channel on top of the substrate. The TSAWs propagating on the substrate penetrate into the sample fluid flow, where the human protein-conjugated microparticles are suspended, inside the PDMS microchannel. The microparticles are subjected to the TSAW-driven ARF with varying magnitude depending on their size and thus flow along different streamlines, leading to triseparation of the proteins. In this work, we used two different-sized streptavidin-functionalized polystyrene (PS) microparticles to capture two kinds of aptamers (apt15 and aptD17.4), which were labeled with a respective biotin molecule at one end. The biotin ends of the aptamers were attached to the microparticles through streptavidin-biotin linkage, whereas the free ends of the aptamers were used to capture their target proteins of thrombin (th) and immunoglobulin E (IgE). The resultant PS-apt15-th and PS-aptD17.4-IgE complexes, as well as mCardinal2, were used for experimental demonstration of acoustofluidic triseparation of the human proteins. We achieved simultaneous separation of proteins of three kinds (th, IgE, and mCardinal2) for the first time via the TSAW-driven ARF in the proposed acoustofluidic device.

Tunable patterning of microscale particles using a surface acoustic wave device with slanted-finger interdigital transducers

Tunable patterning of microscale particles using a surface acoustic wave device with slanted-finger interdigital transducers

Acoustic tweezers based on circular, slanted-finger interdigital transducers for dynamic manipulation of micro-objects.

Acoustic tweezing technologies are gaining significant attention from the scientific communities due to their versatility and biocompatibility. This study presents acoustic tweezers based on circular, slanted-finger interdigital transducers (CSFITs), which can steer the propagation direction of surface acoustic waves (SAWs) by tuning the excitation frequency. The CSFITs based acoustic tweezers enable dynamic and reconfigurable manipulation of micro-objects using multi-tone excitation signals. Compared to traditional interdigital transducers that generate and control SAWs along one axis, the CSFITs allow for simultaneously generating and independently controlling SAWs propagating along multiple axes by changing the frequency composition and the phase information in a multi-tone excitation signal. Moreover, the CSFITs based acoustic tweezers can be used for patterning cells/particles in various distributions and translating them along complex paths. We believe that our design is valuable for cellular-scale biological applications, in which on-chip, contactless, biocompatible handling of bioparticles is needed.

Correction: Acoustic tweezers based on circular, slanted-finger interdigital transducers for dynamic manipulation of micro-objects.

Correction for 'Acoustic tweezers based on circular, slanted-finger interdigital transducers for dynamic manipulation of micro-objects' by Putong Kang et al., Lab Chip, 2020, 20, 987-994, DOI: .

Measurement of locally resonant band gaps in a surface phononic crystal with inverted conical pillars

In this paper, we numerically and experimentally study locally resonant (LR) band gaps for surface acoustic waves (SAWs) in a honeycomb array of inverted conical pillars grown on the surface of a 128°YX lithium-niobate substrate. We show that the inverted conical pillars can be used to generate lower LR band gaps below the sound cone. This lowering effect is caused by the increase in the effective pillar mass without increasing the effective stiffness. We employ the finite-element method to calculate the LR band gaps and wideband slanted-finger interdigital transducers to measure the transmission of SAWs. Numerical results show that SAWs are prohibited from propagating through the structure in the lowered LR band gaps. Obvious LR band-gap lowering is observed in the experimental result of a surface phononic crystal with a honeycomb array of inverted conical pillars. The results enable enhanced control over the phononic metamaterial and surface structures, which may have applications in low-frequency waveguiding, acoustic isolation, acoustic absorbers, and acoustic filters.

Acoustothermal tweezer for droplet sorting in a disposable microfluidic chip.

Precise control over droplet position within a microchannel is fundamental to droplet microfluidic applications. This article proposes acoustothermal tweezer for the control of droplet position, which is based on thermocapillary droplet migration actuated by acoustothermal heating. The proposed system comprises an acoustothermal heater, which is composed of a slanted finger interdigital transducer patterned on a piezoelectric substrate and a thin PDMS membrane, and a PDMS microchannel. In the proposed system, droplets moving in a droplet microfluidic chip experience spatiotemporally varying thermal stimuli produced by acoustothermal heating and thus migrate laterally. In comparison to previous methods for droplet sorting, the acoustothermal tweezer offers significant advantages: first, the droplet position can be manipulated in two opposite directions, which enables bidirectional droplet sorting to one of three outlets downstream; second, precise control over the droplet position as well as improved droplet lateral displacement on the order of hundreds of micrometers can be achieved in a deterministic manner, thereby enabling multichannel droplet sorting; third, the PDMS microfluidic chip is disposable and thus can be easily replaced since it is attached to the substrate by reversible bonding, which allows the acoustothermal heater to be reused. Given these advantages, the proposed droplet sorting system is a promising droplet microfluidic lab-on-a-chip platform for tunable, on-demand droplet position control.

Qualitative and quantitative analysis of surface-acoustic-wave-based ultraviolet photodetectors

Surface-acoustic-wave (SAW) based ultraviolet (UV) photodetectors with multiple optical sensing areas is presented by depositing patterned ZnO films on a LiNbO3 substrate. Two different types of electrodes, uniform interdigital transducer (IDT) and slanted finger interdigital transducer (SFIT), are designed and fabricated to analyze the performances of SAW array UV photodetectors. By tuning ultraviolet wavelengths, qualitative characteristics of SAW UV photodetectors are observed through the measurements of SAW attenuation. Quantitative analyses of the photodetectors are presented by varying light intensities. Moreover, dynamic responses of SAW array UV photodetectors under the illumination of different optical wavelengths are also demonstrated. Ultraviolet light of 350 nm with power densities of 8.46 and 18.34 μW/cm2 is detectable from a UV photodetector with uniform IDT and SFIT, respectively. This study not only indicates that a SAW-based UV sensor is a potential candidate of array UV photodetectors but also provides a comprehensive analysis of SAW UV photodetectors.

Tunable patterning of microparticles and cells using standing surface acoustic waves.

We have developed an acoustic-based tunable patterning technique by which microparticles or cells can be arranged into reconfigurable patterns in microfluidic channels. In our approach, we use pairs of slanted-finger interdigital transducers (SFITs) to generate a tunable standing surface acoustic wave field, which in turn patterns microparticles or cells in one- or two-dimensional arrays inside the microfluidic channels--all without the assistance of fluidic flow. By tuning the frequency of the input signal applied to the SFITs, we have shown that the cell pattern can be controlled with tunability of up to 72%. This acoustic-based tunable patterning technique has the advantages of wide tunability, non-invasiveness, and ease of integration to lab-on-a-chip systems, and shall be valuable in many biological and colloidal studies.

Analysis of UV light photodetectors using slanted finger interdigital surface acoustic wave devices

In this study, an ultraviolet array photodetector with lithium-based slanted finger interdigital transducers (SFITs) and ZnO optically active areas is developed. The array photodetector can obtain multiple ultraviolet light intensities within a single measurement and is proven to be capable of measuring a distributed light field. The acoustoelectric effect is introduced in the SFIT coupling-of-mode simulation to evaluate the performance of the array photodetector. The simulated results have good agreement with the experimental responses. An optimum design of the SFIT and ZnO for the performance of the array photodetector is analyzed. To evaluate the performance of a surface acoustic wave photodetector, several design parameters such as working frequency, size of optically active areas, and separation gaps between active areas, are discussed in detail. Important results of design parameters from the detailed analysis are summarized in this study which are useful for the application of the proposed photodector.

Application of slanted finger interdigital transducer surface acoustic wave devices to ultraviolet array photodetectors

An ultraviolet array photodetector is developed by the application of a LiNbO3-based slanted finger interdigital transducer (SFIT) and several ZnO optically active areas. This array photodetector can obtain ultraviolet light intensities within a single measurement. In this analysis, the acoustoelectric interaction between the photogenerated carriers in several active areas and a SFIT surface acoustic wave device is simulated. In the experiment, the photodetector with one to three optically active areas are fabricated. Ultraviolet light with a wavelength of 380 nm and an intensity of 65 μW cm−2 is detectable from the photodetector developed in this study. The minimum size of the optically active areas is 0.04 mm2 that can provide good spatial resolution. The ultraviolet array photodetector is capable of measuring a distributed light field.

Power Flow Angles for Slanted Finger Surface Acoustic Wave Filters on Langasite Substrate

Power flow angles (PFAs) on a langasite (LGS) substrate with Euler angles of (0°, 138.5°, ψ), ψ=25.7 to 27.7° are investigated for slanted finger interdigital transducer (SFIT) surface acoustic wave (SAW) filters by an electrical and optical methods. In the electrical method, several tilted SFIT SAW filters with different tilt angles for (0°, 138.5°, ψ) LGS substrates were designed, and the frequency responses of the filters were measured. In the optical method, the PFAs were directly measured by optical probing for a parallel interdigital transducer (IDT) with wide propagation area on the substrate. As a result, a good correlation between electrical and optical measurements of the PFAs is obtained, but the calculated PFAs are slightly different from the measured PFAs. A good frequency response of a tilted 380 MHz SFIT SAW filter with an appropriate tilt angle corresponding to the PFA on the substrate is obtained even though the aperture is small.

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.

Actuating and detecting of microdroplet using slanted finger interdigital transducers

Using surface acoustic wave (SAW) to drive droplets on a piezoelectric chip is a distinctive microfluidic technique developed recently. In the previous research, uniform interdigital transducers (IDTs) were used to radiate SAW amplitude and move the droplets within the same delay line simultaneously. In this paper, we employ slanted finger interdigital transducers (SFITs) to replace uniform IDTs. The merit is that we can move individual droplets at will by varying the frequency. This is because SAW amplitude profile actuated by a SFIT is not uniform along the aperture. The position of the maximum amplitude can be changed by adjusting the input frequency. In addition, we apply the coupling-of-modes model to simulate the amplitude profile and the frequency response of SFITs. Since the transmitted SAW intensity is strongly attenuated by the liquid when SAW hits the droplet, we can use the measured frequency response to detect the position of the droplet and verify the validity of the proposed method; we fabricated a SFIT on an Y128X-LiNbO3 substrate coated with hydrophobic film. The experimental results show good agreement with those predicted by the simulations. We note that the proposed method can further be used to construct a multichannel microfluidic chip.

Frequency Responses of Surface-Acoustic-Wave Filters with Two-Track Slanted Finger Interdigital Transducers

In this study, the scattering characteristics of a novel surface-acoustic-wave (SAW) filter with two-track slanted finger interdigital transducers (SFITs) are investigated. The novel SAW filter consists of two-track input and output SFITs, with each track configured in many subfilters in parallel connection. Because the arrival time of a SAW may differ among subfilters, passband ripples appear. However, by properly controlling the gap length between the input and output SFITs of each track, passband ripples can be eliminated evidently. Moreover, due to differences in radiation conductance among subfilters, a curve fitting technique is used to design the overlap width of each subfilter to maintain the passband responses of the SAW filter as flat as possible. A Chebyshev polynomial and a two-step weighting technique are employed to decrease sideband level. Simulation results show that by tailoring SAW SFITs and the gap difference of each track, a high-performance broadband IF filter can be obtained.

Frequency band-gap measurement of two-dimensional air/silicon phononic crystals using layered slanted finger interdigital transducers

In this paper, we investigate the frequency band-gap features of micromachined air/silicon phononic band structures using layered slanted finger interdigital transducers (SFIT). In order to achieve the applications of phononic crystals on the microelectromechanical system related components, the frequency band-gap widths of surface waves are studied both theoretically and experimentally in micrometer scale phononic crystals. For further integration with the complementary metal-oxide semiconductor processing techniques, silicon is chosen as the base material of the two-dimensional phononic crystals in this study. To cover the frequency band-gap width of the phononic crystal, the wideband SFIT- and the SFIT∕ZnO∕Si-layered structures in the measurement are analyzed and discussed. For layered structures, the dispersive relation is calculated by the effective permittivity approach, and the frequency response of the layered SFIT is then simulated by the coupling-of-modes model. The frequency band-gap width and the frequency range of two-dimensional air/silicon phononic crystals in micrometer scale are measured, and the result agrees well with the theoretical evaluation.

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.

Analysis of SFIT saw filters using coupling of modes model

This paper presents a study on using the coupling‐of‐model (COM) model to analyze wide‐band SAW filters with slanted finger interdigital transducers (SFIT). In contrast to other design methods, the COM model is more convenient and more simple to use for designing different types of electrodes by just changing the elements of a P‐matrix. In this paper, we give a complete description of how to use the COM approach to analyze SFIT SAW filters and show a comparison of simulated and experimental frequency responses of the SFIT SAW filter. Two SFIT SAW filters with 30% bandwidth, 60 MHz center frequency, and different numbers of SFIT pairs were designed. On the experimental side, SFIT SAW filters using YZ‐LiNbO3 as the substrate material were fabricated and the on‐wafer frequency responses were measured. Comparisons made between the simulated and measured frequency responses showed good agreement.

123°LiNbO3기판을 이용한 기울인 빗살변환기 구조의 70MHz IF 필터 구현

In this paper, surface acoustic wave(SAW) bandpass filters using slanted finger interdigital electrode transducers(IDTs) are investigated. The slanted finger IDTs are used to design SAW filters with good shape factor, a flat passband, and good out-of-band rejection characteristic. For the filter design, input-output IDT structure was simulated with modified impulse model; uniform-withdrawal weighting IDTs, withdrawal-withdrawal weighting IDTs. SAW filters of uniform-withdrawal weighting IDTs structure were designed and fabricated on <TEX>$128^{\circ}LiNbO_3$</TEX> piezoelectric substrates. Implemented SAW filter has a fractional bandwidth of 30%, center frequency of 70MHz and shape factor of <TEX>$1.12\pm0.01$</TEX>.

Diffraction analysis of slanted-finger interdigital transducers

For analysis, slanted-finger interdigital transducers (SFITs) are usually divided into many channels in parallel to the propagation direction. Every channel is considered to be a subfilter. This procedure neglects the diffraction completely. This paper introduces the diffraction in the SFIT analysis by use of the angular spectrum of plane waves (ASPW) approach. The analysis is based on a one-component bulk-wave model. The integrals of an ASPW are carried out by summing up M wave components with various wave vectors. Two wave modes propagating toward both forward and backward directions within SFIT are assumed to take the coupling effect into account. The amplitudes of both wave modes are expressed by M-dimensional vectors. By means of the boundary conditions at every finger edge, the transfer matrix and transduction vector are found. As a result, coupling-of-modes-like equations that link wave components at adjacent finger and gap regions are obtained. By matrix-matrix and matrix-vector multiplication, the complex amplitudes of all the wave components in all the finger and gap regions are calculated. These amplitudes yield the total wave field including reflection at all fingers. The wave field yields the piezoelectric part of the input and output transducer currents as functions of the filter input and output voltages representing the piezoelectric part of the Y matrix. The model is used for analyzing a unidirectional SFIT filter, the transducers of which are composed of SPUDT cells on 37/spl deg/ YX quartz. The simulated transmission behavior is compared with experimental results. Good agreement is found. Especially for the case of a large slanted angle, the profound deformation appearing within the passband and the high-frequency transition band can be clearly identified as the most important influence of the diffraction on the transmission behavior.

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.