Single-phase Unidirectional Transducers Research Articles

Full extraction of the COM parameters for Rayleigh type surface acoustic wave

Extraction of Coupling of Modes (COM) parameters is important in the design of modern high-performance surface acoustic wave (SAW) devices. In this study, we established an effective connection between the COM parameters and the results of stationary and eigenfrequency studies based on the finite element method. Based on this connection, we proposed an approach for fast and full extraction of the COM parameters including the phases of coupling reflection and transduction coefficient. The COM parameters of the commonly used electrode width controlled single-phase unidirectional transducer and bidirectional interdigital transducer cells are computed and compared. It shows that this is a fast, accurate, general, and full extraction of the COM parameters for Rayleigh type SAW.

Analysis and Design of Single-Phase Unidirectional Transducers with High Directivity

Electrode-width-controlled (EWC) single-phase unidirectional transducers (SPUDT) contribute to reduction of insertion loss of surface acoustic wave (SAW) devices due to their strong unidirectional properties. In this work, we propose a method to optimize the unidirectionality of EWC-SPUDT based on our research results that the unidirectionality of the EWC-SPUDT cell is strongly related to its reflectivity and its unidirectional angle. Furthermore, in order to ensure strong unidirectionality to achieve low insertion loss, a simulator based on the finite element method (FEM) is used to study the relationship between geometrical configuration of the EWC-SPUDT cell and its reflection coefficient, as well as its transduction coefficient. Simulation results indicate that the reflection coefficient of the optimized EWC-SPUDT cell composed of 128° YX lithium niobite (LiNbO3) substrate and Al electrodes with thickness of 0.3μm reaches the optimal value of 5.17% when the unidirectional angle is designed to be −90°. A SAW delay line is developed with the optimized EWC-SPUDT cell without weighing, and the simulation results are verified by experiments. The experimental results show that the directivity exceeds 30 dB at the center frequency and the insertion loss is just 6.7 dB.

Gigahertz Low-Loss and High Power Handling Acoustic Delay Lines Using Thin-Film Lithium-Niobate-on-Sapphire

In this work, we present the first group of gigahertz low-loss, wideband, and high power handling delay lines (ADLs) using a thin-film lithium niobate (LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> )-on-sapphire platform. The ADLs leverage a single-phase unidirectional transducer (SPUDT) to efficiently excite the shear horizontal surface acoustic wave (SH-SAW) in the film stack. The fabricated miniature SH-SAW ADL at 1.1 GHz shows a low insertion loss (IL) of 2.8 dB, a wide fractional bandwidth (FBW) of 6.14%, and a fast phase velocity of 5127 m/s. The device also features a high 1-dB compression point (P1dB) of 30.4 dBm. The temperature coefficient of frequency is -45 ppm/K. ADLs with delays between 12 and 172 ns have been implemented, with IL between 2.8 and 8.3 dB. SH-SAW propagation characteristics are extracted, showing a group velocity of 4747 m/s and a propagation loss of 6.73 dB/mm or 31.9 dB/μs. The simultaneous low-loss and high power handling illustrate the great potential of LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -on-sapphire for RF and cross domain applications at gigahertz.

The Notch Filter on Surface Acoustic Wave on Base of Single Phase Unidirectional Transducers

High requirements on the electrical parameters of the filters are made in radio engineering systems. For bandpass filters, these requirements apply to the squareness coefficient of the amplitude frequency response and rejection. Therefore, the notch filter can be used as a complement to bandpass filters, since it serves to suppress signals in a certain frequency range. Notch filters on surface acoustic waves are not widely used, but they have a number of interesting features and advantages over types of filters that work on other principles. The purpose of the development is to produce a notch filter on surface acoustic waves at 40.05 MHz with 40 dB suppression, a bandstop at the level of -3 dB at least 135 kHz (relative value - 0.34 %), a bandstop at the level of -40 dB at least 35 kHz (relative value - 0.0875 %). Results: a brief classification of notch filters is given. Various variants of passive notch filters are considered. Based on the method of coupling of modes and the finite element method, key elements on surface acoustic waves were calculated and manufactured for a notch filter with the required parameters on a substrate of a 36°YX Quartz piezoelectric material. The key element is a unidirectional transducer of DART type. The scheme is based on a bridge scheme with additional passive tuning elements. It is shown that in order to implement this filter, it is necessary not only to select all the topology parameters of individual key elements included in the filter very precisely, but also to perform precise configuration and selection of passive LC-elements. These calculations are confirmed by the results of experiments.

Low-Loss 5-GHz First-Order Antisymmetric Mode Acoustic Delay Lines in Thin-Film Lithium Niobate

In this work, we present the low-loss acoustic delay lines (ADLs) at 5 GHz, using the first-order antisymmetric (A1) mode in 128° Y-cut lithium niobate thin films. The ADLs use a single-phase unidirectional transducer (SPUDT) design with a feature size of quarter acoustic wavelength. The design space is analytically explored and experimentally validated. The fabricated miniature A1 ADLs with a feature size of show a high operating frequency at 5.4 GHz, a minimum insertion loss (IL) of 3 dB, a fractional bandwidth (FBW) of 1.6%, and a small footprint of 0.0074 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The low IL and high operating frequency have significantly surpassed the state-of-the-art performance of ADLs. The propagation characteristics of A1 acoustic waves have also been extracted. The demonstrated designs can lead to low-loss and high-frequency transversal filters for future 5G applications in the sub-6-GHz bands.

Power transmission characteristics of EWC-SPUDT SAW filters fabricated for multiplex transmission system of inverter gate drive circuits

To increase the electrical power transmission of surface acoustic wave (SAW) filters for gate drive systems of power conversion circuits, we fabricated unidirectional SAW filters based on a SiO2/Al/Pt/LiNbO3 structure in the frequency range of 380–950 MHz and measured their power durability characteristics. A transversal-type electrode-width-controlled single-phase unidirectional transducer (EWC-SPUDT) design was used to reduce the insertion loss of the SAW filters. Our experimental results showed that the transmitted electric power of the output port exceeded 560 mW, which was sufficient for the gate drive circuits. The results obtained demonstrate the validity of the SiO2/Al/Pt/LiNbO3 structure for the proposed EWC-SPUDT design.

A Unidirectional Transducer Design for Scaling GHz AlN-Based RF Microsystems.

In this work, we present a novel unidirectional transducer design for frequency scaling aluminum nitride (AlN)-based radio frequency (RF) microsystems. The proposed thickness-field-excited single-phase unidirectional transducers (TFE-SPUDT) adopt 5/16 wavelength electrodes and, thus, enable efficient piezoelectric transduction with better frequency scalability. The design space of the TFE-SPUDT is theoretically explored and validated using the acoustic delay line (ADL) testbeds. The ADL testbeds with a large feature size of [Formula: see text] show a center frequency of 1 GHz, a minimum insertion loss (IL) of 4.9 dB, and a fractional bandwidth (FBW) of 5.3%, significantly surpassing the IL and frequency scalability of the previously reported AlN transducers. The design approach can potentially contribute to various AlN-based RF microsystems for signal processing, physical sensing, optomechanical interaction, and quantum acoustic applications, and are readily extendable to other piezoelectric platforms.

Aluminum Nitride Lamb Wave Delay Lines With Sub-6 dB Insertion Loss

We present a group of low-loss Lamb mode acoustic delay lines in an aluminum nitride (AlN) thin film. The low-loss acoustic delay lines are enabled by the thickness-field-excited single-phase unidirectional transducers. The fabricated miniature acoustic delay lines show a fractional bandwidth of 4.5%, a minimum insertion loss of 5.9 dB, outperforming the previously reported aluminum nitride delay platforms. The demonstrated delay ranges from 105 ns to 165 ns with center frequencies from 175 MHz to 255 MHz. The design approach and the significantly lower insertion loss described herein are expected to open new horizons for hybridized signal processing based on AlN and CMOS. [2019-0094]

Gigahertz Low-Loss and Wideband S0 Mode Lithium Niobate Acoustic Delay Lines.

We present the first group of gigahertz S0 mode low loss and wideband acoustic delay lines (ADLs). The ADLs use a single-phase unidirectional transducers (SPUDT) design to launch and propagate the S0 mode in an X-cut lithium niobate thin film with large electromechanical coupling and low damping. In this work, the theoretical performance bounds of S0 mode ADLs are first investigated, significantly surpassing those in state-of-the-art. The design tradeoffs of S0 mode ADLs, when scaled to the gigahertz frequency range, are also discussed. The fabricated miniature ADLs show a fractional bandwidth (FBW) of 4% and a minimum insertion loss (IL) of 3.2 dB, outperforming the incumbent surface acoustic wave (SAW) counterparts, and covering a wide range of delays from 20 to 900 ns for digitally addressable delay synthesis. Multiple ADLs with center frequencies from 0.9 to 2 GHz have been demonstrated, underscoring their great frequency scalability. The propagation properties of S0 waves in lithium niobate at the gigahertz range are experimentally extracted. The demonstrated ADLs can potentially enable wide-range and high-resolution delay synthesis that is highly sought after for the self-interference cancellation in full-duplex radios.

A Radio Frequency Nonreciprocal Network Based on Switched Acoustic Delay Lines

This work demonstrates the first non-reciprocal network based on switched low-loss acoustic delay lines. The 4-port circulator is built upon a recently reported frequency-independent, programmable, non-reciprocal framework based on switched delay lines. The design space for such a system, including the origins of the insertion loss and harmonic responses, is theoretically investigated, illustrating that the key to better performance and low-cost modulation signal synthesis lies in a large delay. To implement a large delay, we resort to in-house fabricated low-loss, wide-band lithium niobate (LiNbO3) SH0 mode acoustic delay lines employing single-phase unidirectional transducers (SPUDT). The 4-port circulator, consisting of two switch modules and one delay line module, has been modularly designed, assembled, and tested. The design process employs time-domain full circuit simulation and the results match well with measurements. A 18.8 dB non-reciprocal contrast between insertion loss (IL = 6.6 dB) and isolation (25.4 dB) has been achieved over a fractional bandwidth of 8.8% at a center frequency 155 MHz, using a record low switching frequency of 877.19 kHz. The circulator also shows 25.9 dB suppression for the intra-modulated tone and 30 dBm for IIP3. Upon further development, such a system can potentially lead to future wide-band, low-loss chip-scale nonreciprocal RF systems with unprecedented programmability.

Surface Acoustic Wave Gyroscopic Effect in an Interdigital Transducer.

The surface acoustic wave (SAW) gyroscopic effect in an interdigital transducer (IDT) deposited on a piezoelectric substrate is different from that in the piezoelectric substrate due to a reflection induced by IDT. In this work, an extended coupling-of-mode (COM) model including the gyroscopic effect and the reflection was developed to analyze the SAW gyroscopic effect. First, dispersion characteristics parameters of SAW were fitted according to the data derived using the finite element method (FEM). Then, variations of stop band edge frequency were calculated using the extended COM theory by integrating dispersion characteristics parameters into the COM model. We compared its results with those obtained via FEM analysis to confirm the proposed model’s validity. We found that the variation in stop band edge frequency related to gyroscope effect reached the maximum value with a zero reflectivity value. For split IDT, the sensitivity of gyroscope effect is 0.036 Hz/rad/s with a lower than 1% normalized thickness. Conversely, the value of sensitivity was almost zero for bidirectional IDT and electrode width controlled single-phase unidirectional transducer (EWC/SPUDT).

Enhanced Sensitivity of a Hydrogen Sulfide Sensor Based on Surface Acoustic Waves at Room Temperature

In this contribution, a new surface acoustic wave (SAW)-based sensor was proposed for sensing hydrogen sulfide (H2S) at room temperature (30 °C), which was composed of a phase discrimination circuit, a SAW-sensing device patterned with delay line, and a triethanolamine (TEA) coating along the SAW propagation path of the sensing device. The TEA was chosen as the sensitive interface for H2S sensing, owing to the high adsorption efficiency by van der Waals’ interactions and hydrogen bonds with H2S molecules at room temperature. The adsorption in TEA towards H2S modulates the SAW propagation, and the change in the corresponding phase was converted into voltage signal proportional to H2S concentration was collected as the sensor signal. A SAW delay line patterned on Y-cut quartz substrate with Al metallization was developed photographically, and lower insertion and excellent temperature stability were achieved thanks to the single-phase unidirectional transducers (SPUDTs) and lower cross-sensitivity of the piezoelectric substrate. The synthesized TEA by the reaction of ethylene oxide and ammonia was dropped into the SAW propagation path of the developed SAW device to build the H2S sensor. The developed SAW sensor was characterized by being collecting into the phase discrimination circuit. The gas experimental results appear that fast response (7 s at 4 ppm H2S), high sensitivity (0.152 mV/ppm) and lower detection limit (0.15 ppm) were achieved at room temperature. It means the proposed SAW sensor will be promising for H2S sensing.

Wave Propagation Direction and c-Axis Tilt Angle Influence on the Performance of ScAlN/Sapphire-Based SAW Devices.

Some previously reported surface acoustic wave (SAW) devices using bulk piezoelectric substrates showed higher acoustic power radiated in either forward or backward wave propagation direction depending on their crystal orientations and are called natural single-phase unidirectional transducers (NSPUDT). While these reports were based on bulk piezoelectric substrates, we report directionality in the c-axis tilted 44% scandium doped aluminum nitride thin piezoelectric film-based SAW devices on sapphire. It is worth noting that our observance of directionality is specifically in Sezawa mode. We produced a c-axis tilt up to 5.5° over the single wafer and examined the directionality by comparing the forward and backward insertion loss utilizing split finger electrodes as a receiver. The wave propagation direction and c-axis tilt angle influence on the performance of SAW devices is evaluated. Furthermore, return loss and insertion loss data are presented for various SAW propagation directions and c-axis tilt angles. Finally, the comparison for both acoustic modes, i.e., Rayleigh and Sezawa, is reported.

Development of a Wireless and Passive SAW-Based Chemical Sensor for Organophosphorous Compound Detection.

A new wireless and passive surface acoustic wave (SAW)-based chemical sensor for organophosphorous compound (OC) detection is presented. A 434 MHz reflective delay line configuration composed by single phase unidirectional transducers (SPUDTs) and three shorted reflectors was fabricated on YZ LiNbO3 piezoelectric substrate as the sensor element. A thin fluoroalcoholpolysiloxane (SXFA) film acted as the sensitive interface deposited onto the SAW propagation path between the second and last reflectors of the SAW device. The first reflector was used for the temperature compensation utilizing the difference method. The adsorption between the SXFA and OC molecules modulates the SAW propagation, especially for the time delay of the SAW, hence, the phase shifts of the reflection peaks from the corresponding reflectors can be used to characterize the target OC. Prior to the sensor fabrication, the coupling of modes (COM) and perturbation theory were utilized to predict the SAW device performance and the gas adsorption. Referring to a frequency-modulated continuous wave (FMCW)-based reader unit, the developed SAW chemical sensor was wirelessly characterized in gas exposure experiments for dimethylmethylphosphonate (DMMP) detection. Sensor performance parameters such as phase sensitivity, repeatability, linearity, and temperature compensation were evaluated experimentally.

On-chip temperature-compensated Love mode surface acoustic wave device for gravimetric sensing

Love mode surface acoustic wave (SAW) sensors have been recognized as one of the most sensitive devices for gravimetric sensors in liquid environments such as bio sensors. Device operation is based upon measuring changes in the transmitted (S21) frequency and phase of the first-order Love wave resonance associated with the device upon on attachment of mass. However, temperature variations also cause a change in the first order S21 parameters. In this work, shallow grooved reflectors and a “dotted” single phase unidirectional interdigitated transducer (D-SPUDT) have been added to the basic SAW structure, which promote unidirectional Love wave propagation from the device's input interdigitated transducers. Not only does this enhance the first-order S21 signal but also it allows propagation of a third-order Love wave. The attenuation coefficient of the third-order wave is sufficiently great that, whilst there is a clear reflected S11 signal, the third-order wave does not propagate into the gravimetric sensing area of the device. As a result, whilst the third-order S11 signal is affected by temperature changes, it is unaffected by mass attachment in the sensing area. It is shown that this signal can be used to remove temperature effects from the first-order S21 signal in real time. This allows gravimetric sensing to take place in an environment without the need for any other temperature measurement or temperature control; this is a particular requirement of gravimetric biosensors.

A stable and highly sensitive strain sensor based on a surface acoustic wave oscillator

A stable and highly sensitive strain sensor based on a surface acoustic wave (SAW) oscillator is presented in this article. A 151MHz SAW delay line was fabricated on a 128° YX LiNbO3 substrate as the oscillator element. A cantilever beam with a tapered cross section was designed to provide a quasi-uniform strain testing platform and analyzed with a finite element method (FEM) model. Single-mode selection capability was achieved by using a comb configuration, and the triple transition interference (TTI) was suppressed by using a single phase unidirectional transducer (SPUDT). Prior to fabrication, a FEM/boundary element method (BEM) simulation was used to simulate and optimize the parameters of the SAW delay line structure. The RF test results of the fabricated SAW devices agree well with the simulation results. Further, the output frequency fluctuation of the developed SAW oscillator was shown to be only approximately 0.7ppm in a temperature-controlled laboratory environment for stability tests. The measured sensitivity was evaluated to be 126Hz/μɛ, and good linearity was observed at strain values ranging from 0 to 400μɛ.

Enhanced Sensitivity of Surface Acoustic Wave-Based Rate Sensors Incorporating Metallic Dot Arrays

A new surface acoustic wave (SAW)-based rate sensor pattern incorporating metallic dot arrays was developed in this paper. Two parallel SAW delay lines with a reverse direction and an operation frequency of 80 MHz on a same X-112°Y LiTaO3 wafer are fabricated as the feedback of two SAW oscillators, and mixed oscillation frequency was used to characterize the external rotation. To enhance the Coriolis force effect acting on the SAW propagation, a copper (Cu) dot array was deposited along the SAW propagation path of the SAW devices. The approach of partial-wave analysis in layered media was referred to analyze the response mechanisms of the SAW based rate sensor, resulting in determination of the optimal design parameters. To improve the frequency stability of the oscillator, the single phase unidirectional transducers (SPUDTs) and combed transducer were used to form the SAW device to minimize the insertion loss and accomplish the single mode selection, respectively. Excellent long-term (measured in hours) frequency stability of 0.1 ppm/h was obtained. Using the rate table with high precision, the performance of the developed SAW rate sensor was evaluated experimentally; satisfactory detection sensitivity (16.7 Hz·deg·s−1) and good linearity were observed.

Design and fabrication of low loss and high suppression monolithic inverse wavelet transform processor

This paper presents a low loss and high suppression monolithic inverse wavelet transform processor (MIWTP) using surface acoustic wave (SAW) devices. New functions for achieving the MIWTP have been derived. Its structure combines a withdrawal weighted single phase unidirectional transducer (SPUDT) with an apodized SPUDT. The experimental device is achieved with the aluminum electrode and the 128 degree rotated Y-cut lithium niobate (LiNbO3) substrate. Measured results demonstrate the center frequency 68.1 MHz, the minimum insertion loss (IL) 5.4 dB, the 3-dB fractional bandwidth 1.3% and the sidelobe suppression over 40 dB.

Battery-Free Love-Wave-Based Neural Probe and Its Wireless Characterizations

A wireless Love-wave-based neural probe that utilizes a one-port reflective delay line was developed for both reading and stimulating neurons in the brain. Poly(methyl methacrylate) (PMMA) as a waveguide layer and gold (Au) electrodes were structured on the top of a 41° YX LiNbO3 piezoelectric substrate, following the parameters extracted from coupling-of-mode (COM) modeling. For a one-port reflective delay line, single-phase unidirectional transducers (SPUDTs) and three shorted grating reflectors were employed, which made possible the implementation of a wireless and battery-free neural probe. The fabricated Love-wave-based neural probes were wirelessly measured using two antennas with a 440 MHz central frequency and a network analyzer. Sharp reflection peaks with a high signal-to-noise ratio were observed from the reflection peaks. The probe was immersed in 0.9% saline solution while applying input DC voltages. Good linearity, high sensitivity, and reproducibility were observed depending on DC applied voltage, in the range from 0 to 500 mV. The sensitivity obtained from the DC firings (artificial neural firings) was ∼0.04 µs/VDC, indicating that this prototype probe is very promising for the wireless reading and stimulation of neural firings in in vivo animal testing.

Design and implementation of high data capacity RFID tag using eight-phase encoding

Enhancing the data capacity of surface acoustic wave (SAW) radio frequency identification (RFID) tags using the combination of time position and phase encoding with fewer number of reflectors is investigated in this work. Simulation and fabrication of SAW RFID tags are performed using COMSOL Multiphysics and standard photolithography process. Lithium Tantalite (LiTaO3) is used as a substrate and Aluminium metal electrodes are used to realise the interdigital transducer (IDT) electrodes. The data capacity enhancement is tested using a SAW RFID tag consisting of a single-phase unidirectional transducer (SPUDT) transducer and four code reflectors. By designing the width of SPUDT electrodes and floating electrodes and proper positioning of reflectors, eight phases (i.e. 0°, −45°, −90°, −135°, −180°, −225°, −270° and −315°) are detected which correspond to 3 bits of data per reflector. These eight phases would result to have one more data bit per reflector. Therefore, eight phase states along with four time positions lead to 32 different states (i.e. 5 bits of data per reflector). To facilitate the realisation of our idea, the fabrication process has been performed at 250 MHz; however, it can easily be implemented at higher frequencies (i.e. GHz range).