Wen Wang

A novel wireless, passive CO2 sensor incorporating a surface acoustic wave reflective delay line

A 440 MHz wireless and passive surface acoustic wave (SAW) chemical sensor was developed for CO2 detection. The developed SAW gas sensor is composed of single phase unidirectional transducers (SPUDTs), three shorted grating reflectors, and CO2-sensitive polymer film on 41° YX LiNbO3 substrate. Coupling of modes (COM) modeling was used to find optimal design parameters. Using the extracted design parameters, the SAW device was fabricated. Teflon AF 2400 was used as the sensitive film because it provides high CO2 solubility, permeability and selectivity. In wireless device testing using a network analyzer, four sharp reflection peaks with high signal-to-noise (S/N) ratio, small signal attenuation, and few spurious peaks were observed in the time domain. The time positions of the reflection peaks were well matched with the predicted values from the simulation. Infusion of CO2 into the chamber induced large phase shifts of the reflection peaks. Good linearity and repeatability were observed for a CO2 concentration of 0–450 ppm. The obtained sensitivity was 1.98° ppm−1. Temperature and humidity effects were also investigated during the sensitivity evaluation process.

A novel 440 MHz wireless SAW microsensor integrated with pressure?temperature sensors and ID tag

This paper presents the development of a 440 MHz range surface acoustic wave (SAW)-based microsensor integrated with pressure–temperature sensors and ID tag. Two piezoelectric substrates were bonded, in which a ~150 µm air gap was structured by metal poles. The pressure sensor was placed on the top substrate, whereas the ID tag and temperature sensor were located on the bottom substrate. Coupling of modes (COM) modeling was used to find optimal design parameters. Using the extracted optimal design parameters, the SAW device was fabricated. In wireless device testing using a network analyzer, sharp reflection peaks with high S/N ratio, small signal attenuation and small spurious peaks were observed in the time domain. All the reflection peaks were well matched with the predicted values from the simulation. With 10 mW RF power from the network analyzer, a ~1 m readout distance was observed. Depending on applied external pressure, the phase shifts of the reflection peaks were linearly varied. The evaluated sensitivity was about ~2.9° kPa−1. Eight sharp ON reflection peaks were observed for the ID tag. The temperature sensor was characterized from 20 °C to 200 °C. A large phase shift per unit temperature change was observed.

Advances in SXFA-Coated SAW Chemical Sensors for Organophosphorous Compound Detection

A polymer-coated surface acoustic wave (SAW)-based chemical sensor for organophosphorous compound sensing at extremely low concentrations was developed, in which a dual-delay-line oscillator coated with fluoroalcoholpolysiloxane (SXFA) acted as the sensor element. Response mechanism analysis was performed on the SXFA-coated chemical sensor, resulting in the optimal design parameters. The shear modulus of the SXFA, which is the key parameter for theoretical simulation, was extracted experimentally. New designs were done on the SAW devices to decrease the insertion loss. Referring to the new phase modulation approach, superior short-term frequency stability (±2 Hz in seconds) was achieved from the SAW oscillator using the fabricated 300 MHz delay line as the feedback element. In the sensor experiment on dimethylmethylphosphonate (DMMP) detection, the fabricated SXFA-coated chemical sensor exhibited an excellent threshold detection limit up to 0.004 mg/m3 (0.7 ppb) and good sensitivity (∼485 Hz/mg/m3 for a DMMP concentration of 2∼14 mg/m3).

Advances in SAW Gas Sensors Based on the Condensate-Adsorption Effect

A surface-acoustic-wave (SAW) gas sensor with a low detection limit and fast response for volatile organic compounds (VOCs) based on the condensate-adsorption effect detection is developed. In this sensor a gas chromatography (GC) column acts as the separator element and a dual-resonator oscillator acts as the detector element. Regarding the surface effective permittivity method, the response mechanism analysis, which relates the condensate-adsorption effect, is performed, leading to the sensor performance prediction prior to fabrication. New designs of SAW resonators, which act as feedback of the oscillator, are devised in order to decrease the insertion loss and to achieve single-mode control, resulting in superior frequency stability of the oscillator. Based on the new phase modulation approach, excellent short-term frequency stability (±3 Hz/s) is achieved with the SAW oscillator by using the 500 MHz dual-port resonator as feedback element. In a sensor experiment investigating formaldehyde detection, the implemented SAW gas sensor exhibits an excellent threshold detection limit as low as 0.38 pg.

Enhanced Sensitivity of Novel Surface Acoustic Wave Microelectromechanical System-Interdigital Transducer Gyroscope

In this paper, we present a novel microelectromechanical system-interdigital transducer (MEMS-IDT) surface acoustic wave (SAW) gyroscope with an 80 MHz central frequency on a 128 YX LiNbO3 wafer. The developed MEMS-IDT gyroscope is composed of a two-port SAW resonator, a dual delay line oscillator, and metallic dots. The SAW resonator provides a stable standing wave, and the vibrating metallic dot at an antinode of the standing wave induces the second SAW in the normal direction of its vibrating axis. The dual delay line oscillator detects the Coriolis force by comparing the resonant frequencies between two oscillators through the interference effect. The coupling of mode (COM) modeling was used to extract the optimal design parameters prior to fabrication. In the electrical testing by the network analyzer, the fabricated SAW resonator and delay lines showed low insertion loss and similar operation frequencies between a resonator and delay lines. When the device was rotated, the resonant frequency differences between two oscillators linearly varied owing to the Coriolis force. The obtained sensitivity was approximately 119 Hz deg 1 s 1 in the angular rate range of 0 –1000 deg/s. Satisfactory linearity and superior directivity were also observed in the test. # 2009 The Japan Society of Applied Physics DOI: 10.1143/JJAP.48.06FK09

The development of a wireless Love wave biosensor on 41° YX LiNbO3

This paper presents a novel wireless Love-wave-based biosensor using a polymethyl methacrylate (PMMA) waveguide and protein A receptor layers on a 41? YX LiNbO3 piezoelectric substrate for immunoglobulin G (IgG) detection. A 440?MHz reflective delay line composed of single-phase unidirectional transducers (SPUDTs) and three shorted grating reflectors was fabricated as the sensor element. A theoretical modeling was performed to describe the wave propagation of Love wave devices on a 41? YX LiNbO3 piezoelectric substrate with large piezoelectricity. The fabricated devices were wirelessly characterized by using the network analyzer as the reader unit. The resultant reflection peaks showed large signal/noise ratio, sharp peaks, and few spurious signals. The binding of the IgG to the protein A receptor layer induced large phase shifts of the reflection peaks due to the mass loading effect. Good linearity, reproducibility, and high sensitivity were observed in the IgG concentration range 1?65?nM. Unique advantages such as high sensitivity and a simple wireless measurement method over other currently available biosensors are also presented.

High frequency stability oscillator for surface acoustic wave-based gas sensor

This paper presents a 158 MHz surface acoustic wave (SAW) oscillator used for a gas sensor. As the oscillator element, a SAW delay line on ST-X quartz substrate with low insertion loss (<8 dB) and single mode selection capability was developed. Low insertion loss was achieved by an electrode width control single phase unidirectional transducer (EWC/SPUDT) configuration. Single mode selection was simply accomplished by a comb transducer which is a means of combining the frequency selectivity of two interdigital transducers (IDTs). Coupling of modes (COM) simulation was performed to predict device performance prior to fabrication. The measured frequency response S12 showed a good agreement with simulated results. The effect of the oscillator circuit system temperature shift upon frequency stability was observed in detail. The experimental results showed that the baseline noise was typically up to ~0.7 × 10−7 in a laboratory environment with temperature control. The oscillator was successfully applied to a gas sensor coated self-assembled composite monolayer as a sensor material for dimethyl-methyl-phosphonate (DMMP). The sensitivity for low DMMP concentration detection was evaluated as ~25 Hz mg−1 m−3, and the threshold detection limit was up to 0.5 mg m−3.

Surface Acoustic Wave Based Pressure Sensor with Ground Shielding over Cavity on 41° YX LiNbO3

A surface acoustic wave (SAW)-based pressure sensor was fabricated for stable mechanical compression force measurement. A single phase unidirectional transducer (SPUDT) and two acoustic tracks were employed to minimize inherent insertion loss and improve reflectivity from the reflectors. The coupling of modes (COM) theory and finite element methods (FEMs) were used to determine optimal design parameters. A LiNbO3 diaphragm was bonded to a heavily doped silicon substrate with a cavity of ~250 µm deep, in which gold was lined all over the inner cavity to reduce the coupling loss of SAW energy to the surrounding atmosphere. As a mechanical compression force was applied to the diaphragm, the diaphragm bent, resulting in phase shifts of the reflected peaks. The phase shifts were modulated depending on the amount of mechanical compression applied. The measured reflection coefficient S11 showed good agreement with simulated results.

Enhanced sensitivity of a surface acoustic wave gyroscope using a progressive wave

A surface acoustic wave (SAW)-based gyroscope with an 80 MHz central frequency was developed on two different piezoelectric substrates (128° YX LiNbO3 and ST-X quartz). A sensor was developed that contained two SAW oscillators. One oscillator was used as the sensing element and had metallic dots in the cavity between the input and output interdigital transducers (IDTs). The other oscillator was used as a reference element. Two oscillators were formed to extract the Coriolis effect by comparing the oscillation frequencies between these two delay lines, and metallic dots were used to induce a Coriolis force. Three different IDT structures were used to obtain a stable progressive SAW. Coupling of modes modeling was conducted prior to fabrication for determining the optimal device parameters. The device was fabricated and then measured on a rate table in accordance with the results of simulation. When the device was subjected to an angular rotation, the oscillation frequencies of the two oscillators were observed to differ. Depending on the angular velocity, the frequency difference was linearly modulated. The obtained sensitivity was approximately 62.57 Hz deg−1 s−1 at angular rates in the range 0–1000 deg s−1 in the case of the LiNbO3 substrate and single-phase unidirectional transducer and combed electrode structure. The dependence of the device performance on the piezoelectric substrate, IDT structure, and temperatures was also characterized. The developed device has good resistance to mechanical shock and stability to temperature.

A New Micro-rate Sensor Based on Shear Horizontal Surface Acoustic Wave Gyroscopic Effect

In this paper, we present a new micro-rate sensor based on the shear horizontal surface acoustic wave (SH-SAW) gyroscopic effect. The new SH-SAW propagating along the ST-90°X quartz substrate with heavy metallization exhibits excellent temperature stability, large acoustic velocity, high electromechanical coupling factors, and very small propagation attenuation. The response mechanism of such an SH-SAW micro-rate sensor was established using partial-wave analysis methods. The angular detection sensitivity in the propagation path of SH-SAW was evaluated and the effect of the metal interdigital transducer (IDT) electrode thickness on the sensor performance was also studied, resulting in the realization of the optimized design parameters prior to fabrication. Two SH-SAW delay lines with a reverse direction and an operation of 80 MHz on the same chip are fabricated as the feedback of SAW oscillators. The single-phase unidirectional transducer (SPUDT) was used to structure the delay lines to decrease the insertion loss. The Coriolis force from the external rotation acts on the particles along the SAW propagation path, then a pseudo SAW was induced, and couples with the initial SH-SAW; thus, the SAW velocity was deviated. Meanwhile, the differential oscillation frequency was changed linearly and used to characterize the input angular rate. Then, using the precise rate table, the performance of the fabricated SH-SAW rate sensor was evaluated experimentally. A sensitivity of 1.268 Hz deg-1 s-1 at angular rates of up to 2000 deg s-1, good linearity, and excellent temperature stability are observed.