Wencheng Xu

A contour-mode film bulk acoustic resonator of high quality factor in a liquid environment for biosensing applications

This letter reports an acoustic resonator of high quality factors (Qs) operating in liquid media. The film bulk acoustic resonator (FBAR) is made of a ring-shaped piezoelectric aluminum nitride thin film, and is excited in a contour mode. By having a low motional resistance upon coupling with liquids, the contour mode FBAR achieved Qs up to 189, more than 12× over the state-of-the-art FBARs in liquids. The resonator was characterized by an aptamer—thrombin binding pair for a biosensor and showed a mass resolution of 1.78 ng/cm2.

A High-Quality-Factor Film Bulk Acoustic Resonator in Liquid for Biosensing Applications

We report a high-quality-factor (Q) film bulk acoustic resonator (FBAR) operating in liquid environments. By integrating a microfluidic channel to a longitudinal-mode FBAR, a Q of up to 150 is achieved with direct liquid contacting. A transmission line model is used to theoretically predict the Q behavior of the FBAR. The model suggests an oscillatory pattern of Q as a function of the channel thickness and the acoustic wavelength in the liquid, which is experimentally verified by precisely controlling the channel thickness. This FBAR biosensor is characterized in liquids for the real-time in situ monitoring of the competitive adsorption/exchange of proteins, the Vroman effect. The FBAR offers a minimum detectable mass of 1.35 ng/cm2 and is successfully implemented in a Pierce oscillator as a portable sensing module.

A Fully Passive Wireless Microsystem for Recording of Neuropotentials Using RF Backscattering Methods

The ability to safely monitor neuropotentials is essential in establishing methods to study the brain. Current research focuses on the wireless telemetry aspect of implantable sensors in order to make these devices ubiquitous and safe. Chronic implants necessitate superior reliability and durability of the integrated electronics. The power consumption of implanted electronics must also be limited to within several milliwatts to microwatts to minimize heat trauma in the human body. In order to address these severe requirements, we developed an entirely passive and wireless microsystem for recording neuropotentials. An external interrogator supplies a fundamental microwave carrier to the microsystem. The microsystem comprises varactors that perform nonlinear mixing of neuropotential and fundamental carrier signals. The varactors generate third-order mixing products that are wirelessly backscattered to the external interrogator where the original neuropotential signals are recovered. Performance of the neurorecording microsystem was demonstrated by wireless recording of emulated and in vivo neuropotentials. The obtained results were wireless recovery of neuropotentials as low as approximately 500 microvolts peak-to-peak (μVpp) with a bandwidth of 10 Hz to 3 kHz (for emulated signals) and with 128 epoch signal averaging of repetitive signals (for in vivo signals).

Real-Time Monitoring of Whole Blood Coagulation Using a Microfabricated Contour-Mode Film Bulk Acoustic Resonator

We report a microelectromechanical systems contour-mode film bulk acoustic resonator (C-FBAR) to monitor in-vitro whole blood coagulation in real time. The C-FBAR has a suspended ring made of piezoelectric aluminum nitride excited in the radial-extensional mode. It operates at its resonant frequency of 150 MHz and possesses a quality factor of 77 in citrated human blood. The C-FBAR is characterized using aqueous glycerine solutions showing that it accurately measures the viscosity in the range of 1 to 10 centipoise. The C-FBAR, then, is used to monitor in-vitro blood coagulation processes in real time. Results show that its resonant frequency decreases as viscosity of the blood increases, during the fibrin generation process after the coagulation cascade. The coagulation time and the start/end of the fibrin generation are quantitatively determined. The C-FBAR has the potential to become a low-cost, portable, yet reliable tool for hemostasis diagnostics.

In-Liquid Quality Factor Improvement for Film Bulk Acoustic Resonators by Integration of Microfluidic Channels

We demonstrate a significant improvement in the quality factor (Q) of film bulk acoustic resonators (FBARs) in liquid environments via the integration of microfluidic channels. Our device consists of a longitudinal-mode excited zinc oxide (ZnO) FBAR and parylene-encapsulated microfluidic channels. Considerable enhancement in the Q of the resonant system is obtained by confining the liquid in the microfluidic channels of thickness comparable to the acoustic wavelength. The improved FBAR achieves Q values of up to 120, which represents an improvement of a factor of eight over those of current state-of-the-art devices.

Oscillatory

When a film bulk acoustic resonator (FBAR) is coupled to a liquid layer with thickness comparable to the acoustic wavelength, the Q factor varies in a damped oscillatory pattern with the liquid thickness. This letter reports an analytical modeling and experimental demonstration of this behavior by integrating microfluidic channels to MEMS-based FBARs. It is found that Q assumes its maxima and minima when the channel thickness is an odd multiple of quarter-wavelength and a multiple of half-wave-length, respectively. The microfluidics integrated FBARs achieve a 10 × improvement of Q over fully immersed FBARs, showing the potential of use as high-resolution sensors involving liquids.

Q

When a film bulk acoustic resonator (FBAR) is coupled to a thin liquid layer, the quality factor (Q) of the resonator varies in a damped oscillatory pattern versus the liquid thickness. This paper reports this behavior with an analytical modeling and experimental demonstration of it. The thin liquid layer with thickness comparable to the acoustic wavelength is realized by integrating a microfluidic channel to the FBAR. Q assumes its maxima (up to 150) and minima (as low as 50) when the channel thickness is an odd multiple of quarter-wavelength and a multiple of half-wavelength, respectively. We also present a bio-molecular detection test for aptamer immobilization and thrombin binding as a proof-of-concept sensing application.

Factor in Film Bulk Acoustic Resonators With Integrated Microfluidic Channels

We present a temperature compensation technique of a film bulk acoustic resonator (FBAR)-based oscillator by tuning the supply voltage of the oscillator. The FBAR-based oscillator uses a high-Q FBAR that is made of a thin ZnO piezoelectric film sandwiched by two electrodes. The FBAR is significantly sensitive to temperature change, consequently resulting in large temperature sensitivity of the FBAR-based oscillator. In this paper, we present a temperature compensation technique that improves the temperature coefficient (TCfosc) of a 1.625-GHz FBAR-based oscillator from -118 ppm/K to less than 1 ppm/K by tuning the supply voltage of the oscillator. The tuning technique has a large frequency tunability of -4305 ppm/V.

Oscillating behavior of quality factor of a film bulk acoustic resonator in liquids

This paper presents the thermal analysis and characterization of a zinc oxide (ZnO) based film bulk acoustic resonator (FBAR) having a high quality factor (Q) in liquid environments for biosensing applications. Q of up to 120, an improvement of at least 8× greater than state-of-the-art devices in liquids, are achieved by integrating microfluidic channels with heights comparable to the acoustic wavelength in FBAR. In order to achieve temperature stability in the highly sensitive FBAR sensor, we analyze sources of thermal effects and characterize FBAR in a Pierce oscillator. Measurements show a temperature coefficient of oscillation frequency (TCF) of -112 ppm/K for the uncompensated circuit. We show that this thermal drift can be reduced to less than 1ppm/K by applying a properly chosen bias to the oscillator, which suggests the possibility of a feedback approach to achieve thermal stability.

A Temperature Compensation Concept for a Micromachined Film Bulk Acoustic Resonator Oscillator

We report a MEMS contour-mode film bulk acoustic resonator (C-FBAR) to monitor whole blood coagulation in-vitro in real time. The C-FBAR has a suspended ring of piezoelectric aluminum nitride excited in the radial-extensional mode. It operates at its resonant frequency of 150 MHz and possesses a quality factor (Q) of 140 in water. The C-FBAR is characterized using aqueous glycerine solutions showing that it accurately measures the viscosity in the range of 1 to 10 centipoises. The C-FBAR, then, is used to monitor blood coagulation in-vitro in real time. The resonant frequency decreases as viscosity of the blood increases during the fibrin creation after the coagulation cascade. The coagulation time and the start/end of the fibrin creation are determined, suggesting the C-FBAR as a potentially low-cost, portable tool for hemostasis diagnostics.