Yunjia Li

Very high Q-factor in water achieved by monolithic, resonant cantilever sensor with fully integrated feedback

We present a novel, monolithic, mass-sensitive cantilever sensor for measurements in liquids, which achieves a high quality factor (Q-factor) by closed-loop actuation. The cantilever is the frequency-determining element in the feedback system, its resonance frequency being a function of the mass-change on the surface. While cantilever-based sensors generally suffer from low quality factors in liquids due to the strong damping, our device uses an internal feedback loop circuitry to enhance the Q-factor. This allows to increase Q-factor from 23 to 19,000 at a resonance frequency of 221 kHz. The cantilever is electromagnetically actuated by Lorentz force while the oscillation is detected by piezoresistive MOS-transistors. A fully differential feedback circuitry with amplitude control is integrated together with the cantilever on the same chip. Thanks to the high Q-factor and the resulting frequency stability, even small frequency (and mass) changes can be precisely measured by this fully integrated system. Therefore, active, external actuation or readout instrumentation, such as a laser for optical detection, is not required. The sensor is an excellent candidate for biosensing applications in liquids such as biomolecule hybridization and illustrates the advantage of integrated circuitry for resonant sensors.

Large Stroke Staggered Vertical Comb-Drive Actuator for the Application of a Millimeter-Wave Tunable Phase Shifter

This paper presents the design, fabrication, and characterization of a MEMS actuator with large static deflection as a waveguide-mounted variable millimeter-wave phase shifter. The actuator is composed of a pair of interdigitated microplates actuated by vertical comb-drives and suspended by SU-8 torsional springs. The SU-8 spring possesses a thin metallization top layer and a reverse-T-shaped cross-section enabling low torsional stiffness and high in-plane stability. A maximum mechanical deflection of 10.3° is obtained under a dc actuation voltage of 35 V. The dynamic characterization of the device shows that the resonance frequency of the torsional mode is well separated from the other three bending modes, confirming the designed low torsional stiffness and high in-plane stability. The torsional viscoelastic creeping is measured as a function of time at different loads and shows a maximum of 0.5° for an applied voltage of 27.5 V. A high operation cycle test is conducted and the metalized SU-8 spring withstands 800 million cycles without showing fatigue. RF measurements show that a variable mechanical deflection angle between 0° and 8.2° results in a variable transmission phase shift up to 58.0°. The measured insertion loss is always below 5.1 dB at 98 GHz, corresponding to a figure of merit of 11.5°/dB.

Magnetically actuated CMOS resonant cantilever gas sensor for volatile organic compounds

We present a novel design of a resonant cantilever gas sensor for Volatile Organic Compounds (VOCs) in air. The cantilever is electromagnetically actuated, and the oscillation is detected with piezoresistive MOS-transistors. A fully differential feedback circuit is integrated together with the cantilever on the same chip. The whole system is fabricated in an industrial 0.8-/spl mu/m CMOS (Complementary Metal Oxide Semiconductor) process combined with subsequent micromachining. The cantilever is the frequency-determining element in the feedback system. Polymers are used as sensitive layers to assess VOC concentrations. The device exhibits less power dissipation of the cantilever as compared to earlier designs, which feature electrothermal actuation and readout via p-diffused resistors. The low power consumption enables the use of the sensor in portable devices.

A microdevice with large deflection for variable-ratio RF MEMS power divider applications

In this paper, we present the design and fabrication of a large deflection MEMS actuator directly integrated as a variable-ratio radio frequency (RF) power divider in a waveguide. The device is based on a tilting microplate suspended by SU-8 springs and driven by a vertical comb drive actuator. The fabricated device is characterized by laser Doppler vibrometer and white light interferometer measurements. The dynamic measurement confirms the resonance frequency of the torsional mode being around 370 Hz, from which the spring stiffness has been extracted and used for static modeling of the device. The fabricated microplate devices achieved deflection angles of 5.9° with a dc actuation voltage of 30 V. First RF transmission measurements show good agreement with results from electromagnetic field simulations. A variable power split ranging from equal division up to a ratio of more than 1:2 is measured at 82.5 GHz whilst keeping the amount of dissipated power below 25%. This is the first reported actuated RF MEMS device integrated in a metal waveguide operating at such high frequency.

Low-cost fabrication of PMMA and PMMA based magnetic composite cantilevers

We present a simple and low-cost fabrication process for microcantilevers from both pure Polymethylmethacrylate (PMMA) and magnetic PMMA composites with maghemite nanoparticle concentrations up to 3 vol.% (12 wt.%). The fabricated cantilevers have lengths of 30 – 300 µm, widths of 14 µm and a thickness of 3 µm. The presented fabrication process consists of structural layer patterning by hot embossing, removal of the residual layer by a deep ultraviolet (DUV) exposure step through a partially transparent mold and a sacrificial layer etch to release the suspended microstructures. Process parameters including hot embossing temperature and pressure have been studied. The optimized parameters for pure PMMA are 180° C and 2370 N/cm2 and for magnetic composite with 3 vol.% nanoparticle concentration 195° C and 2660 N/cm2. The molded cantilevers show a well transferred pattern and a sharp side wall profile.

W-band tunable reflective type phase shifter based on waveguide-mounted RF MEMS

A novel concept for a continuously variable W-band phase shifter is proposed. It is based on a ridge waveguide resonator tuned by MEMS-actuated conductive fingers that interact with the fields beneath the ridge. The rotation of anti-parallel oriented MEMS conductive fingers of about half wavelength size realize a distributed interaction with the purposely structured waveguide ridge. In such a way, large transmission phase variations are obtained. By cascading an additional resonator and a waveguide short, a reflection type phase shifter is created with a single MEMS. For MEMS finger deflection angles between 0° and 7.5°, this device realizes reflection phase shift variation of 381° and a reflection coefficient magnitude of better than −1.65 dB at a frequency of 106.5 GHz. This corresponds to a figure of merit of 230°/dB.

Monolithic CMOS multi-transducer gas sensor microsystem

This paper presents a monolithically integrated multi-transducer microsystem to detect organic and inorganic gases. The system comprises two polymer-based sensor arrays, a metal-oxide-based sensor array, driving and signal processing electronics and a digital communication interface. The chip is fabricated in industrial 0.8 /spl mu/m CMOS-technology with subsequent post-CMOS micromachining and operates at a supply voltage of 5 volt. The monolithic integration of different transducer types with associated driving and read-out circuitry significantly improves the discrimination capability of the system and reduces the packaging efforts.

Sensing Properties of a Novel Temperature Sensor Based on Field Assisted Thermal Emission

The existing temperature sensors using carbon nanotubes (CNTs) are limited by low sensitivity, complicated processes, or dependence on microscopy to observe the experimental results. Here we report the fabrication and successful testing of an ionization temperature sensor featuring non-self-sustaining discharge. The sharp tips of nanotubes generate high electric fields at relatively low voltages, lowering the work function of electrons emitted by CNTs, and thereby enabling the safe operation of such sensors. Due to the temperature effect on the electron emission of CNTs, the collecting current exhibited an exponential increase with temperature rising from 20 °C to 100 °C. Additionally, a higher temperature coefficient of 0.04 K−1 was obtained at 24 V voltage applied on the extracting electrode, higher than the values of other reported CNT-based temperature sensors. The triple-electrode ionization temperature sensor is easy to fabricate and converts the temperature change directly into an electrical signal. It shows a high temperature coefficient and good application potential.

Equivalent-Circuit Model for CMOS-Based Resonant Cantilever Biosensors

We present the modeling and characterization of a CMOS-based resonant cantilever biosensor operating in fluid environments. The device, which is fabricated in a 0.8-mum CMOS process with subsequent CMOS-compatible micromachining, consists of four resonant cantilevers (resonance frequency about 200 kHz in water) monolithically integrated with dedicated analog and digital signal processing and conditioning circuitry. The integrated cantilevers feature electromagnetic actuation and piezoresistive readout. In the first part of the paper an equivalent circuit model of the resonant cantilever is developed. The model allows a full characterization of the complete system comprising the cantilevers and the circuitry using commercial circuit simulation software. As will be shown, the presented model shows good agreement with actual measurements. At the end of the paper the application of the monolithic sensor system to the detection of prostate specific antigen (PSA) at clinically significant concentration levels will be presented.

SU-8 as a Torsional Spring: Viscoelastic Creeping and Closed Loop Control

The torsional viscoelastic creeping and recovery of an SU-8 spring is reported by studying the deflection angle variation of a vertical comb-drive actuator suspended by an SU-8 spring at three different stress levels. A maximum creep of 0.43° (corresponding to a relative change of strain of 5.7%) is observed under a constant stress of 8.3 MPa in 1 h (V = 22.7 V). A capacitive readout circuit is designed to sense the torsional deflection of the comb-drive actuator. The circuit is capable of actuating and sensing the capacitance change by the same combdrives with a resolution of 0.03° (corresponding to a capacitance resolution of 0.05 pF). The readout capacitance is then used as the input variable for a proportional-integral (PI) controller in order to eliminate the time and load dependent drift by creeping of the SU-8 spring. The PI controller is tuned by the Ziegler-Nichols method and has a settling time of ~2.3 s for the output. Compared with the open loop deflection of 0.43°, the closed-loop deflection is reduced by a factor of 7 down to 0.06°. This controller error is prominent at high actuation voltages, and is attributed to the actuators lateral and vertical cross-sensitivity.