Kemal Safak Demirci

Liquid-Phase Chemical Sensing Using Lateral Mode Resonant Cantilevers

Liquid-phase operation of resonant cantilevers vibrating in an out-of-plane flexural mode has to date been limited by the considerable fluid damping and the resulting low quality factors (Q factors). To reduce fluid damping in liquids and to improve the detection limit for liquid-phase sensing applications, resonant cantilever transducers vibrating in their in-plane rather than their out-of-plane flexural resonant mode have been fabricated and shown to have Q factors up to 67 in water (up to 4300 in air). In the present work, resonant cantilevers, thermally excited in an in-plane flexural mode, are investigated and applied as sensors for volatile organic compounds in water. The cantilevers are fabricated using a complementary metal oxide semiconductor (CMOS) compatible fabrication process based on bulk micromachining. The devices were coated with chemically sensitive polymers allowing for analyte sorption into the polymer. Poly(isobutylene) (PIB) and poly(ethylene-co-propylene) (EPCO) were investigated as sensitive layers with seven different analytes screened with PIB and 12 analytes tested with EPCO. Analyte concentrations in the range of 1-100 ppm have been measured in the present experiments, and detection limits in the parts per billion concentration range have been estimated for the polymer-coated cantilevers exposed to volatile organics in water. These results demonstrate significantly improved sensing properties in liquids and indicate the potential of cantilever-type mass-sensitive chemical sensors operating in their in-plane rather than out-of-plane flexural modes.

Mass-sensitive detection of gas-phase volatile organics using disk microresonators.

The detection of volatile organic compounds (VOCs) in the gas phase by mass-sensitive disk microresonators is reported. The disk resonators were fabricated using a CMOS-compatible silicon micromachining process and subsequently placed in an amplifying feedback loop to sustain oscillation. Sensing of benzene, toluene, and xylene was conducted after applying controlled coatings of an analyte-absorbing polymer. An analytical model of the resonators chemical sensing performance was developed and verified by the experimental data. Limits of detection for the analytes tested were obtained, modeled, and compared to values obtained from other mass-sensitive resonant gas sensors.

Geometrical optimization of resonant cantilevers vibrating in in-plane flexural modes

The influence of the beam geometry on the quality factor and resonance frequency of resonant silicon cantilever beams vibrating in their fundamental in-plane flexural mode has been investigated in air and water. Compared to cantilevers vibrating in their out-of-plane flexural mode, utilizing the in-plane mode results in reduced damping and reduced mass loading by the surrounding fluid. Quality factors as high as 4,300 in air and 67 in water have been measured for cantilevers with a 12 µm thick silicon layer. This is in comparison to Q-factors up to 1,500 in air and up to 20 in water for cantilevers vibrating in their fundamental out-of-plane bending mode. Based on the experimental data, design guidelines are established for beam dimensions that ensure maximal Q-factors and minimal mass loading by the surrounding fluid.

Temperature compensation method for resonant microsensors based on a controlled stiffness modulation

A strategy to compensate for frequency drifts caused by temperature changes in resonant microstructures is presented. The proposed compensation method is based on a controlled stiffness modulation of the resonator by an additional feedback loop to extract the frequency changes caused by temperature changes. The feasibility of the suggested method is verified experimentally by compensating for temperature-induced frequency fluctuations of a micromachined resonator. The developed compensation scheme requires only one additional feedback loop and is applicable to any resonant microstructure featuring excitation and detection elements.

Gas and liquid phase sensing of volatile organics with disk microresonator

The sensing of volatile organic compounds (VOCs) using a MEMS resonator with an in-plane vibrational mode is reported. VOCs are detected in both the gas and liquid phases by a polymer-coated disk microresonator, which is operated as the frequency determining element in an amplifying feedback loop. The functionalized disk microresonators exhibit a short term frequency stability of 1.1 times 10-7 in air and 3.4 times 10-6 in water. Using polymer membranes as chemically sensitive layers, different concentrations of o-xylene, benzene, octane, trichloroethane, and toluene have been detected in the gas phase, with the limit of detection for o-xylene being 2.2 ppm. M-xylene has been detected in the liquid phase with a limit of detection of 1.9 ppm.

Exploring the resolution of different disk-type chemical sensors

Chemical sensitivities and limits of detection to volatile organic compounds (VOCs) for polymer-coated disk resonators in the gas phase are reported. The dependence of the sensor performance on the device geometry is investigated. The use of rotational inplane modes results in high frequency stabilities, with Allan variances as low as 7.8×10−9 being achieved for 25µm thick resonators. A limit of detection of 18.3 ppm for toluene was obtained for a 5 µm thick device coated with an only 20 nm thick polymer layer.

Frequency Drift Compensation in Mass-Sensitive Chemical Sensors based on Periodic Stiffness Modulation

The successful compensation of frequency drift in a mass-sensitive chemical microsensor is demonstrated. The proposed compensation method uses a periodic stiffness modulation, generated by a second feedback loop, to monitor the microresonators quality factor (Q-factor). The Q-factor is solely obtained from frequency measurements and monitored along with the measurand-induced frequency shift during normal closed-loop sensor operation. This simultaneous measurement of Q-factor and frequency shift enables the compensation of frequency drift induced by environmental disturbances using the extracted Q-factor. The feasibility of drift compensation has been demonstrated by implementing the compensation scheme into a closed-loop chemical sensing system and performing gas-phase chemical measurements.

Assessing polymer sorption kinetics using micromachined resonators

This paper introduces a new approach to investigate polymer sorption kinetics by weighing the polymer films using micromachined in-plane resonators. A custom gas-testing setup enables fast analyte concentration changes, which are necessary to study analyte diffusion into thin polymer coatings deposited on top of the microresonators. Short-term frequency stabilities of the microresonators in the 10−8 range in air yield sub-picogram mass resolution and enable real-time measurement of analyte uptake into µm-thick films with time constants ranging from seconds to minutes. As an example, the diffusion of alcohols and aromatic hydro-carbons into poly(epichlorohydrin) and poly(isobuty-lene) films, which are of interest for chemical sensing applications, has been investigated.

A Low-Leakage Body-Guarded Analog Switch in 0.35-

A low-leakage body-guarded analog switch (BGswitch) for slow switched-capacitor (SC) circuits is presented. The improvement of accuracy in SC circuits employing BG-switches is demonstrated by comparing their performance with counterparts employing conventionally biased CMOS switches in three applications: sample-and-hold (S/H) amplifier, SC amplifier, and highvoltage drain-extended MOSFET (DEMOS). The leakage currents of BG-switch-enabled circuits are characterized across process variations and different operation voltages in all demonstrated applications. With nominal output voltages at room temperature, the average absolute leakage current of BG-switch-enabled S/H amplifier (12.02 aA), SC amplifier (54.52 aA), and DEMOS (53.71 fA) show leakage current improvement of 21, 28, and 17 dB, respectively, compared with equivalent circuits utilizing transmission gates (TGs). BG-switch-enabled S/H circuits and SC amplifiers with average performance exhibit lower leakage currents up to 100 °C compared with TG-enabled circuits. The demonstrated applications utilizing BG-switches were fabricated in a standard 0.35-μm BiCMOS process.

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This paper presents a chemical microsystem based on the integration of a silicon-based resonant microsensor and a CMOS ASIC for portable sensing applications in air. Cantilever-based microresonators coated with chemically-sensitive polymer films are used as microsensors. The CMOS ASIC utilizes the self-oscillation method, which uses an amplifying feedback loop. For improved short- and long-term frequency stabilities, the ASIC also includes several control circuits such as an automatic gain control loop, an automatic phase control loop, and a frequency drift compensation loop. The performance of the implemented microsystem has been evaluated experimentally by detecting various volatile organic compounds.