Tahereh Arezoo Emadi

Multiple Moving Membrane CMUT With Enlarged Membrane Displacement and Low Pull-Down Voltage

A multiple moving membrane capacitive micromachined ultrasonic transducer ( M3-CMUT) has been fabricated and is shown to exhibit a significantly enlarged total membrane displacement ( ~ 280 nm) compared with the displacement of a conventional CMUT ( ~ 85 nm) for the same bias voltage. The M3-CMUT exhibits a significant reduction of the device pull-down voltage, while showing a much higher capacitance change, 100 fF for the M3-CMUT compared with only 20 fF for CMUT, at a dc bias of 25 V. The device performance, sensitivity, and acoustic power generation capability are primarily associated with the magnitude of the displacement of the membrane, and therefore, are enhanced through employing multiple deflectable membranes in M3-CMUT device. This high performance M3-CMUT is a promising candidate for high resolution ultrasonic imaging application.

Design and Fabrication of a Novel MEMS Capacitive Transducer With Multiple Moving Membrane,

A novel capacitive micromachined ultrasonic transducer is designed and fabricated. This transducer employs a stack of two deflectable membranes suspended over a fixed bottom electrode. In this configuration, the two moving membranes deflect simultaneously in response to a bias voltage, which results in a smaller effective cavity height compared with the conventional capacitive transducers. Electromechanical and acoustic analyses are conducted to investigate the transducer properties. A set of seven transducers with radii ranging from 30 to 55 μm were fabricated utilizing a sacrificial microelectromechanical system fabrication technology. Electrical measurements were performed and were compared with results from physical deflection measurements utilizing an optical vibrometer system. The results have been compared with analytical models as well as characterization of a set of five conventional, single membrane, transducers fabricated with the same technology. These experiments indicate a good agreement between the model and measured data. A larger membrane deflection and smaller cavity height are achieved from the double membrane devices. Therefore, this type of device may enhance the transducer acoustic power generation capability as well as increasing its sensitivity both of which result from the reduction in the transducer effective cavity height.

{\rm M}^{3}

Multiple moving membrane capacitive micromachined ultrasonic transducers (M3-CMUTs) employ a multiple vibrating plate configuration. The presence of an additional plate improves the transducer properties. In this letter, the device displacement amplitude is further enhanced through eliminating the transducer fixed electrode and employing a deflectable plate as the bottom electrode. A set of air-coupled transducers with the effective plate radius of 65 μm has been fabricated. Electrical and optical measurements were conducted. It is demonstrated that the transducer with a deflectable bottom electrode exhibits larger top plate displacement amplitude compared with the conventional CMUTs as well as the M3-CMUT with a fixed bottom electrode. A maximum of a 57% and 4% increase in the plate displacement is achieved at a dc bias level of 27 V for the transducer with vibrating bottom electrode compared with the conventional CMUT and M3-CMUT with a fixed bottom electrode, respectively.

-CMUT

A robust capacitive micromachined ultrasonic transducer has been developed. In this novel configuration, a stack of two deflectable membranes are suspended over a fixed bottom electrode. Similar to conventional capacitive ultrasonic transducers, a generated electrostatic force between the electrodes causes the membranes to deflect and vibrate. However, in this new configuration the transducer effective cavity height is reduced due to the deflection of two membranes. Therefore, the transducer spring constant is more susceptible to bias voltage, which in return reduces the required bias voltage. The transducers have been produced employing a MEMS sacrificial technique where two different membrane anchoring (curved- and flat- anchors) structures, with similar membrane radii were fabricated. Highly doped polysilicon was used as the membrane material. The resonant frequencies of the two transducers have been investigated. It was found that the transducers with curved membrane anchors exhibits a larger resonant frequency shift compared to the transducers with flat membranes for a given bias voltage. Comparison has been made between the spring constant of the flat membrane transducer and that of a conventional single membrane transducer. It is shown that the multiple moving membrane transducer exhibits a larger reduction in the spring constant compared to the conventional transducer, when driven with the same bias voltage. This results in a transducer with a higher power generation capability and sensitivity.

Design and Characterization of a Capacitive Micromachined Transducer With a Deflectable Bottom Electrode

In this paper, the temperature dependency of conventional polymer-based gas sensors is addressed and a novel polymer-based chemicapacitor sensor is presented for application in early detection of deterioration of grain in storage facilities. 3-D heat transfer simulations are used to investigate several heated platforms with the sensing area in millimeter range dimensions to enhance spoilage-induced analyte detection. A resistive heater is employed to heat and maintain the sensor at the desired operating temperature. The platform is optimized to achieve a uniform heat distribution throughout the sensing area with a temperature variation of ΔT ≤ 1°C. Capacitive measurements are performed as a more sophisticated technique for analyte detection, where mechanisms other than swelling are involved. In this method, the need for conductive filler is eliminated, resulting in an improvement in sensor reproducibility and repeatability. 1-octanol and relative humidity measurements are performed, as they are the two key volatiles due to grain deterioration. The results verify functionality of the fabricated sensors at different temperatures, in ppm range for 1-octanol, and up to 75% RH. Responses at the elevated temperature of 40 °C, above the grain bins ambient temperature, are insensitive to ambient temperature fluctuations. Sensitivity measurements demonstrate that an array of detectors, each held at different temperatures and operating at different frequencies, can be utilized to further enhance the sensor sensitivity and selectivity to a desired analyte.

Development of a novel configuration for a MEMS transducer for low bias and high resolution imaging applications

A major drawback in conventional polymer-based gas sensors is their temperature dependency, which causes drift in sensor response patterns in the real environment applications. Sensor repeatability and reproducibility are also a concern due to the difficulties associated with polymer composite film preparation. To enhance sensor performance for spoilage-induced analyte detection in grain storage facilities and to eliminate the ambient temperature dependency, a new sensor structure is proposed. The new design employs a temperature-controlled cantilever that maintains a uniform temperature throughout the polymer-based sensor. Capacitive measurements are performed to characterize the sensing material properties while exposed to different analytes. The performed frequency spectroscopy and temperature modulation demonstrate that an array of detectors, each held at different temperatures and operating at different frequencies, can be utilized to further enhance the sensor sensitivity and selectivity to a desired analyte.

Polymer-Based Chemicapacitor Sensor for 1-Octanol and Relative Humidity Detections at Different Temperatures and Frequencies

In this paper a 1-D micromachined-based transducer array is proposed for fault detection in high power transmission power cables. The proposed array benefits from silicon microfabrication techniques, and includes N = 101 capacitive micromachined cells that are located 5μm from each other. Transducer cells are actuated simultaneously, creating an acoustic wave that travels in the surrounding water domain. COMSOL acoustic simulations indicate that this array has a bandwidth of 7 MHz at -6 dB, resulting in a fractional bandwidth of BW/f0 = 70%. This array generates a directive beam pattern with a beam width of 3° at the central frequency of f = 10 MHz, resulting in high-resolution fault detection.