The pressure level inside microsystem packages can significantly affect the performance of various microsystems, including resonators and inertial sensors. We are presenting the theory and experimental verification for using bondwires as sensors to measure in-package pressure. This approach is of particular interest for developing microsystems that need to operate under stable ambient conditions. The proposed technique covers the pressure range needed for most microsensors without imposing any additional processing steps and hence is applicable to a wide range of systems and applications. The device can be created from unused pads on a package during the wire bonding step in any packaging process. Relying on a standard step in packaging process, our approach adds no cost or space requirements to the overall system.
In this paper, we propose two structures for electrostatically actuated torsional microresonators in order to improve their mechanical quality factor (Q). In order to attain higher Q-factors, various mechanisms that limit Q-value of microresonators are investigated. The dominant sources of energy loss were minimized through structural design considerations. Since anchor and support losses are known to be the dominant energy dissipation mechanisms for most resonators, including the torsional microresonators of this work, our main focus is to come up with design structures that lead to anchor loss reduction. Finite element analyses (FEA) were employed to verify the design principle. It is demonstrated that the results are in agreement with our expectations, validating the effectiveness of presented structures from the view point of anchor loss reduction and Q-factor boosting. To compare analytic and experimental results, the proposed structures are fabricated and will be tested.
A timing reference incorporating a single-crystal-silicon micromechanical resonator with a quality factor of larger than one million and a resonant frequency of 2.18 MHz is demonstrated. The resonator is excited in the square extensional bulk acoustic mode at 4 mtorr, and it has been fabricated in a foundry SOI MEMS process. The silicon microresonator is adapted as a timing element for a precision oscillator with a measured short-term Allan deviation of 0.6 ppb.
This paper describes a new type of electric field sensor fabricated using micromachining technology. This micromachined sensor is dramatically smaller than conventional field mills, possessing a field chopping shutter measuring only 1 mm 2 . The shutter is moved using thermal actuators, thereby eliminating the wear and tear associated with rotating and moving elements of field mills. The sensor requires minimal operating power, with the shutter being driven by a 75 mV drive signal while consuming only 70 muW. The field chopping shutter operates at ~4200 Hz, enabling the measurement of both ac and dc fields. Two sets of sense electrodes enable differential field measurement, thereby not requiring a reference ground potential. The sensor has a linear response to electric field amplitude and has demonstrated capable of measuring a dc field as small as 42 V/m. This miniature sensor is the smallest sensor with such a resolution for use in power engineering applications.
A simulator is developed to estimate the electrical conductivity of polymer/nanotube composite layers as well as the change in their resistances under an applied strain. Simulation results are verified using experimental data on SU-8/Multiwall Carbon NanoTube composites. The model is based on conduction through a polymer body due to percolation between the conductive nanotubes. The simulator predicts the nanocomposite conductivity normalized by contact resistance between different filler concentrations. Several devices with different filler concentrations were fabricated on silicon substrates and studied. Experimental results agree with the performance trend that is predicted by the simulator as filler concentration and applied strains were varied independently. The simulator is capable of accounting for nanotube dimensions, polymer physical properties, conduction channel shape, and unevenly distributed forces in the polymer body.
An interface circuit for differential capacitive sensing applications with tunable dynamic range is presented. Capacitive microsensors are ubiquitously employed in many applications pertaining to all aspects of modern life. The proposed circuit requires a small chip area and offers a good signal-to-noise ratio as well as adjustable sensitivity and dynamic range. Reference signals are produced on chip and used for the synchronous demodulation of current signals through a differential capacitive sensor. In addition to the normal open-loop operation mode of the circuit, it can be operated within a novel closed-loop configuration in order to extend its dynamic range. The circuit was designed and fabricated in a standard 0.35-μm CMOS technology from Austriamicrosystems. Experimental and simulation results are presented and discussed. The circuit is capable of resolving 0.4 fF of variation in capacitance with a 50-kHz measurement bandwidth. Reducing the bandwidth to 1 kHz for signal frequencies around 10 kHz increases the dynamic range of the closed-loop circuit to 99 and 110 dB for open-loop and closed-loop circuits, respectively.
This paper presents a study on the application of piezojunction transduction for the detection of vibrations of resonant microdevices. The piezojunction effect refers to the dependence of the electrical characteristics of a p-n junction to mechanical stress. It is shown that the piezojunction signal is proportional to the bias current of the diode, which can be adjusted as needed. A simple model that accounts for both capacitive and piezojunction currents and the equivalent electrical representations of the microdevice are developed and verified. A bulk-mode extensional resonator with an integrated p-n junction was designed and fabricated to serve as a proof-of-concept device. The static and dynamic responses of the fabricated devices were measured and compared against the models. The extensional-mode frequency of the resonator was measured to be ~7 MHz with a mechanical quality factor of ~1400. Capacitive and piezojunction signals at the output of the device were isolated and studied. It is shown that even with diode bias currents on the order of a few microamperes, the piezojunction and capacitive currents are comparable. Experimental verification demonstrates that piezojunction effect is a promising addition to the existing detection techniques in the resonance-based applications, where small chip area, integration, and power consumption are key requirements.
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