Micro-machined variable capacitors for power generation

Accepted versio

A MEMS Dual Vertical Electrometer and Electric Field-Mill

Presented is the first iteration of a Microelectromechanical System (MEMS) dual vertical electrometer and electric field-mill (EFM). The device uses a resonating structure as a variable capacitor that converts the presence of a charge or field into an electric signal. Previous MEMS electrometers are lateral electrometers with laterally spaced electrodes that resonate tangentially with respect to each other. Vertical electrometers, as the name suggests, have vertically spaced electrodes that resonate transversely with respect to each other. The non-tangential movement reduces damping in the system. Both types demonstrate comparable performance, but the vertical electrometer does so at a fraction of the size. In addition, vertical electrometers can efficiently operate as an electric field sensor. The electric field sensor simulations did not compare as well to other MEMS electric field sensors. However, the dual nature of this device makes it appealing. These devices can be used in missiles and satellites to monitor charge buildup in electronic components and the atmosphere [11]. Future iterations can improve these devices and give way to inexpensive, high-resolution electrostatic charge and field sensors

Doctor of Philosophy

dissertationThis thesis presents the design, fabrication and characterization of a microelectromechanical system (MEMS) based complete wireless microsystem for brain interfacing, with very high quality factor and low power consumption. Components of the neuron sensing system include TiW fixed-fixed bridge resonator, MEMS oscillator based action-potential-to-RF module, and high-efficiency RF coil link for power and data transmissions. First, TiW fixed-fixed bridge resonator on glass substrate was fabricated and characterized, with resonance frequency of 100 - 500 kHz, and a quality factor up to 2,000 inside 10 mT vacuum. The effect of surface conditions on resonator's quality factor was studied with 10s of nm Al2O3 layer deposition with ALD (atomic layer deposition). It was found that MEMS resonator's quality factor decreased with increasing surface roughness. Second, action-potential-to-RF module was realized with MEMS oscillator based on TiW bridge resonator. Oscillation signal with frequency of 442 kHz and phase noise of -84.75 dBc/Hz at 1 kHz offset was obtained. DC biasing of the MEMS oscillator was modulated with neural signal so that the output RF waveform carries the neural signal information. Third, high-efficiency RF coil link for power and data communications was designed and realized. Based on the coupled mode theory (CMT), intermediate resonance coil was introduced and increased voltage transfer efficiency by up to 5 times. Finally, a complete neural interfacing system was demonstrated with board-level integration. The system consists of both internal and external systems, with wireless powering, wireless data transfer, artificial neuron signal generation, neural signal modulation and demodulation, and computer interface displaying restored neuron signal

Cavallo's Multiplier for in situ Generation of High Voltage

A classic electrostatic induction machine, Cavallo's multiplier, is suggested for in situ production of very high voltage in cryogenic environments. The device is suitable for generating a large electrostatic field under conditions of very small load current. Operation of the Cavallo multiplier is analyzed, with quantitative description in terms of mutual capacitances between electrodes in the system. A demonstration apparatus was constructed, and measured voltages are compared to predictions based on measured capacitances in the system. The simplicity of the Cavallo multiplier makes it amenable to electrostatic analysis using finite element software, and electrode shapes can be optimized to take advantage of a high dielectric strength medium such as liquid helium. A design study is presented for a Cavallo multiplier in a large-scale, cryogenic experiment to measure the neutron electric dipole moment.Comment: 9 pages, 10 figure

CHARACTERIZATION OF ELECTROMAGNETIC VIBRATION BASED ENERGY HARVESTING SYSTEM

This work presents the characterization of electromagnetic vibration based energy harvesting system. Energy harvesting is the process which energy is derived from external sources, captured, and stored for low power application devices. Vibration energy is an alternative energy sources beside solar, wind, and thermal energies. Energy harvesting is crucially important as it is environmental friendly and renewable sources. Overall, the electromagnetic vibration based energy harvesting converted vibration energy into electrical energy based on electromagnetic principle and second order mass spring system. It mainly consists of proof mass, vibrating membrane (spring), coil and also permanent magnet. The main objective of this project is to perform the characterization for electromagnetic vibration based energy harvester. The characterizations are divided to two parts which cover both the electrical and mechanical characterizations. Under the electrical characterization, the open loop, closed loop, impedance matching and the effect of difference gravitational acceleration to output voltage are investigated. For mechanical characterization, the resonance frequency, bandwidth frequency and damping ratio are analyzed. From the electrical and mechanical characterization, it was found that the open loop voltage produced by the device is 0.6759V, while the close loop voltage is 0.3073V at 260 matched impedance. Device resonance occurs at 22.7 Hz with damping ratio of 0.001862 and 10Hz operating bandwidth. From the characterization results, this device is found to be suitable for low power and autonomous applications such as the wireless sensor network, memory cell for portable electronic device and others

Development of a Prototype Miniature Silicon Microgyroscope

A miniature vacuum-packaged silicon microgyroscope (SMG) with symmetrical and decoupled structure was designed to prevent unintended coupling between drive and sense modes. To ensure high resonant stability and strong disturbance resisting capacity, a self-oscillating closed-loop circuit including an automatic gain control (AGC) loop based on electrostatic force feedback is adopted in drive mode, while, dual-channel decomposition and reconstruction closed loops are applied in sense mode. Moreover, the temperature effect on its zero bias was characterized experimentally and a practical compensation method is given. The testing results demonstrate that the useful signal and quadrature signal will not interact with each other because their phases are decoupled. Under a scale factor condition of 9.6 mV/°/s, in full measurement range of ± 300 deg/s, the zero bias stability reaches 15°/h with worse-case nonlinearity of 400 ppm, and the temperature variation trend of the SMG bias is thus largely eliminated, so that the maximum bias value is reduced to one tenth of the original after compensation from -40 °C to 80 °C

Optimization studies of thermal bimorph cantilevers, electrostatic torsion actuators and variable capacitors

In this dissertation, theoretical analyses and optimization studies are given for three kinds of MEMS devices: thermal bimorph cantilevers, electrostatic torsion actuators, and variable capacitors. Calculation, simulation, and experimental data are used to confirm the device behavior and demonstrate the application of the design approaches. For thermal bimorph cantilevers, an analytical model is presented which allows theoretical analysis and quantitative optimization of the performance based on material properties and device dimensions. Bimorph cantilevers are divided into two categories for deflection optimization: either the total thickness is constant, or the cantilever has one constant and one variable layer thickness. The optimum equations are then derived for each case and can be used as design rules. The results show that substantial improvements are possible over existing design approaches. Other parameters like static temperature distribution, power consumption, and dynamic behavior are also discussed, as are design tradeoffs such as feature size, application constraints, fabrication feasibility, and cost. The electrostatic torsion actuator studies are conducted for two device types: round and rectangular. The first case describes an analytical study of the pull-in effect in round, double-gimbaled, electrostatic torsion actuators with buried, variable length electrodes, designed for optical cross-connect applications. It is found that the fractional tilt at pull-in for the inner round plate in this system depends only on the ratio of the length of the buried electrode to the radius of the plate. The fractional tilt at pull-in for the outer support ring depends only on the ratio of the length of the buried electrode to the outer radius of the ring and the ratio of the ring\u27s inner and outer radii. Expressions for the pull-in voltage are determined in both cases. General relationships are also derived relating the applied voltage to the resulting tilt angle, both normalized by their pull-in values. Calculated results are verified by comparison with finite element MEMCAD simulations, with fractional difference smaller than 4% for torsion mode dominant systems. For the second case, a fast, angle based design approach for rectangular electrostatic torsion actuators based on several simple equations is developed. This approach is significantly more straightforward than the usual full calculation or simulation methods. The main results of the simplified approach are verified by comparing them with analytical calculations and MEMCAD simulations with fractional difference smaller than 3% for torsion mode dominant actuators. Also, good agreement is found by comparison with the measured behavior of a micro-fabricated full-plate device. In the last topic, ultra-thin silicon wafers, SU-8 bonding and deep reactive ion etching technology have been combined for the fabrication of folded spring, dual electrostatic drive, vertical plate variable capacitor devices with displacement limiting bumpers. Due to the presence of the bumpers, the variable capacitor with parallel plate drive electrodes has two tuning voltage regimes: first a parabolic region that achieves roughly a 290% tuning range, then a linear region that achieves an additional 310%, making the total tuning range about 600%. The variable capacitor with comb drive electrodes has a parabolic region that achieves roughly a 205% tuning range, then a linear region that achieves an additional 37%, making its total tuning range about 242%. The variable capacitors have Q factors around 100 owing to the use of silicon electrodes other than lower resistivity metal

1.05-GHz CMOS Oscillator Based on Lateral-Field-Excited Piezoelectric AlN Contour-Mode MEMS Resonators

This paper reports on the first demonstration of a 1.05-GHz microelectromechanical (MEMS) oscillator based on lateral-field-excited (LFE) piezoelectric AlN contour-mode resonators. The oscillator shows a phase noise level of −81 dBc/Hz at 1-kHz offset frequency and a phase noise floor of −146 dBc/Hz, which satisfies the global system for mobile communications (GSM) requirements for ultra-high frequency (UHF) local oscillators (LO). The circuit was fabricated in the AMI semiconductor (AMIS) 0.5-μm complementary metal-oxide-semiconductor (CMOS) process, with the oscillator core consuming only 3.5 mW DC power. The device overall performance has the best figure-of-merit (FoM) when compared with other gigahertz oscillators that are based on film bulk acoustic resonator (FBAR), surface acoustic wave (SAW), and CMOS on-chip inductor and capacitor (CMOS LC) technologies. A simple 2-mask process was used to fabricate the LFE AlN resonators operating between 843 MHz and 1.64 GHz with simultaneously high Q (up to 2,200) and kt2 (up to 1.2%). This process further relaxes manufacturing tolerances and improves yield. All these advantages make these devices suitable for post-CMOS integrated on-chip direct gigahertz frequency synthesis in reconfigurable multiband wireless communications