Rebecca L. Peterson

Harvesting traffic-induced vibrations for structural health monitoring of bridges

This paper discusses the development and testing of a renewable energy source for powering wireless sensors used to monitor the structural health of bridges. Traditional power cables or battery replacement are excessively expensive or infeasible in this type of application. An inertial power generator has been developed that can harvest traffic-induced bridge vibrations. Vibrations on bridges have very low acceleration (0.1‐0.5 m s −2 ), low frequency (2‐30 Hz), and they are non-periodic. A novel parametric frequency-increased generator (PFIG) is developed to address these challenges. The fabricated device can generate a peak power of 57 μW and an average power of 2.3 μW from an input acceleration of 0.54 m s −2 at only 2 Hz. The generator is capable of operating over an unprecedentedly large acceleration (0.54‐9.8 m s −2 ) and frequency range (up to 30 Hz) without any modifications or tuning. Its performance was tested along the length of a suspension bridge and it generated 0.5‐0.75 μW of average power without manipulation during installation or tuning at each bridge location. A preliminary power conversion system has also been developed. (Some figures in this article are in colour only in the electronic version)

Microsystems for energy harvesting

This paper reviews the state of the art in miniature microsystems for harvesting energy from external environmental vibration, and describes two specific microsystems developed at the University of Michigan. One of these microsystems allows broadband harvesting of mechanical energy from extremely low frequency (1–5 Hz) random vibrations abundant in civil infrastructure, such as bridges. These parametric frequency increased generators have a combined operating range covering two orders of magnitude in acceleration (0.54–19.6 m/s2) and a frequency range spanning up to 60Hz, making them some of the most versatile harvesters in existence. The second of these systems is an integrated microsystem for harvesting energy from periodic vibrations at moderate frequencies (50–400 Hz) typically present in devices such as motors or transportation systems. This harvester utilizes a thinned-PZT structure to produce 2.74 µW at 0.1 g (167 Hz) and 205 µW at 1.5 g (154 Hz) at resonance. Challenges in the design of electronic circuitry (integrated or hybrid) for regulating the scavenged energy are briefly discussed.

Thinned-PZT on SOI process and design optimization for piezoelectric inertial energy harvesting

This paper presents the design, fabrication, and testing of a thinned-PZT/Si unimorph for vibration energy harvesting. It produces a record power output and has state-of-the-art efficiency. The harvester utilizes thinning of bulk-PZT pieces bonded to an SOI wafer, and takes advantage of the similar thermal expansion between PZT and Si to minimize beam bending due to residual stress. Monolithic integration of a tungsten proof mass lowers the resonance frequency and increases the power output. The harvester dimensions, including the PZT/Si thickness ratio and the proof-mass/total-beam length ratio, are optimized via parametric multi-physics FEA. Additionally, a fabrication process for hermetic packaging of the harvester is introduced. It uses vertical Si vias for electrical feed-throughs. An unpackaged harvester with a tungsten proof mass produces 2.74 µW at 0.1 g (167 Hz), and 205 µW at 1.5 g (154 Hz) at resonance (here, g = 9.8 m/s2). The active device volume is 27 mm3 (7 × 7 × 0.55 mm3). We report the highest power output, Normalized Power Density (N.P.D.), and Figure of Merit (N.P.D. × Bandwidth) amongst reported microfabricated vibration energy harvesters.

Fused-Silica Micro Birdbath Resonator Gyroscope (

We present a 3-D fused-silica micro-scale shell gyroscope, called the birdbath resonator gyroscope (BRG). The BRGs axisymmetric geometry leads to a good frequency and Q symmetry. The birdbath resonator can be fabricated with a good structural symmetry because its anchor is self-aligned to the rest of the structure. The BRG has n=2 wine-glass modes at 10.5 kHz and has a large frequency separation between the n=2 wine-glass modes and the closest parasitic mode (|fparasitic-fn=2|/fn=2=0.3), which will potentially lead to a low vibration sensitivity. The equations of motion for 3-D shell gyroscopes are derived and the effective mass and angular gain of the BRG is estimated using finite element method (FEM). The BRG is fabricated using a 3-D micrometer-blow-torching process and assembly on an electrode substrate made with the silicon-on-glass process. The BRG is operated in the force-rebalance mode at vacuum at room temperature and has a scale factor of 27.9 mV/(deg/s), a full-scale range , an angle random walk of 0.106 deg/√h, and a bias stability of 1 deg/h. A large angular gain (0.317) is measured, which is close to the estimated value of 0.25 obtained via FEM.

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Ultrathin, strained-silicon-on-insulator (s-SOI) structures without a residual silicon-germanium (SiGe) underlayer have been fabricated using stress balance of bi-layer structures on compliant borophosphorosilicate glass (BPSG). The bi-layer structure consisted of SiGe and silicon films, which were initially pseudomorphically grown on a silicon substrate and then transferred onto BPSG by a wafer bonding and SmartCut process. The viscous flow of the BPSG during a high-temperature anneal then allowed the SiGe/Si bi-layer to laterally coherently expand to reach stress balance, creating tensile strain in the silicon film. No dislocations are required for the process, making it a promising approach for achieving high-quality strained-silicon for device applications. To prevent the diffusion of boron and phosphorus into the silicon from the BPSG, a thin nitride film was inserted between the bi-layer and BPSG to act as a diffusion barrier, so that a lightly doped, sub-10-nm s-SOI layer (0.73% strain) was demonstrated. N-channel MOSFETs fabricated in a 25-nm silicon layer with 0.6% strain showed a mobility enhancement of 50%.

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Fully-depleted strained Si n-channel MOSFETs were demonstrated on a compliant borophosphorosilicate insulator (BPSG) without an underlying SiGe buffer layer. Stress balance of a SiGe/Si structure, transferred onto BPSG by wafer bonding and Smart-cut processes, is utilized for the first time to make strained-Si on insulator (sSOI) by a process that does not involve the introduction of misfit dislocations. Strained-Si n-channel MOSFETs with a strain level of 0.6%, equivalent to that of a conventional strained Si layer grown on a relaxed Si/sub 0.85/Ge/sub 0.15/ buffer, exhibit 60% mobility enhancement over the control, in good agreement with theory. This approach to fabricating strained Si overcomes any potential process or device complexity due to the presence of a SiGe layer in the final devices.

Ultrathin strained-SOI by stress balance on compliant substrates and FET performance

Harvesting energy from ambient vibrations is a promising technology for fully autonomous wireless sensor nodes, which can give birth to new applications in biomedical, industrial, and environmental monitoring. There have been independent solutions in increasing the harvesting efficiency either on the mechanical harvester [1] or on its power management circuitry [2,3]. Recently, a piezoelectric MEMS harvester using AlN was demonstrated to generate enough energy to autonomously power a wireless temperature sensor with a full-bridge rectifier built with off-the-shelf components [1]. Meanwhile, AC-DC converters for piezoelectric harvesters have been designed to enable efficient power extraction [2], or efficient rectification of low-voltage outputs [3], and have been tested with commercial meso-scale piezoelectric beams. However, to realize an efficient stand-alone energy generator platform, it is necessary to integrate these efforts into a single low-volume system. This paper presents a self-supplied energy generator, which includes a MEMS harvester hybridly integrated with its power management circuitry for autonomous charging of an energy reservoir (Fig. 6.9.1). The proposed packaging of the generator is <0.3cm3. Initial testing results are obtained with an unpackaged MEMS harvester.

Fully-depleted strained-Si on insulator NMOSFETs without relaxed SiGe buffers

This paper presents a novel 3-D fabrication process of microstructures using high-Q materials. The key feature of this process is the use of a blow torch that can provide intensive localized heat up to 2500°C in a very short time (<;10 sec), above the melting temperatures of many high-Q materials such as fused silica. Surface roughness of 5.3 Å is realized in fused silica, which is crucial for high optical and mechanical quality factors (Q). We demonstrate the fabrication of micro hemisphere and half-toroid (birdbath) geometries from 100μm-thick fused silica substrates. We also create micro birdbath resonators by batch-level releasing the birdbath shells and demonstrate one of the best mechanical Q and low stiffness and damping anisotropies among existing micro mechanical resonators. The birdbath resonator is promising for emerging applications such as micro rate-integrating gyroscope (μ-RIG).

A self-supplied inertial piezoelectric energy harvester with power-management IC

In this paper, we report on high-performance piezoelectric-on-silica micromechanical resonators for integrated timing applications. Fused silica is used as the resonator structural material for its excellent material properties, and thin film aluminum nitride is used as the piezoelectric transduction layer. A silica resonator is demonstrated with a high quality factor (QU~25,841), low motional impedance (Rm ~350 Ω), and good power handling capability. The measured f×Q product of this resonator is the highest amongst reported micromachined silica/fused quartz resonators.

High-Q fused silica birdbath and hemispherical 3-D resonators made by blow torch molding

This paper presents a new and simple microfabrication process for creating various 3-D microstructures with high-aspect ratios . The key feature of this process is the use of a blow torch, which provides very intense localized heat for a short amount of time . The flame temperature can be up to 2500 °C for a propane-oxygen torch, above the melting temperatures of many high- Q materials. We demonstrate the fabrication of hemispherical and half-toroid (birdbath) shells from 100- μm-thick fused silica substrates. The structures have an rms surface roughness of 5.3 Å, which is crucial for achieving both high mechanical and optical Q. We create microbirdbath resonators by batch-level releasing the birdbath shells. We verify the resonance mode shapes of the degenerate n=2 wineglass modes using laser vibrometry and measure the frequency and Q of these modes using laser vibrometry and a capacitive measurement method. We demonstrate one of the best mechanical Q and smallest frequency split between the n=2 wineglass modes among existing micromechanical resonators. The birdbath resonator is promising for emerging applications such as the microrate-integrating gyroscope.