Dennis Hohlfeld

VIBRATION ENERGY HARVESTING WITH ALUMINUM NITRIDE-BASED PIEZOELECTRIC DEVICES

This paper describes the measurement results of piezoelectric energy harvesters with aluminum nitride (AlN) as a piezoelectric material. AlN was chosen for its material properties and for its well-known sputter deposition process. For AlN devices a high optimum load resistance is required, which is favorable due to the high resulting voltage level. The output power harvested from mechanical vibrations has been measured on micromachined harvesters with different geometries. The resonance frequencies ranged from 200 up to 1200 Hz. The packaged devices had limited output powers and quality factors due to air damping caused by the package. A maximum output power of 60 µW has been measured on an unpackaged device at an acceleration of 2.0 g and at a resonance frequency of 572 Hz. The package of the harvester requires special attention, since air damping can significantly decrease the maximum power output.

Vacuum-packaged piezoelectric vibration energy harvesters: Damping contributions and autonomy for a wireless sensor system

This paper describes the characterization of thin-film MEMS vibration energy harvesters based on aluminum nitride as piezoelectric material. A record output power of 85 μW is measured. The parasitic-damping and the energy-harvesting performances of unpackaged and packaged devices are investigated. Vacuum and atmospheric pressure levels are considered for the packaged devices. When dealing with packaged devices, it is found that vacuum packaging is essential for maximizing the output power. Therefore, a wafer-scale vacuum package process is developed. The energy harvesters are used to power a small prototype (1 cm\3 volume) of a wireless autonomous sensor system. The average power consumption of the whole system is less than 10 μW, and it is continuously provided by the vibration energy harvester.

Micro-machined tunable optical filters with optimized band-pass spectrum

A novel MEMS-based tunable optical filter structure is presented which for the first time combines the advantages of an optimized filter shape function with tunability. Such a filter is essential for monitoring and reconfiguration of optical communication networks. The device is based on a Fabry-Perot interferometer employing multiple solid-state silicon cavities and dielectric Bragg mirrors. It is fabricated as a self-supporting membrane with thin film metal resistors using silicon MEMS technology.

Modeling and characterization of MEMS-based piezoelectric harvesting devices

Vibrational piezoelectric harvesting devices (PHD) provide an autonomous power source for various types of sensors, actuators and MEMS devices. There have been several examples of vibrational energy harvesters published in the literature over the years. However, for many applications the generated power is not yet sufficient. In this paper, a physical model for predicting the generated electric power from piezoelectric harvesting devices is introduced. The model is based on estimating the total charge generated on a piezoelectric material when it is subjected to mechanical strain as a result of bending at the fundamental resonance frequency. Based on Euler–Bernoulli beam theory, the strain can be determined in terms of the beam deflection at purely mechanical excitation. The proposed model extends the current state of the art by consideration of the strain distribution due to the presence of an extended mass volume at the end of the beam. The constitutive equations of piezoelectricity in the sensing mode correlate the strain and the induced charge in the piezoelectric element. Using the device design parameters and the beam deflection as inputs, the power output can be calculated. The results of the model were experimentally verified for MEMS-based PHDs. The model was found to give an accurate prediction of the electrical parameters under various damping conditions. After model validation, a subsequent device optimization has been made to improve the power generation.

An all-dielectric tunable optical filter based on the thermo-optic effect

A MEMS based, thermally tunable optical filter, hybridly assembled with a fibre based input/output, is presented. The filter is based on a Fabry?Perot interferometer employing a silicon cavity and silicon based dielectric Bragg reflectors and is fabricated as a free-standing membrane. The filter membrane is fixed to the substrate through micromachined suspension arms, which act as a thermal isolation. Wavelength tuning is achieved through thermal modulation of the cavitys optical thickness using thin film resistors. The filter characteristics measured were a full width at half-maximum value (FWHM) of 1.19?nm at a wavelength of 1530?nm and a finesse exceeding?1000. The tuning efficiency of a single-cavity filter with Bragg mirrors based on silicon nitride and silicon dioxide was measured to be 51.9?pm?K?1. Measurements of static and transient electrothermal behaviour were performed on filter membranes, resulting in maximum temperatures of 700??C and thermal time constants of 5.14?ms. The assembly technique relied on the alignment of optical fibres with the filter array using a novel silicon optical bench approach.

A thermally tunable, silicon-based optical filter

Abstract A novel silicon MEMS-based concept for tunable optical filters used in wavelength-division multiplexing (WDM) systems is presented. Such a filter is essential for monitoring and reconfiguring optical networks. The device is based on a Fabry–Perot interferometer employing a silicon cavity and silicon-based dielectric Bragg mirrors and is fabricated as a free-standing membrane. Wavelength tuning is achieved through thermal modulation of the resonator’s optical thickness. Measurement of optical filter performance and tunability as well as numerical simulation results of its steady-state and transient thermal behavior are presented.

First autonomous wireless sensor node powered by a vacuum-packaged piezoelectric MEMS energy harvester

This paper describes the experimental characterization of piezoelectric harvesters with different dimensions. We present a record level of generated power of 85 µW obtained from an unpacked device. We have developed a wafer-scale vacuum package which shows a 100–200 fold increase in power output compared with packaged devices under atmospheric pressure. A wireless sensor node was fully powered by a piezoelectric harvester. The average power consumption was less than 10 µW while it was operating at 15 seconds duty cycle.

Efficient extraction of thin-film thermal parameters from numerical models via parametric model order reduction

In this paper we present a novel highly efficient approach to determine material properties from measurement results. We apply our method to thermal properties of thin-film multilayers with three different materials, amorphous silicon, silicon nitride and silicon oxide. The individual material properties are identified by solving an optimization problem. For this purpose, we build a parameterized reduced-order model from a finite element (FE) model and fit it to the measurement results. The use of parameterized reduced-order models within the optimization iterations speeds up the transient solution time by several orders of magnitude, while retaining almost the same precision as the full-scale model.

Fabrication of gap-optimized CMUT

A recently introduced set up of capacitive micromachined ultrasonic transducers (cMUT) combines a conductive membrane above a structured sacrificial layer. All previous approaches either require an additional metallic electrode or do not possess a structured sacrificial layer and, consequently, may make exact adjustment of the membrane dimensions difficult. The present set ups are especially suited for the fabrication of cMUT with gap heights ranging between 50 nm and 2 /spl mu/m between the electrodes. Large gaps are a prerequisite to enabling sufficient deflections of the membrane and, therewith, to generating high pressure gradients. On the other hand, small gap sizes are desirable for detecting weak ultrasonic sources. This paper focuses on the fabrication process of cMUT to realize electrode separation above 500 nm and, in addition, on the manufacturing of cMUT with gaps below 500 nm. The successful realization has been proven by some basic experimental investigations. Finally, the fundamental equations of a frequently chosen simulation model are documented, as a number of ambiguities exist in the common literature.

Thermal and Optical Characterization of Silicon-Based Tunable Optical Thin-Film Filters

In this paper, we report on the successful fabrication and comprehensive optical and electrothermal characterization of a silicon-based tunable optical filter. Of particular interest here are the static and transient electrothermal characteristics of the filter membrane. The filter is configured as a Fabry-Perot resonator with dielectric Bragg reflectors and is fabricated as a stress-compensated multilayer of dielectric thin-films. Tunability is achieved by changing the refractive index of the solid-state cavity material through thermal modulation, accomplished by Joule heating in metal thin-film resistors. The filter layers are configured as a membrane to provide good thermal isolation and its electrothermal behavior was characterized in steady-state and dynamically. A maximum stable temperature difference of 450 K and a heating efficiency of 13.4 K mW-1 were measured with a thermal time constant for the filter hotplate of 3 ms. The filters spectral width is 0.28 nm at a wavelength of 1550 nm and through thermal modulation it was possible to shift the filter peak over more than 29 nm.