Integration of structures and functions has permitted the electricity consumption of sensors, actuators and electronic devices to be reduced. Therefore, it is now possible to imagine low-consumption devices able to harvest energy from their surrounding environment. One way to proceed is to develop converters able to turn mechanical energy, such as vibrations, into electricity: this paper focuses on electrostatic converters using electrets. We develop an accurate analytical model of a simple but efficient cantilever-based electret energy harvester. We prove that with vibrations of 0.1g (~1 m s−2), it is theoretically possible to harvest up to 30 µW per gram of mobile mass. This power corresponds to the maximum output power of a resonant energy harvester according to the model of William and Yates. Simulation results are validated by experimental measurements, raising at the same time the large impact of parasitic capacitances on the output power. Therefore, we 'only' managed to harvest 10 µW per gram of mobile mass, but according to our factor of merit, this is among the best results so far achieved.

https://iopscience.iop.org/article/10.1088/0964-1726/20/10/105013/pdf

Cantilever-based electret energy harvesters

It is demonstrated that the Bargmann representation of quantum mechanics is ideally suited for semiclassical analysis, using as an example the WKB method applied to the bound-state problem in a single well of one degree of freedom. For the harmonic oscillator, this WKB method trivially gives the exact eigenfunctions in addition to the exact eigenvalues. For an anharmonic well, a self-consistent variational choice of the representation greatly improves the accuracy of the semiclassical ground state. Also, a simple change of scale illuminates the relationship of semiclassical versus linear perturbative expansions, allowing a variety of multidimensional extensions.

Wentzel-Kramers-Brillouin method in the Bargmann representation. [of quantum mechanics

Thanks to miniaturisation, it is today possible to imagine self-powered systems that use vibrations to produce their own electrical energy. Many energy-harvesting systems already exist. Some of them are based on the use of electrets: electrically charged dielectrics that can keep charges for years. This paper presents an optimisation of an existing system and proves that electret-based electrostatic energy scavengers can be excellent solutions to power microsystems even with low-level ambient vibrations. Thereby, it is possible to harvest up to 200\muW with vibrations lower than 1G of acceleration (typically 50\mumpp at 50Hz) using thin SiO2 electrets with an active surface of 1 cm^{2} and a mobile mass of 1g. This paper optimises such a system (geometric, electrostatic and mechanical parameters), using FEM (Finite Element Method) software (Comsol Multiphysics) and Matlab to compute the parameters and proves the importance of such an optimisation to build efficient systems. Finally, it shows that the use of electrets with high surface potential is not always the best way to maximise output power.

Optimization of an electret-based energy harvester

Wet anisotropic etching based silicon micromachining is an important technique to fabricate freestanding (e.g. cantilever) and fixed (e.g. cavity) structures on different orientation silicon wafers for various applications in microelectromechanical systems (MEMS). {111} planes are the slowest etch rate plane in all kinds of anisotropic etchants and therefore, a prolonged etching always leads to the appearance of {111} facets at the sidewalls of the fabricated structures. In wet anisotropic etching, undercutting occurs at the extruded corners and the curved edges of the mask patterns on the wafer surface. The rate of undercutting depends upon the type of etchant and the shape of mask edges and corners. Furthermore, the undercutting takes place at the straight edges if they do not contain {111} planes. {100} and {110} silicon wafers are most widely used in MEMS as well as microelectronics fabrication. This paper reviews the fabrication techniques of convex corner on {100} and {110} silicon wafers using anisotropic wet chemical etching. Fabrication methods are classified mainly into two major categories: corner compensation method and two-steps etching technique. In corner compensation method, extra mask pattern is added at the corner. Due to extra geometry, etching is delayed at the convex corner and hence the technique relies on time delayed etching. The shape and size of the compensating design strongly depends on the type of etchant, etching depth and the orientation of wafer surface. In this paper, various kinds of compensating designs published so far are discussed. Two-step etching method is employed for the fabrication of perfect convex corners. Since the perfectly sharp convex corner is formed by the intersection of {111} planes, each step of etching defines one of the facets of convex corners. In this method, two different ways are employed to perform the etching process and therefore can be subdivided into two parts. In one case, lithography step is performed after the first step of etching, while in the second case, all lithography steps are carried out before the etching process, but local oxidation of silicon (LOCOS) process is done after the first step of etching. The pros and cons of all techniques are discussed.

/pdf/a-comprehensive-review-on-convex-and-concave-corners-in-5f1mlgjztz.pdf

A comprehensive review on convex and concave corners in silicon bulk micromachining based on anisotropic wet chemical etching

Thanks to miniaturization, it is now possible to imagine self-powered systems that use vibrations to produce their own electrical energy. Many energy harvesting systems already exist. Some of them are based on the use of electrets: electrically charged dielectrics that can retain charge for years. This paper presents an optimization of an existing system and proves that electret-based electrostatic energy scavengers can be excellent solutions to power microsystems even with low-level ambient vibrations. Thereby, it is possible to harvest up to 200??W with vibrations of acceleration lower than 1g (typically 50? ?mpp at 50?Hz) using thin SiO2 electrets with an active surface of 1?cm2 and a mobile mass of 1?g. This paper optimizes such a system (geometric, electrostatic and mechanical parameters), using FEM (finite element method) software (Comsol Multiphysics) and Matlab to compute the parameters, and proves the importance of such an optimization to build efficient systems. Finally, it is shown that the use of electrets with high surface potential is not always the best way to maximize output power.

https://iopscience.iop.org/article/10.1088/0964-1726/19/7/075015/pdf

This paper reports a new category of high-Q edge-suspended inductors (ESI) that are fabricated using CMOS-compatible micromachining techniques. This structure was designed based on the concept that the current was crowded at the edges of the conducting metal wires at high frequencies due to the proximity effect. The substrate coupling and loss can be effectively suppressed by removing the silicon around and underneath the edges of the signal lines. Different from the conventional air-suspended inductors that have the inductors built on membranes or totally suspended in the air, the edge-suspended structures have the silicon underneath the center of the metal lines as the strong mechanical supports. The ESIs are fabricated using a combination of deep dry etching and anisotropic wet etching techniques that are compatible with CMOS process. For a three-turn 4.5-nH inductor, a 70% increase (from 6.8 to 11.7) in maximum Q-factor and a 57% increase (from 9.1 to 14.3 GHz) in self-resonance frequency were obtained with a 11-/spl mu/m suspended edge in 25-/spl mu/m-wide lines.

CMOS-compatible micromachined edge-suspended spiral inductors with high Q-factors and self-resonance frequencies

This paper presents a very simple micro-Pirani vacuum gauge made of single crystal silicon. It was fabricated by conventional bulk micromachining technology with only three masks. The gauge has 80 μm thick bulk silicon as structure layer and dual heat sinks, which is very robust with very little internal stress and also provide large heat transfer area. Highly phosphorus ion doping at bottom of the anchors forms ohmic contact with Cr/Au leads. I–V curve shows good linear resistance characteristic of the single crystal silicon heater. The dynamic range of the Pirani vacuum gauge is 1–1000 Pa, and the average sensitivity in the dynamic range is 117 K/W/Pa. With an uncertainty of 1%, the resolution of the gauge is 12 Pa at 1000 Pa and 0.2 Pa at 4 Pa.

A single crystal silicon micro-Pirani vacuum gauge with high aspect ratio structure

This paper reports the micromachining techniques for fabricating edge-suspended RF/microwave passive components, which are proposed to deliver enhanced performance without sacrificing their mechanical strength and reliability. The fabrication incorporates ICP-DRIE dry etching and TMAH anisotropic etching techniques, which are both CMOS compatible. The edge-suspended structures were realized by a TMAH solution consisting of 5 wt% TMAH, 1.6 wt% Si and 0.5 wt% (NH4)2S2O8. This solution offers effective etching of silicon along the {100} and {110} planes, while having negligible etching on aluminum and {111} planes. The layout requirement for achieving edge-suspended passive components is also outlined on the basis of the analysis of the anisotropic etching along different crystal orientations. Using the techniques described here, high performance spiral inductors and coplanar waveguides (CPW) with significantly reduced loss are demonstrated. For a three-turn 4.5 nH inductor, a 70% increase (from 6.8 to 11.7) in maximum Q-factor and a 57% increase (from 9.1 GHz to 14.3 GHz) in self-resonance frequency are obtained with an 11 µm suspended edge in 25 µm wide lines compared to the conventional inductors without being micromachined. A 50 Ω edge-suspended CPW exhibits a reduction in insertion loss, from 2.4 dB mm−1 in a conventional CPW to 0.4 dB mm−1 at 39 GHz.

CMOS-compatible micromachining techniques for fabricating high-performance edge-suspended RF/microwave passive components on silicon substrates

This paper presents detailed characterization of a category of edge-suspended coplanar waveguides that were fabricated on low-resistivity silicon substrates using improved CMOS-compatible micromachining techniques. The edge-suspended structure is proposed to provide reduced substrate loss and strong mechanical support at the same time. It is revealed that, at radio or microwave frequencies, the electromagnetic waves are highly concentrated along the edges of the signal line. Removing the silicon underneath the edges of the signal line, along with the silicon between the signal and ground lines, can effectively reduce the substrate coupling and loss. The edge-suspended structure has been implemented by a combination of deep reactive ion etching and anisotropic wet etching. Compared to the conventional silicon-based coplanar waveguides, which show an insertion loss of 2.5dB/mm, the loss of edge-suspended coplanar waveguides with the same dimensions is reduced to as low as 0.5 dB/mm and a much reduced attenuation per wavelength (dB/lambdag) at 39 GHz. Most importantly, the edge-suspended coplanar waveguides feature strong mechanical support provided by the silicon remaining underneath the center of the signal line. The performance of the coplanar waveguides is evaluated by high-frequency measurement and full-wave electromagnetic (EM) simulation. In addition, the resistance, inductance, conductance, capacitance (RLGC) line parameters and the propagation constant of the coplanar waveguides (CPWs) were extracted and analyzed

Characterization and attenuation mechanism of CMOS-compatible micromachined edge-suspended coplanar waveguides on low-resistivity silicon substrate

In this paper, a very simple micro vacuum pirani gauge made of single crystal silicon is presented. It was fabricated by standard bulk micromachining with only three masks. The high aspect ratio structure of the pirani gauge has large efficient gaseous heat transfer area, thus can increase the sensitivity of the pirani gauge. The dynamic range of the pirani gauge is 2Pa to 400Pa which should be larger if the power of the current source be larger with high sensitivity. And the average sensitivity is 184 K/W/Pa.

Jinwen Zhang

Papers

CMOS-compatible micromachined edge-suspended spiral inductors with high Q-factors and self-resonance frequencies

A single crystal silicon micro-Pirani vacuum gauge with high aspect ratio structure

CMOS-compatible micromachining techniques for fabricating high-performance edge-suspended RF/microwave passive components on silicon substrates

Characterization and attenuation mechanism of CMOS-compatible micromachined edge-suspended coplanar waveguides on low-resistivity silicon substrate

A simple micro pirani vasuum gauge fabricated by bulk micromachining technology