FEASIBILITY OF A MULTI-LAYER ELECTROSTATIC MICRO ENERGY HARVESTER FOR HIGHER POWER OUTPUT

G. Zhang and Z. Ye

Echemics, Monterey Park, USA

Abstract: This paper presents a novel electrostatic micro energy harvester for higher power output based on the concept of actual achievable power density (AAPD). Its core mechanical component is a silicon-based multi-layer in-plane micro energy converter that comprises an embedded high-density material as a moving mass. Compared with a conventional single-layer silicon electrostatic energy converter made with DRIE for the same assigned space, the proposed design has larger capacitance, much greater converted power, and higher AAPD. A novel silicon multi-layer microfabrication technology is also proposed to build this device. The preliminary results on the feasibility of the microfabrication technology are reported in this paper. Keywords: energy harvester, electrostatic, multi-layer, actual achievable power density, microfabrication INTRODUCTION

With advances in MEMS technologies and ultra-low power IC design, the size of electronic circuits and devices and the energy needed to drive them have been being reduced significantly. Harvesting energy from the environment to power micro-devices has become a practical and alternative solution to replace batteries.

Vibration-to-electric energy conversion is one type of energy harvesting where ambient vibration energy is extracted and converted by a vibration energy harvester into electrical energy. Electrostatic energy conversion is one conversion mechanism. One remarkable benefit of electrostatic energy harvesters is that they can be fabricated into micro-sized devices with MEMS technologies. In addition, micromachined harvesters can be easily integrated with micro-electronics, which makes it possible to realize completely autonomous self-powered microsystems.

As the power harvested from vibrations is relatively low, which is typically in the μW to mW range, outputting electric energy as much as possible is the primary development goal of vibration-based micro energy harvesters.

DEVICE DESIGN CONCEPT Actual Achievable Power Density

In vibration energy harvesting, power density defined in Equation 1 is a useful indicator for characterizing and comparing energy harvesters. However, it should be used cautiously to avoid being misled. For example, for building a self-powered microsystem using an electrostatic micro energy harvester as a power source, a certain space is assigned to be occupied by the harvester. In theory, the maximum converted power would be the product

of the assigned volume and the power density of the harvester. This maximum power can be reached only when the microfabricated harvester can fully occupy the assigned space. However, this is usually not the case because of the limitations of current silicon microfabrication technologies. To more precisely describe this situation, the authors present a term called actual achievable power density (AAPD) defined in Equation 2. Substitution of Equation 1 into Equation 2 yields Equation 3. As Vh is usually less than (in some cases, much less than) Va, pa is therefore less than p. The ratio of Vh /Va represents the usage ratio of a given space by a micro harvester.

hVPp = (1)

aa V

Pp = (2)

a

h

a

h

aa V

Vp

VVp

VPp ×=

×==

(3)

where p is harvester power density; pa is actual achievable power density; P is converted power; Vh is harvester volume; and Va is assigned volume.

Equation 3 implies an approach to increase power output, which is to increase AAPD by making use of an assigned space as much as possible, i.e., increasing Vh. Efficiently using an assigned space is extremely vital to the application of micro energy harvesters in microsystems where space is precious and desired to be made the best use of to its maximum. In some cases, not only is a space for a micro-harvester is restricted, but its footprint area is also restricted as well. Thus, a flexible microfabrication technology is strongly required to make complex, arbitrary micro

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energy harvesters that can occupy a given volume as much as possible. Key Design Issues

The proposed dynamic models for the vibration energy harvester show that for a given vibration source the converted power is proportional to moving mass in the harvester. For the electrostatic energy harvester, vibration energy is converted by an energy converter (i.e., variable capacitor) in the harvester. The converted energy per cycle is proportional to capacitance change that is related to the geometry of the variable capacitor. A large capacitance change is highly desired as it is capable of converting more energy.

Currently reported in-plane electrostatic energy converters comprise a simple single-layer capacitor structure. Fig. 1a schematically illustrates a typical variable capacitor layer that comprises two comb capacitors. To increase its converted power, its mass and capacitance need to be increased. The solution is to increase its thickness and the aspect-ratio of the comb capacitor. To fulfill the above requirements, DRIE (deep reactive ion etching) becomes the only suitable micro-fabrication process for making silicon-based electrostatic harvesters as it can produce high-aspect-ratio silicon structures.

(a) (b) Fig. 1: Schematic views of an one-layer (a) and a three-layer (b) energy converter in an assigned space.

However, even using DRIE, the increase of

thickness and the aspect-ratio is still restricted by DRIE capability and comb-finger microstructures. In addition, DRIE is not a three-dimensional (3-D) fabrication process. Rather it is a 2.5-D process, meaning that either the cross-section of a silicon structure along z-axis is exactly the same or its geometry has very limited change along z-axis. Lacking of geometry change along z-axis restricts DRIE to form complex silicon structures which are essential for an energy harvester to fill up a given volume. Therefore, unless an assigned volume in a microsystem is exactly the same as a harvester that can be made with DRIE (a very rare situation), the

fabricated harvester cannot usually fully use the given volume. Fig. 1(a) shows such a situation where the single-layer energy converter only occupies a portion of a given volume, meaning a low actual achievable power density and therefore a low converted power in terms of this given volume. Stacking more one-layer capacitor structures to form a multi-layer structure such as a three-layer example shown in Fig. 1b is a logical solution for an energy converter to increase its thickness and aspect-ratio, and occupy more space.

As the converted power is proportional to moving mass, the silicon-based energy converter has an intrinsic drawback due to the low density of silicon (2.33 g/cm3). Therefore, integrating a piece of high-density material such as gold (19.3 g/cm3) into the silicon harvester is highly desirable.

Proposed Multi-layer Electrostatic Device

As discussed above, to convert more vibration energy, an in-plane electrostatic energy converter must have a greater thickness and aspect-ratio, and also have a high-density material embedded in the device structure. To realize these objectives, we have designed a unique multi-layer energy converter shown in Fig. 2a. It comprises three layers. The top and bottom are two identical comb capacitor layers each of which comprises two fixed electrodes at two ends and one moving electrode in the middle. The middle layer is a main moving mass layer that comprises a high density metal embedded in a silicon cavity shown in Fig. 2b.

(a) (b) Fig. 2. Schematic views of the proposed multi-layer energy converter.

This proposed three-layer energy converter offers significant advantages over the simple single-layer energy converter (Fig. 1a) made with DRIE. Assume that the thickness of the single capacitor layer is 200 µm. The top and bottom capacitor layers of the proposed design are the same as the single capacitor layer. As the middle layer of the design does not have the comb finger microstructures, its thickness can be

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made much thicker with DRIE, e.g., 500 µm for this comparison purpose. The capacitance of our design is two times larger than that of the single layer one. If gold is embedded into the silicon cavity in the middle layer, from a moving mass point view, our design could covert vibration energy up to 20 times greater than the single layer converter.

Note that for further increasing power output, the above three-layer energy converter can be extended to comprise more layers (>3) by repeating the same structure pattern. Furthermore, an array of multi-layer converter could also be made on the same substrate simultaneously for even higher power output. DEVICE FABRICATION CONCEPT

As our design is a multi-layer silicon device, its fabrication would be best realized with multi-layer silicon microfabrication technology. Bonding pre-micromachined silicon structures to be a multi-layer structure has become a standard practice in MEMS microfabrication [1-2]. However, the difficulty of forming the proposed multi-layer energy converter is that its each layer contains discrete silicon features. For example, each of the two capacitor layers includes two fixed electrodes and one moving electrode that are separate and do not have mechanical connections between them (Fig. 2). Obviously this kind of layer cannot be made separately as discrete features would fall down. In addition, our energy converter comprises a metal embedded in its multi-layer silicon structure. Therefore, standard bulk micromachining plus silicon bonding cannot be used to make our device directly.

To fabricate our device, we have developed a novel multi-layer silicon microfabrication process. Its process flow can be schematically illustrated by fabricating an arbitrary multi-layer silicon structure comprising an embedded foreign material shown in Fig. 3. One innovative feature of this process is that layers comprising discrete silicon features can be easily made. Figs. 3-1 to 3-6 schematically show the process steps for making a single layer comprising discrete silicon features. In Fig. 3-1, a silicon wafer is fixed on a temporary substrate (substrate 1). A DRIE mask is patterned on the wafer (Fig. 3-2). Then the silicon wafer is etched through to form a silicon mold by using DRIE (Fig. 3-3). Formed silicon discrete features are supported by the substrate. In Fig. 3-4, a sacrificial material is filled into the silicon mold. The top surface of the two-material layer is planarized so that both materials are exposed (Fig. 3-5). Then the substrate is removed (Fig. 3-6). Now the discrete silicon features are held together by the sacrificial material that functions as an “adhesive”. For making a

multi-layer silicon structure, the above steps are repeated to build all required single layers (Fig. 3-7). These layers are then bonded together to form a multilayer structure with a proper silicon direct bonding technology. Fig. 3-8 shows that all the layers are bonded on a permanent substrate (substrate 2). A multi-layer silicon structure is formed after the sacrificial material is etched (Fig. 3-9).

Fig. 3. Multilayer fabrication process flow.

Another innovative feature of this process is that it can easily embed a foreign material into a multi-layer silicon structure in the same process. This is especially useful for our energy converter that requires a high-density material as a moving mass. Fig. 3-9 shows one example where a portion of the sacrificial material is sealed in a silicon cavity after bonding so that it is left in the silicon structure while the rest of the sacrificial material is etched away. Note that although only one material is filled in the silicon mold shown in Fig. 3, we may fill two or more materials into the mold to realize different technical goals. For example, if a required embedded material is not as the same as the sacrificial material, we can embed it into the silicon mold together with the sacrificial material.

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FABRICATION PROCESS FEASIBILITY To demonstrate if the fabrication process can

indeed make the multi-layer micro energy converter, a test version of the three-layer electrostatic micro energy converter was designed, which is shown schematically in Fig. 4. The designed thicknesses of the two identical capacitor layers and the moving mass layer were 250 µm and 350 µm, respectively.

Fig. 4. The test version design of the three-layer energy converter shown in Fig. 2.

In this feasibility test, silver was selected as both a moving mass material and a sacrificial material as it has a high density (10.49 g/cm3) and is compatible with the sub-processes of the fabrication process.

The three separate single layers of the energy converter were first made following the sub-processes of DRIE, silver-filling, and layer planarization (refer to Figs. 3-1 to 3-6). Fig. 5 shows one example of a fabricated single silicon comb-capacitor layer. Note that its moving electrode and fixed electrodes are held together with the sacrificial material of silver. This result proved the feasibility of making a single silicon layer comprising discrete silicon features with the fabrication process.

Fig. 5. A fabricated single silicon comb-capacitor layer having a die size of 30 mm x 30 mm x 250 µm.

After the fabrication of the required three single layers, each of them comprised two materials, silicon and silver. Considering of the melting point of silver (962°C) and thermal stress resulting from the mismatch in the coefficient of thermal expansion between silicon and silver, low temperature plasma activation silicon direct bonding was selected to bond the three layers and a silicon substrate together to form a monolithic multi-layer structure.

Finally, the sacrificial material silver was etched

away from the bonded multi-layer silicon structure to release the energy converter.

Fig. 6 shows the preliminary testing result of the the fabrication process from our very first trial for making a test design. Though this built test device had various defects, the feasibility of the silicon multi-layer microfabrication technology was indeed proved.

Fig. 6. The released energy converter after dissolving the sacrificial material. CONCLUSION AND FUTURE WORK

This paper presents a configuration of a three-layer silicon-based electrostatic energy converter with an embedded high density metal as a moving mass for converting more vibration energy into electrical energy. To fabricate this device, a multi-layer silicon microfabrication process is proposed. The preliminary testing results proved the feasibility of the fabrication process.

Our future work will optimize the fabrication process and design and build a working micro energy converter prototype. ACKNOWLEDGEMENTS

This research was supported by the National Science Foundation (NSF) through its SBIR Phase I grant #0711677. REFERENCES [1] Fréchette L G, Jacobson S A, Breuer K S,

Ehrich F F, Ghodssi R, Khanna R, Wong C W, Zhang X, Schmidt M A, and Epstein A H 2000 Demonstration of a Microfabricated High-Speed Turbine Supported on Gas Bearings Proc. Solid-State Sensor and Actuator Workshop (Hilton Head Island, SC, USA, 4-8, June 2000) 43-47

[2] Miki N, Zhang X, Khanna R, Ayon A A, Ward D, Spearing S M 2003 Multi-Stack Silicon-Direct Wafer Bonding for 3D MEMS Manufacturing Sensors and Actuators A 103 194-201

Sacrificial material

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