In this study, we fabricated a flexible 3D mesh structure and applied it to the elastic layer of a bimorph cantilever-type vibration energy harvester for the purpose of lowering resonance frequency and increasing output power.
In this study, we fabricated a flexible 3D mesh structure with periodic voids by using a 3D lithography method and applying it to a vibration energy harvester to lower resonance frequency and increase output power. The fabrication process is mainly divided into two parts: three-dimensional photolithography for processing a 3D mesh structure, and a bonding process of piezoelectric films and the mesh structure. With the fabricated flexible mesh structure, we achieved the reduction of resonance frequency and improvement of output power, simultaneously. From the results of the vibration tests, the meshed-core-type vibration energy harvester (VEH) exhibited 42.6% higher output voltage than the solid-core-type VEH. In addition, the meshed-core-type VEH yielded 18.7 Hz of resonance frequency, 15.8% lower than the solid-core-type VEH, and 24.6 μW of output power, 68.5% higher than the solid-core-type VEH. The advantage of the proposed method is that a complex and flexible structure with voids in three dimensions can be relatively easily fabricated in a short time by the inclined exposure method. As it is possible to lower the resonance frequency of the VEH by the mesh structure, use in low-frequency applications, such as wearable devices and house appliances, can be expected in the future.
In recent years, VEHs have drawn much attention as an electric power supply of sensor nodes for implementing wireless sensor networks and Internet of Things (IoT) applications1,2,3,4,5,6,7,8. Among several types of energy conversion in VEHs, piezoelectric-type conversion presents high output voltage. This type of conversion is also suitable for miniaturization because of its high affinity with micromachining technology. Because of these attractive features, many piezoelectric VEHs have been developed using piezoelectric ceramic materials and organic polymer materials9,10,11,12,13.
In ceramic VEHs, cantilever-type VEHs using high-performance piezoelectric material PZT (lead titanate zirconate) are widely reported14,15,16,17,18, and the VEHs often use resonance to obtain high-efficiency power generation. In general, as the resonance frequency increases with the miniaturization of the device size, it is difficult to achieve miniaturization and low-resonance frequency simultaneously. Thus, although PZT has high-power-generation performance, it is difficult to develop small-sized PZT-based devices that work in a low-frequency band without special processing, such as nanoribbon assemblies19,20, because PZT is a high-rigidity material. Unfortunately, our surrounding vibrations such as household appliances, human motion, buildings, and bridges are mainly at low frequencies, less than 30 Hz21,22,23. Therefore, VEHs with its high-power-generation efficiency at low frequencies and small size are ideal for the low-frequency applications.
The easiest way to lower the resonance frequency is to increase the mass weight of the tip of the cantilever. As attaching a high-density material to the tip is all that is required, the fabrication is simple and easy. However, the heavier the mass is, the more fragile the device becomes. Another way of lowering the frequency is to lengthen the cantilever24,25. In the method, the distance from the fixed end to the free end is extended by a two-dimensional meandered shape. The silicon substrate is etched using a semiconductor manufacturing technique to fabricate a meandered structure. Although the method is effective for lowering resonance frequency, the area of the piezoelectric material decreases and, thus, the obtainable output power decreases. In addition, there is a disadvantage that the vicinity of the fixed end is fragile. Regarding some polymer devices, such as the low-frequency VEH, flexible piezoelectric polymer PVDF is often used. As PVDF is usually coated by a spin-coating method and the film is thin, the resonance frequency can be reduced because of the low rigidity26,27. Although the film thickness is controllable in the range of sub-micron to several microns, the attainable output power is small because of the thin thickness. Therefore, even if the frequency can be reduced, we cannot obtain sufficient power generation, and so, practical application is difficult.
Here, we propose a bimorph-type piezoelectric cantilever (consisting of two layers of piezoelectric layers and one layer of elastic layer) with two flexible piezoelectric polymer sheets, which have already been subjected to stretching treatment for improvement of piezoelectric characteristics. Furthermore, we adopt a flexible 3D mesh structure in the elastic layer of the bimorph cantilever to reduce the resonance frequency and improve the power simultaneously. We fabricate the 3D mesh structure by utilizing the backside inclined exposure method28,29 because it is possible to fabricate fine patterns with high precision in a short time. Although 3D printing is also a candidate to fabricate 3D mesh structure, the throughput is low, and the 3D printer is inferior to photolithography in machining accuracy30,31. Therefore, in this study, the backside inclined exposure method is adopted as the method for micromachining the 3D mesh structure.
1. Fabrication of the 3D mesh structure
2. Preparation of piezoelectric film
3. Preparation of substrate for bonding mesh structure and piezoelectric film
4. Fabrication of bimorph vibration energy harvester
We fabricated a bimorph-type VEH composed of two layers of PVDF films and an intermediate layer composed of an SU-8 mesh structure, as shown in Figure 4. The electrodes of the upper and lower PVDF are connected in series to obtain output voltage. The optical image and the two SEM images are elastic layers with a mesh structure. According to the images, the elastic layer processed by the backside inclined exposure appears to have fine 3D mesh patterns without development failure.
Figure 5 shows the results of vibration tests. In the vibration tests, two VEHs—one with a meshed core and the other with a solid-core structure—as the elastic layer are evaluated to verify the validity of meshed-core-type VEH. The VEHs are set on a vibration shaker and excited with a vibration acceleration of 1.96 m/s2 (0.2 G). Both the meshed-core-type and solid-core-type VEHs showed sinusoidal output synchronized with a sinusoidal input. The meshed-core-type VEH exhibited a 42.6% higher output voltage than the solid-core type VEH. Figure 5b shows the frequency response of the maximum output power. The meshed-core-type VEH exhibited a resonance frequency of 18.7 Hz, which is 15.8% lower than the solid-core-type VEH, and an output power of 24.6 μW, which is 68.5% higher than the solid-core-type VEH.
Figure 1: Photomask layout for photolithography to fabricate elastic layer with a 3D meshed-core structure. The photomask has two parts. One is the area for clamping, and the other contains the line and space patterns for mesh-structure patterning. Please click here to view a larger version of this figure.
Figure 2: Set-up for inclined exposure. UV light is exposed vertically to the inclined substrate with a Cr pattern placed on angle adjustment table. Please click here to view a larger version of this figure.
Figure 3: Schematic of a proposed piezoelectric vibration energy harvester with a 3D meshed-core structure and the fabrication process of the harvester. The fabrication process can be divided into 3 sections: (a)-(d) represent the fabrication process of the 3D mesh structure, (e)-(g) represent the preparation of the PVDF film on a glass substrate, and (h)-(j) represent the bonding process to form a bimorph cantilever. (These figures are published under gold Open Access, Creative Commons license and have been modified from [21].) Please click here to view a larger version of this figure.
Figure 4: (a) Photograph of the fabricated bimorph meshed-core vibration energy harvester, (b) cross-sectional optical image of the 3D meshed-core structure, (c) and (d) SEM images of SU-8 meshed-core elastic layer. (These figures are published under gold Open Access, Creative Commons license and have been modified from [21].) Please click here to view a larger version of this figure.
Figure 5: (a) Sinusoidal output voltage of load resistance under each resonance condition (meshed-core 18.7 Hz, solid-core 22.2 Hz) and (b) Maximum output power as a function of vibration frequency under optimum load resistance (meshed-core 17 MΩ, solid-core 13 MΩ) and 0.2 G acceleration. (These figures are published under gold Open Access, Creative Commons license and have been modified from [21].) Please click here to view a larger version of this figure.
The successful fabrication of the 3D mesh structure and the proposed bimorph VEH described above is based on four critical and distinctive steps.
The first critical step is processing using backside inclined exposure. In principle, it is possible to fabricate a mesh structure by inclined exposure from the upper surface using the contact lithography technique. However, backside exposure presents a more accurate processing precision than contact lithography, and defects during development are less likely to occur28,29. This is because the gap between the photomask and the photoresist could arise due to the waviness of the photoresist surface. Hence, light diffraction occurs and processing precision is lowered because of the gap. Therefore, in this study, we fabricated a mesh structure using the backside inclined exposure method. In addition, the measured value of the structural angle of the fabricated mesh structure is about 65°, with just a 1% error as compared with the designed value of 64°. From the result, we conclude that it is appropriate to apply the backside inclined exposure method to fabricate the mesh structure.
The second critical step is the development process of SU-8. If a developing defect occurs, the mesh structure loses inherent flexibility. To develop the thick SU-8 film, typically 10-15 min is used. However, this developing time is insufficient for the development of a 3D mesh structure. The 3D mesh structure differs from the 2D pattern fabricated by photolithography because it has many internal voids inside the membrane. If the developing time is short, development does not progress to the interior of the mesh structure, causing patterning failure. That is why, it is necessary to apply a relatively long development time, 20-30 min32. If finer patterns are required, even longer developing time may be necessary. However, at that time, we have to consider the swelling caused by long development time33.
Next, the method to exploit a PDMS-formed substrate in the bonding process of PVDF film and SU-8 mesh structure is unique. It makes spin coating possible and, as a result, PVDF and SU-8 can be easily adhered using a spin-coated SU-8 thin adhesive layer. PVDF and SU-8 can be bonded, even by using a commercially available instant glue. However, the adhesive material hardens after the adhesive is solidified. Moreover, it is difficult to form a thin film with the instant glue. If the thickness of the instant glue is larger, it will increase the rigidity of the entire device. An increase in rigidity leads to an increase in the resonance frequency (i.e., it prevents lowering the resonance frequency, which is the main purpose of this study). On the other hand, using the SU-8 thin film formed by spin coating as an adhesion layer does not greatly affect the increase in rigidity because the formed SU-8 film is thin. In addition, as the mesh structure is made of SU-8, it is possible to increase the adhesive strength by using the same material for the adhesion layer. That is why the SU-8 adhesion has enough adhesive strength to bond an SU-8 mesh structure and PVDF films. Furthermore, from the aspect of reproducibility of the device, it would be useful to use the SU-8 thin film as an adhesion layer, as a constant film thickness can be realized by spin coating film formation.
Fourth, the coating method of SU-8 is distinctive. We have selected a spray multilayer coating method for the SU-8 thick film. Although it is possible to form a thick film by spin coating, large surface waviness occurs, and it is difficult to coat the film uniformly34. On the other hand, using the spray multi-coating method reduces the waviness and suppresses the error of film thickness in the substrate34. Particularly, attention needs to be given to large waviness because when the thickness of the 3D mesh structure becomes nonuniform, the vibration characteristics and rigidity of the device is changed by the partially increased or decreased thickness.
In principle, as photolithography uses UV light, the fabricable shapes are limited. It is true that we can fabricate complex structures such as a 3D mesh structure by using inclined exposure. However, arbitrary shapes such as a three-dimensional structure with a curved shape in the film thickness direction are difficult to form35,36. The 3D printing can produce arbitrary three-dimensional shapes, and the design is flexible. However, the throughput of the fabrication is low, and the processing precision and mass production are inferior to photolithography. Thus, it is not suitable for fabricating structures with fine patterns in a short time. In addition, processing 3D CAD data is necessary, and it takes time to create the 3D model. On the other hand, in the case of photolithography, especially in the inclined exposure method, the CAD data necessary for the photomask is two-dimensional, and the design is relatively easy. For example, the oriented design for a 3D mesh structure is just the 2D line and space patterns, as shown in Figure 3. Considering these facts, in this research, we exploited the 3D lithography technique to develop a flexible 3D mesh structure.
In this study, we fabricated a flexible 3D mesh structure and applied it to the elastic layer of a bimorph cantilever type VEH for the purpose of lowering resonance frequency and increasing output power. Since the proposed method is useful in lowering resonance frequency, it will be useful for vibration energy harvester targeted for low-frequency application such as wearable devices, monitoring sensors for public buildings and bridge, house appliances, etc. Further improvement of output power would be expected by combining the trapezoidal shape, triangle shape, and thickness optimization which is previously proposed in other papers37,38,39.
The authors have nothing to disclose.
This research was partially supported by JSPS Science Research Grant JP17H03196, JST PRESTO Grant Number JPMJPR15R3. The support from MEXT Nanotechnology Platform Project (The University of Tokyo Microfabrication Platform) to the fabrication of photomask is greatly appreciated.
SU-8 3005 | Nihon Kayaku | Negative photoresist | |
KF Piezo Film | Kureha | Piezoelectric PVDF film, 40 mm | |
Vibration Shaker | IMV CORPORATION | m030/MA1 | Vibration Shaker |
Spray coater | Nanometric Technology Inc. | DC110-EX | |
Sputtering equipment | Canon Anelva Corporation | E-200S | |
PDMS | Dow Corning Toray Co. Ltd | SILPOT 184 W/C | Dimethylpolysiloxane |
Spin coater | MIKASA Co. Ltd | 1H-DX2 | |
Digital oscilloscope | Teledyne LeCroy Japan Corporation | WaveRunner 44Xi-A | |
SEM | JEOL Ltd. | JCM-5700LV | |
Digital microscope | Keyence Corporation | VHX-1000 |