Our protocol demonstrates the fabrication steps of a flexible mesh structure and the bonding process to make a polymer based vibration energy harvester. The advantage of this technique, is that 3D photography can easily fabricate the mesh structure, that is effective in vibration energy harvester for low frequency applications To begin, prepare 30 millimeter by 40 millimeter glass substrates. Set a glass substrate in a Teflon jig for cleaning.
Wear protective glasses, clothing and gloves. Then, emerge the jig in piranha solution for one minute. Set the glass substrate in the chamber of an RF magnetron sputtering machine.
Set the RF power to 250 watts, the sputtering time to 11 minutes, the flow rate of Argon gas to 12 sccm, and the chamber pressure to 0.5 pascals. Now, form 100 to 200 nanometers of chromium film on the glass substrate, by RF magnetron sputtering. Next, set the substrate on a fixing stage in a spin coated chamber.
Drop positive photo resist as 1A13 on the chromium film, and coat the one to two micron thin film by spin coating at 4000 RPM for 30 seconds. After baking the substrate as described in the text protocol, contact the photo resist coated substrate with a photo mask. Expose UV light vertically to the photo mask.
Ensure, that the exposure dose is 80 millijoules per square centimeter, and the wavelength is 405 nanometers. Immerse the substrate in 150 milliliters of TMAH solution, and develop the photo resist for 30 seconds to one minute. After rinsing the substrate with pure water, immerse the substrate in the 150 milliliters of chromium etching solution, and etch chromium for about one to two minutes.
Then, remove the photo resist as described in the text protocol. Now, set the substrate on the fixing stage in the spin coater chamber. Drop approximately one milliliter of acryclic resin solution on the chromium pattern side of the substrate to release the fabricated structure as a sacrificial layer.
Then, form a thin film by spin-coating at 2000 RPM for 30 seconds. After baking the substrate at 100 degrees Celsius for 10 minutes, set the substrate on an attached plate in the spray coater. Cover the substrate with an edge cover to prevent edge beat.
Pour the negative photo resist SU8-3005 into the syringe. Set the nozzle diameter, nozzle movement speed, atomization pressure, fluid pressure, pitch distance, and interval time for each layer as listed in the text protocol. Also, set the distance between the nozzle and substrate to 40 millimeters.
Spray SU8 multilayers on the substrate. Repeat the coating 10 times in the same way. Then, bake the substrate on a hot plate at 95 degrees Celsius for 60 minutes.
After determining the film thickness per layer, as described in the text protocol, spray the multilayer to achieve the target film thickness. In this research, 40 layers are applied for a 200 micron thickness. Now, place the substrate on an angle adjustment table by flipping the substrate over.
Tilt the angle of the adjustment table to 45 degrees. Place the angle adjustment table under the UV light source. Apply UV light vertically to the substrate at an exposure dose of 150 millijoules per square centimeter, and a wavelength of 365 nanometers.
After the exposure, return the angle of adjustment table to zero degrees, and tilt it to 45 degrees in the opposite direction. Apply UV light vertically in the same way, before performing the post-exposure bake as described in the text protocol. Now, develop the substrate for approximately 20 to 30 minutes in SU-8 developer.
If the developing time is not enough, it leads to insufficient opening of the mesh voids. After rinsing in IPA, as described in the text protocol, immerse the substrate in Toluene solution for approximately three to four hours. Ensure, that the sacrificial layer of acrylic resin is edged and the SU-8 structure with the mesh structure is released from the substrate.
To prepare the piezoelectric film, cut out the PVDF sheet to the device shape with a 360 square millimeter sheet. Place the cut PVDF films on a Petri dish with a cellular swiper. Store them in a desiccator.
Now, pour 10 milliliters of the main agent of PDMS, and one millileter of curing agent, into a centrifuge tube. Set the centrifuge tube in a planetary storing and defoaming machine, and mix both solutions for one minute. Now, prepare two 30 millimeter by 40 millimeter glass substrates.
Set the glass substrate on a fixing stage in the spin coater chamber. Drop PDMS solution onto the glass substrate. Then, form the PDMS film by spin coating at 4000 RPM, and bake the substrate as described in the text protocol.
Place the cut PVDF films one by one onto two different PDMS substrates. Ensure, that just by placing PVDF films on the surface of PDMS, they adhere to each other. If wrinkles are seen on the PVDF films, extend them with a roller.
Drop SU-8 onto PVDF film one, placed on PDMS substrate one. Then, form the SU-8 thin film by spin coating at 4000 RPM. Place the SU-8 mesh structure on PVDF film one and bond them.
Now, drop SU-8 onto PVDF film two, placed on PDMS substrate two. Form the SU-8 thin film by spin coating at 4000 RPM. Peal off PVDF film two from PDMS substrate two, and then place on top of the SU-8 mesh structure, placed on PVDF film one.
Store the adhered device with the bonded state in a container with low humidity, such as a desiccator for about 12 hours. After 12 hours, put the tweezers into the bottom side of the lowest layer, PVDF film one. Then, simultaneously peel off the bonded three layers of PVDF film one, SU-8 mesh structure, and PVDF film two from the substrate.
A bimorph type vibration energy harvester, composed of two layers of PVDF films and an intermediate layer, composed of an SU-8 mesh structure, is shown. The electrodes of the upper and lower PVDF are connected in series to obtain output voltage. The optical image and the two SEM images show 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. In the vibration tests, two devices, one with a meshed-core elastic layer, and the other with a solid-core structure elastic layer, are evaluated to verify the validity of meshed core type device. When the devices are set on a vibration shaker and excited, both, the meshed-core type and the solid-core type devices, showed sinusoidal output synchronized with sinusoidal input.
The meshed-core type device exhibited a 42.6%higher output voltage than the solid-core type device. Shown here is the frequency response of the maximum output power. The meshed-core type device exhibited a resonance frequency of 18.7 hertz, which is 15.8%lower than the solid-core type device.
It also exhibited an output power of 24.6 microwatts, which is 68.5%higher than the solid-core type device. On inclined exposure, from backside of the substrate, and sufficient developing time are important to make fine prints of the mesh structure. In the thin bonding process, we can also use an instant glue.
However, the glue will fill the void of the mesh structure, and cause an increase in device rigidity. Therefore, the resonance frequency also increases. All using energy harvesting systems, we can apply the 3D photography to as a micro nano applications, such as biological, optical and microfluidic systems.
