The overall goal of this procedure is to fabricate 3D carbon structures using conventional and nonconventional carbon microelectromechanical system processes with carbon sources of SU-8 polymer and human hair, respectively. 20 years ago, we actually started a complete new process where we are replacing silicon as the major material for making MEMS devices, microelectromechanical systems. Our idea is that carbon is a better material than silicon for making these three-dimensional shapes.
Specifically, we take a polymer precursor, pattern it, and then convert it to carbon at high temperature, around 900 to 1, 000 degrees C, in an inert atmosphere. What happens in that process is the shape is preserved, but it shrinks. In order words, you have isometric shrinkage, and you end up with any 3D carbon shape you might imagine.
That is called carbon MEMS. It is much less expensive because all I do is take a carbon precursor, a polymer, and pattern it in three dimension and then convert it to carbon. Carbon comes in many allotropes, from diamond to graphite to glassy carbon.
Depending on how I pretreat the polymer's chains, I can actually end up with a 3D carbon structure that's more graphitic or more diamond-like or glassy carbon-like. Today, the process of carbon MEMS will be demonstrated for you by my graduate student, Esham Shomloo. To begin the polymer structure fabrication procedure, first set a hotplate to 95 degrees Celsius.
Turn on a spin coater and the connected vacuum pump. Program a two-step spin coating sequence with a first step of 10 seconds at 500 rpm and a second step of 30 seconds at 1, 000 rpm, both with a ramp rate of 100 rpm per second. Place a silicon wafer or other substrate on the vacuum holder of the spin coater.
Start the vacuum to fix the substrate to the holder. Apply sufficient epoxy resin-based negative photoresist to cover the surface of the substrate. Run the two-step spin coating sequence to achieve a final photoresist thickness of one to 250 micron.
Release the substrate from the holder. Using tweezers, carefully transfer the substrate to the 95 degrees Celsius hotplate without disturbing the thin film. Bake the substrate at 95 degrees Celsius for one to 40 minutes, depending on the thickness of the photoresist.
During the soft bake, turn on the photolithography UV system and set the exposure time to 12 seconds. Once the soft bake has completed, transfer the substrate to the UV system. Clamp the patterned mask with a vacuum mask holder.
Place the mask holder with the mask on the UV exposure system. Put the photoresist-coated substrate on the stage of the UV exposure system. Cover the substrate with the patterned mask, and gently press the mask against the substrate.
Expose the substrate to UV light for the set duration, and then separate the mask from the substrate, Perform a post-exposure bake at 95 degrees Celsius for one to 15 minutes, depending on the thickness of the photoresist. Immerse the substrate in the appropriate developer solution for one to 20 minutes while gently shaking the container. Dry the patterned wafer under a stream of nitrogen gas or compressed air.
Inspect the wafer under a microscope at 50 times magnification to verify that the desired pattern is well shaped. Then, place the patterned wafer in a pressurized, open-ended tube furnace. Set the flow rate for the first 15 minutes of the sequence to 1.5 times the volume of the furnace tube per minute and then to the furnace tube volume per minute for the remainder of the sequence.
Set a sequence of ramping to 300 degrees Celsius at five degrees Celsius per minute, holding at 300 degrees Celsius for one hour, ramping to 900 degrees Celsius at the same rate and holding for one hour, and then cooling to 300 degrees Celsius at 10 degrees Celsius per minute. Then, run the heating and cooling sequence. Once the sequence has completed, wait for the pyrolyzed sample to cool to room temperature under a stream of nitrogen gas.
Then, turn off the furnace and gas flow. Then, carefully remove the pyrolyzed sample from the furnace and proceed to characterization. To begin fabricating a hair-derived 3D carbon structure, wash a sample of human hair with deionized water.
Dry the hair sample under a stream of nitrogen gas. Obtain a silicon wafer or a ceramic boat. Arrange the individual hairs in the desired configuration on the wafer or in the boat.
If using a silicon wafer, fix the hairs to the wafer with epoxy-based polymer. Then, place the sample in a pressurized, open-ended tube furnace. Run the pyrolysis sequence to first stabilize and then carbonize the sample.
Allow the sample to cool to room temperature under a flow of nitrogen. Then, carefully remove the sample from the furnace, using tweezers when necessary, and proceed to characterization. Hair-derived carbon microfibers were prepared in various patterns, depending on the initial arrangement of the hair sample.
Energy-dispersive X-ray spectroscopy and Raman spectroscopy were performed before and after pyrolysis. Only two peaks corresponding to the D and G bands appeared in the Raman spectrum after pyrolysis. The intensity ratio between the peaks indicated that the microstructures were primarily composed of glassy carbon.
Electrochemical sensors were fabricated using bare carbon, carbon nanotubes, or the hair-derived carbon fibers. Cyclic voltammetry of dopamine and ascorbic acid solutions showed that the electrode incorporating the hair-derived carbon fibers showed improved performance compared to the bare carbon, though they did not match the performance of carbon nanotubes. I hope that this short video on carbon MEMS inspires many of you to explore the process by yourselves.
Indeed, there is many, many more applications that I can see coming out of this simple process. This can range from novel batteries, new fuel cells, new sensors, et cetera. Perhaps, overtaking silicon as the most important MEMS material.
