Fabrication of 3D Carbon Microelectromechanical Systems (C-MEMS)

Summary

Long and hollow glassy carbon microfibers were fabricated based on the pyrolysis of a natural product, human hair. The two fabrication steps of carbon microelectromechanical and carbon nanoelectromechanical systems, or C-MEMS and C-NEMS, are: (i) photolithography of a carbon-rich polymer precursor and (ii) pyrolysis of the patterned polymer precursor.

Abstract

A wide range of carbon sources are available in nature, with a variety of micro-/nanostructure configurations. Here, a novel technique to fabricate long and hollow glassy carbon microfibers derived from human hairs is introduced. The long and hollow carbon structures were made by the pyrolysis of human hair at 900 °C in a N₂ atmosphere. The morphology and chemical composition of natural and pyrolyzed human hairs were investigated using scanning electron microscopy (SEM) and electron-dispersive X-ray spectroscopy (EDX), respectively, to estimate the physical and chemical changes due to pyrolysis. Raman spectroscopy was used to confirm the glassy nature of the carbon microstructures. Pyrolyzed hair carbon was introduced to modify screen-printed carbon electrodes ; the modified electrodes were then applied to the electrochemical sensing of dopamine and ascorbic acid. Sensing performance of the modified sensors was improved as compared to the unmodified sensors. To obtain the desired carbon structure design, carbon micro-/nanoelectromechanical system (C-MEMS/C-NEMS) technology was developed. The most common C-MEMS/C-NEMS fabrication process consists of two steps: (i) the patterning of a carbon-rich base material, such as a photosensitive polymer, using photolithography; and (ii) carbonization through the pyrolysis of the patterned polymer in an oxygen-free environment. The C-MEMS/NEMS process has been widely used to develop microelectronic devices for various applications, including in micro-batteries, supercapacitors, glucose sensors, gas sensors, fuel cells, and triboelectric nanogenerators. Here, recent developments of a high-aspect ratio solid and hollow carbon microstructures with SU8 photoresists are discussed. The structural shrinkage during pyrolysis was investigated using confocal microscopy and SEM. Raman spectroscopy was used to confirm the crystallinity of the structure, and the atomic percentage of the elements present in the material before and after pyrolysis was measured using EDX.

Introduction

Carbon has many allotropes and, depending on the particular application, one of the following allotropes can be chosen: carbon nanotubes (CNTs), graphite, diamond, amorphous carbon, lonsdaleite, buckminsterfullerene (C₆₀), fullerite (C₅₄₀), fullerene (C₇₀), and glassy carbon^1,2,3,4. Glassy carbon is one of the most widely used allotropes because of its physical properties, including high isotropy. It also has the following properties: good electrical conductivity, low thermal expansion coefficient, and gas impermeability.

There has been a continuous search for carbon-rich precursor materials to obtain carbon structures. These precursors can be manmade materials or natural products that are available in particular shapes, and even include waste products. A wide variety of micro/nanostructures are formed via biological or environmental processes in nature, resulting in unique features that are extremely difficult to create using conventional manufacturing tools. As patterning took place naturally in this case, the synthesis of nanomaterials using natural and waste hydrocarbon precursors could be carried out using an easy, one-step process of thermal decomposition in an inert or vacuum atmosphere, called pyrolysis⁵. High-quality graphene, single-walled CNTs, multi-walled CNTs, and carbon dots have been produced by thermal decomposition or the pyrolysis of plant-derived precursors and wastes, including seeds, fibers, and oils, such as turpentine oil, sesame oil, neem oil (Azadirachta indica), eucalyptus oil, palm oil, and jatropha oil. Also, camphor products, tea-tree extracts, waste foods, insects, agro wastes, and food products have been utilized^6,7,8,9 Recently, researchers have even used silk cocoons as a precursor material to prepare porous carbon microfibers¹⁰. Human hair, usually considered a waste material, was recently used by this team. It is made up of approximately 91% polypeptides, which contain more than 50% carbon; the rest are elements such as oxygen, hydrogen, nitrogen, and sulphur¹¹. Hair also comes with several interesting properties, such as very slow degradation, high tensile strength, high thermal insulation, and high elastic recovery. Recently, it has been used to prepare carbon flakes employed in supercapacitors¹² and to create hollow carbon microfibers for electrochemical sensing¹³.

The machining of a bulk carbon material to fabricate three-dimensional (3D) structures is a difficult task, as the material is very brittle. Focused ion beam^14,15 or reactive ion etching¹⁶ may be useful in this context, but they are expensive and time-consuming processes. Carbon microelectromechanical system (C-MEMS) technology, which is based on the pyrolysis of patterned polymeric structures, represents a versatile alternative. In the past two decades, C-MEMS and carbon nanoelectromechanical systems (C-NEMS) have received much attention because of the simple and inexpensive fabrication steps involved. The conventional C-MEMS fabrication process is carried out in two steps: (i) patterning a polymer precursor (e.g., a photoresist) with photolithography and (ii) pyrolysis of the patterned structures. Ultraviolet (UV)-curable polymer precursors, such as SU8 photoresists, are often used to pattern structures based on photolithography. In general, the photolithography process includes steps for spin coating, soft bake, UV exposure, post bake, and development. In the case of C-MEMS; silicon; silicon dioxide; silicon nitride; quartz; and, more recently, sapphire have been used as substrates. The photo-patterned polymer structures are carbonized at a high temperature (800-1,100 °C) in an oxygen-free environment. At those elevated temperatures in a vacuum or inert atmosphere, all of the non-carbon elements are removed, leaving only carbon. This technique allows for the attainment of high-quality, glassy carbon structures, which are very useful for many applications, including electrochemical sensing¹⁷, energy storage¹⁸, triboelectric nanogeneration¹⁹, and electrokinetic particle manipulation^20. Also, the fabrication of 3D microstructures with high aspect ratios using C-MEMS has become relatively easy and has led to a wide variety of carbon electrodes applications^18,21,22,23, often replacing noble metal electrodes.

In this work, the recent development of a simple and cost-effective way to fabricate hollow carbon microfibers from human hair using non-conventional C-MEMS technology¹³ is introduced. The conventional SU8 polymer-based C-MEMS process is also described here. Specifically, the fabrication procedure for high-aspect ratio solids and hollow SU8 structures is described^24.

Protocol

1. 3D Human Hair-derived Carbon Structure Fabrication

NOTE: Use personal protective equipment. Follow laboratory instructions to use the instruments and to work inside the laboratory.

1. Prepare collected human hair by washing it with DI water and drying it with N₂ gas.
2. Arrange the hairs as desired, such as in parallel strands, cross over, with two hairs wound together, etc.
3. Attach the hairs to a silicon substrate using SU8 or keep them directly in a ceramic boat.
4. Place the hair-attached silicon substrate or boat into a furnace.
5. Turn on the furnace and open the valve of an inert gas (N₂) tank.
   NOTE: The optimal gas flow rate is dependent upon the volume of the furnace tube. A 6 L/min flow rate was applied for a tube volume of 6 L. To establish a completely inert environment in the furnace tube, a gas flow rate 1.5 times higher than the optimal gas flow rate was applied for the first 15 min.
6. Set the parameters, including maximum pyrolysis temperature, temperature ramp rate, and inert gas flow rate, and run the furnace.
   1. For example, increase temperature from room temperature to 300 °C at a 5 °C/min ramp rate. Keep it at 300 °C for 1 h for stabilization. Further increase the temperature to 900 °C and maintain it for 1 h more for carbonization.
   2. Cool the furnace down to 300 °C at a rate of 10 °C/min and turn off the heater of the furnace, as the controlled cooling is not necessary after 300 °C. Leave the samples in the furnace until the temperature reaches room temperature by N₂ flow only.
7. Turn off the furnace and gas flow upon the completion of the pyrolysis process.
8. Take the samples out of the furnace.

2. 3D Polymer Structure Fabrication: Photolithography

1. Design a 2D layout of the desired 3D photoresist structure using an appropriate software package and prepare the printed mask (i.e., a polyethylene photofilm mask).
   NOTE: A commercial service was used to get the design printed. The size of the mask generally depends on the design.
2. In a clean laboratory facility, turn on two hot plates and set the temperatures to 65 °C and 95 °C, respectively.
3. Switch on a spin coater and a vacuum pump. Ensure that the vacuum pump is connected through a tube to the spinner head.
4. Set the parameters of the two-step spin, such as the spinning speed, ramp, and duration. For the first step, set the spinning speed to 500 rpm, the ramp to 100 rpm/s, and the spin time to 10 s to begin the spin cycle. For the next step, set the spinning speed to 1,000 rpm, the ramp to 100 rpm/s, and the spin time to 30 s to evenly spread the photoresist.
5. Place a substrate (i.e., a 4 inch x 4 inch and 550 µm ± 25 µm-thick Si wafer with a 1 µm-thick SiO₂ layer) at the center of the holder.
6. Deposit photosensitive polymer (i.e., SU8 photoresist) directly onto the center of the substrate. Use enough to cover the surface.
7. Push the "vacuum" button to hold the substrate.
8. Push the "run" button to coat the substrate with SU8 and to achieve a final thickness of 250 µm.
9. After the completion of spinning process, press the "vacuum" button again to release the coated substrate from the holder.
10. Hold the coated substrate carefully using a tweezer to keep the surface smooth and clean. Transfer the substrate directly onto the hot plate at 65 °C temperature for 6 min and then onto the hot plate at 95 °C temperature for 40 min (soft bake).
    NOTE: Baking at 65 °C is required to ensure the slow evaporation of the solvent, resulting in the better coating and better adhesion to the substrate, while baking at 95 °C further densifies the SU8.
11. In the meantime, press the switch to turn on the UV-exposure system and set the time of exposure to "12 s" using the set button in the system.
    NOTE: For a 250 µm-thick SU8 layer, the exposure energy must be 360 mJ/cm².
12. Once the baking step (step 2.10) is completed, put the substrate into the UV-exposure system and place the printed side of a photomask (from step 2.1) onto it. Use the whole mask area to cover the coated substrate and press gently to ensure that there is no gap between the mask and substrate.
13. Expose the SU8-coated substrate to UV radiation through the photomask using predefined UV settings.
14. Heat the substrate again by placing it directly on the hotplate at 65 °C for 5 min and at 95 °C for 14 min for a post-exposure bake (PEB).
    NOTE: The PEB increases the degree of cross-linking in the UV-exposed areas and makes the coating more resistant to solvents in the development step.
15. Remove the unexposed photoresist regions by dipping the substrate in the dedicated developer solution, placed in a beaker, for 20 min. Continuously shake the solution (carefully) to ensure the complete removal of the un-exposed resist areas.
16. Dry the developed structures by holding the substrate and blowing nitrogen or compressed air onto it.
17. Inspect the wafer under a microscope with 50X magnification to compare the patterns transferred to the photoresist with the desired patterns.

3. 3D Carbon Structure Fabrication: Pyrolysis

1. Place the samples prepared using photolithography (steps 2.1-2.17) inside a pressurized, open-ended tube furnace.
2. Turn on the furnace and set the parameters for pyrolysis, as mentioned above in step 1. Repeat the process from step 1.6-1.8.
3. Handle the samples carefully using tweezers and proceed to characterization.

Results

A schematic of the fabrication process for human hair-derived hollow carbon microfibers is shown in Figure 1. The carbonized human hair was characterized using SEM to estimate the shrinkage. The hair diameter shrank from 82.88 ± 0.003 µm to 31.42 ± 0.003 µm due to the pyrolysis. Scanning electron microscopic (SEM) images of various patterns made using hair-derived carbon microfibers are shown in Figure 2. The atomic percentages o...

Discussion

In this paper the methods for manufacturing a variety of carbon microstructures based on the pyrolysis of natural precursor materials or photo-patterned polymer structures were reported. The carbon materials resulting from both the traditional and non-conventional C-MEMS/C-NEMS processes are typically found to be glassy carbons. Glassy carbon is a widely used electrode material for electrochemistry and also for high-temperature applications. The microstructure of glassy carbon is composed of both crystalline and amorphou...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
SU8-2100	Microchem	Product number-Y1110750500L
Spinner	Laurell Technologies Corporation	Model-WS650HZB-23NPP/UD3
Hotplate	Torrey Pines Scientific	HS61
UV-exposer	Mercury Lamp, SYLVANIA	H44GS-100M, P/N-34-0054-01
Photomask	CAD/Art		No number
Developer	Microchem	Y020100 4000L
DI water system	Milli Q	ZOOQOVOTO
IPA	CTR Sientific	CTR 01244
N2 gas	AOC Mexico		No number
Furnace	PEO 601, ATV Technologie GMBH	Model-PEO 601, Serial no.-195
Si/SiO2	Noel Technologies