Pyrolysis Process Optimization for SU8-Derived Carbon Structures
Authors: Pramanick Bidhan , Perez-Gonzalez Victor Hugo , Martinez-Chapa Sergio Omar , Madou Marc J 
Published: February 2020

For the last 15 years, the Carbon-MEMS (C-MEMS) process has gained widespread interest due to its low cost, reliability and simplicity for the fabrication of 2-D or 3-D carbon structures. Briefly, one step photolithography is used to pattern the structures onto a photosensitive layer and the patterned structures are then converted to carbon using thermochemical decomposition (also known as Pyrolysis). During pyrolysis, an organic material is heated in an inert atmosphere to temperatures higher than 800°C. This step is as important as photolithography to fabricate 3-D carbon structures because it defines the physico-chemical properties of the resulting carbon structures (e.g., electrical conductivity, thermal conductivity, reactivity, and mechanical stiffness, among others). Several parameters of the pyrolysis process can be adjusted in order to fine-tune the properties of the carbon structures. For example, a flow of Nitrogen (N2), Argon (Ar), or even a mixture of gases like N2 and Hydrogen (H2) is commonly used to create an inert atmosphere, with each gas yielding carbon structures with different properties. Other critical parameters are the gas flowrate, final temperature, and heating and cooling ramps. Therefore, optimization of these process parameters is of utmost importance for the carbon structures to meet the requirements of the application at hand. Sometimes, although pyrolysis parameters have been optimized, a very small amount of Oxygen (O2) can invade the pyrolysis chamber, rendering the structures unusable. The source of O2 may be a defect in the pyrolysis chamber or the sample itself (we found that the aging effect of the polymeric sample being pyrolyzed can significantly affect the result of the pyrolysis step). We discuss here about the effect of N2 flowrate, sample aging, final temperature and temperature ramps on carbon yield, and report a few guidelines for the operation of a newly installed pyrolysis system. In this work, samples for pyrolysis consisted on SU-8-based nanofibers electrospun atop 3-D SU-8 walls through electromechanical spinning (EMS). We found that the required N2 flow volume (per minute) is equal to, or a little more than, the tube volume. If a reasonably higher flowrate is required to properly pyrolyze the polymeric structure, then O2 is accessing the pyrolysis chamber through the exhaust tube or through a defect in the chamber itself. We employed an N2 flowrate 1.5 times the volume of the tube at the beginning of the process (for at least 15minutes) to ensure that the atmosphere within the pyrolysis chamber is inert and reduced the flow to slightly more than the volume of the tube for the rest of the process. We often obtain carbon structures even at lower N2 flow than the volume of the tube. However the average pore size is bigger in that case. We made a comparative study of average pore size at different flowrates. For the aging effect study we noticed that small structures are less affected with time than bigger structures. Finally, we optimized the pyrolysis process parameters for SU8 derived carbon structures and compared them with those of an existing, already-optimized facility.