Fabrication and characterization of suspended pyrolytic carbon microstructures in various pyrolysis temperatures

The aim of this Master’s thesis is to fabricate and study the issues related to the fabrication of suspended C-MEMS microstructures, as well as to investigate the properties of unpatterned pyrolytic carbon films in relation to the pyrolysis temperatures. In recent years, suspended pyrolytic carbon microstructures have started to emerge as part of the next generation C-MEMS devices. Although the use of such structures can greatly improve the quality and expands the application of C-MEMS devices, suspended pyrolytic carbon microstructures are far more susceptible to fabrication issues than substrate-bound structures. So, in order to further advance the C-MEMS process we must first understand the underlying fabrication issues that these structures face. Suspended SU-8 microstructures with varying shapes and sizes were prepared with the use of sacrificial layers and pyrolyzed in an inert atmosphere, in order to obtain suspended pyrolytic carbon microstructures. The structures were then analyzed in terms of their structural stability (optical microscope, SEM) and contraction (profilometer). The pyrolytic carbon films were prepared by pyrolyzing unpatterned SU-8 films at four different pyrolysis temperatures between 800 and 1100 °C. The films were characterized in terms of their electrical resistivity (4-point probe), crystallinity (Raman spectroscopy) and surface roughness (AFM). During the fabrication process various issues were observed. This allowed us to determine a correlation between the shape and size of the microstructures with the specific fabrication issue, a potential reasoning as to why these issues would occur and how they can be avoided in the future. Based on the obtained results, a new analysis of the pyrolysis process was performed from a structural standpoint of SU-8 microstructures. Novel microstructures were also presented in the form of pyrolytic carbon cups, which show great promise as structures used for the trapping of micro and nanoparticles. Analysis of the pyrolytic carbon films show an increase in the electrical conductivity, surface roughness and crystallinity of the material with higher pyrolysis temperatures. The electrical resistivity drops from 1.29·10-4 to 2.92·10-5 Ωm as the pyrolysis temperature is increased from 800 to 1100 °C. At the same time, the surface roughness of the pyrolytic carbon films increases from 0.33 to 1.27 nm. The Raman spectra indicate a very high level of structural disorder and small crystallinity of the material. The crystallite size was calculated to increase from 6.45 to 9.15 nm with higher pyrolysis temperatures. Furthermore, detailed analysis of the Raman spectra also indicates a buildup of intrinsic stress at temperatures up to 1000 °C. Upon increasing the pyrolysis temperature further, the stress is gradually reduced from the material as the structure begins to anneal