Pyrolytic carbon from Novolac Epoxy resin compressed before photocrosslinking and pyrolysis

Abstract

The so-called SU-8 formulation, originally designed as a negative photoresist, has become the gold standard for the fabrication of microelectromechanical systems (MEMS) due to its relatively versatile processability and to the unique set of properties of the carbon obtained from pyrolysis of the patternable Bis-Phenol A Novolac Epoxy (BPNE) oligomer contained therein. Here, we investigate the possibility of increasing the degree of graphitization and electrical conductivity of BPNE-derived carbon by mechanically compressing the uncured oligomer pre-pyrolysis. BPNE sheets were casted, compressed and photocrosslinked to “freeze” the material in a strained configuration before carbonization. The electrical sheet resistance was determined using the four-point probe technique. The relative sp2/sp3 content and the degree of crystallinity were compared by Raman and XRD spectroscopy. It was hypothesized that applying compressive loads of up to 2000 kg at 90°C would suffice to increase the crystallite size and the mesoscale order of the percolated graphenic carbon network by promoting the alignment of the aromatic segments from the phenolic groups before pyrolysis. However, negligible variations in the resistivity and sp2-carbon microstructural features were observed when compressing millimetric sheets of BPNE as compared to uncompressed samples, suggesting that higher forces and temperatures may be required to synthesize BPNE-derived glass-like carbon materials with a slightly more graphitic character.