M. Majidian, C. Grimaldi, M. Baibarac, I. Baltog, L. Forró, A. Magrez
Keywords: graphene, composite, SU8, conductive polymer
Summary:SU8 epoxy is a photo-resist with great intrinsic properties such as high thermal stability, chemical and mechanical robustness along with high sensitivity to UV. Having eight reactive epoxy sites in each monomer molecule, a high degree of cross-linking is attainable for SU8 after photo-activation and thermal treatment. These characteristics make SU8 one of the most favorable polymers for MEMS applications, pattern transfer and fabrication of high aspect ratio structures. However, besides the attractive features, SU8 has few drawbacks: it is an electrically insulating material with very low thermal conductivity; it is brittle and has high internal stresses which can lead to bending of the structures. Addition of fillers has been shown to partially overcome these drawbacks. Graphene, due to its advantageous electronic and mechanical properties as well as good chemical stability, is considered as a promising material for nanoelectronic and optoelectronic devices. Having a very high specific surface area (2600m2 g-1) enables high electrical conductivities for composites with a low loading of graphene fillers. In this work, we present SU8-graphene-based nanocomposite containing reduced graphene oxide (RGO) flakes, as a new patternable conductive polymer with electrical properties superior to other graphene-based polymer composites for MEMS application. Our composites preserve the photo-patternability after optimization of UV lithography parameters and well-defined structures can be drawn at wafer scale. Scanning and transmission electron microscopy were used to analyze the microstructures of the composites. Chemical contribution of the RGO fillers within the matrix, before and after crosslinking of the polymer matrix, was investigated by FTIR and FTraman spectroscopy. We believe that the functional groups on the surface of the RGO flake, facilitate the chemical reaction between the RGO fillers and the epoxy by providing additional protons required for opening of the epoxy ring, therefore, increase the kinetics of cationic crosslinking. Electrical and mechanical properties of these composites can be controlled by tuning the content of graphene flakes within the SU8 matrix. Electrical conduction of the composite samples containing various volume fractions of RGO was measured using two and four probe methods. We showed that the conductivity behavior of the composites as a function of volume fraction is not consistent with the classical percolation model of transport. Instead, we suggest that the composite conductivity results from tunneling processes between the graphene flakes High electrical conductivities compare to pure SU8 and other graphene-based composites, even at very low filler loadings, is a result of homogeneous dispersion of the flexible graphene flakes in the SU8 matrix.