Carbonization of 3D printed polymer structures for CMOS-compatible electrochemical sensors

Abstract

Carbon based electrodes suitable for integration with CMOS readout electronics are of great importance for a variety of emerging applications. In this study, we have looked into the prerequisites for the optimized pyrolytic conversion of 3D printed polymer microstructures and nanostructures with the goal of developing sensing electrodes for a lab-on-CMOS electrochemical system. As a result, we identified conditions for a sequence of anneals in oxidative and inert environments that yield carbonized structures on metallized substrates with improved shape retention, while also providing electrical insulation of the surrounding metal stack. We demonstrated that titanium metal layers can be conveniently used to form electrically insulating titanium oxide on the substrate outside the carbonized structures in a self-aligned fashion. However, significant shrinkage of polymer structures formed by 3D printing or stereolithography is inevitable during their pyrolysis. Furthermore, the catalytically active titanium oxide present during initial stages of carbonization leads to additional loss of carbon and significant artifacts in the resulting structures. To minimize these adverse effects of titanium oxide on the shape retention of the carbonized structures, we developed an optimized processing sequence. Various processing steps in this sequence were characterized in terms of their effects on titanium oxide growth and geometrical changes in the 3D printed structures, while impedance and Raman spectroscopy were performed to evaluate their degree of pyrolytic conversion and, therefore, potential for electrochemical sensing.