CMOS Compatible Carbonization of Polymer for Elctrochemical Sensors

Carbon-based electrodes that are integrable with CMOS readout electrodes possess great potential in a wide range of cutting-edge applications. The primary scientific contribution is the development of a processing sequence which can be implemented on CMOS chips to fabricate pyrolyzed carbon microelectrodes from 3D printed polymer microstructures to develop lab-on-CMOS monolithic electrochemical sensor systems. Specifically, optimized processing conditions to convert 3D printed polymer micro- and nano-structures to carbonized electrodes have been explored in order to obtain sensing electrodes for lab-on- CMOS electrochemical systems. Processing conditions have been identified, including a sequel of oxidative and inert atmosphere anneals to form pyrolyzed microstructures on metallized substrates. Developed processing sequence was optimized to improve shape retention alongside maintaining an electrical insulation over the metallized substrate and around the carbonized precursors. It has been demonstrated that oxidation of titanium metal layer can be exploited to develop electrically insulating titanium oxide around the pyrolyzed precursors in a self-aligned manner. Nonetheless, substantial shrinkage of 3D printed polymeric precursors in unavoidable during their carbonization process. Moreover, primary steps of carbonization produces catalytically active titanium oxide which causes further reduction of carbon and serious artifacts in the carbonized microstructures. As such, an optimized processing sequence has been developed to reduce these detrimental effects of titanium oxide on reserving the shape of pyrolyzed polymers. The processing sequences were characterized in terms of their influence on the growth of titanium oxide and dimensional changes in the 3D printed polymers. Besides, the degree of carbonization of pyrolyzed precursors was evaluated using impedance and Raman spectroscopy. Electrical current-based localized Joule heating was used to overcome incomplete pyrolysis and analytical approximation and FEM-based COMSOL simulation were performed to determine the required processing conditions and potential outcomes. Increase in electrical conductivity of partially pyrolyzed polymer was observed by increasing anneal time within CMOS compatible temperature windows and by increasing ambient temperature of the partially pyrolyzed polymer. Using a high electrical bias and elevated ambient temperatures, an estimated average temperature of 727 °C was obtained within the active section of the precursor using localized Joule heating. This approach experimentally verified the feasibility of achieving complete carbonization of 3D printed precursors using CMOS compatible partial pyrolysis followed by localized Joule heating.