Impact of O2-based plasma strip chemistries on the electrochemical behavior of TiN electrodes for biomedical applications
In this work, we present the impact of standard CMOS-based patterning techniques and more specifically the impact of strip chemistries on the electrochemical behavior of TiN electrodes. As TiN is a material that is both CMOS- and biocompatible, these electrodes are of interest for in vitro and in vivo biomedical applications to interface with electrogenic cells. When scaling down the TiN electrodes to cellular and sub-cellular dimensions, the use of optical lithography in combination with RIE-based (Reactive Ion Etch) patterning will be required. This can be done with standard CMOS processes and tools. However, the TiN etch and more important the subsequent strip of the photoresist are often O2-based. It will be demonstrated that these O2-based chemistries need to be avoided in order to keep the electrode impedance low and its charge injection limit high. The increased surface oxide after exposure to these chemistries decreases the double layer capacitance and reduces the ability of the electrode to transfer charge to the medium and cells. Screening of different strip chemistries, that can be used in the patterning of micrometer sized electrodes, was achieved by electrochemical measurements in combination with XPS analysis on large TiN electrodes. Strip plasmas based on N2/H2 are interesting alternatives to keep the electrode surface intact by reducing the amount of oxygen present at the electrode surface. Moreover, H2O vapor can reduce the surface oxide even further and can lead to lower impedance values and an increased charge storage capacity, beyond the values of the as-deposited layers. The results presented in this paper demonstrate that the scalability of TiN electrodes can be improved significantly when O2-based plasma chemistries are removed from the electrode patterning process. Optimized cleaning after etch and during strip can be beneficial to the overall electrochemical characteristics of the electrode.
electrode oxidation titanium nitride