Passivation and corrosion of microelectrode arrays

The application of silicon based microsensors in aqueous environments is hi ndered by unsatisfactory barrier properties and poor corrosion resistance o f common passivation layers which give insufficient protection to electroni c microstructures. This paper reports on investigations of the protective e ffect of various types of layers (compatible to silicon planar technology) against 1 M NaCl at pH 2 to 10. Failures of the passivation layers were det ected by leak current and conductivity measurements with subsequent investi gations of failure mechanisms by scanning electron microscopy (SEM). Both organic and inorganic films were tested with chips which were complete ly covered with the passivation layer. Organic films had a time to failure of at best 500 h, achieved by Probimer(R) and plasma treated polyimide. The poor barrier properties of PECVD-SiO2 and Si3N4 monolayers (only a few hou rs) were clearly surpassed by combining the monolayers to SiO2/Si3N4-duplex and SiO2/Si3N4/SiO2 triplex (ONO) layers. The most promising barrier prope rties were achieved by the tripler (ONO) layer which yielded a time to fail ure of 1200 h compared to 500 h for the duplex layer on non-buried conducti ng tracks. Burying the conducting tracks into the thermal SiO2 layer signif icantly improved the performance of the duplex (2000 h) and the SiC layer ( 1000 h compared to 700 h on non-buried tracks) once again. In the case of open electrodes the Si3N4 layer quickly failed, whereas the duplex and the SiC layer revealed better protective properties. Organic fil ms failed due to swelling and the formation of blisters. Intrinsic mechanic al stress with chemical interaction resulted in stress corrosion cracking ( SCC) and finally lead to the failure of the inorganic PECVD layers.