Failure Analysis of Microfabricated Iridium Ultramicroelectrodes in Chloride Media
 

Melissa A. Nolan and Samuel P. Kounaves*
Department of Chemistry, Tufts University, Medford, MA 02155 U.S.A.

*Author to whom correspondence should be addressed (skounave(at)tufts.edu).

This study investigates the failure mechanisms for an electrochemical sensor consisting of an array of microlithographically fabricated iridium ultramicroelectrodes on a silicon wafer.   It has been noted that the use of these sensors during voltammetric determination of heavy metal ions in the presence of  >0.1 M chloride ions results in loss of response and general failure of the sensor.  The addition of mercury or silver to the chloride containing solution caused massive accumulations on the exposed iridium surfaces, which were identified using auger scanning electron microscopy.  This failure was attributed to the penetration of chloride ions through pin-holes in the insulating layer and reacting with the aluminum interconnect traces on the chip.  Consequently, the aluminum traces dissolved and hydrogen gas was evolved.  This reaction generates a potential which is transmitted to exposed iridium and consequently metal ions present in the solution are then reduced onto these surfaces.  Several insulating layers, silicon dioxide (5000 Å) and silicon nitride (1500 Å, 2500 Å, and 5000 Å) were investigated, but failure occurred with all.   It would appear that it is critical that sub-micron sized pin-holes during the fabrication process be better controlled.  In most cases this would require extra steps during the fabrication process and could substantially increase the cost.  A lower cost solution is to use gold for the interconnect traces.  Thus, even though the pin-holes may still exist, the insignificant amount of electroactive surface exposed should not to interfere with the typical techniques used for electroanalysis.