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.