Properties of Ag/AgCl electrodes fabricated with IC-compatible technologies

The purpose of this work is to fabricate and characterize Ag/AgCl electrodes made on a silicon chip at the wafer level with integrated circuit-compatible fabrication techniques. Such electrodes are useful as reference electrodes in several kinds of chemical sensors. Two types of electrode were investigated. The first type uses an evaporated AgCl layer that is patterned with lift-off photolithography. The second type is formed by exposing a selected part of the silver substrate to a KCrO3Cl solution. Both types of electrode give the thermodynamically expected potential response to variations of Cl− ion concentration. The potential generated by the KCrO3Cl-formed electrodes was more stable, however. Auger electron spectroscopy depth profiles indicate that immersion in a KCrO3Cl solution produces a thin layer of AgCl on top of a layer of AgO. The low electronic resistance of AgO then reduces the measured series resistance of the KCrO3Cl-formed electrodes. Impedance plane plots and the impedance as a function of frequency were measured for both types of electrode, and the impedance of the evaporated AgCl electrodes was indeed considerably higher. The impedance measurements could be successfully modelled by assuming a Randles equivalent circuit for the AgCl/electrolyte interface. For the KCrO3Cl-formed electrodes, the impedance was modified by the porosity these electrodes manifested.