Fabrication and Modeling of On-Chip Integrated Miniaturized Ag/AgCl Reference Electrode for Solid-State Chemical Detector

This work proposes a fabrication and optimization process of miniaturized Ag/AgCl reference electrodes (REs) at a micrometer (μm) scale for the miniaturized chemical sensors and it provides a stable reference voltage for ion-sensitive field effect transistor (ISFET) to detect pH in biochemical environments. The thin silver films are grown by magnetron sputtering and chemically chlorinated to form the Ag/AgCl microelectrodes. Different chlorination durations and current densities have been applied to investigate the best fabrication recipe. The electrical performance of the proposed microelectrode has been compared with a standard saturated calomel electrode (SCE), and it is observed that the electrode fabricated with an optimized recipe could provide less than 4-mV accuracy within the pH range from 4 to 10 and lifetime exceeding 200 min even with only 1.95 × 10⁵ μm² area. The correlation between the microelectrode area and both impedance and noise is expressed by equations, serving as a theoretical guide for designing and fabricating the Ag/AgCl RE on the chip.