A micro-volume open liquid-junction reference electrode for pH-ISFETs

In this paper we present a study of two different types of micro reference electrodes with respect to electrochemical performance, stability and whole wafer processing compatibility. The first reference electrode consists of an already reported porous silicon membrane as quasi open liquid junction, a micro-machined cavity in the silicon substrate filled with a KCl solution and a Ag/AgCl wire which is in contact with this solution. It exhibits promising results with respect to stability and lifetime due to the very small pores. However, a sufficient uniformity over the wafer requires a particular care due to the critical control of the electrochemical HF etching procedure. The second reference electrode is a less complicated design but involves special requirements on the internal electrolyte. In this structure, an anisotropically etched cavity is made through the wafer, thus contacting the solutions at both sides. The efflux of the internal electrolyte is in this case limited partly by the small opening at one side of the wafer, and by the incorporation of the internal electrolyte in a hydrogel, which strongly reduces its diffusion. Theoretical calculations and electrochemical measurements are used to correlate the electrolyte leakage, electrode potential drift and electrode impedance with each other. With the reduction of the diffusion coefficient of the inner electrolyte ions in the hydrogel, sufficient stability can be obtained. Our results indicate that an integrated reference electrode with an internal volume of 0.1 μl and with a drift of less than 10 μV/h is feasible. The technologies of these two designs are compared in terms of process compatibility and electrochemical behavior.