Development of EIS capacitor pH sensor for highly corrosive environment

In order to develop pH sensors which can be used in highly corrosive solutions having pH values lower than \or higher than 13, electrolyte-insulator-semiconductor (EIS) capacitor pH sensors were examined. A thin oxide film was used as an insulator layer and a p-type Si semiconductor was used as a substrate. Ta₂O₅ and ZrO₂ thin films were formed by low pressure CVD technique at several deposition temperatures and used as the insulator layer. Capacitance, C, versus voltage, V, curves of the capacitors were measured in solutions of pH= - 1.9 to 15.5. A gate voltage value, V_R, at a given capacitance was obtained from a measured C-V curve and a V_R-pH plot was made for each insulator oxide. The pH response characteristics of the sensor were evaluated from the gradient of the V_R-pH plot. Corrosion resistances of the insulator oxides were measured by immersion tests in 10 kmol · m^-3 H₂SO₄ and 10 kmol · m^-3 NaOH. It has been found that a Ta₂O₅ thin film deposited at 723 K shows the large gradient of V_R-pH plot, a low limit pH for response in acid solutions,and a high corrosion resistance against acid. A ZrO₂ thin film deposited at 573 K shows a high limit pH for response in alkali solutions and relatively high corrosion resistance against alkali. EIS capacitor pH sensors using the Ta₂O₅ film as the insulator layer are suitable for measurement in strong acid and those using the ZrO₂ film are suitable for measurement in strong alkali.