1997 Volume 46 Issue 4 Pages 243-250
In order to develop pH sensors which can be used in highly corrosive solutions having pH values lower than 1 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, VR, at a given capacitance was obtained from a measured C-V curve and a VR-pH plot was made for each insulator oxide. The pH response characteristics of the sensor were evaluated from the gradient of the VR-pH plot. Corrosion resistances of the insulator oxides were measured by immersion tests in 10kmol·m-3 H2SO4 and 10kmol·m-3 NaOH. It has been found that a Ta2O5 thin film deposited at 723K shows the large gradient of VR-pH plot, a low limit pH for response in acid solutions, and a high corrosion resistance against acid. A ZrO2 thin film deposited at 573K 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.
