ISFET COMBINATION PH PF FOR THE FAST DETERMINATION OF VERY-LOW FLUORIDE CONCENTRATIONS USING ACID-SOLUTIONS

Two types of pH-ISFETs were used as the basis for F--FETs with Si3N4 or Al2O3 as sensitive layers, respectively. Deposition of LaF3 on the gate isolator results in pF-ISFETs. A Nernstian sensitivity was reached with both kinds of fluoride ISFETs. While the ISFETs with the Si3N4 interlayer were very stable, a drift of 5-10 mV/h was observed for those using Al2O3 in contact with the LaF3 layer. We suppose that insertion of fluoride into the Al2O3 is the reason for that effect. A combination of pH- and pF-ISFETs was investigated in acid solutions. It was found that the limit of detection can be improved by one order of magnitude in acid solutions. Impedance spectroscopy and tracer measurements with F-18 were used to identify the rate determining step of the potential forming process.