Ion-sensitive oxide films were deposited on a Si wafer to make a MOS-diode structure in a biased RF (radio frequency) sputtering system using Ar+O
2 as reactant gas. Various kinds of oxide films such as SiO
2, Ta
2O
5, Mg-Al-Ag-P-O, Pb-Al-Ag-P-O and Si-N-O of about 1, 000Å in thickness, were formed. The adsorption of ion on the MOS-diode surface in an electrolytic solution was induced the change of the surface capacitance. The capacitance change of the MOS-diode in the standard pH buffer solutions and some anion solutions were closely related to the adsorption properties of those ions on the oxide film surface. In the standard pH solutions, the adsorption-layer capacitance of SiO
2 and Ta
2O
5 film increases with increasing the values of pH. The capacitance of the alkali metal-free phosphate glass films, however, was independent of the pH in the electrolyte. On the other hand, the capacitance of these glass films increases with increasing anion concentration. In relation to the variation of nitrogen and oxygen in the Si-N-O film, the slope of the capacitance-pH relationship decreses with decresing oxygen content in the film.
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