The influence of surface oxidation on the pH-sensing properties of silicon nitride

Abstract

The pH-sensing properties of silicon nitride were studied with special emphazise on surface oxidation. Simulations of the surface potential and the sensitivity were performed using the site binding theory to show the influence of an oxygen rich surface and surface oxidation of silicon nitride. Samples were produced by depositing silicon nitride in a LPCVD process with flow ratios of the processing gases NH 3 and SiCl 2H 2 ranging from 2 to 20. The samples were characterized by XPS and capacitance–voltage measurements of EIS-structures. Silicon nitride etched in buffered HF before measurement showed a pH-sensitivity of up to 58 mV pH −1, and an increasing hysteresis was found with increasing NH 3/SiCl 2H 2 gas-flow ratios during deposition. Samples not etched had only a pH-sensitivity between 45 and 52 mV pH −1 because of their oxygen rich surface layer. The oxygen rich layer was found to be approx. 4 nm deep and soluble in buffer electrolyte. Surface oxidation and dissolution of the oxygen rich layer is claimed to be decisive for the pH-sensing properties of silicon nitride.