An atomic force microscope estimation of the point of zero charge of silicon insulator

The authors measured electrostatic interaction between the silicon nitride tip of an atomic force microscope (AFM) and different silicon insulators surfaces: silicon oxide; the silicon nitride layer of a light addressable potentiometric sensor (LAPS) and the silicon nitride surface of an ion sensitive field effect transistor (ISFET) device produced by a modified process with an additional annealing step, immersed in electrolyte solutions at different pH values. Measured interactions are compared with computer simulations carried out using a theoretical model based on the site-binding theory for the development of the double-layer structure of the surface charge and an analytical approximated solution of the Poisson-Boltzmann equation. An estimation of the surface charge developed at different pH values is then obtained for the sample under study and the AFM tip material and, consequently, their point of zero charge are determined. The proposed procedure could become a valuable routine characterization test for the production of silicon-based chemical sensors. Results suggest that an additional surface charge, independent from pH, was introduced by the annealing process