AN ATOMIC-FORCE MICROSCOPE ESTIMATION OF THE POINT OF ZERO CHARGE OF SILICON INSULATORS

The authors measured electrostatic interaction between the silicon nit ride tip of an atomic force microscope (AFM) and different silicon ins ulators surfaces: silicon oxide; the silicon nitride layer of a light addressable potentiometric sensor (LAPS) and the silicon nitride surfa ce 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 ar e compared with computer simulations carried out using a theoretical m odel based on the site-binding theory for the development of the doubl e-layer structure of the surface charge and an analytical approximated solution of the Poisson-Boltzmann equation. An estimation of the surf ace charge developed at different pH values is then obtained for the s amples under study and the AFM tip material and, consequently, their p oint of zero charge are determined. The proposed procedure could becom e a valuable routine characterization test for the production of silic on-based chemical sensors. Results suggest that an additional surface charge, independent from pH, was introduced by the annealing process. (C) 1998 Elsevier Science S.A. All rights reserved.