STUDY OF PECVD SILICON OXYNITRIDE THIN-LAYERS AS ISFET SENSITIVE INSULATOR SURFACE FOR PH DETECTION

In order to prepare insulators for pH sensing ISFETs, silicon oxynitride thin layers have been grown in a PECVD process on c-pSi/SiO2 structures. The bulk composition and the hydrogen incorporation are controlled and chosen in order to obtain a given surface density of = SiNH2 sites which are responsible for the Nernstian pH sensitivity of the silicon nitride and can be chemically grafted with alkyl silane in order to obtain a pH insensitive surface. Two types of samples have been prepared varying gas flows (N2O, NH3, and SiH4) and one with helium dilution (sample 2). The surface (NH + NH2) site densities vary from 16.4-mu-M . m-2 to 4.4-mu-M . m-2 (sample 3). They all present a Nernstian behavior in pH range 2-10; this behavior is preserved for two months in water. The pH point of zero charge of sample 3, calculated after an exposure to water through the site-binding model previously developed by Harame et al., is 5.64; in agreement with a good stability of this material in water.