Sensitization of dielectric surfaces by chemical grafting: application to pH ISFETs and REFETs

Abstract

The aim of this work is to prepare a pH sensing device working in a differential arrangement between an ISFET and a REFET. Two types of insulator surfaces have been tested: the pH sensitivity of silica is 36 mV pH −1 and that of the PECVD silicon oxynitride is nernstian. The grafting of these surfaces with long-alkyl-chain silanes has taken place. The surface preparation must be well controlled in order to obtain dense, grafted, long-alkyl-chain monolayers on both insulators. The pH sensitivity of grafted silica depends on the chain length; it is 10 mV pH −1 for C22 grafted silica. For a silicon oxynitride with a surface density of SiNH/NH 2 groups of 2.6 nm −2, the pH sensitivity of the C18 grafted surface is 15 mV pH −1. PECVD silicon oxynitride seems to be well adapted as a new insulator for pH ISFETs.