A new technique used in order to realize fluoride ion-sensitive membranes is presented. This method consists of the ion implantation technique which is highly compatible with integrated circuit (i.c.) technology. This technique can be used for realizing various inorganic membranes for sensing purposes. In this paper, we studied two thin film membranes based on lanthanum and calcium fluoride compounds for fluoride detection, through the ion implantation technique. The electrical and chemical properties of the firms obtained are studied on electrolyte/oxide/semiconductor structures (EOS). The fluoride ion sensitivity is determined using capacitance voltage measurements for the EOS structures. The response obtained is 44 mV per decade in the range of pF(-) 4 to 2 for LaF3 membrane, and 52 mV per decade in the range of pF(-) 3 to 1 for CaF2 membranes. Finally, we present data for fluoride sensitive ISFET (ion sensitive field effect transistor) microsensors using the developed CaF2 membrane with a corresponding REFET (reference field effect transistor) in individual and differential mode.