Herstellung und Charakterisierung von Hafniumoxidschichten für pH-Sensoren

The need for process control in chemical and biochemical industries has forced the development of sensor systems, that combine precise measurement with a good lifetime and/or very low cost, which ususally is accomplished by integrated sensors. Especially in chemical processes there's a need for precise regulation of acid/alkaline-controlled systems for quality reasons. The way for both integration and good sensing characteristics is the ISFET technology, which is used with several metal-oxide pH-sensing layers nowadays. The problem, however, is either sensing range or sensing stability and linearity of the sensor. In this work Hafniumoxide and its preperation as protective and sensitive layer by reactive RF-Magnetron-Sputtering is proposed and discussed. It was found that particularly the stability is strongly dependend of the sputter parameters. Especially a high substrate temperature during the sputtering process seems to strongly increase crystallinity, which is a key parameter for alkaline stability. While crystallinity can be guaranteed by temper processes, the thermal expansion coefficient of hafniumoxide combined with temperature during the proces causes very high stresses and, at a point microporosity, which strongly reduces sensor lifetimes in both acid and alkaline environments. Other important sputter parameters are the pressure, mainly leading to ultra-compact layers with up to 2 GPa compressive stress while flattening the surface which is very good for withstanding acids, and ozone/argon ratio, which increases stoichiometry. A good stoichiometry though seems to be important for both linearity and stability reasons. Other parameters does not or hardly not change the layer parameters, but can be adapted to meet process time and edge covering needs. Experiments were carried out in IPMS Fraunhofer Institute in Dresden, GER, with support by the Dresden University of Technology, TU-Dresden, GER