Title: Sensing film materials development for FET-based pH sensors

Abstract

Ion-Sensitive Field-Effect-Transistor (ISFET)-based chemical/biochemical sensors are used for measuring ion concentrations analyte solutions. Most commonly, they are used as pH sensors, where the current through the transistor changes with respect to the ion concentration at the sensing region (gate). ISFET is a solid-state device, which makes it rugged for the measurement of ion activities in analytes, with the aid of a reference electrode and it has been widely used for pH measurements. PH is one of the most important parameters in agricultural, environmental, bio-medical and potable water studies. ISFET has attracted much interest due to the feasibility of facile integration with readout circuits. It is an alternative to the conventional glass membrane electrode-based pH sensors, where it enables measurements in ultra-small volumes with fast response. This work deals with the design, modelling, fabrication and characterization of ISFET-based pH sensors. The most critical element in ISFET-based pH sensors is the sensing film deposited over the gate region of the sensor. In this work, the sensing films are deposited using pulsed-DC magnetron-assisted reactive sputtering technique. Various process parameters for deposition of the film were optimized through several experiments. The deposition process for three sensing films were optimized, namely, AlN, Al2O3 and TiN, and their material characterization was carried out using various techniques. ISFET-based pH sensors were fabricated using well established, CMOS compatible unit processes. In this work, three variants of ISFET-based pH sensors have been fabricated with AlN, Al2O3 and TiN as the sensing films, using self-aligned process. The devices were encapsulated using the Dam-and-Fill technique and packaged using thick-film alumina technology. The sensors were characterized for pH sensing applications. Moreover, drift and hysteresis studies were performed for each sensor and low drift and hysteresis values were obtained for the fabricated ISFETs.