Exploration of New Materials and Processes for Ion Sensitive Field Effect Transistors

Abstract

Ion Sensitive Field Effect Transistors (ISFETs) find applications as front end transducers in the biomedical domain as a biosensor. The favourable characteristics of ISFETs like sensitivity, speed, miniaturization, easy integration, high reliability, low cost and precise process control make ISFETs a promising candidate for the same. However, future applications in the bio medical field require bio sensors with faster response time, which are stable and bio compatible such that it can be used for both in vitro and in vivo applications. For ISFET based technology to drive the biosensor market, it should have excellent sensitivity to pH. Even though the concept of ISFETs as a tool for electrophysiology was first introduced in the 1970and#8223;s its practical applications fell behind over the years as the pH sensitivity of classical ISFETs were limited by the Nernst limit of 59mV/pH. This work aims at the exploration of new materials and processes for ISFETs to enhance their pH sensitivity. Engineering the sensory surface, engineering the structural dimensions, reducing the screening of counter ion charge, electromechanical coupling techniques etc. are some of the methods suggested in literature to improve the pH sensitivity of ISFETs. In the quest to engineering the sensory surface, the possibility of improving sensitivity by various gate dielectrics was evaluated first. A process flow for fabrication of ISFETs was developed and optimised for experimental studies. First, ISFETs with classical SiO2 as dielectric, were fabricated using the optimised process flow with additional post fabrication modification of acrylic well to contain the electrolyte. Encapsulation which is a challenge in ISFET technology was addressed with SU-8 encapsulation on the fabricated devices. A platinum electrode just touching the electrolyte was used to apply gate bias. The pH response of the fabricated devices was studied and was found to adhere to classical ISFET nature. To study the effect of dielectrics on pH response of ISFETs, a comparative..