Exploration of New Materials and Processes for Ion Sensitive Field Effect Transistors
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 1970‟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 analysis of three different sensing layers SiO2, SiO2/Si3N4 stack and SiO2/Al2O3 stack was done. The focus was to simplify fabrication process including SU-8 encapsulation. A new concept of SU-8 well for containment of the electrolyte in order to enable efficient interaction with biological fluid was attempted. The pH response was evaluated from the I-V characteristics and ISFETs with SiO2/Si3N4 stack and SiO2/Al2O3 stack showed better pH response with sensitivity of 59.33mV/pH and 55.68mV/pH respectively as against sensitivity of 48.9mV/pH in ISFETs with SiO2 as gate dielectric. The analysis proved enhancement of pH sensitivity nearer to the ideal value of Nernst limit. To evaluate the feasibility of ISFET pH response beyond Nernst vii limit a simulation analysis was carried out on double gate (DG) ISFETs. The study proved that engineering the structural dimensions in the form of dual gate architecture indeed improved the sensitivity beyond Nernst limit. It was observed that the top gate pH sensitivity in terms of voltage at constant drain current was within the Nernst limit, whereas the bottom gate sensitivity was much above the Nernst limit with average sensitivity around 280mV/pH. The results were promising enough to consider fabricating the device with a new material but with dual gate architecture. A new material is essential, as scaling trends for future technologies demand an alternative to silicon for transistor fabrication. Two dimensional layered materials are touted to replace silicon technology because of their high charge mobility and ultrathin structures. Naturally 2D layered material Molybdenum di sulphide (MoS2) belonging to the Transition Metal Dichalcogenide class is known to have intrinsic band gap unlike the more popular graphene and is biocompatible. Hence, an attempt was made to fabricate and characterize 2D material based (MoS2) double gate ISFETs, to provide sensitivity beyond Nernst limit. HSQ encapsulation was done on devices instead of SU-8 encapsulation along with SU-8 well for electrolyte containment. The cumbersome platinum reference electrode was replaced by an extended gate within the SU-8 well. The top gate and bottom gate pH sensitivity of the fabricated device were computed from the obtained transfer characteristics and was estimated to be 20-46mV/pH and 1-3V/pH respectively. The pH response analysis agreed with the general observation on double gate architecture that though the top gate pH sensitivity is much below the Nernst limit, the bottom gate sensitivity shows enhancement by a large extent. The super Nernstian behavior of the fabricated MoS2 double gate ISFETs is encouraging for it to find applications as front end transducers in the biomedical field especially in diagnostics where high precision sensing is necessary.