High-sensitive Low Power Tunneling Field Effect Transistor Biosensor for Multiplexed Sensing Application

Abstract

As fabrication technology has continued to develop, various nano-size biomedical sensors have been widely researched since the size of biological entities, such as DNA, proteins, and viruses are similar to their size. Especially, chemical and biomedical sensors using fluorescent labeling and parallel optical detection techniques have received much attention for high sensitivity. However, they have a number of drawbacks such as expensive and time-consuming processes for sample preparation and data analysis. To overcome these limitations, silicon nanowire (SiNW) Ion Sensitive Field Effect Transistors (ISFET) have been proposed as one of the most promising chemical/biomedical sensors since it has good characteristics such as label-free, real-time detection, and excellent sensitivity caused by high surface-to-volume ratio. In terms of the fabrication process, SiNW ISFETs also have compatibility with CMOS technology. Various fabrication methods of SiNW ISFETs have been reported. Recently, our group demonstrated a novel SiNW MOSFET sensor which can be integrated with a CMOS device by using top-down fabrication process. However, the proposed top-down process cannot avoid the damage of the sensing material by the plasma damage because the sensing area is formed by the dry-etch process, which leads to the degradation of the sensitivity and the current drift. Furthermore, it is impossible to fine-tune the various threshold voltages (Vth) of the circuit devices by the implant process, which results in circuit malfunction and reduction in amplification factor. In this thesis, the novel top-down approached fabrication method using top SiO2-SiN-bottom SiO2 (ONO) dielectric stacks is proposed and implemented to obtain sensors with defect-free sensing oxide and Vth-tunable devices in CMOS read-out circuits. By wet-etching the top SiO2 and the SiN, the sensor with defect-free sensing oxide is obtained. Also, the Vth-tunable circuit devices with the ONO stacks are simultaneously achieved by protecting the ONO stacks from the wet-etching. Through the measurements of pH response and current drift in the sensor and program operations in the circuit device, it is confirmed that the pH/biomolecule response and the current drift of the sensor are improved and the Vth of the circuit device can be fine-controlled. Although the defect-free sensing material improves the drift and the sensitivity, the MOSFET sensor has a theoretical limitation on the maximum sensitivity because MOSFETs cannot implement sub-60mV/dec SS at room temperature. To achieve the higher sensitivity, a TFET sensor is proposed and fabricated since it can achieve sub-kT/qS at room temperature by using band-to-band tunneling as carrier injection mechanism. From TCAD simulations, it is revealed that the TFET sensor has two ID saturations by the saturation of the source-to-channel tunneling width and the drain-side carrier injection. Moreover, it is experimentally confirmed that the TFET sensor is the superior sensitivity up to the first ID saturation region and there is no difference in sensitivity from the second ID saturation region as compared to the MOSFET sensor. Finally, the possibility of multiplexed sensing is verified with the fabricated MOSFET and TFET sensors. To form two different sensing materials reacted with GBP-Ala/Anti-AI and SBP-H1N1/Anit-H1N1 for the multiplexed-sensing, gold is partially covered on the SiO2 by a lift-off process. Then, the changes of saturation and GIDL current are monitored in the MOSFET sensor after the reactions of GBP-Ala/Anti-AI (SBP-H1N1/Anit-H1N1) to the gold (SiO2). Two different biomolecules are independently detected by the changes in the saturation and the GIDL currents. To solve the problems of the MOSFET sensor by the dependence of the gold formation position on the sensitivity and the large current difference between the saturation and the GIDL currents, the changes of tunneling and ambipolar currents are measured in the TFET sensor. As a result, it is revealed that two different biomolecules can be detected without interference regardless of the position of the gold layer by the changes of the tunneling and ambipolar currents with almost equivalent current level.