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Title: Engineering robust CMOS ISFET smart sensor systems
Author: Liu, Yan
ISNI:       0000 0004 2716 569X
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2012
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The development of biomedical research and fast point-of-care diagnostics require large-scale sensing equipment and smart portable healthcare devices. Therefore, integrating chemical sensors into solid state platforms becomes the most popular solution in modern chemical sensing applications. As a result, the continuous trend of scaling the transistor in semiconductor engineering and sensors' feature size in biomedical or biochemical areas, converge into the concept of Lab-On-Chip (LOC). By combining LOC and the well developed fabrication process, Complementary Metal Oxide Semiconductor transistor (CMOS), a high level integration incorporating sensors and processing circuitry can be realized with minimal fabrication costs and convenient data processing ability. This work focuses on the engineering chemical sensing systems based on the CMOS ISFET, which provides high scalability and integration ability. An extended model for CMOS ISFETs is proposed to create an accurate model for robust sensors design. The origins of threshold variation and transconductance reduction are explained in detail by using this model. A complete study on the electrolyte-insulator interface across the sensing membrane is provided to qualitatively explain the non-ideal effects such as drift and noise. Based on the study of both electronic and chemical sides, a design strategy is presented and indicates that large sensors are better for accurate measurements and small sensors are suitable for large-scal parallel sensing. Using this knowledge, we investigate the interface circuit with capabilities to reduce the non-linear effects of ISFETs. To reduce the trapped charge effect in the device, an auto-offset-removal approach is presented and demonstrated in complimentary sensing pairs based on autozeroing techniques. The trapped charge effects and the transistor low frequency noise are attenuated to provide a larger dynamic range. Moreover, by using pH-to-Time sensors and Time-to-Digital converters, a highly compact readout scheme is designed with a minimized analogue biasing and processing. This scheme shows a great potential in large scale sensing platforms, which require low power consumption and high sensors density. Finally, a novel system, which feedbacks the electrical signal into the chemical environment, is developed to reduce the non-ideal characteristics. By incorporating the principle of a Sigma- Delta modulator, we develop the chemical sigma-delta modulator. By using the ISFETs as the quantizer, a titrator as the feedback delta modulator, and the chemical diffusion characteristics as the Sigma modulator, a chemical noise shaping is realized to minimize the drift and low frequency noise.
Supervisor: Toumazou, Christofer Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral