Investigation of CMOS Front-End Architectures for C60 Single-Molecule Sensor

Starting from the twentieth century, climate change have progressively become the most dangerous threat to life on Earth. Some of its effects, like air pollution, have led to the development of devices, called gas sensors, that are able to monitor the presence of substances that could be harmful or toxic for human health. From a technological and electrical point of view, a constraint about sensors is their difficult compatibility with CMOS electronic systems which they interact with. Since full integration on CMOS circuits is complicated to achieve, the best compromise consists in designing circuits that have the task of properly conditioning the sensor and convert the detection information into electrical signals. In this manner, one can easily interface the sensor to instruments that will acquire the generated signals and recover the response of the sensor to gas exposure. In this context, the aim of this thesis is to implement a CMOS interfacing front-end for a molecular gas sensor based on fullerene (in particular C60), used as sensing element, connected to two gold (Au) electrodes. Operating in amperometric mode, the sensor is exposed to a gas whose presence modulates the current that flows through the Au-C60-Au structure with respect to the case in which no gas is present. After electrical simulations of its behavioural model, the C60 sensor has shown good capability of being selective in detection of gases like nitric oxide (NO) and nitrogen dioxide (NO2). In order to distinguish between the multitude of gases and their different configurations (molecule orientations etc.), a signature is needed for their acknowledgement. In particular, frequency is a quantity that can be acquired (for example by means of a micro-controller) and measured with good accuracy. For this reason, I have implemented a current-to-frequency converter that can provide a reliable indication about the gas that interacts with the sensor. In Chapter 1, I will explore the state-of-the-art of gas sensing devices and their interfacing circuitry. In particular, I will highlight the main technologies and applications in which they operate and the various hardware architectures for analog front-ends and gas recognition circuits. In Chapter 2, I will describe the features and sensing capabilities of C60-based sensor in order to identify the requirements for the implementation of the analog front-end. According to them, the architecture of the integrated circuit will be derived and shown by means of block schemes whose characteristics will be briefly described. In Chapter 3, I will go on with the detailed implementation of each stage of the analog front-end; I will focus on the design choices we have taken to adapt the CMOS architecture to the requirements of the molecular sensor. In Chapter 4, I will test the electrical behaviour of the circuit by means of simulations that have the purpose of validating the final results. In Chapter 5, the conclusions of this thesis work will be drawn by discussing final outcomes, showing that the implemented circuit is promising to act as a conditioning circuit for the gold-C60-gold gas sensor. At the end, I will provide a brief perspective on future developments.