Micro-for-Nano: A Low-Power Platform for Nanomaterial Integration and Nanosensors Interface on 0.13μm CMOS Technology.

During the last years, material science has been focused on the exploration of the material characteristics at nanoscale. In fact, some materials show different properties only if they are designed with a nanometer structure. Even if they can be used to build macro devices (e.g., tactile surface, strain sensors), the nanostructured materials can reach high sensitivity or accuracy. Thin films [1], nanoparticles [2] and nanowires composites [3] have been widely used thanks to their sensitivity to mechanical strengths [4] or light stimuli [5–7]. In these cases, a large number of nanostructured elements have been merged in a single device to transduce macro-phenomena (e.g., strain, bending, pressure, temperature). Although nanomaterials can be used for standard sensor applications, the aim of nanotechnology is to exploit the dimension of the basic elements (e.g., nanoparticles) to conceive innovative applications at nanoscale. In order to exploit the ultra-small dimension of these materials, researchers addressed the development of nanodevices including only a single nanostructured element to increase sensitivity and accuracy. Nanomaterials, such as nanowires (NWs), bridging molecules or nanoparticles, are considered the basis for a new generation of bio-sensors able to interact with gases [8, 9], molecules (e.g., DNA molecules) or other bio-substances at nanoscale. Some examples are the lab-on-chip designed to implement drug detection using functionalized CNT [10] and the Electronic Nose able to identify different gas molecules [11]. The fabrication process of a nanosensor (or nanodevice) mainly consists in the integration of nanomaterials (previously synthesized for achieving the desired functionality) with metal electrodes. The fabrication process is actually complex and implies high costs. Different techniques can be used to connect nanomaterial with metal electrodes and, then, to the custom electronic interface. The most used methods for integration involve a stochastic deposition upon interdigitated electrodes [12] or chemical processes to directly grow the nanomaterials in-situ [13] or an electrically controlled deposition of nanomaterials dissolved in liquid solution [14]. The fabricated nanodevice is a passive component and it needs to be connected to a measurement system, involving long cables and therefore high parasitics. Fundamentally, when a nanomaterial is exposed to specific molecules or physical phenomena, its resistance or capacitance changes proportionally to the sensed quantity. Thus, the larger the variation of the resistance or capacitance of nanomaterials, the higher the sensitivity to specific phenomena. The electronic interface for passive nanosensors should be able to stimulate the nanomaterial and convert the large variation of its electrical characteristics to analog or digital signals compliant with commercial electronics. The nanomaterial signal is usually a current in the pA-μA range and the noise coupling, due to long interconnections, can easily affect the whole nanodevice sensitivity. Hence, a new approach for the nanosensor fabrication and for the read-out is strictly required to cut fabrication costs and improve measurement accuracy. The electronic interface needs to be placed as close as possible to avoid interferences at the interconnection cables. Anyway, the read-out system has also to overcome flicker-noise effects during DC or low-frequency measurements. In addition to the issues related to the measurement accuracy, a single nanosensor is not sufficient to produce reliable results because of the process variation in nanomaterial synthesis and nanodevice fabrication. Thus, an array of nanosensors is strongly suggested because a large number of nanodevices compensates the defects in single nanosensor fabrication. The measurement system provides the final results performing an average calculation of the nanosensor outputs. Actually, if the final aim is a complex system as the Electronic Nose [15] (i.e., an integrated multi-sensors system) or a bio-sensors for blood analysis [16], an array of nanosensors is strictly required given that different molecules have to be detected and average measurements are mandatory. This PhD thesis reports about a flexible platform implemented in CMOS technology for conceiving a Micro-for-Nano (M4N) system where nanosensors and microelectronics coexist on the same chip. The nanomaterial integration process (Chapter 2, Chapter 3), the read-out circuits for nanosensor interface (Chapter 4, Chapter 5) and the architecture to handle large number of integrated nanosensors (Chapter 6) will be described in the following chapters. The M4N project has been developed in collaboration with the Italian Institute of Tecnology (IIT@PoliTO), which has supported all the experiments needed to set-up the integration process and to characterize the designed CMOS circuits.