Electrostatic discharge (ESD) can easily damage integrated circuits (IC) and electronics systems [1-6]. It is hence imperative to investigate the ESD fundamentals and develop robust ESD protection solutions for semiconductors and ICs. Conventional on-chip ESD protection structures for ICs rely on in-Si PN-junction-based devices, which have many inherent disadvantages, making them unsuitable for future ICs at nano nodes.
Both novel ESD device structures and robust ESD interconnects are critical to on-chip ESD protection designs. In this dissertation, I report research outcomes of transient and systematic characterization of graphene ribbon (GR) used as interconnects for on-chip ESD protection circuits for future ICs. A large set of GR wires with varying and practical dimensions were fabricated using chemical vapor deposition (CVD) method and characterized by transmission line pulsing (TLP) and very fast TLP (VFTLP) measurements. This research indicates that, with its unique properties, e.g., high thermal conductivity and high current handling capability, graphene ribbons may be used as interconnects for on-chip ESD protection circuits, replacing existing aluminum and copper metal interconnects. I also report a novel above-IC graphene-based nano-electromechanical system (gNEMS) transient switch ESD protection mechanism and structure. TLP testing confirms the new gNEMS ESD protection concept, showing dual-polarity transient ESD switching effect with a response time down to 200ps. This novel gNEMS switch is a potential ESD protection solution to realize above-Si ESD protection designs through 3D heterogeneous integration in the back end of line (BEOL) of ICs.
The third part of my PhD research was to explore using graphene to make novel ultrasound transducers. Ultrasound imaging utilizes ultrasonic acoustic waves to monitor organs and tissues, which has been widely used in biomedical diagnosis, such as in obstetrics and gynecology, cardiology, urology and cancer detection areas. Capacitive micro-machined ultrasound transducers (CMUT) are recently used for ultrasound imaging systems. CMUTs have advantages over piezoelectric micro-machined ultrasound transducers (PMUTs). CMUT can be fabricated by standard IC fabrication processes, making it possible to make high-performance, lost-cost and compact ultrasounic system-on-s-chip (SoC) for hand-held ultrasound imaging systems for various biological and medical applications. One key disadvantage of existing CMUT structures is the relatively low frequency, well below 100MHz as reported, hence, poor imaging resolution. I report the first concept of a graphene CMUT structure with a resonant frequency around/beyond 110MHz, making it possible to develop ultra-high resolution hand-held ultrasound imaging products for healthcare applications.