Carbon Nanotube Capacitors

Capacitors are important components in many integrated circuits. They serve numerous roles in analog and mixed signal circuits, including switched capacitor filters and sampleand-hold circuits. In addition, capacitors provide a vital role in the decoupling of microprocessors, digital signal processors, and microcontrollers from power supply variations. Traditional integrated circuit capacitors use a parallel plate structure. The capacitance of these parallel plate structures is fundamentally limited by the die area they consume and the thickness of the dielectric material between the parallel plates. However, increasing the capacitance of traditional parallel plate capacitors by increasing the die area (resulting in a more expensive component) or decreasing the dielectric thickness (resulting in a higher leakage current) contradicts two of the fundamental tenements of integrated circuit design (ITRS, 2008). Carbon nanotubes (CNTs) are nanotechnology materials that have been in prominence for the last several years. They are cylinders of graphene that can exhibit radii on the order of nanometers. CNTs exhibit a number of properties that make them attractive as potential horizontal and vertical interconnects in future integrated circuits (Naeemi et al., 2004, Naeemi et al., 2005, Raychowdhury & Roy, 2004). The same properties allow CNTs to be used to create a new capacitor structure suitable for use in future integrated circuits (Budnik et al., 2006). In this chapter we introduce these CNT capacitor structures and compare their capacitance per unit area with existing technologies. The chapter is organized as follows. Section 2 reviews the structure and limitations of traditional parallel plate capacitors in integrated circuits. Section 3 briefly introduces CNTs, their varieties, and how they can be manufactured. Section 4 reviews the electrical models that have been developed for CNTs for use in the development of a CNT capacitor model. Section 5 formally introduces CNT capacitor devices as an extrapolation of the currently proposed CNT interconnects. Three different CNT capacitor structures and their electrical models are presented in this section. Finally, the conclusions are presented in Section 6.