1.2 Three-Dimensional Integration

Continuous miniaturization along Moore's law has enabled the integration of different digital functional components, such as logic and memory storage, on a monolithic single chip through 2D-system-on-chip (SoC). The continued shrinking of physical feature sizes of the digital functionalities, referred as "more Moore" research domain, provides improved density, performance, and reliability values of the IC to the applications. The "more than Moore" approach leverages the scaling capabilities derived from the "more Moore" developments to incorporate digital functionalities into compact systems. A prominent strategy to continue along this trend is provided by the integration of a large number of functions of the chip by efficiently using the third dimension. System integration is the method of integrating together different circuit blocks of a chip and is one of the major applications of 3D integration [150]. 3D integration is a new design paradigm which simply consists of the process of vertically stacking multiple layers of active devices of the chip and forming electrical connections between them, either through the silicon die or through the multilayer interconnect with an embedded die [9]. It provides a space transformation of the traditional 2D planar IC implementation to 3D stacking, as illustrated in 1.4.

Figure 1.4: Conceptual view of the space transformation of (a) 2D planar SoC to (b) 3D-SoC.

Using short vertical metal interconnects in the novel 3D design, the distance between different device layers becomes relatively small when compared to 2D-ICs and allows for faster communication between different circuits blocks. These shorter electrical connections employed in 3D integration technologies provide many benefits, such as increased interconnect density and bandwidth which enable the reduction of the average load capacitance and resistance, resulting in higher speed and lower power consumption when compared to a typical 2D chip [122,131]. Furthermore, the ability of these systems to include disparate circuit blocks with different performance requirements enables the unique opportunity for heterogeneity and diversification of technologies.