A three-dimensional integrated circuit (3D IC) is a MOS (metal-oxide semiconductor) integrated circuit (IC) manufactured by stacking as many as 16 or more ICs and interconnecting them vertically using, for instance, through-silicon vias (TSVs) or Cu-Cu (copper-copper) connections, so that they behave as a single device to achieve performance improvements at reduced power and with a smaller footprint than conventional two dimensional processes. The 3D IC is one of several 3D integration schemes that exploit the z-direction to achieve electrical performance benefits in microelectronics and nanoelectronics. 3D integrated circuits can be classified by their level of interconnect hierarchy at the global (package), intermediate (bond pad) and local (transistor) level. In general, 3D integration is a broad term that includes such technologies as 3D wafer-level packaging (3DWLP); 2.5D and 3D interposer-based integration; 3D stacked ICs (3D-SICs); 3D heterogeneous integration; and 3D systems integration; as well as true monolithic 3D ICs. International organizations such as the Jisso Technology Roadmap Committee (JIC) and the International Technology Roadmap for Semiconductors (ITRS) have worked to classify the various 3D integration technologies to further the establishment of standards and roadmaps of 3D integration. As of the 2010s, 3D ICs are widely used for NAND flash memory and in mobile devices.
Types
3D ICs vs. 3D packaging 3D packaging refers to 3D integration schemes that rely on traditional interconnection methods such as wire bonding and flip chip to achieve vertical stacking. 3D packaging can be divided into 3D system in package (3D SiP) and 3D wafer level package (3D WLP). 3D SiPs that have been in mainstream manufacturing for some time and have a well-established infrastructure include stacked memory dies interconnected with wire bonds and package on package (PoP) configurations interconnected with wire bonds or flip chip technology. PoP is used for vertically integrating disparate technologies. 3D WLP uses wafer level processes such as redistribution layers (RDLs) and wafer bumping processes to form interconnects. 2.5D interposer is a 3D WLP that interconnects dies side-by-side on a silicon, glass, or organic interposer using through silicon vias (TSVs) and an RDL. In all types of 3D packaging, chips in the package communicate using off-chip signaling, much as if they were mounted in separate packages on a normal printed circuit board. The interposer may be made of silicon, and is under the dies it connects together. A design can be split into several dies, and then mounted on the interposer with micro bumps. 3D ICs can be divided into 3D stacked ICs (3D SIC), which refers to advanced packaging techniques stacking IC chips using TSV interconnects, and monolithic 3D ICs, which use fab processes to realize 3D interconnects at the local levels of the on-chip wiring hierarchy as set forth by the ITRS, this results in direct vertical interconnects between device layers. The first examples of a monolithic approach are seen in Samsung's 3D V-NAND devices. As of the 2010s, 3D IC packages are widely used for NAND flash memory in mobile devices.
3D SiCs
The digital electronics market requires a higher density semiconductor memory chip to cater to recently released CPU components, and the multiple die stacking technique has been suggested as a solution to this problem. JEDEC disclosed the upcoming DRAM technology includes the "3D SiC" die stacking plan at "Server Memory Forum", November 1–2, 2011, Santa Clara, CA. In August 2014, Samsung Electronics started producing 64 GB SDRAM modules for servers based on emerging DDR4 (double-data rate 4) memory using 3D TSV package technology. Newer proposed standards for 3D stacked DRAM include Wide I/O, Wide I/O 2, Hybrid Memory Cube, High Bandwidth Memory.
Monolithic 3D ICs True monolithic 3D ICs are built in layers on a single semiconductor wafer, which is then diced into 3D ICs. There is only one substrate, hence no need for aligning, thinning, bonding, or through-silicon vias. In general, monolithic 3D ICs are still a developing technology and are considered by most to be several years away from production. Process temperature limitations can be addressed by partitioning the transistor fabrication into two phases. A high temperature phase which is done before layer transfer followed by a layer transfer using ion-cut, also known as layer transfer, which has been used to produce Silicon on Insulator (SOI) wafers for the past two decades. Multiple thin (10s–100s nanometer scale) layers of virtually defect-free Silicon can be created by utilizing low temperature (<400 °C) bond and cleave techniques, and placed on top of active transistor circuitry, followed by permanent finalization of the transistors using etch and deposition processes. This monolithic 3D IC technology has been researched at Stanford University under a DARPA-sponsored grant. CEA-Leti also developed monolithic 3D IC approaches, called sequential 3D IC. In 2014, the French research institute introduced its CoolCube™, a low-temperature process flow that provides a true path to 3DVLSI. At Stanford University, researchers designed monolithic 3D ICs using carbon nanotube (CNT) structures vs. silicon using a wafer-scale low temperature CNT transfer processes that can be done at 120 °C.
Manufacturing technologies for 3D SiCs There are several methods for 3D IC design, including recrystallization and wafer bonding methods. There are two major types of wafer bonding, Cu-Cu connections (copper-to-copper connections between stacked ICs, used in TSVs) and through-silicon via (TSV). 3D ICs with TSVs may use solder microbumps, small solder balls as an interface between two individual dies in a 3D IC. As of 2014, a number of memory products such as High Bandwidth Memory (HBM) and the Hybrid Memory Cube have been launched that implement 3D IC stacking with TSVs. There are a number of key stacking approaches being implemented and explored. These include die-to-die, die-to-wafer, and wafer-to-wafer.
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