While 3D architectures elevate workload efficiency and efficacy, 3DIC design does introduce new challenges. Because of the distinct physical characteristics of 3D design and stacking, traditional tools and methodologies are not sufficient to solve these limitations and require a more integrated approach. In addition, there is a need to look at the system in a much more holistic way, compared to a typical flat 2D design. Simply thinking about stacking 2D chips on top of each other is insufficient in dealing with the issues related to true 3D design and packaging.
Since the designs must be considered in three dimensions, as opposed to the typical x, y aspects of a flat 2D design, everything must be managed with the addition of the z dimension – from architectural design to logic verification and route connection – including bumps and through-silicon vias (TSVs), thermal, and power delivery network (PDN) opportunities for new tradeoffs (such as interposer based versus 3D stacks, memory on logic or logic on memory, and hybrid bonding versus bumps). Optimization of the ‘holy grail’ of PPA is still a critical guiding factor; however, with 3DICs, it now becomes cubic millimeter optimization, because it’s not just in two directions, but also the vertical dimension that must be considered in all tradeoff decisions.
Further complicating matters, higher levels of integration available with 3DICs obsolete traditional board and package manual-level techniques such as bump layout and custom layout for high-speed interconnects, which cause additional bottlenecks. Most importantly, interdependency of previously distinct disciplines now needs to be considered in a co-design methodology (both people and tools), across all stages of design IP, chip package, architecture, implementation, and system analysis.
Let’s look at an example of a specific design challenge – the goal to improve memory bandwidth. Traditionally, designers would look at how to connect the memory and CPU to get the highest possible bandwidth. But with 3DICs, they need to look at both the memory and CPU together to figure out the optimal placement in the physical hierarchy, as well as how they connect, through direct die bonds or silicon vias, for example. While performance is critical, designers need a way to evaluate the power and thermal impact by stacking these types of functions together in different ways, introducing new levels of complexities and design options.