Comparative analysis of two 3D integration implementations of a SAR processor

When designing 3DICs there are five major issues that differ from 2D that must receive special attention: power delivery, thermal density, design for test, clock tree design and floorplanning. Power delivery in 3D must receive special attention as 3D designs have larger supply currents flowing through the package power delivery pins, along with a longer power delivery path than in comparable 2D system. Thermal density is an issue as 3D integrated chips will have more heat density and less capacity to remove heat than a comparable 2D chip. 3D clock tree distribution is much more difficult than in 2D because the most commonly used methodologies and design tools are geared towards 2D designs and process variation between the different tiers makes it harder to keep skew, jitter and power consumption down. Design for test is harder in 3D because 3D vias provide another point of failure and post fabrication repairs such as focused ion beam are more difficult to perform in 3D. Finally, floorplanning is drastically different in 3D than in 2D, and the four aforementioned issues must all be taken into account during 3D floorplanning. In this paper, all five design issues are explored in the context of a high-resolution memory-on-logic synthetic aperture radar (SAR) processor. The SAR processor is chosen specifically as it requires a significant amount of memory bandwidth that is best met with the high I/O bandwidth afforded by a 3D process. The issues are examined in the context of two implementations for two different 3D integration processes. The first implementation was done in MIT Lincoln Laboratory's 3D FDSOI 1.5 V three tier process and is currently in fabrication. The second design is currently in the design stage, and will be fabricated in two tiers of Chartered Semiconductor's 130 nm process 3D integrated with two tiers of high bandwidth DRAM using Tezzaron Semiconductor's vertical interconnection technology.