Abstract:
For about a decade CEA-Leti researchers have been working on a solution for monolithic 3D integration. Three key areas have been addressed; the process and transistor technology, the design flow enablement and the power, performance and cost (PPC) benchmarking for various logic partitioning schemes.
The fundamental technology has been demonstrated on silicon with transistor performance on two tiers that rival their 2D counterparts. A key development area has been low temperature transistor processing needed for the 3D transistor manufacturing.The process technology leverages CEA-Leti’sknow-how on wafer bonding and lift-off for the formation of nanometer device quality Si layers.
Because the alignment is done thru the transparent thin silicon layers of the second tier.The technology is truly monolithic in the sense that the via density connecting the metals and second transistor tier are compatible with the pitches of the technology node being used. No other 3D stacking technology can achieve this result to date.
With two transistor tiers, the technology reduces the die area by about 50% and provides significant reductions in the metal interconnect lengths, which results in benefits on all three aspects of the PPC figure of merit.
Two partitioning schemes have been studied, namely cell-on-cell partitioning and transistor level partitioning. Both methods show great promise. In the cell-on-cell scheme, a detailed cost study was conducted. It shows the area reduction more than balances out the process complexity and accompanying additional defects and results in cost savings of 50% or more for large die. In the case of transistor partitioning, the area reduction is a bit more modest, but with the inherently simpler manufacturing process, the cost savings can be similar or even bigger. The additional benefits of transistor level partitioning are fabrication cycle times, use of standard 2D P&R tools and independent optimization of NMOS and PMOS transistors.
CoolCubeTM technology has been demonstrated on silicon and is ready to initiate development of the next phase to transition the technology to large scale and mass production.