I/O Architecture, Substrate Design, and Bonding Process for a Heterogeneous Dielet-Assembly based Waferscale Processor

Demand for large amounts of parallelism is growing rapidly for today's computing systems. This is due to the proliferation of applications such as graph processing, data analytics, machine learning, etc. which require a large number of processing cores and a large amount of memory bandwidth. Often systems comprising of many individual packaged chips are employed to run these applications. However, inter-package communication has not scaled well and this bottleneck threatens the performance scaling of these applications. One way to alleviate this bottleneck is to build waferscale processors where many compute cores and memory blocks can communicate efficiently at very high bandwidths. In this work, we attempt to build a many-core waferscale processor using heterogeneous dielet assembly on the Silicon Interconnect Fabric (Si-IF) technology. The design and implementation of a dielet based waferscale processor have their own set of challenges. Some of the challenges include (1) design of area and energy efficient highly parallel I/O cells, (2) Si-IF substrate design and its impact on signaling and power delivery, and (3) reliable and efficient dielet-to-wafer bonding process. In this work, we will discuss the solutions to these three challenges that we employed in our dielet and Si-IF substrate design. Our custom-designed I/O cell is only $157.8\mu m^{2}$ , which is 95% smaller than the standard cell I/Os and consumes only about 0.075pJ/bit. We co-designed the dielets with the Si-IF substrate to ensure that we can achieve on-chip like communication characteristics for inter-dielet communication. This helps us to seamlessly partition a large design into fine-grained dielets. For delivering power to the dielets across an entire wafer, we use power delivery from the edge of the wafer. This scheme results in large resistive power loss, and as a result, we designed a novel power management unit on each tile to provide reliable power to the core circuitry in the dielets. Lastly, we briefly discuss the copper-gold bonding process and the heterogeneous dielet assembly scheme we developed for an efficient and reliable assembly process. Shear tests show that the bond strength achieved with this process is ∼113.3 MPa which is >5x compared to the previously reported bond strength for gold-gold bonding.