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A Wide I/O Memory-on-Logic Product Prototype Enabled by Through Silicon Stacking Technology
Keywords: Through Silicon Stacking, Memory on Logic, 3DIC
In recent years, Through Silicon Stacking (TSS) has garnered much attention as a candidate technology to break through the “memory wall” between a processor/digital die and off-chip system memory. Using Through Silicon Vias (TSVs) to electrically connect the frontside circuits of a digital silicon die to its backside, and bonding a wide I/O memory die to the backside of this digital die promises significantly reduced interconnect parasitics and markedly improved memory access bandwidth. Such a digital-die-to-memory-die back-to-face configuration continues to facilitate the strong drive power demanded by the digital die directly through the package substrate. In this paper, we report on a 28 nm High-K metal gate (HKMG) product prototype assembled back-to-face with a 4Gb 3x nm wide I/O DRAM chip using TSS technology in an integrated fabless supply chain [1]. The digital chip interfaces to the wide I/O memory chip using close to 1200 interconnects that are enabled by micro-bump joints with pitch as small as 40um. This micro-bump mediated interface connects to the frontside of the digital chip using TSVs with sub-10um diameter and >8:1 aspect ratio fabricated using a via-mid integration processed concomitantly with the digital die frontside circuits. Our data shows high continuity yields and good signal integrity through the die-to-die interconnects. We show that, while the effect of TSVs on transistors is well-understood [1], with appropriate chip floor-planning, it is possible to obviate or account for these effects, and re-use 2D circuit IP in a 3D product with minimal die size growth. Moreover, our analyses show, successful integration of TSS-enabling features into the 28nm HKMG die leads to yields comparable to a die without these features. A 3D assembly process discussed elsewhere [2] allows for a compact form-factor package with <1mm thickness. We present the impact of this assembly process to Wide I/O DRAM performance as well as digital IP performance. Functional integration of the wide I/O memory via a memory buffer on the digital chip allows for memory access rates as high as 200 Mhz. A DRAM BIST interface on the digital chip will be used to test the memory through the wide I/O interface between the two chips. As part of an integrated product solution, we include a test methodology with test stages distributed throughout the integration flow which allow for selection of known good die to minimize assembly yield loss. For the first time, micro-probing of the wide I/O micro-bump interface to confirm through-Si interconnect integrity is reported.
V. Ramachandran,
Qualcomm Technologies, Inc.
San Diego, CA

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