Sony/imec
Breakthrough in highly dense backside interconnects
Sony and imec have developed a new technology for high-density backside interconnects in 3D chips, enabling dense front-to-back connectivity while avoiding the high aspect-ratio challenges typically associated with through-silicon vias (TSVs).
Imec and Sony Semiconductor Solutions Corporation (Sony) jointly developed a novel integration module for highly dense backside interconnects – key components of 3D stacking and backside functionalization technologies. The module is structured around a self-aligned local backside dielectric isolation (local BDI) step, resulting in low-resistance and low-leakage highly dense front-to-back through-Si vias (TSVs), with 3x larger overlay window compared to conventional TSV approaches. The local BDI TSV approach is an integration method, which will enable new 3D integration schemes for a variety of use cases, including logic and memory devices.
Backside functionalization and 3D stacking are key enabling technologies for next-generation semiconductor devices. They require high-density backside interconnects to ensure connectivity between the fine-grained active frontside and the less densely structured wafer backside. One attractive approach for making backside interconnects is the via-middle TSV process. While this approach enables high-density front-to-back connectivity, the TSVs typically have a high aspect ratio, which presents challenges in terms of both metallization and electrical performance.
Sony and imec now jointly present an alternative module for integrating backside TSVs, referred to as local BDI. At the heart of this module is a self-aligned isolation structure that is formed locally at the overlap between the TSVs and the active part of the wafer frontside (see also Figure). This new approach to backside connectivity has significant advantages over the conventional via-middle TSV process.
Zsolt Tokei, imec fellow and program director of 3D system integration: »Starting from highly dense and narrow via connections already present in the wafer frontside (i.e., the middle-of-line (MOL) vias), our module for the first time allows to transition to much wider TSV connections between the active frontside and the wafer backside. Compared to a via-middle TSV approach, the local BDI TSVs have 50% larger bottom and top critical dimension (CD), simplifying the TSV metallization process and improving its resistance threefold. The process also enlarges the tolerance for misalignment between the TSVs and the narrow MOL vias to up to 30nm – demonstrated for a standard cell configuration with 115nm cell height. In addition, within this improved overlay window, the self-aligned structures provide very good isolation with the surrounding Si substrate as derived from leakage current measurements.«
The process flow starts with conventional FEOL, MOL and BEOL processing, followed by wafer bonding and Si thinning. Local BDI formation at the TSV/active overlap regions consists of conformal dielectric deposition and isotropic etching and is followed by TSV metallization. »The local BDI module will enable new 3D integration schemes for a variety of use cases – including advanced logic and memory applications,« adds Zsolt Tokei. »In addition, as opposed to backside integration schemes that rely on removing the remaining bulk Si, our module allows TSVs to connect through bulk Si with up to 500nm thickness. This is of interest for applications such as DRAM that make use of the relatively thick Si layer remaining at the wafer backside.«
Takushi Shigetoshi, Senior Manager at Sony and lead author of the work adds: »3D integration is becoming increasingly important across a wide range of semiconductor applications, and it is highly meaningful to develop a variety of backside connectivity schemes that can be selected according to the target application.«
