The bigger Phase 1 change is moving the memory controller from the XPU to the custom HBM base die. Han estimated controllers account for 5 to 10% of an XPU's area — space that, refilled with compute, could yield a 10–20% performance gain. Moving the controller next to memory also enables a new SRAM-based repair scheme in which failed C-die addresses can be redirected to SRAM on the base die, avoiding the need to sacrifice an entire spare row or column for a single defective cell.
Even with the controller moved in, Samsung says a substantial portion of the base-die area remains unused. Phase 2 fills that space with more functions, first with some relatively straightforward additions. The company proposes SoC-like telemetry and reliability features, including thermal, voltage, process, and aging sensors, as well as more advanced self-test hardware.
It also wants to use the edge of the base die for direct memory expansion, arguing that capacity is becoming as important as bandwidth. Dedicated controllers and PHYs could connect a secondary tier of external memory directly to custom HBM, rather than going through conventional PCIe expansion . Han said that extra memory could be LPDDR or even HBM, offering higher bandwidth and lower latency than PCIe-based memory extension.
Last in Phase 2 is compute — right on the base die. Samsung wants to place selected processing elements (PEs) under the DRAM, offloading memory-bound work while compute-heavy operations remain on the GPU. It calls this broader 2.5D architecture advanced HBM (aHBM), citing benefits such as less traffic across the interposer and reduced latency and I/O power draw.
Phase 3 appears to be Samsung's most radical step, with the company halting HBM architecture optimization and rebuilding it instead. Introducing zHBM, Samsung's “ultimate solution” for maximizing bandwidth under future AI's brutal power limits.
The zHBM concept eliminates the conventional side-by-side arrangement of XPU and HBM across an interposer. Instead, the processor sits directly beneath the DRAM stack in a true 3D structure. This architecture allows Samsung to replace the large edge PHY with distributed I/Os spread across the die. Data no longer has to travel laterally across an interposer, thereby shortening the physical path and eliminating the need for conventional HBM PHY and D2D link interfaces.
Samsung says the biggest payoff is power. Its projections show zHBM cutting I/O power by around 70% compared with HBM5. In another example, Samsung models roughly 2.3X more DRAM bandwidth while reducing memory power by about 100W compared to a four-stack HBM4E system.
On the flip side, thermals are the obvious complication. Han said Samsung is targeting roughly four-high zHBM stacks, compared with the much taller 12-high or 16-high configurations possible with conventional HBM, specifically because of heat. Distributed I/O helps by spreading the circuitry rather than concentrating it into hotspots, but zHBM is a balancing act involving capacity, bandwidth, heat, and physical integration.
Manufacturing zHBM will also require advanced wafer-on-wafer bonding and hybrid copper bonding to meet the required I/O density, with a much tighter co-design process between the DRAM and SoC teams. Samsung did not provide a firm launch date or timeline for the phases. However, HBM4’s 4nm logic base die is the concrete starting point, while cHBM and aHBM are nearer-term extensions, with zHBM as the long-term endpoint.
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TOPICS HBM HBM4 memory Etiido Uko Social Links Navigation News Contributor Etiido Uko is a news contributor for Tom's Hardware covering the latest updates in big tech and the PC industry. He is a mechanical engineer and senior technical writer with over nine years of experience in documentation and reporting. He is deeply passionate about all things engineering and technology, and is an expert in gadgets, manufacturing, robotics, automotive, and aerospace.
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