Samsung has unveiled a three-phase roadmap to progressively transform high-bandwidth memory (HBM) into an integrated memory-and-compute system, culminating in the company's zHBM architecture, which places the processor directly beneath the DRAM stack and eliminates the conventional 2.5D interposer link between the two. Detailing the roadmap at Hot Chips 2026, Samsung's Sangwook Han, of the company's DRAM design team, identified the base die as the key enabler of the evolution, which began with the company’s decision to manufacture the HBM base die on an advanced logic process.

In conventional HBM, the base die (B-die) was fabricated on the same DRAM process node as the core dies (C-dies) in the stack above. Starting with HBM4, Samsung moved the base die to a 4nm logic process, primarily to reduce power draw and minimize die area. Additionally, it gave Samsung a much more capable piece of silicon.

The company contends that a die built on the same class of logic process as XPUs could do much more than serve as a data interface. Samsung now plans to progressively offload more functions into the base die, eventually removing the physical gap between memory and the XPU entirely.

The current state of HBM and its growing constraints

The current HBM architecture comprises multiple DRAM core dies stacked vertically on a base die and connected through thousands of TSVs. The stack sits beside an XPU on an interposer, with the base die bridging the memory and compute silicon.

Bandwidth has been the main driver of HBM’s evolution. The current HBM4 stack has roughly 1 to 5 TB/s of bandwidth obtained through 1,000 to 2,000 I/Os running at about 8 to 16 Gbps each. These figures are expected to rise with upcoming HBM generations. The problem is that conventional ways of scaling bandwidth present significant challenges.

TSV signaling speed is difficult to increase, so HBM generations have added more TSVs instead. However, this consumes area and forces tighter TSV pitches. The PHY has also grown more demanding. HBM4 doubled the data I/O count from 1,024 to 2,048 DQs, and signaling speed keeps rising. Power is an even bigger issue. While energy per bit is improving, total HBM power continues to rise as bandwidth is scaling faster. Samsung says this is why HBM4 moves the base die to an advanced logic process, as the denser, more efficient logic reduces power draw.

This move underpins and enables the three-phase plan. An advanced logic node shrinks the interface circuitry while enabling the HBM base die to perform functions previously handled by the processor. Samsung calls this direction custom HBM, or cHBM, which keeps the conventional DRAM stack but customizes the logic underneath it for a specific accelerator.

Phase 1: Reclaim XPU area

The first phase is about handing processor area back to compute in what Samsung calls “XPU area reclamation.” AI accelerators are hitting familiar scaling walls, such as slowing process scaling and dies pressing against reticle and interposer limits. To expand compute, Samsung plans to evict non-compute blocks, moving their functions to the base die’s underutilized silicon.

The first target is the HBM Physical Interface (PHY), one of the largest blocks on the base die. Samsung proposes replacing the traditional interface with a much smaller die-to-die (D2D) link. On an 11 × 12.8mm HBM4 base die, the conventional PHY occupies more than 8 × 4mm, while the custom HBM D2D block is about 8.5 × 1.5mm, with channel depth cut from 5.5mm to 2mm. Because the matching interface on the XPU shrinks too, Samsung also reclaims processor silicon.

Conversely, shrinking the same power into less silicon increases power density and creates hotspots. Samsung’s answer is a Heat Path Block (HPB) that provides an alternative route for heat to exit the concentrated interface region. The company says an HPB covering more than half of the PHY can slash peak temperature by more than 35%.

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.

Phase 2: Making the die a more useful smart memory subsystem

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: zHBM goes fully 3D, placing the processor underneath the memory

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.