High Bandwidth Memory Deep Dive

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You will hear a lot about HBM these days, especially if you follow AI accelerators. High Bandwidth Memory (HBM) is a vertically stacked DRAM architecture connected through TSVs (through-silicon vias) to deliver massive bandwidth in a compact footprint. TSVs directly link multiple DRAM dies, drastically improving memory throughput.

🚀 Why HBM Matters

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HBM provides several critical advantages:

• High speed: Bandwidth in the terabytes per second, over 20× faster than DDR.
• Power efficiency: Physical proximity to the logic die reduces energy per bit transferred.
• Area efficiency: Highest capacity per mm² among modern memory technologies.

In the Generative AI era, memory bandwidth—not compute—is often the bottleneck.
Transformers require quadratic memory scaling with sequence length during training, and inference demands large KV caches that grow linearly with tokens. HBM directly addresses this constraint.

🧩 HBM Technology Evolution

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Each generation of AI accelerator depends heavily on HBM improvements—either more stacks, more dies per stack, or next-gen HBM standards. Capacity jumps like:

• H100 → H200 (50% capacity increase)
• B200 → B300
• Rubin → Rubin Ultra (4× jump)

HBM4 doubles channels per stack (8 → 16) and increases I/O speed, boosting total bandwidth.

The HBM market is expanding rapidly—SK Hynix expects 50% CAGR (2024–2028). HBM is also less cyclical than commodity DRAM thanks to long-term contracts.

🏆 SK Hynix: The Current HBM Leader

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SK Hynix leads the HBM market (60%+ share), primarily due to their MR-MUF (Mass Reflow Molded Underfill) technology, used in NVIDIA’s flagship GPUs from Hopper to Blackwell.

Key benefits over Samsung/Micron’s TC-NCF approach:

• ~20% higher yield
• 3× more thermal bumps → better heat dissipation
• Less warpage, thanks to room-temperature, low-force processing

SK Hynix’s advantage is reinforced by:

• Exclusive epoxy molding compounds (via Namics Corp.)
• Earlier commitment to HBM (since 2013)
• Strong partner ecosystem

Samsung, meanwhile, has struggled with front-end yield, particularly at the 1a/1c nm nodes. Yield challenges magnify as stacks grow (8-Hi → 16-Hi). With insufficient progress at 1c nm, Samsung risks falling further behind.

🧱 The Base Die Transition: DRAM → Logic

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HBM4 introduces a major architectural shift:
the base die moves from DRAM process to logic (FinFET) process.

Benefits:

• Lower power
• Higher I/O performance
• Customizable logic (ECC, power management, accelerators)
• Better scaling with advanced nodes (TSMC N3, Samsung 4nm, etc.)

HBM4 base die responsibilities:

1. Control of DRAM stacks (refresh, repair, read/write)
2. 2048-bit I/O PHY (2× HBM3E)
3. Integrated logic features (ECC, PMICs, custom chiplets)

SK Hynix and Micron will outsource to TSMC N3/N12, while Samsung will fabricate in-house at 4nm—a risk given Samsung’s mixed 4nm track record.

🔧 Hybrid Bonding: The Next Big Shift

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Hybrid bonding is poised to transform HBM:

• Cu-to-Cu bonding without microbumps
• <10 µm pitch vs ~40–55 µm microbumps
• Lower resistance, better thermals
• Smaller total stack height
• Higher bandwidth

It requires:

• CMP planarization
• Higher-class cleanrooms
• Expensive bonders ($3M vs $1–2M TCB)

Hybrid bonding is expected to enter mass production around 2028–2029 for HBM5 (20-Hi), aligning with NVIDIA’s roadmap.

🏭 HBM Supply Chain

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Key equipment suppliers:

TCB: Hanmi, Hanwha, ASMPT, K&S, Semes
Hybrid Bonder: Besi, ASMPT, SUSS
Metrology: Camtek, KLA
CMP: Applied Materials, Ebara
TSV: Applied Materials, LAM, TEL
Grinder: Disco
Tester: Advantest, Teradyne

TCB remains mainstream today. Hanmi dominates HBM3/HBM3E for SK Hynix and Micron.

In hybrid bonding, Besi leads with 100 nm precision (Gen 1) and <50 nm (Gen 2).
ASMPT and KLIC are attempting to delay hybrid adoption by promoting next-gen flux-less TCB platforms.

🇨🇳 China’s Progress in HBM

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China (CXMT, YMTC, Tongfu Microelectronics) remains several years behind global leaders due to:

• Export controls (lithography, TSV, etch)
• Limited DRAM maturity
• Slower start on HBM R&D

CXMT:

• Mass-produced HBM2 in 2024 (limited volume)
• Targeting HBM3 production next year
• Ambitious plan for HBM3E in 2027
• Front-end capability at 1y/1z nm, researching 1α nm without EUV

TechInsights confirms CXMT’s DDR5 (1z nm) density comparable to global peers (albeit with larger die and unknown yield).

Back-end:

• Likely to use TC-NCF
• Still receiving TCB equipment from Korean vendors until very recently

Overall:
China trails by ~3–4 years in HBM3/HBM3E.

New US export restrictions (Jan 2025)

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HBM DRAM now restricted if:

• Cell area < 0.0026 µm²
• Density > 0.20 Gb/mm²

• 3000 TSVs per die

(CXMT likely impacted beyond 1z nm.)

🔮 What’s Next?

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Global competition in HBM—across the U.S., Korea, and China—will increasingly hinge on:

• Front-end DRAM maturity (1b/1c/1α nm)
• Back-end packaging leadership
• Hybrid bonding adoption
• Base die logic-node advancement

Hybrid bonding, in particular, could reset competitive positions, rewarding whichever vendor masters the technology first.

HBM has become the most strategic memory technology of the AI era—and its importance will only grow as models scale, context windows lengthen, and accelerators push for more bandwidth and capacity.