monolithic

```svg 3D IC: go vertical — bond two wafers, or build the second tier in placeParallel 3D bonds finished wafers via TSV or Cu-Cu; monolithic 3D grows a new transistor tier on top, in place1 · Two ways to go verticalParallel — bonddie 2die 1TSV / Cu-Cu bondMonolithic — in placetier 2tier 1inter-tier via (nm)Parallel bonds two finishedwafers; monolithic grows tier 2directly on tier 1 — no bond.Inter-tier vias are far denserthan any bonded connection —that is the whole point.2 · Vertical pitch laddermicrobump~30–40 µmTSV~5–10 µmCu-Cu hybrid bond< 1 µmmonolithic inter-tier via~50–100 nmFiner vertical pitch = finer 3Dpartitioning: whole chips, thenblocks, then individual gates.Bonding buys density; monolithicbuys another 100x on top.3 · Thermal budget & payoffsPayoffsShorter global wiresMemory directly over logicCFET: nFET over pFET, tiny cellsNew floorplans across tiersThe thermal wallTier-2 built cold (<~500 °C) sotier-1 devices surviveBuried tiers are hard to coolYield multiplies across tiersHeat removal and low-temp devicequality are the real limits.Bond or build-in-placeStack and bond two finished wafers,or grow a second transistor tiersequentially on the first.Density sets the payoffFrom TSV µm to monolithic-via nm,finer vertical pitch moves you fromchip-level to gate-level 3D.Heat & thermal budget biteBuried tiers are hard to cool, andmonolithic's top tier must be builtcold enough to spare the bottom. ``` **Monolithic 3D VLSI Integration** is **stacking multiple device layers on silicon via sequential processing for extreme integration density** — achieves 3-4x density gain. Monolithic 3D transcends 2D planar limits. **Sequential Processing** grow first layer, insulate, pattern vias, repeat for next layer. Layer-by-layer construction enables vertical integration. **Thermal Budget** second layer processing limited by first layer (interconnects stable to ~500°C for copper). Requires lower-temperature processes for upper layers. **Channel Material Quality** regrown silicon via solid-phase crystallization or transfer maintains crystallinity. **Device Stacking** stack transistors vertically. Significant footprint reduction. **Interlayer Connections** vias through dielectric connect layers. Contact/via resistance critical. **3D Density** theoretical 3x improvement; practical 2-2.5x accounting for overhead. **Prototype Status** demonstrated by MIT, Samsung on research circuits. Not yet production volume. **Power Efficiency** shorter interconnects reduce capacitance, power dissipation. **Thermal Management** lower tiers' heat dissipates through upper layers, challenging. **Stress Control** CTE mismatch between materials; engineering mitigates via films. **Gate Engineering** gate-last compatible with sequential processing. **Yield Challenges** first-tier defects propagate; yield lower than 2D. **Monolithic 3D achieves maximum density** through stacked sequential processing.

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