Hybrid Bonding vs Microbumps: Key Differences
Both technologies connect chips to chips, or chips to wafers, vertically through many terminals. Microbumps join the chips by melting small amounts of solder, for example on the tips of copper pillars, so spacing is needed to keep solder from squeezing out and bridging to neighbors, and the pitch is typically tens of µm. Hybrid bonding uses no solder; it directly bonds polished flat surfaces of copper electrodes and dielectric, so the pitch can be below 10 µm and the interconnect density can be raised by orders of magnitude. In return, the bonding surfaces must be flat and clean at the nanometer level.
| Aspect | Hybrid bonding | Microbumps |
|---|---|---|
| Bonding method | Copper electrodes to copper and dielectric to dielectric, bonded directly surface to surface | Solder bumps, for example on copper pillars, are bonded by heating |
| Solder | Not used | Used |
| Typical pitch | Below 10 µm. Can be narrowed to a few µm | Tens of µm. About 20 µm at the narrowest |
| Interconnect density (per mm²) | About 10,000 (10 µm pitch) to more than 100,000 (3 µm pitch) | Hundreds (40 µm pitch) to thousands (20 µm pitch) |
| Surface flatness and cleanliness | Nanometer-level flatness is required. Even a small particle creates a void that cannot bond | Solder absorbs height variations, so requirements are less strict |
| Between chips | No gap; no underfill needed | A gap forms, which is filled with underfill or a resin film |
| Main process steps | Planarize by CMP → clean and activate with plasma → align and bond → anneal (typically at a few hundred °C) to join copper to copper | Form bumps by plating → align → bond by thermocompression (TCB) or reflow → underfill |
| Bonding unit | Wafer-to-wafer or die-to-wafer | Mainly die level (die-to-wafer, die-to-die) |
| Electrical characteristics | Extremely short joints with small parasitic capacitance. Low energy needed per bit transferred | Bumps add parasitic capacitance, a disadvantage compared with hybrid bonding |
| Typical uses | Stacked image sensors, stacking cache SRAM on top of processors, etc. | Stacking DRAM in HBM, connecting chips on interposers, etc. |
Figures are typical orders of magnitude and vary by process generation and product.