About this document
As HBM stacks reach 12 layers, the dies get thinner and the number of interfaces grows. Bonding and annealing layer by layer, or stacking the dielectric first and annealing the whole assembly in an oven, both put the entire stack through a thermal history. This technical note describes a process that fixes all 12 layers by dielectric bonding first and completes the copper joins in a single 1500 nm photothermal exposure. The full report is available in Korean. An English summary is provided on this page.
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Summary
Taller HBM stacks increase the thermal load of the bonding process. Layer-by-layer processing accumulates thermal history, while batch oven annealing heats the entire stack. In the approach described here, all 12 layers are bonded dielectric-first to secure alignment and fixation, and the copper joins are then completed with a single nanosecond photothermal exposure at 1500 nm. At 1500 nm, interband absorption in silicon is suppressed, so the light passes through the stack and generates heat only at the TiN absorber layer behind each pad. Because the interfaces sit at different depths, the design turns on an optical path that avoids shadowing by the metal wiring and on allocating energy interface by interface. The note sets out the configuration that links all 12 interfaces into one exposure sequence, together with a staged verification plan.
Conclusion
Where the thermal step sits determines how much burden the stack carries. Repeating bond-and-anneal layer by layer accumulates thermal history at the lower interfaces, while stacking the dielectric first and then annealing in an oven turns the heat the bonding pads require into thermal history for the whole stack. As dies grow thinner and interfaces multiply, both paths become more costly. Before the exposure step, the dielectric holds the stack in position. Alignment and fixation are secured first, and the copper bonds are completed last. The TiN absorber takes in the light and delivers heat to the copper bonding interface; the constrained copper expands instantaneously to make contact, and the bond grows from there. Allocating the energy each of the 12 interfaces needs is the center of the design. The energy absorbed at a given interface is the product of incident fluence, path transmittance and absorptance, so an optical path clear of the metal wiring has to be secured at every depth. In process terms, a copper bonding module is attached behind the existing stacking line. The copper anneal window after placement and dielectric pre-bonding is compressed, lowering the thermal load on the whole stack as higher stacks, thinner dies and finer bonds advance together.
Topics
- HBM
- Thermal control
