Hybrid-bond yield starts at CMP

In a hybrid bond, the dielectric surfaces touch and bond first. Then an anneal expands the copper so the pads close the remaining gap and fuse. For that sequence to work, the copper must start out slightly recessed, by a few nanometres, not more.

Chemical-mechanical polishing (CMP) sets that recess. Pad density, pattern, slurry, pad wear and wafer edge effects all shift it, die by die and wafer by wafer. By the time a bad recess shows up as a void or an open, the wafer is bonded, and in HBM, the whole stack is lost.

Three failure modes that start at CMP

• Over-recessed: the copper expands but the pads never meet. Open circuit.
• Protruding: copper touches first and holds the dielectric apart. Voids.
• Rough or non-planar dielectric: weak bond, delamination risk.

All three are set at CMP and all three are measurable before bonding, if you measure enough of the wafer. Particles, the other main cause of voids, need inspection tools, not AFM.

From sampling to control

With today’s AFM throughput, recess is checked on a few sites per lot, which is enough to qualify a recipe but not to control it. Dense, wafer-by-wafer AFM data changes what CMP control can be: correct the polish on the next wafer and hold out-of-window wafers before they are bonded.

CMP already runs closed-loop on film thickness. Measuring recess on every wafer at the tool would close the loop on the number hybrid bonding depends on.