The surfaces are prepared to be flat and clean, then aligned so matching contacts meet. This supports much finer spacing than many connections that use separate bumps.
Small alignment errors and surface defects can spoil connections. Manufacturing yield therefore depends on precise preparation, alignment and inspection.
ELI5
Hybrid bonding attaches chip pieces together while also connecting their tiny electrical contacts. It helps put many connections into a small space.
For example, two stacked chip pieces can communicate through matching copper contacts on their facing surfaces. If those contacts are misaligned, the intended connections may fail.
Why use hybrid bonding?
It can put more connections in a smaller area and shorten the distance signals travel.
What makes it difficult?
The surfaces must be clean and flat, and matching contacts must line up precisely.
