Fine keyed alignment and bonding for wafer-level 3D ICs

Precise wafer-to-wafer alignment accuracy is crucial to interconnecting circuits on different wafers in three dimensional integrated circuits. We discuss the use of fabricated structures on wafer surfaces to mechanically achieve higher alignment accuracy than can be achieved with our existing (baseline) alignment protocol. The keyed alignment structures rely on structures with tapered side-walls that can slide into each after two wafers are "pre-aligned" using our baseline alignment protocol. Results indicate that alignment accuracy is about a quarter micron, well below the one micron alignment accuracy obtained in our baseline alignment procedure using commercial state-of-the-art wafer alignment equipment. In addition to improving alignment, the alignment structures also hinder undesirable bonding-induced misalignment. The keyed alignment structures are also promising for nano-imprint lithography.