The wafer-to-wafer overlay accuracy results were achieved in techniques known as hybrid bonding, and dielectric bonding. Wafer-to-wafer bonding is a technique that potentially enables high-density integration of future ICs through three-dimensional (3D) integration. This is achieved by aligning top and bottom wafers that are then bonded, thus creating a stacked IC. An important advantage is that wafers/ICs with different technologies can be stacked, e.g. memory and processor ICs.
Many of the alignment techniques and bonding methods for 3D integration have evolved from microelectromechanical system (MEMS) fabrication methods. The fundamental difference between MEMS and 3D integration is that the alignment or overlay accuracy has to be improved by 5–10 times. Accurate overlay is needed to align the bonding pads of the stacked wafers and it is essential to achieving a high yield with wafer-to-wafer bonding. It is in this respect that imec and EVG have realized the reported results on overlay accuracy.
Firstly, the hybrid (via-middle) wafer-to-wafer bonding technique was improved by using EVG’s high quality bonding system with “beyond state-of-the-art integration” definition of bonding pads, resulting in a high yield and a 1.8 µm pitch, which is significantly better (imec asserts) compared to recently published results reporting 3.6 µm pad size,.
Secondly, the dielectric (via-last) wafer-to-wafer bonding technique was tackled. This technique requires extremely good overlay accuracy to align the copper pads from both wafers, which are then contacted by through-silicon vias (TSVs). In this case, 300 nm overlay across the wafer was achieved.
“Further improving the overlay accuracy for wafer-to-wafer bonding into the sub-200nm range requires optimization of the interaction between the wafer bonding tool and processes as well as pre-and post-processing and the wafer material,” explains Markus Wimplinger, corporate technology development & IP director at EVG.
Imec’s 3D integration program explores technology options to define innovative solutions for cost-effective realization of 3D interconnect with TSVs. Imec’s 3D integration processes are completely executed on