Surface activated bonding - high density packaging solution for advanced microelectronic system

Surface activated bonding (SAB) was a recently developed high density interconnection method for advanced microelectronic system. SAB bonding was proposed based on the simple principle that two surface activated materials could be easily bonded together at the room temperature or low temperature. The surfaces were activated by fast atom beam or plasma irradiation source which has been proved helpful to remove the oxide and organic layer covering on the material surface. Chip-on-chip (COC), as an important application of SAB flip chip method, is discussed in this paper. To investigate the SAB bonding feasibility, some electrically plated Au microbumps which have a size of 40 × 40 × 30 μm³, were prepared on the silicon chip and bonded with Au, Al or Cu pads with a thickness of about 0.3 μm on another silicon chip. Electrical measurement, shear test and thermal aging (423K) data demonstrated the fine performance for SAB interconnection. It was found even the bonded chip-chip system was die separated, SAB achieved Au bump-metal pad interconnection was remained well. As bonding is carried out at the room temperature or low temperature and the material is surface activated before, no intermediate layer or impurity particle will exist at the as-bonded interface. That, for the SAB bonded Au-Al sample, a homogeneous intermetallic compound layer was finally grown and formed at the Au-Al interface (Au-Al) without any voids after 500hours thermal aging at 423K in air, which was completely different from the performance of Au-Al interface constructed by other interconnection techniques such as wire bonding or soldering flip chip method. The application of SAB flip chip bonding on chip-on-film and chip-on-flexible board (COF) was also investigated. SAB flip chip method realized a high bonding accuracy of about 0.5 μm for the above systems.