Yukiharu Akiyama

Au Bump Interconnection with Ultrasonic Flip-Chip Bonding in 20 µm Pitch

Superfine flip-chip bonding technologies in 20 µm pitch microbumps on copper through-hole electrodes are substantial technologies for three-dimensional (3D) chip stacking LSI. As the advanced interconnection technology to connect the through-hole electrodes at low temperature and low bonding force, the ultrasonic flip-chip bonding (UFB) was verified by the total evaluation and the atomic-level analysis of the bonding interface on the chip-on-chip (COC) structure utilizing electroplated Au microbumps in 20 µm pitch. First, the lower limit bonding conditions were confirmed to be a bonding force of 20 N and an amplitude of 3 µm; the bonding accuracy achieved was within ±2 µm, the electrical interface resistance was stable about 0.57 Ω, and no damage around the interconnection structure was observed. Secondly, the mechanism of solid phase bonding interface formation at the atomic level without solid phase diffusion was confirmed as the Au-Au solid phase UFB bonding mechanism, and the orientation geometry of such bonding was apparently different from that of thermo compression bonding, which showed solid phase diffusion across the boundary. The achievement of this research will enable the realization of the 3D chip stacking LSI in the near future, which is characterized by scalabilities and high-performance. The subjects are the elucidation of the real oscillation contributes to bonding to optimize the process conditions and the establishment of the micro joint reliabilities utilizing UFB process.

Development of Coated-Wire Bonding Technology.

Wire bonding technology is being applied to narrower pad piches and longer spans in LSI packages with the increase in number of input-output pins and shrinkage in chip size. This makes adjoining wires liable to touch during the molding process. To solve fundamentally this problem, the authors have been developing bonding technologies, using coated gold wires which do not suffer from short circuit failures even if the wire touch occurs.In this study, various coated wires were evaluated to select the optimum coating film with improved resistance to the wire touch at high tempratures and improved bonding continuity. Packages were then assembled with the selected wires, and various reliability tests were conducted.The best properties were obtained with the coating film of 0.4 μm thick heat-resistant formal, which showed high insulation reliability in a high temperature wire-crossing test and enabled continual bonding for more than one hundred thousand wire connections. It was confirmed that the assembled packages have sufficient insulation reliability between touching wires and that no failures occurred in various reliability tests including temperature cycling and high temperature high humidity bias tests.