Newly developed low cost, reliable wafer level hermetic sealing using Cu/Sn system

Wafer level hermetic sealing using Cu/Sn system was studied in this paper. On 8 inch wafer, seal rings with the width of 300 μm and length of 11 mm were fabricated. The seal ring was composed of metal and Sn solder layer was patterned. Inside of each seal ring, cavity with area of 6×6 mm² and 250 μm in depth was formed using wet-etching. In order to prevent fast diffusion between solder and Cu, thin Ni buffer layer was deposited on Ni. To prevent metal oxidation, thin Au layer was deposited on Ni and Sn surface. Bonding was conducted It is known that Ni and Au have larger solubility controlled N₂ atmosphere with the bonding force of 5.5MPa and bonding temperatures of 280 °C for 20 min. After bonding, the seal ring joint was composed of (Cu, Ni, Au)₆Sn₅ and thin Cu₃Sn intermetallic compounds (IMC). Non continuous small voids were formed at the bonding line. The hermeticity of as bonded sealing dies was smaller than 5×10^-8 atm·cc/sec. After various reliability tests, most of the dies have the hermeticity smaller than 5×10^-8 atm cc/sec. The shear test results showed that present IMC joint had a robust bonding strength (>75.2MPa). Test under 250C, the shear strength of the seal joint was around 60MPa. Based on present study, new bonding structure was designed and the test results showed that all dies can achieve acceptable hermeticity after bonding and reliability rests.