Wafer level hermetic packaging with IMC-less Cu-Cu bonding for 3D microsystems (invited)

Metallic copper-copper (Cu-Cu) thermo-compression bonding is investigated for potential application as hermetic seal in 3D microsystem packaging. Cavities are etched to a volume of 1.4×10^-3 cm³ in accordance with the MIL-STD-883E standard prescribed for microelectronics packaging. In the case of metal bonding, clean Cu layer with a thickness of 300 nm and an underlying 50 nm of Ti barrier layer are used. The wafer pair is bonded at 300 °C under the application of a bonding force of 5500 N for 1 hr. The bonded cavities are stored in a helium bomb chamber and leak rate is measured with a mass spectrometer. Excellent helium leak rate below 5 × 10^-9 atm.cm³/s is detected for all cases and this is at least 10X better than the reject limit. Based on these results, Cu-Cu thermo-compression bonding is particularly attractive for wafer-level hermetic packaging of 3D microsystems (IC/IC, non-IC/IC, etc) as the same bonding medium can provide for hermetic seal, electrical connection, and mechanical bond. Since Cu-Cu bond is a single metal system, no inter-metallic compound (IMC) is formed hence resulting in excellent electrical, thermal, and mechanical properties of the bond compared with solder-based or eutectic bonding. Cu-Cu bonding is also a low cost solution in comparison with Au-Au bonding.