Vacuum Sealing of MEMS Cavities using Nanoparticle Sintering

Abstract

Many semiconductor sensors, particularly MEMS sensors such as inertial, temperature, pressure, and resonance sensors need a vacuum environment for optimal performance and sensitivity in addition to the improvement in their long-term reliability. With the rise of heterogeneous integration with the MEMS and ICs being integrated together, process compatibility becomes a very big issue. Cu is a desirable bonding and sealing material for low-temperature vacuum packaging of next-generation microsystems, as it is compatible with modern CMOS circuits and 3-D IC fabrication. Cu thermocompression bonding has been reported to achieve vacuum sealing at low temperatures, but these approaches need extremely flat and clean Cu surfaces or additional capping layers, and the overall bonding areas are still rather large.
To mitigate this, the use of copper nanoparticles as the sealing material has been proposed. In this study, the first successful demonstration of die and multi-die level hermetic sealing has been demonstrated using Copper nanoparticle paste as the sealing material for sealing ring widths as small as 8$\mu$ at a minimum temperature and pressure levels of 300$^\circ$C and 10 MPa respectively with average encapsulated pressure of 54.18 mbar. Moreover, the leak rate evaluation was conducted which revealed no discernable leakage over a period of 48 days. Finally, the bond shear test and a subsequent failure analysis revealed high densification of the Copper nanoparticles at the bond interface.