Low-Temperature Sandwich Structure Wafer-Level Hermetic Packaging via Three-Layer Simultaneous Bonding for 3-D Microsystems

A sandwich structure, which is stacked by a wafer-level copper–copper (Cu–Cu) thermocompression bonding and a silicon–silicon (Si–Si) direct bonding, has been investigated for potential application as a hermetic seal in 3-D microsystem packaging. First, the bottom and middle layers are prebonded by hydrophilic Si direct bonding. Oxygen plasma is employed for surface activation. Then, the prebonded wafers and the top wafer are bonded via Cu thermocompression bonding at a bonding temperature of 300 °C and a bonding force of 8000 mbar for a duration of 1 h. The entire bonding process of the three layers has borne only one heating procedure, which could effectively reduce the residual stress after bonding. Cavities are etched to a volume of
$21.4\times 10^{-3}$
cm3 following the MIL-STD-883E standard prescribed for microelectronics packaging. The helium leak test demonstrates that 96.8% of the samples present a superior leak rate smaller than
$1.7\times 10^{-8}$
atm
$\cdot $
cm3/s. Moreover, the interfacial adhesion energy between the Cu–Cu bonding interface and the Si–Si bonding interface was projected at 16.4 ± 4.6 J/
$\text{m}^{2}$
and Si breakage energy. The results hold great promise for 3-D microsystem wafer-level packaging.