CMOS Compatible Hermetic Packages Based on Localized Fusion Bonding of Fused Silica

Hermetic packages compatible with complementary metal-oxide-semiconductor (CMOS) processes and capable of transmitting RF and optical energy are in high demand. A hermetic packaging process based on the localized fusion bonding of patterned and stacked fused silica wafers to encapsulate a CMOS chip has been successfully demonstrated. The process also allows for feedthroughs to be fabricated within the package for wired interconnection to encapsulated devices. A carbon dioxide laser is applied to achieve localized fusion bonding of the fused silica stack. Exploiting vector motion of the laser, simultaneous bonding and dicing is achieved within 2 s. The low thermal conductivity of fused silica thermally isolates the CMOS chip from the bond area. The heat diffusion during the process is examined both experimentally and using finite element analysis. The temperature falls to CMOS compatibility levels (under 400°C) within a distance of 660 μm from the bond line. The viability of the packaging process and the hermeticity of the package are simultaneously evaluated by encapsulating a commercial MEMS humidity sensor within the package and monitoring the internal package humidity as a function of time. The packaged sensor was placed in an external relative humidity environment of 85% at 24°C and the relative humidity change inside the package was measured over a time span of 300 days. A water vapor leakage rate less than 4.6 × 10
^-14
atm·cm
³
/s was observed. These preliminary results suggest that the hermeticity of the package can meet the criterion of implantation in human body for more than 70 years for a package with an inner volume of 0.0012 cm
³
.