Wafer level glass frit bonding for MEMS hermetic packaging

Wafer level bonding is widely applied in the manufacture of sensors, actuators and CMOS MEMS. Bonding technology includes direct bonding, anodic bonding, eutectic bonding, adhesive bonding and glass frit bonding. Glass frit bonding has pattern-able, excellent sealing performances, high bonding strength, don't need apply any voltage during bonding process and less CTE mismatch compared to glass and silicon is more suitable for the hermetic MEMS packages such as gyroscope, accelerometer and RF resonator. To ensure the performances and reliability of assembled parts, the mainly processes both screen printing process and wafer-to-wafer bonding process have to be properly controlled. The limited line width of screen printed glass frit and the optimized bonding conditions of wafer level glass frit bonding process for accelerometer were investigated in this study. Five different line width in a stencil have been designed for confirming the limitation of line width for glass frit screen printing process. To ensure the affect of bonding conditions on sealing performance and bonding strength, a series both of bonding force and temperature for wafer-to-wafer thermal compression bonding were been chosen under reasonable range. Also, the effect of additional ball pin applied during bonding process has been discussed. The bonding performances and reliability were carried out by die shear test and Helium leak test according to the MIL standard 883E 1014.9. The results indicated that the MEMS packaging yield mainly controlled by screen printing process and the finest line width of glass frit is 100 um and 150 um after screen printing and bonding process respectively, when the packaging yield more than 95% can be achieved. The final thickness of glass frit after bonding has been controlled to 10 um for best sealing performance. The bonding temperature is more effective than bonding force both on packaging yield and sealing performance. At the same time, the results shown that applied ball pin on the glass wafer induced the crack occurrence on the center of upper glass wafer during bonding process. From the results of die shear test and Helium leak test, under the optimized bonding conditions, the average bonding strength is up to 36 MPa and the Helium leak value is about 10^-9 atm cc/ sec far smaller than the standard value (10⁸ atm cc/ sec). From the fracture analysis after die shear testing, the fracture occurred on the silicon wafer, which reveals that the bonding strength of interface between glass and wafer is strong enough. These results are useful for employing new bonding designs lead to better reliability and performance for wafer level MEMS package.