Characterization of closed-loop solder-line bonding for hermetic MEMS packaging

Abstract

This thesis investigates closed-loop solder-line bonding for hermetic MEMS packaging with electrical feed-throughs. AuSn solder-line and Ti/Au adhesion metal layers have been used for a low-temperature, high-strength, hermetic MEMS packaging. Leak rate has been used as a parameter to characterize the hermetic sealing of the packaging, and bonding strength has been evaluated by pressurized test. In the theoretical study, to measure the internal cavity pressure change, we use a silicon diaphragm that deflects proportionally to the pressure difference between the external and the internal cavity pressure. Pressurized accelerated test condition has been provided to activate the leakage. Packaging specimens with and without feed-throughs have been designed. Substrate and local heating methods have been used in the bonding process. From the leak rate test, specimen with a feed-through bonded by local heating shows the maximum leak rate of $18.8±9.9×10^{-10} mbar-ℓ/s$. The reject limit of the MIL-STD-883E is the $3.814×10^{-8} mbar-ℓ/s$ and solder-line bonding specimen satisfies the sealing requirements in MIL-STD-883E. Specimens bonded by substrate heating show the lower leak rate than the specimen fabricated by local heating. From an analysis on the fracture surface of bonding by EDS, the specimen by substrate heating has stronger bond than the specimen by local heating. Weaker solder means that it has more sources of leak, that is, cracks, micropores, impurities and etc. The packaging having weaker solder has larger leak rate than others. So, the specimen by substrate heating has lower leak rate than the specimen by local heating. From EDS results, we can find out that feed-throughs do not influence much to the leak rate. Bonding strength of the solder-line bonding is 3.53±0.07 MPa. Fracture bonding strength can be measured by enlarging the applicable pressure in test equipment. In this thesis, proposed AuSn solder-line bonding satisfies the hermetic MEMS packaging...