Development of processes for vacuum packaging of micro-electro-mechanical systems (MEMS)

A laser-assisted bonding (LAB) technique is demonstrated for low temperature region selective processing. A continuous wave (CW) carbon dioxide (CO 2) laser (lambda = 10.6 mum) is used for solder (Pb37/Sn63) bonding of metallized silicon substrates for MEMS packaging applications. Laser-assisted selective heating of silicon led to the reflow of an electroplated or screen-printed intermediate solder layer which produced silicon-solder-silicon joints. The bonding process was performed on the fixtures in a vacuum chamber at an air pressure of 10-3 Torr to achieve fluxless soldering and vacuum encapsulation. The bonding temperature at the sealing ring was controlled to be close to the reflow temperature of the solder, 183°C. Pull test results showed that the joint was sufficiently strong and the bonding strength was believed to be comparable to the tensile strength of Pb37/Sn63 solder of around 50 MPa. SEM photos of the cross-section view showed that the solder was fully reflowed and the condition of the sealed cavity was uniform.; Helium bomb and bubble tests, according to MIL-STD-883E, showed that the leak rate of the package was below 10-8 atm cc/sec under optimized bonding conditions. The results of the Design of Experiment (DOE) method indicated that both laser incident power and scribe velocity significantly influenced bonding results. Although hermetic testing was still effective in the experiment due to large sample size, a gas flow model was used to show that the helium bomb test and bubble test method are not applicable to small volumes (<10-4 cc), typically encountered for MEMS packages.; This process was also successfully demonstrated for sealing MEMS ceramic packages. A CO2 laser-assisted silicon lid sealing process for gas breakdown test structures in ceramic quad flat pack (CQFP) was performed using a Au80/Sn20 solder preform. The MEMS die was sealed and tested (at Coventor Inc.) in the package under various environments such as air, vacuum, nitrogen and helium.; This novel method is especially suitable for encapsulation and vacuum packaging of chips or wafers containing MEMS and other micro devices with low temperature budgets. Further, released and encapsulated devices on the sealed wafers can be diced without damaging the MEMS devices at wafer level.