Selective bonding and encapsulation for wafer-level vacuum packaging of MEMS and related micro systems

A laser-assisted bonding technique is demonstrated for low temperature region selective processing. A continuous wave carbon dioxide (CO₂) laser (λ=10.6 μm) is used for solder (Pb37/Sn63) bonding of metallized silicon substrates (chips or wafers) for MEMS 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 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. Pull test results showed that the joint was sufficiently strong. Helium leak testing showed that the leak rate of the package met the requirements of MIL-STD-883E under optimized bonding conditions and bonded packages survived thermal shock testing. The testing, based on a design of experiments method, indicated that both laser incident power and scribe velocity significantly influenced bonding results. 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, where managing stress distribution is important. Further, released and encapsulated devices on the sealed wafers can be diced without damaging the MEMS devices at wafer level.