Laser-assisted Selective Bonding for Wafer-level Chip-scale Vacuum Packaging of MEMS and Related Micro Systems

摘要

In this work, low temperature selective solder (Pb37/Sn63) bonding of silicon chips or wafers for MEMS applications using a continuous wave (CW) carbon dioxide (CO_2) laser at a wavelength of 10.6μm was examined. The low reflectivity, fair transmittance, and high absorptivity of silicon at the 10.6μm wavelength led to selective heating of the silicon and 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 one milliTorr in order to achieve fluxless soldering and vacuum encapsulation of silicon dies. The bonding temperature at the sealing ring was close to the reflow temperature of the eutectic lead tin solder, 183 °C. The global average temperature was even lower due to local laser heating. Pull test results showed that the joint was sufficiently strong and could not be separated before the silicon die broke. Microscopic view showed the fully reflown solder and condition of the sealed cavities was fine and uniform. This novel method is especially suitable for vacuum bonding wafers containing MEMS and other micro devices with low temperature budgets, for example, for the reason of managing the stress distribution. Further, sealed encapsulated and released wafers can be diced without damaging the MEMS devices at wafer scale.