A Wafer-Level Packaging Approach for MEMS and Related Microsystems using Selective Laser-Assisted Bonding (LAB)

A wafer-level bonding technique for hermetic, near-hermetic & vacuum packaging of MEMS is proposed. Silicon wafers are metallized with an intermediate layer of gold on which a low temperature solder such as Pb37/Sn63 having a thickness of 50-100 mum is deposited. The bonding layer is deposited either by screen printing or eutectic preforms. Two identical wafers are aligned at the intermediate layer and a continuous wave CO₂ laser in combination with an X-Y translation stage is employed for selective heating of the silicon wafer to reflow the solder, bonding the wafers. The laser used had a wavelength of 10.6 mum with a spot size of 0.2-1.0 mm. The bonding temperature at the intermediate layer exceeded the reflow temperature of the eutectic Pb-Sn solder due to local laser heating effects, but the global average temperature was much lower (about 100degC). For a vacuum packaging approach the wafers were mounted on a custom-built wafer-mating fixture housed in a vacuum chamber maintained at a pressure of 10^-3 Torr. Optical, X-ray and acoustic microscopy were performed on the bonded wafer and pull tests on the diced packages to assess the quality of the solder bonds