Laser Bonding of Glass to Silicon Using Polymer for Microsystems Packaging

Laser joining is a promising technique for wafer-level bonding. It avoids subjecting the complete microelectromechanical system (MEMS) package to a high temperature and/or the high electric field associated with conventional wafer-level bonding processes, using the laser to provide only localized heating. We demonstrate that a benzocyclobutene (BCB) polymer, used as an intermediate bonding layer in the packaging of MEMS devices, can be satisfactorily cured by using laser heating with a substantial reduction of curing time compared with an oven-based process. A glass-on-silicon (Si) cavity bonded with a BCB ring can be produced in a few seconds at a typical laser intensity of 1 W/mm² resulting in a local temperature of ~300degC. Hermeticity and bond strength tests show that such cavities have similar or better performance than cavities sealed by commercial substrate bonders. The influence of exposure time, laser power, and applied pressure on the degree of cure, bond strength, and hermeticity is investigated. The concept of using a large area uniform laser beam together with a simple mirror mask is tested, demonstrating that such a mask is capable of protecting the center of the cavity from the laser beam; however, to prevent lateral heating via conduction through the Si, a high-conductivity heat sink is required to be in good thermal contact with the rear of the Si.