Investigation of localized laser bonding process for ceramic MEMS packaging

The packaging poses a critical challenge for commercialization of MEMS (micro-electro-mechanical system) products. It provides structural and environmental protection for MEMS devices to enhance their reliability. Major problems with the packaging process include degraded reliability caused by the excess stress due thermal mismatch in the die attach, wire bonding, flip chip, and lid sealing phases of the process, and altered performance of the MEMS device after packaging caused by thermal exposure. As a consequence, local heating and rapid heating approaches, such as resistive micro-heaters, rapid thermal processing (RTP), and laser bonding, have been developed for MEMS packaging. This paper investigates the localized laser bonding technique for ceramic MEMS packaging. A continuous wave CO₂ laser (wave length = 10.6 mum) is applied as a heat source to locally heat ceramic (Al₂O₃) MEMS packages for lid to substrate bonding. To determine the laser power density and scanning speed, a FEA (finite element analysis) thermal model is constructed to simulate the localized laser bonding process. The modeling results provide a guideline for selection of the laser bonding parameters and assist optimization of the packaging process. Further, the effect of external pressure at sealing ring on the forming of bond is studied. To evaluate the laser bonded packages, pull tests are conducted. It is found that the scanning speed and external pressure have significant influence on the pull strength at the bonding interface. The packages are then cross-sectioned and polished, and the bonding interface is inspected under a microscope and scanning electron microscope with energy dispersive spectrometer. Experimental results show that packages bonded with relatively low scanning speed and external pressure conditions have higher bonding quality. This research demonstrates the potential of localized laser bonding process for ceramic MEMS packaging