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Anisotropic Conductive Adhesive for Wafer-to-Wafer Bonding
Keywords: Wafer bonding, Anisotropic conductive adhesive (ACA), Metal coated polymer spheres (MPS)
A new bonding method based on an anisotropic conductive adhesive (ACA) is developed. The ACA is composed of benzocyclobutene (BCB) filled with 3µm sized metal covered polymer spheres (MPS). The BCB assures mechanical strength whereas the MPS result in electrical conductivity at interconnection points. The amount of MPS is below the percolation limit to assure electrical contact between adjacent pads whereas there is electrical insulation in between. The ACA is spin or spray coated onto 100mm silicon wafers and bonded at 250oC with various tool pressures. Anisotropic conductive adhesives and films (ACF) are widely used for flip-chip bonding of semiconductor chips. Flip-chip bonding of silicon chips onto glass substrates for display applications is a common application. Advantages of ACA/ACF compared to e.g. solder interconnects are reduced height, finer pitch, lower assembly process temperature, and reduced cost. Most commonly, both deposition of ACA or ACF and assembly are performed on chip level, although application of ACF on wafer level before singulation has been demonstrated as feasible. Wafer bonding using BCB is well established in the microsystems community. BCB is a robust thermoset polymer with attractive properties such as a glass transition temperature above 350oC, a low dielectric constant and good coating properties. In microsystems fabrication, BCB has traditionally been used for non-hermetic encapsulation of sensors and RF-switches. A low bonding temperature and a high tolerance for surface topography are among the advantages of wafer bonding with BCB. We demonstrate how the advantages of an ACA based on MPS and adhesive bonding using BCB is combined into a wafer level bonding method where mechanical and electrical connection, as well as a well controlled stand-off height, is achieved in a single step. The demonstrated technology will be a low cost process with possible applications within MEMS fabrication and 3D-integration of MEMS and/or IC.
M.M.V. Taklo, Senior Research Scientist
SINTEF IKT
Oslo N-0373,
Norway


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