Benzo-cyclo-butene bonding process with ´stamp´ printing for wafer level package

Presented is a novel process of benzo-cyclo-butene (BCB) bonding for a wafer level package with `stamp´ printing, in which the BCB structure is transferred and patterned by pressing the cap wafer towards another wafer with a liquid BCB layer. Compared with the conventional BCB bonding process, this new method not only avoids the residual polymer inside the cavity to extend the application of BCB bonding in the complex three-dimensional structures but also raises the bonding strength because of the non-treatment before bonding. The key process parameters, such as the auxiliary wafer material, pre-curing time and bonding pressure were optimised after a series of contrast experiments. The bonding quality was evaluated qualitatively by manual separation and quantitatively by a tensile bonding test. This new method has a minimum bonding strength of 5.9 MPa, which is more than ten times the conventional one. Besides, the bonding yield reaches 80% in five groups of 4-inch wafers. Finally, this process was successfully utilised in an acceleration microswitch.