Mechanics of direct wafer bonding

Direct wafer bonding, also known as fusion bonding, has emerged as a key process in the manufacture of microelectromechanical systems (MEMS). The use of wafer bonding increases design flexibility, allows integration of dissimilar materials, and permits wafer-level packaging. While direct wafer bonding processes are becoming more prevalent in the fabrication of MEMS devices, failure during the bonding process is often a problem and is not completely understood. A modeling framework, based on the mechanics of the bonding process, has been on the mechanics of the bonding process, has been developed to correlate bonding failure to wafer geometry, surface condition, and etch patterns. The modeling approach is based on an energy balance between the reduction in surface energy as the bond is formed and the strain energy that is stored in the wafers as they conform to each other. The model allows the effect of flatness deviations, wafer geometry (i.e. thickness, diameter), wafer mounting, and etched features on the bonding process to be shown. Modeling results demonstrate that wafer bow, wafer thickness, and certain types of etch patterns are critical factors in controlling bonding success. Bonding experiments, in which specific flatness deviations and etch patterns have been introduced on wafers prior to bonding, have been carried out and compared to the modeling results. The understanding of the process gained through the modeling can be used to set tolerances on wafers, assist in mask layout, and guide the design of bonding equipment to ensure success in direct wafer bonding processes