Wafer-to-Wafer-Bonding

Wafer bonding is a process for temporary or permanent joining of two or more wafers with or without an intermediate layer. Wafer bonding has various applications: packaging (e.g. for sensors and actuators), assembly (e.g. for RF components), integration (e.g. for electrical/micromechanical/optical component), substrate and device fabrication.The System Packaging department offers standard wafer level bonding technologies such as silicon direct, anodic, glass frit and adhesive bonding. These technologies can be adapted according to the specific requirements of target applications.

Other bonding techniques including thermos-compression, solid-liquid inter-diffusion, eutectic, surface activated and reactive bonding are also available in the department and can be optimized for different requirements (e.g. process temperature, yield and bond strength). Other approaches for wafer bonding are primarily driven by the emergence of new materials and processes in micro systems technology. The department has particularly studied bonding methods for plastics, metals and ceramics; considering process temperature constraints, substrate chemical resistance and low cost processes. The examples of these processes are thermo-compression bonding or reactive bonding, both based on forming compounds between two metals.

Different techniques are also available in the department for improving the bond quality; some of these pre-treatment processes are: wet or dry surface modification, plasma activation (particularly for direct bonding) and chemical mechanical polishing.

A relatively new trend in wafer bonding is temporary bonding. This has gained a great deal of interest due to the increasing demand for wafer thinning. Wafer thinning (for thicknesses down to 50 µm) facilitates new applications such as 3D interconnections (based on through silicon vias or other stacking technologies), reduced device thickness (thus flexible devices). Temporary wafer bonding/deboning assists with the handling of ultra-thin wafers which is otherwise challenging or not practical. The department has developed an adhesive based temporary bonding/debonding process for the handling of thin wafers with sufficient mechanically stability. Once the thinned wafer is processed it can be removed from its carrier wafer using a room temperature peel-off debond process.

For both permanent and temporary bonding, clean rooms with fully integrated process line including characterization equipment for the processing of 4"to 8" substrates are available at Fraunhofer ENAS