Wafer Bonding - Problems and Possibilities

Doctoral thesis, 1998

The wafer bonding technology offers a unique opportunity to combine different materials. This has been used for the realisation of novel silicon on insulator (SOI) structures. By replacing the buried silicon dioxide layer with a polycrystalline diamond film the thermal properties of the SOI structure are improved. Also experiments using aluminium nitride film as the buried insulator are presented. It is also showed that wafer bonding can be used for the formation of a semi-insulating substrate. Since wafer bonding is adhesion between surfaces, specific surface requirements for successful bonding have been investigated. Surface roughness and chemical termination are key issues for successful bonding. Various methods for measuring the surface roughness are discussed. Focus has been held at Atomic Force Microscope (AFM) for determining the surface roughness. The influence of the measuring technique on the result has been investigated. It is found essential to determine the smallest unique dominating surface feature, which sets the scale for measuring the surface roughness for every material. Then the surface roughness can be related to the bondability of the materials. The use of wafer bonding in other applications like in micro-mechanics or for bonding III-V materials are also discussed. Some examples of what have been done are given. Finally a few reflections about the future are made.