Investigation of Anodic Bond Formation Process and Potential Use of the Results

Anodic bonding of glass to silicon wafers is an essential step in microsystems manufacturing, combining reliable process setup with robust process control and providing very strong and hermetic wafer bonds. It is general accepted that the bond formation is based on anodic oxidation processes, but it is still under debate where the oxygen originates from and how far the oxidation can be driven. The oxidation mechanism during anodic bonding has been studied using sequential and one-step bonding procedures. It can be concluded that the oxygen for the oxidation originates from the glass. It is therefore possible to completely oxidise sputtered aluminium between two glass wafers during bonding, resulting in a nearly optically transparent bond of both wafers. By understanding the oxidation processes, it is possible to predict which surface layers will bond well (silicon, silicon dioxide, aluminium) and which will be hardly or impossible to bond (silicon nitride, gold). These predictions have been tested and confirmed by experimental results.