Impacts of Wafer-Bow and Surface Contamination on Low Temperature Wafer Direct Bonding

In recent years, low temperature wafer direct bonding has become one of the key fabrication methods for preparing integrated semiconductor materials (such as SOI, GaAs/InP integration) and packaging MEMS as well as optic-electro devices, especially for those with 3D structures. Mechanical quality, surface chemistry and manufacturing process flow are main aspects that determine the bond quality. When applied, commercially-available wafers with mechanical quality variations, including surface roughness, wafer-bow, total thickness variations etc. may cause many problems that greatly reduce the bond strength. Besides, surface contamination which is induced during complicated process is inevitable sometimes and may cause voids across the interface and instability in process. In this paper, the impacts of wafer-bow and surface contamination on the low temperature wafer direct bonding are estimated and calculated based on the thin plate elastic mechanics, energy conservation theory and basic criterion for wafers to bond under low temperature. On the other hand, finite element analytical tools are also employed to simulate those impacts. Many aspects concerning the wafer´s initial bow and thickness are revealed from the results. The amount of deflection, to which two wafers are finally deform to and reach the minimum strain energy, are described in detail. Wafers pairs with different thicknesses are found to have lower strain energy and easy to bond than those with the same thickness. Moreover, thinner pairs are less sensitive to particles than thicker ones. Proper force loaded during bonding which is beneficial to enhance bond quality can be estimated based on the original wafer-bow and other material proprieties