(Invited) Surface Activated Wafer Bonding; Principle and Current Status

The surface activated bonding method originally uses cleaning of material surfaces by sputter etching using high energy ion/atom beam of inert gases, typically Ar. [1] The cleaning process removes adsorbed atoms and compound layers, typically oxides, which stabilize the surface. Therefore, after the cleaning process the surfaces become unstable “active” states. Mating two such activated surfaces in vacuum enables strong bond formation even at room temperature. This process is quite suitable for the wafer direct bonding. Because intimate contact is automatically achieved at the bonding interface by attractive force between two atomically smooth surfaces of polished wafers, the process requires not only heating process but also pressure application. [2,3] This method has been successfully applied to various semiconductor wafers such as Si, GaAs, SiC, etc. [4,5] Most of the metals can be also bonded by the method in case of atomically smooth surfaces of, for example, metal layers finished by CMP and/or thin metal films deposited on well-polished wafers. [6,7] On the other hand, materials such as SiO 2 and polymers cannot be directly bonded by the method. To bond such materials, deposition of very thin intermediate layers of metals or Si has been proposed. [8] The deposited metal atoms firmly adhere to the surface of SiO 2 and/or polymers and simultaneously form active metal surface. The thin film deposition is regarded as a new process for the surface activation. This means that the concept of the surface activated bonding has been extended in order to apply the method to wide range of materials. Various applications of the surface activated bonding have been developed in the field of wafer-level packaging and engineered substrates. In wafer-level packaging field, various MEMS devices have been already commercialized and 3D-integration of heterogeneous devices are under investigation. In engineered substrate application, RF filters have been already commercialized and still succeeding developments continue. Engineered substrates for micro-electronics, power-electronics, solar cells, etc. are now extensively developed by the method. We believe the surface activated bonding will be used in wide range of technological fields in the near future. [1] T. Suga, et al., Acta Metall. Mater. 40 (1992) S113. [2] H. Takagi, et al., Appl. Phys. Lett. 68(1996) 2222. [3] H. Takagi, et al., Sens. Actuat. A 105 (2003) 98. [4] M. M. R. Howlader, et al., J. Vac. Sci. Technol. B 19(2001) 2114. [5] J. Suda, et al., Proc ICSCRM 2013, Miyazaki, Japan, (2013) 358. [6] A. Shigetou, et al., J. Mater. Sci. 40 (2005) 3149 [7] T. Shimatsu, et al., J. Vac. Sci. Technol. B 28(2010) 706. [8] R. Kondou, et al., Scripta Materialia 65(2011) 320.