Surface Activated Bonding -from the Standard SAB to Modified SAB
Authors: Suga Tadatomo 
Published: February 2020

The surface activated bonding (SAB) was proposedin thelate 1980’s for bonding of metal to metal and to ceramics at room temperature. The standard SAB method is based on surface activation process by bombardment of the surfaces by Ar ions or neutral atoms in ultra-high vacuum to clean and activate them so that they can be bonded spontaneously only by contact without heat treatment.The term of the SAB was used for the first time in the article of F. S. Ohuchi and T. Suga (1994) [1],to describe the bonding process which “utilizes the fundamental nature of atomically clean surfaces created by energetic particle bombardment leading to bond formation when two surfaces are brought into contact.” The standard SAB process for Al-Al and Al-Si3N4 was described in 1992[2],and Cu-Cu for micro-bonding in 1993[3]. The standard SAB method was applied successfully to the direct wafer boding of Si-Si in 1996[4]and then expanded to heterogeneous bonding between semiconductors and metals at room temperature. The SAB method has attracted increasing interest due to its simple process flow, no need for additional intermediate materials for bonding, and compatibility with CMOS technology.
Simulations of the electronic structure of direct bonded interfaces reveal that certain electron transfer may occur across the bonded interface depending on the nature of the activated surfaces, resulting into covalent, metallic, or ionic bonding at the interfaces.
The surface activation process of the ion bombardment induces various effects on the activated surfaces. For metals, the native oxide is removed and point defects are induced with nano-grain formation observed in some cases. For semiconductors, dangling bonds are formed at the surface which however sometimes contains certain electronic defects and becomes amorphous structure. For ionic materials such as oxides, carbides, and nitrides, certain preferential sputtering effect is observed: for instance, SiC surface after the surface activation shows a Si depletion layer. Those effects could contribute the strong bonding formation at the bonded interface, or sometime cause some restriction in the utilization of the bonded interface.
The standard SAB method has failed in bonding SiO2, carbon materials and organic polymers because SiO2 and carbon materials may hardly form dangling bonds, the later becomes amorphous, and polymer materials are carbonized and inert to bond. To overcome the limit of the standard SAB, modified SAB method has been developed in which metal nano-intermediate layer is inserted into the bonded interface so that the electronic polarization is sealed by the metallic layer and the week adhesion is enhanced by boding to the metal layer. By the modified SAB, bonding of glass, SiC, GaN, diamond, and polymer films as well as their combinations have been demonstrated.
The conventional wafer bonding technique which is called as fusion bonding, oxide bonding, or hydrophilic bonding, requires a reaction with water and OH- group formation as a prior condition and belongs to a different category of the SAB. However, its combination with the modified SAB provides also a new possibility for low temperature bonding.

References

[1] F. S. Ohuchi and T. Suga, “Electronic roomtemperaturestructure of metal/ceramic interfaces fabricated by "Surface Activated Bonding", in Advanced Materials '93, Ill / B: Composites, Grain Boundaries and Nanophase Materials, edited by M. Sakai, et al. Trans. Mat. Res. Soc. Jpn., Volume 16B (1994) pp. 1195-1199.
[2] T. Suga, Y. Takahashi, H. Takagi, B. Gibbesch, G. Elssner, “Structure of Al-Al and Al-Si3N4 interfaces bonded at room temperature by means of the surface activation method”, Acta metall. mater., 40, Suppl. (1992) pp. S133-S137
[3] T. Suga, T. Fujiwaka, G. Sasaki, “Surface activated bonding and its application on micro-bonding at room temperature", European Hybrid Microelectronics Conf., 9th, Proceedings., Nice, Jun. 2-4 (1993) pp.　314-321.
[4] H. Takagi, K. Kikuchi, and R. Maeda, T. R. Chung, T. Suga, “Surface activated bonding of silicon wafers at room temperature”, Appl. Phys. Lett. 68, 2222 (1996) doi: 10.1063/1.115865