Combined Surface-Activated Bonding (SAB) Technologies for New Approach to Low Temperature Wafer Bonding
Authors: He Ran , Fujino Masahisa , Yamauchi Akira , Suga Tadatomo 
Published: February 2020

Surface activating/cleaning is of great importance for low temperature wafer bonding and various surface activating methods have been studied. Fast atom bombardment activation removes oxides and contaminations on bonding surface and it is efficient for room temperature wafer bonding for semiconductors and metals, etc. Plasma activated bonding has been developed for low temperature hydrophilic bonding for silicon and silicon oxide wafers[1] and Au bonding. Formic acid gas treatment reduces Cu oxide to Cu and then the Cu can be bonded at temperature below 200°C [2]. Water vapor-assisted SAB method introduces water vapor onto the FAB activated surfaces at atmospheric pressure, and then homogeneous and heterogeneous bonding of Cu, SiO2, and polyimide can be achieved at 150°C and atmospheric pressure [3]. SAB with a modification using nano-adhesion layer at the interface realized room temperature bonding of polymer films and glass[4]. However, every activating method has its advantages and limitations depending the bonding materials and applications. Thus, there is a need to combine various surface activating methods for novel low temperature wafer bonding approaches development. The objectives of this study are to develop new approach for low temperature wafer bonding by using combined surface-activated bonding (SAB) technologies and to explore the mechanisms of the bonding processes.
We used Ar plasma and Ar fast atom bombardment (FAB) to activate the Cu/polymer hybrid surface for removal of Cu native oxide and organic contaminations on Cu surface. X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical composition and state of the activated surfaces. The results show that Ar FAB is more effective for Cu oxides and organic contaminations removal than Ar plasma. The XPS spectra (Figure 1) show that Cu hydroxide, Cu oxides and organic contaminations on Cu surface could be removed by FAB activating for 9 minutes. However, Cu contamination on polymer surface is observed after both Ar plasma and Ar FAB activation.
To develop new low temperature bonding approaches, we designed a combined surface-activated bonding system, which consists of two main subsystems: a surface activating subsystem and a wafer bonding subsystem. The surface activating subsystem could employ combined surface activating processes, including oxygen, nitrogen, and fluorine containing plasma, formic acid gas, water vapor and FAB surface treatments. The wafer bonding subsystem could perform wafer alignment and bonding process in ultra high vacuum.
In this paper, we report the design and results of a combined surface-activated bonding system that we have constructed to explore the effects of combined surface activating processes on bonding surfaces, and conclude with prospects for the future.

References

[1] C. Wang and T. Suga, “Room-Temperature Direct Bonding Using Fluorine Containing Plasma Activation,” J. Electrochem. Soc., vol. 158, no. 5, pp. H525–H529, May 2011.
[2] W. Yang, M. Akaike, M. Fujino, and T. Suga, “A New Combined Process of Formic Acid Pretreatment for Low-temperature Bonding of Copper Electrodes,” ECS Trans., vol. 50, no. 7, pp. 133–138, Mar. 2013.
[3] A. Shigetou and T. Suga, “Vapor-Assisted Surface Activation Method for Homo- and Heterogeneous Bonding of Cu, SiO2, and Polyimide at 150°C and Atmospheric Pressure,” J. Electron. Mater., vol. 41, no. 8, pp. 2274–2280, Aug. 2012.
[4] T. Matsumae, M. Nakano, Y. Matsumoto, and T. Suga, “Room Temperature Bonding of Polymer to Glass Wafers Using Surface Activated Bonding (SAB) Method,” ECS Trans., vol. 50, no. 7, pp. 297–302, Mar. 2013.