Surface activated bonding

Surface activated bonding (SAB) is a non-high-temperature wafer bonding technology with atomically clean and activated surfaces. Surface activation prior to bonding by using fast atom bombardment is typically employed to clean the surfaces. High-strength bonding of semiconductors, metals, and dielectrics can be obtained even at room temperature.

Overview In the standard SAB method, wafer surfaces are activated by argon fast atom bombardment in ultra-high vacuum (UHV) of 10−4–10−7 Pa. The bombardment removes adsorbed contaminants and native oxides on the surfaces. The activated surfaces are atomically clean and reactive for formation of direct bonds between wafers when they are brought into contact even at room temperature.

Researches on SAB The SAB method has been studied for bonding of various materials, as shown in Table I. +Table I. Studies of standard SAB for various materialsSiGeGaAsSiCCuAl2O3SiO2SiJ. Liang, T. Miyazaki, M. Morimoto, S. Nishida, N. Watanabe, and N. Shigekawa, "Electrical Properties of p-Si/n-GaAs Heterojunctions by Using Surface-Activated Bonding," Appl. Phys. Express, vol. 6, no. 2, p. 021801, Feb. 2013. Available H. Takagi, J. Utsumi, M. Takahashi, and R. Maeda, "Room-Temperature Bonding of Oxide Wafers by Ar-beam Surface Activation," ECS Trans., vol. 16, no. 8, pp. 531–537, Oct. 2008. Available GeGaAsSiCCuAl2O3SiO2Failure The standard SAB, however, failed to bond some materials such as SiO2 and polymer films. The modified SAB was developed to solve this problem, by using a sputtering deposited Si intermediate layer to improve the bond strength. +Table II. Modified SAB with Si intermediate layerBonding intermediate layerReferencesSiO2-SiO2Sputtered Fe-Si on SiO2R. Kondou and T. Suga, "Room temperature SiO2 wafer bonding by adhesion layer method," presented at the Electronic Components and Technology Conference (ECTC), 2011 IEEE 61st, 2011, pp. 2165–2170. Available Polymer filmsSputtered Fe-Si on both sidesT. Matsumae, M. Fujino, and T. Suga, "Room-temperature bonding method for polymer substrate of flexible electronics by surface activation using nano-adhesion layers," Japanese Journal of Applied Physics, vol. 54, no. 10, p. 101602, Oct. 2015. Available Si-SiCSputtered Si on SiCSi-SiO2Sputtered Si on SiO2K. Tsuchiyama, K. Yamane, H. Sekiguchi, H. Okada, and A. Wakahara, "Fabrication of Si/SiO2/GaN structure by surface-activated bonding for monolithic integration of optoelectronic devices," Japanese Journal of Applied Physics, vol. 55, no. 5S, p. 05FL01, May 2016. Available The combined SAB has been developed for SiO2-SiO2 and Cu/SiO2 hybrid bonding, without use of any intermediate layer. +Table III. Combined SAB using Si-containing Ar beamBond interfaceReferencesSiO2-SiO2Direct bond interfaceCu-Cu, SiO2-SiO2, SiO2-SiNxdirect bond interface

Technical specifications Materials Semiconductor: Si-Si, Ge-Ge, GaAs-SiC, SiC-SiC, Si-SiC, etc. Metal: Al-Al, Cu-Cu, etc. Dielectric: Polymer films, SiO2, etc. Cu/Dielectric Hybrid: Cu/SiO2 and Cu/SiO2/SiNx Advantages Low process temperature: room temperature–200 °C No concerns of thermal stress and damages High bonding quality Semiconductor and metal bonding interfaces without oxides Completely dry process without wet chemical cleaning Process compatibility to semiconductor technology Drawbacks High vacuum level (10−4–10−7 Pa)

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