Microscopic Picture of Direct Bonding Via Surface Activation for Low-Resistance Si/Wide-Gap Semiconductor Heterointerface
Authors: Ohno Yutaka , Liang Jianbo , Shigekawa Naoteru , Yoshida Hideto , Miyagawa Reina , Shimizu Yasuo , Nagai Yasuyoshi 
Published: December 2020

Surface-activated bonding (SAB) [1], that is a direct wafer bonding process without additional buffer layers, is a promising method to fabricate tough and steep heterointerfaces at low cost. SAB can fabricate any heterointerfaces free from dislocations and cracks, even for dissimilar materials with different crystal structures and lattice constants, without high-temperature annealing. Recently, SAB is applied to the next-generation semiconductors such as diamond, SiC, and GaN, as well as to the basic semiconductors such as Si and GaAs, towards low-resistance semiconductor-to-semiconductor heterointerfaces free from adherent layers. Even though such bonding is successfully demonstrated, the principle of the SAB is still controversial due to the difficulty of analyzing their non-equilibrium heterostructures at an atomistic level.
In the present work, we have clarified the bonding mechanism in Si/GaAs [2] and Si/diamond [3] heterointerfaces fabricated by SAB at room temperature (RT), by using high-angle annular dark-field (HAADF) and energy dispersive x-ray spectroscopy (EDX) under cross-sectional scanning transmission electron microscopy (STEM) and time-of-flight secondary ion mass spectrometry (TOF-SIMS), combined with low-temperature focused ion beam (LT-FIB) technique that can suppress the structural modification during FIB processes [4]. In the SAB process, wafer surfaces are activated at RT by the irradiation of inert atoms in a high vacuum, and the surfaces are then bonded by the contact under a specific pressure. In the bonding process at RT, atomic intermixing across the interfaces, due to the transient enhanced diffusion assisted by the point defects introduced in the surface activation process, forms an intermediate layer of 4-5 nm thick having gradient composition (see Fig. 1 for Si/GaAs interface). Interestingly, no structural defect, such as cracks and dislocations, is introduced at the heterointerfaces even after high-temperature annealing, presumably due to the gradient nanolayer acting as a buffer layer that can reduce the interface energy, as well as forming strong chemical bonds. Spontaneous formation of the gradient nanolayer is, therefore, the key concept of SAB, by which we can create tough and steep heterointerfaces of dissimilar materials at low cost. On the other hand, since the point defects, as well as their agglomerates [5], can degrade the electronic properties such as interface resistances, the decrease of the defects would play a crucial role in fabricating electronic devices with SAB techniques. The degraded properties can be recovered by an appropriate annealing after the SAB processes, although the atomistic structure around the heterointerfaces would be modified during the annealing [6]. By controlling SAB and subsequent annealing conditions, we can obtain low-resistance heterointerfaces via the optimization of the trade-off relationship between the chemical bonding strength and the electronic properties, determined by the distribution of point defects beneath the activated surfaces before bonding.
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Fig. 1 (a) HAADF-STEM image of an as-bonded GaAs/Si heterointerface. The exact location of the interface is indicated with the broken line. A thin amorphous layer is formed in the Si side, while a defective crystalline layer, with small vacancy clusters [5], are formed in the GaAs side. (b) Density profiles across the interface for As, Ga, Si, and all kinds of atoms obtained by STEM-EDS. A slight atomic intermixing across the interface takes place in the bonding process at RT, via the transient enhanced diffusion assisted by the point defects introduced in the surface activation process. The total density depletes at the interface, indicating the introduction of vacancies, mainly on As sites in GaAs. The ratio between the As density and the Ga one in the GaAs side is below 1.0 down to the depth of 2 nm from the interface, and it is above 1.0 in the depth range of 2 to 6 nm, due to Frenkel-type defects (vacancy-interstitial pairs) on the As sites. Those defects would induce a high interface resistance of the order of 10-1 Ωcm2 [2].

Figure 1