Table of contents

In this work, we fabricated Ga₂O₃(001)/Si(100) and Ga₂O₃(010)/Si(100) heterointerfaces by surface activated bonding at room temperature and investigated the effect of Si thickness on the thermal stability of the heterointerfaces by heating the bonding samples at different temperatures. The heterointerface with a thin Si exhibited a good thermal stability at 1000 °C. A 4 nm thick intermediate layer with a uniform thickness was formed at the as-bonded Ga₂O₃(001)/Si(100) heterointerface, but for the as-bonded Ga₂O₃(010)/Si(100) heterointerface, an intermediate layer with a non-uniform thickness was formed. The thickness of both intermediate layers ranged from 3.6 to 5.4 nm and decreased after annealing at 500 °C, followed by an increase after annealing at 1000 °C. The component of the intermediate layer includes Ga, O, and Si atoms.

Amorphous Si films with a low surface roughness of 0.13 nm were used to examine the bonding performance of atomic diffusion bonding of quartz glass wafers at room temperature in vacuum. The high bonding strength was achieved for films with thickness δ of 2–50 nm: the blade could not be inserted between the bonded wafers. Using a vacuum chamber with a base pressure of 1 × 10^–6 Pa, the great bonding strength was maintained even with waiting time in vacuum of as long as 3.6 × 10³ s from film deposition to bonding. The excellent bonding performance was almost equal to that achieved using Ti films.

Atomic diffusion bonding (ADB) of wafers at room temperature in air was studied using Ag films. Using an ultra-high vacuum magnetron sputtering system, Ag (20 nm) films with Ti (5 nm) underlayers were deposited. The propagation speed of crystal lattice rearrangement in the bonding process decreased with an increased exposure time of film surfaces to air (t_exp). Propagation did not occur at t_exp of 500 s. The cohesion of Ag film surfaces by film surface exposure to air and reduction of the Ag film surface energy by Ag oxide or sulfide formation probably cause ADB performance degradation.

Surface-activated bonding (SAB) of Cu by gas cluster ion beam (GCIB) irradiation with acetic acid vapor was studied. GCIB irradiation realizes surface smoothing and surface reaction enhancement without severe damage. Therefore, it is promising for SAB. In this study, acetic acid vapor was introduced during Ar-GCIB irradiation to assist the removal of surface oxides on the Cu surface. XPS results showed that Cu(OH)₂ was effectively removed by reaction with adsorbed acetic acid, and there was no residue by acetic acid adsorption. In addition, surface roughness decreased by Ar-GCIB irradiation with acetic acid because of the preferential removal of protrusion. Preliminary bonding experiments showed an increase of Cu–Cu bond strength by Ar-GCIB irradiation with acetic acid vapor.

GaN substrates were directly bonded with Si substrates by wet treatments using H₂SO₄/H₂O₂ and NH₃/H₂O₂ mixtures. Under the optimized condition, the tensile strength reached 7.36 MPa, and a part of the Si substrate was fractured within the bulk instead of the bonding interface. There is an amorphous intermediate layer with a thickness of 1.7 nm, which mainly consists of Si oxides, at the bonding interface. It is remarkable that wafer-scale GaN/Si integration was successfully achieved by using common cleaning methods. It is believed that the proposed direct bonding technique would contribute to future heterogeneous integration because the GaN and Si substrates can be bonded through the atomically thin intermediate layer without vacuum processes.

Chemical composition around diamond/silicon heterointerfaces fabricated by surface activated bonding (SAB) at room temperature is examined by energy-dispersive X-ray spectroscopy under scanning transmission electron microscopy. Iron impurities segregate just on the bonding interfaces, while oxygen impurities segregate off the bonding interfaces in the silicon side by 3–4 nm. Oxygen atoms would segregate so as to avoid the amorphous compound with silicon and carbon atoms, self-organized at the bonding interfaces in the SAB process. When the bonding interfaces are annealed at 1000 °C, the amorphous compound converts into cubic silicon carbide (c-SiC), and nano-voids 5–15 nm in size are formed at the region between silicon and c-SiC, at which the oxygen density is high before annealing. The nano-voids can act as the gettering sites in which metal impurities are preferentially agglomerated, and the impurity gettering would help to improve the electronic properties of the bonding interfaces by annealing.

We propose the use of a laminated wafer with a conductive diamond layer for forming cavities as an alternative silicon-on-insulator wafer for micro-electro mechanical system (MEMS) sensors. Since this wafer has no insulator such as a buried oxide (BOX) layer but a conductive layer, it is not charged during plasma treatment in MEMS sensor fabrication processes. The conductive diamond layer was formed on a base wafer doped with boron of more than 2 × 10²¹ atoms cm⁻³ by microwave-plasma-enhanced chemical vapor deposition. The resistivity of this layer was 0.025 Ω cm, and this layer can be selectively etched to a base wafer made of silicon crystal, such as a BOX layer. In addition, a silicon wafer can be bonded to its layer without voids with gaps of more than 2 nm by surface-activated bonding. Therefore, we believe that the laminated wafer studied here is useful for the fabrication processes for MEMS sensors that may otherwise be damaged by plasma treatment.

Coplanar waveguides (CPWs) are fabricated by directly bonding 17 μm Al foils to Si substrates with different resistivities and wet etching. Their RF characteristics are compared with those of CPWs fabricated on 0.8 μm thick evaporated Al layers. A lower insertion loss is observed for Si substrates with higher resistivity irrespective of the thickness of conductors because the substrate loss is decreased. For a 10000-Ω · cm Si substrate, Al-foil-based CPWs reveal a ∼0.7 dB cm⁻¹ lower insertion loss in comparison with evaporated-Al-based CPWs at 1 GHz. These features agree with the calculation, which implies the superiority of direct bonding of metal foils for realizing low-loss passive components.

We evaluate the current–voltage (I–V) and temperature-dependent I–V characteristics of p⁺-Si/p-diamond heterojunction diodes (HDs) fabricated using surface-activated bonding and compare their characteristics with those of Al/p-diamond Schottky barrier diodes (SBDs) fabricated on the same diamond substrate. The ideality factor, reverse-bias current, and on/off ratio of HDs are improved by annealing them at temperatures up to 873 K, which is in good contrast to the characteristics of SBDs. The barrier height at Si/diamond bonding interfaces is decreased by annealing. The difference in response to annealing between HDs and SBDs implies that the density of interface states formed during the surface activation process is decreased by annealing HDs. The characteristics of HDs are degraded by annealing them at 1073 K, which is assumed to be due to the formation of intermediate layers or the occurrence of local strain at Si/diamond bonding interfaces.

Bond strength is the most reliable criterion of the wafer bonding quality. Water stress corrosion affects the bond strength, corresponding to the measurement atmosphere and residual moisture at the bonding interface. In this study, we developed a new methodology to measure the wafer bond strength including the water stress corrosion effect under the controlled atmospheres, namely, dry atmosphere, wet atmosphere, and vacuum. The developed method experimentally demonstrates the evaluation of the water stress corrosion by the surrounding air and the interfacial water separately. Furthermore, it is also indicated that the water stress corrosion depends on the bonding methods, such that the surface activated bonding using Si intermediate layer has high durability for corrosion and the hydrophilic bonding has low durability. This study will provide a new understanding of the relation between the bonding process and the water stress corrosion effect, especially for the interfacial water.

Atomic diffusion bonding with oxide underlayers using Al and a-Si films was examined to create a bonded interface with Al₂O₃ and Si-oxides having large band gaps for high optical density applications. Surface free energy of the bonded interface greater than 2 J m⁻² and 100% light transmittance were achieved after annealing at 300 °C in the range of film thicknesses δ on both sides from 0.3 to 0.5 nm using Al films and with δ of around 0.5 nm using a-Si films. Structural analyses revealed that the bonded interface consists of Al₂O₃ and Si-oxides with oxygen dissociated from oxide underlayers.