Hetero-Integration of β-Ga2O3 and Diamond Substrates by Hydrophilic Bonding Technique
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2020-11-23 |
| Journal | ECS Meeting Abstracts |
| Authors | Takashi Matsumae, Yuichi Kurashima, Hideki Takagi, Hitoshi Umezawa, Koji Tanaka |
Abstract
Section titled “Abstract”Beta-phase gallium oxide (β-Ga 2 O 3 ) has a wide bandgap of ~4.9 eV and a high breakdown voltage of 8 MV/cm. Moreover, high-quality, large-size, and low-cost β-Ga 2 O 3 substrates can be obtained by melt-growth techniques, such as Czochralski (CZ), Floating Zone (FZ), Edge-defined film-fed growth (EFG) techniques. Owing to these features, β-Ga 2 O 3 is a possible candidate for the next-generation power electronics material. However, the primary obstacle for this application is the heat dissipation problem due to low thermal conductivity of ~10-30 W/m/k. A possible solution is to integrate β-Ga 2 O 3 power electronics with high-thermal-conductivity materials. Our research group has developed a hydrophilic direct bonding technique of semiconductor material and diamond heat spreader[1], which has extraordinary high thermal conductivity. A diamond (111) surface treated with a H 2 SO 4 /H 2 O 2 mixture was functionalized with OH groups, and the OH-terminated diamond substrate can form direct bonding with a OH-terminated Si substrate by a dehydration reaction. OH termination using oxygen plasma irradiation allows a strong bonding of semiconductor substrates, such as Si, SiO 2, and Al 2 O 3 (the oxide of group 13 element as Ga 2 O 3 )[2]. The purpose of this study is to form direct bonding of an oxygen-plasma-activated β-Ga 2 O 3 substrate and the H 2 SO 4 /H 2 O 2 -treated diamond substrate by the hydrophilic direct bonding technique, as illustrated in Fig. 1. For the bonding experiment, thin β-Ga 2 O 3 films were exfoliated along the (100) plane using β-Ga 2 O 3 bulk crystal (from Novel Crystal Technology) with a thermal release tape. The thickness of the β-Ga 2 O 3 film was ~10 µm. The cleaved β-Ga 2 O 3 surface was irradiated by oxygen plasma at 200 W and 60 Pa for 30 s using our reactive ion etching (RIE) equipment. Meanwhile, diamond substrates with a 4° off-angled (111)-oriented surface (from EDP) were treated with a H 2 SO 4 /H 2 O 2 (4:1) mixture at 75 °C for 10 min. Subsequently, they were cleaned with an NH 3 /H 2 O 2 /H 2 O (1:1:5) mixture at 75 °C for 10 min. The β-Ga 2 O 3 and diamond surfaces were contacted with each other under atmospheric conditions. The contacted specimens were stored with desiccant for around three days and then annealed at 250 °C for 24 h for bond formation. Figure 2 shows the photograph of the β-Ga 2 O 3 film bonded on the diamond substrate. As the β-Ga 2 O 3 film is transparent, Newton’s rings were observed where the surfaces were not bonded. The nano-structure of the β-Ga 2 O 3 /diamond interface was investigated by a transmission electron microscope (TEM). The focused ion beam (FIB) was introduced from the β-Ga 2 O 3 side to fabricate the ultra-thin cross-sectional specimen of the bonding interface. Figure 3 shows the TEM image around the bonding interface. The β-Ga 2 O 3 [010] and diamond [110] directions were slightly twisted because of manual alignment. It revealed that the β-Ga 2 O 3 and diamond surfaces were atomically bonded without interfacial voids or cracks. The β-Ga 2 O 3 and diamond lattices were rarely damaged except for the outermost surface of the β-Ga 2 O 3 film; The distortion is possibly due to oxygen plasma irradiation, interfacial deformation, and/or thermal stress. Generally, oxide layers having low thermal conductivity were generated at the bonding interface when semiconductor substrates formed direct bonding under atmospheric conditions. However, this study realized the direct bonding without any intermediate oxide layers because β-Ga 2 O 3 is an oxide material and diamond never develops an oxide layer. Thus, the bonding interface is almost ideal for efficient heat dissipation from a β-Ga 2 O 3 power device. Moreover, it can facilitate the development of future electronic devices using the β-Ga 2 O 3 /diamond interface. [1]. T. Matsumae, Y. Kurashima, H. Umezawa, and H. Takagi, Scr. Mater. , 175 , 24-28 (2020). [2]. T. Suni, K. Henttinen, I. Suni, and J. Mäkinen, J. Electrochem. Soc. , 149 , G348 (2002) http://jes.ecsdl.org/cgi/doi/10.1149/1.1477209. Figure 1