Recently, β-Ga₂O₃ has caught attentions in the field of high-voltage power-device applications due to its excellent material properties and suitability for mass production¹⁾. Thermal conductivity of Ga₂O₃ is, however, low, and bonding Ga₂O₃ to high-thermal conductivity substrate material is desired to improve heat dissipation of power devices. For the bonding between dissimilar materials, low temperature process is desirable to avoid the problems caused by thermal expansion mismatch. Surface activated bonding (SAB), which can achieve strong bonding without any heat treatments, is one of the candidates for the purpose, however bonding strength of oxide materials is low compared to metals and semiconductors²⁾. In this paper, we propose bonding process using in-situ deposition of Si thin film on oxide materials, which can be performed by a standard SAB apparatus without any modification. By the process, we tried bonding of Ga₂O₃ and Al₂O₃ to Si.

In the bonding experiments, a 15 x 10-mm², 500-μm-thick, single-side polished chip of (010) β-Ga₂O₃ (TAMURA Corporation) was used. 100-mm-diameter, 650-μm-thick, single-side-polished, c-face sapphire wafers and 100-mm-diameter, 400-μm-thick, double-side-polished, (100) Si wafers were also used. A pair of Ga₂O₃ and Si specimens were introduced into an apparatus for the surface activated bonding (MWB-12ST, Mitsubishi Heavy Industries Machine Tool Co. Ltd.). After setting specimens on stages in the bonding chamber, Ar fast atom beam was irradiated to the Ga₂O₃ chip for 10 seconds to remove adsorbed surface contaminants. Then, the Ar fast atom beam was irradiated to the Si wafer for 60 seconds to remove surface contaminants and an oxide layer. During this surface activation step for Si, some of sputtered Si atoms were deposited on the Ga₂O₃ surface and form a thin Si layer. Immediately after the surface activation and Si layer deposition step, Ga₂O₃ and Si specimens were mated in the vacuum and pressed. Sapphire and Si wafers were bonded by the same procedure.

The Ga₂O₃ chip was successfully bonded to the Si wafer except the area near the edge of the chip as shown in figures. The unbonded area is supposed to be caused by the edge-sagging formed during polishing process. Sapphire wafers were also successfully bonded to Si wafers. The Ga₂O₃/Si bonded specimen was cut into two pieces at the center of the Ga₂O₃ chip by dicing saw, and strength of Ga₂O₃/Si bonded specimen was evaluated by the dicing test from Si side. No cleavage from the bonding interface was observed after dicing test (lower-left in figures). Sapphire/Si bonding strength was evaluated by the blade test ³⁾ as well as the dicing test. The bonding was so strong that no crack initiated at the bonding interface and Si wafer was cracked by blade insertion. Micro structure of the Ga₂O₃/Si and sapphire/Si interface was investigated by TEM. No cleavage was observed in low magnification images. In a HRTEM image of Ga₂O₃/Si interface (lower-right in figures), an amorphous intermediate layer was observed. Considering bonding step, the intermediate layer was resulted from deposited Si layer on Ga₂O₃ surface and damaged layer on Si surface formed during Ar fast beam irradiation⁴⁾.

In-situ deposition of Si thin film on oxide wafer is proved to be effective for strong bonding formation at room-temperature. Ga₂O₃ and sapphire were successfully bonded to Si wafers. This process is expected to provide a promising method to improve heat dissipation in Ga₂O₃ devices, as well as integration of oxides to other materials with large thermal expansion mismatch.

This work is partly supported by International Joint Research Program for Innovative Energy Technology, METI.

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