【Member News】Preparation of Wafer Grade β-Ga₂O₃/SiC Heterostructure Integrated Material by High Temperature Hydrophilic Bonding and Ion Beam Stripping

Introduction

  The XOI Research Team of Heterogeneous Integration, Shanghai Institute of Microsystems and Information Technology, Chinese Academy of Sciences, adopted the high temperature hydrophilic bonding combined with ion beam stripping technology under atmospheric conditions to successfully integrate the 2-inch high quality (- 201) β-Ga₂O₃ single crystal film to a substrate of 4H-SiC. The research team  systematically studied the evolution, of which hydrogen injection β-Ga₂O₃ bubbles on the surface, and the pressure change inside the bubbles during this process. The relevant research results was published online in the journal Science China Materials sponsored by the Chinese Academy of Sciences and the National Natural Science Foundation, entitled “Wafer-scale single-crystalline β-Ga₂O₃ thin film on SiC substrate by ion-cutting technique with hydrophilic wafer bonding at elevated temperature”. The co-first authors of the thesis are Shen Zhenghao, a doctoral student of the Shanghai Institute of Microsystems, Chinese Academy of Sciences, and Xu Wenhui, a postdoctoral fellow. The corresponding authors of the paper are researcher Ou Xin and researcher You Tiangui of the XOI Research Group of Heterogeneous Integration, Shanghai Institute of Microsystems, Chinese Academy of Sciences, and professor Han Genquan of Xi'an University of Electronic Science and Technology.

Research background

  β-Ga₂O₃ is one of ultra-wide band-gap semiconductors with the most significant to practical application, whose band gap width is up to~4.9 eV. Thanks to the ultra-high breakdown field strength of~8 MV/cm, good intrinsic electron mobility of 300 cm²/V · s, and low cost and high quality, the feasibility of β-Ga₂O₃ wafer has great application potential in the next generation of power devices. However, the thermal conductivity of β-Ga₂O₃ is very low, only 9.9-29.0 W/m · K, which will bring serious self-heating effect, leading to decreasing the stability of β-Ga₂O₃ power devices during operating at a high power. Therefore, effective thermal management is important for β- Ga₂O₃ power device. One of the ideal solutions is to heterogeneously integrate β- Ga₂O₃ thin film with high thermal conductivity substrates such as SiC (340 W/m · K) and diamond (2200 W/m · K), thus improving heat dissipation capacity of β-Ga₂O₃ power device. However, owing to the large lattice mismatch and thermal mismatch, the quality of β-Ga₂O₃ thin film, whose heteroepitaxial growth is on SiC and diamond substrates, is difficult to meet the requirements of preparing high-performance power devices. In contrast, wafer bonding technology can break through the limitation of material lattice mismatch, which is a possible realization to integrate β-Ga₂O₃ with heterogeneous substrates.Through the combination of surface activated bonding (SAB) and ion beam stripping technology, we have been able to transferred β-Ga₂O₃ film to SiC substrate. Although SAB technology has been implemented high-strength bonding between β-Ga₂O₃ and SiC substrate, and single crystal quality β-Ga₂O₃ thin film transfer, but the bonding conditions of ultra-high vacuum limit the wide application of this technology. In addition, the rapid Ar ion bombardment in the SAB process will lead to the formation of defects on the bonding interface. Compared with SAB technology, hydrophilic bonding is a more practical scheme that can bond heterogeneous wafers under atmospheric conditions, and this technology has little damage to the bonding interface. However, due to the mismatch of thermal expansion coefficient between β-Ga₂O₃ and SiC, and the thermal stress of heterogeneous interface caused by the high temperature annealing process of ion beam stripping will destroy the bonding pair of β-Ga₂O₃/SiC hydrophilic bonding at room temperature. Therefore, it is necessary to improve the conventional hydrophilic bonding at room temperature to realize the β-Ga₂O₃ film transfer of high quality through ion beam stripping technology. In addition, the surface bubbling process caused by hydrogen implantation in β-Ga₂O₃ has not been well clarified. Analyzing this process is important for understanding the physical mechanism of ion beam stripping β-Ga₂O₃ film.

Highlights of the Research

  By the high temperature hydrophilic bonding combined with ion beam stripping technology, the research team successfully integrated 2 inches of high quality (- 201) β-Ga₂O₃ single crystal film to 4H-SiC substrate (Fig. 1a and b). To understand the physical mechanism of ion beam stripping β-Ga₂O₃ thin film, the research team systematically studied the evolution process of bubbles on the surface of β-Ga₂O₃, as well as the pressure change inside the bubbles during this process (see Fig. 1c, d and e).

Figure 1. (a) β- High temperature hydrophilic bonding diagram of Ga₂O₃ and 4H-SiC wafer; (b) 2 inches transferred to 4-inch SiC substrate by ion beam stripping technology β- Ga₂O₃ thin film; (c) H implanted under optical microscope β- The surface morphology of Ga₂O₃ after annealing at different temperatures; (d) Physical process of surface blistering; (e) Pressure in bubbles at the same bubble height Δ P and stress in the bubbling layer σ 0 changes with bubble diameter.

  The finite element simulation is used to predict the appropriate bonding temperature. The high temperature hydrophilic bonding of β-Ga₂O₃ and 4H-SiC wafers at 96 ℃ effectively reduces the thermal stress on the heterogeneous interface during the ion beam stripping process, and prevents the debonding of the β-Ga₂O₃/4H-SiC bonding pair to realize the transfer of the film (Fig. 2a and b). Final β-Ga₂O₃ thin films have excellent quality of single crystal, and the FWHM of diffraction peak is 79.2 arcsec. After chemical-mechanical polishing of the film, an extremely smooth surface was obtained with a RMS roughness only 0.1 nm (in Fig. 2c, d, e and f).

Figure 2. (a) When annealing at 300 ℃ β- Thermal stress distribution at the edge of Ga₂O₃/4H-SiC heterostructure interface; (b) The maximum thermal stress of finite element simulation changes with annealing temperature and bonding temperature; (c)  β- Ga₂O₃ bulk single crystal and its properties before and after annealing at 900 ℃ β- XRD curve of Ga₂O₃ thin film; (d) Polished β- AFM surface morphology of Ga₂O₃ film; (e), (f)  β- The longitudinal section TEM image of Ga₂O₃/4H-SiC heterogeneous interface and the Ga, Al, Si EDS scan near the bonding interface.

Summary and prospect

  In this operation, high-quality (- 201) β- Ga₂O₃ single crystal film of 2 inches was transferred to 4H-SiC substrate. After chemical-mechanical polishing of the film, a smooth surface with RMS roughness only 0.1 nm was obtained. Prepared by high temperature hydrophilic bonding combined with ion beam stripping technology, β- Ga₂O₃/4H-SiC heterostructure integrated material will provide a practical platform for  β-Ga₂O₃ power devices with high performance.

Article link：https://doi.org/10.1007/s40843-022-2187-2