Surface Activated Bonding of LiNbO3 and GaN at Room Temperature
Authors: Takigawa Ryo , Higurashi Eiji , Asano Tanemasa 
Published: February 2020

Lithium niobate (LiNbO3: LN) is a unique ferroelectric material with excellent electro-optical (EO), non-linear optical (NLO), acousto-optical, piezo-electrical, and pyro-electrical characteristics, in addition to a wide transmission curve ranging from the ultraviolet to the mid-infrared. Thus, it is widely utilized for numerous applications, including ultra-fast optical modulators, wavelength convertors, surface acoustic wave filter, and pyro-imaging devices. At the same time, Gallium Nitride (GaN) film is a direct widegap semiconductor commonly used for light emitting diode light, power devices, and radio frequency components. Therefore, wafer bonding of LN and GaN has the potential for creating a new platform capable of realizing multi-functional micodevices that can fully exploit the unique properties mentioned above.
Generally, a GaN sample is prepared as a structure of an as-grown GaN film on a sapphire. The coefficient thermal expansion (CTE) of LN is 14.4 (a-, b-axes)-7.5 (c-axis) × 10-6/K at room temperature, whereas that of GaN is 5.59 (a-axis)-3,17 (c-axis) × 10-6/K and that of sapphire is 5 × 10-6/K. Therefore, it is desirable for low-temperature bonding of LN and as-grown GaN film for overcoming the large CTE mismatch. Conventional plasma activated bonding requires relatively high temperature annealing process at above 200 °C to achieve a high bond strength. The high-temperature process causes a large thermal stress, which may often generate cracks.
Surface activated bonding (SAB) is a powerful method for low temperature bonding of dissimilar materials. By a surface activation process with argon (Ar) fast atom beam (FAB) or ion beam bombardment, a high bond strength can be achieved at room temperature due to the strong attractive forces between atomically cleaned surfaces. Recently, room-temperature bonding of a GaN and a Si has been demonstrated using ion beam bombardment [F. Mu et al., Applied Surface Science, 2017]. It is interesting to apply the SAB to bond a LN wafer and a GaN. To date, little work has focused on the application of the SAB method to the direct bonding of LN and GaN.
In this paper, we report surface activated bonding of a LN and a GaN at room temperature. In this experiment, we prepared a commercially available 3-inch LN (Z-cut) wafers and a 2 inch as-grown GaN wafer which is a 2-μm-thick GaN epitaxial layer grown on a 430-μm-thick sapphire wafer. In direct bonding, sub-nm RMS roughness of the bonding surface is typically required to achieve strong bonding at room temperature. Using atomic force microscopy, the measured RMS roughness of LN and GaN epitaxial layer was approximately 0.1 nm and 0.3 nm, respectively. For surface activation process, Ar FAB bombardment on each bonding surface was performed. Figure 1 (a) shows a schematic diagram of our SAB process. When the background pressure fell below 1×10-5 Pa, the two wafers were activated by Ar FAB bombardment. After that, the surfaces of LN and GaN epitaxial layer were brought into contact with an applied load at room temperature. The Ar FAB bombardment time was 60 s, and the applied load was approximately 10 MPa. Figure 1 (b) shows the photograph of the bonded LN and GaN wafers at room temperature. As shown, LN/GaN hybrid structure without generating serious cracks was demonstrated in wafer scale. The void was formed by particle contaminations and non-bonding at the peripheral area may be responsible for the thickness distribution of the GaN epitaxial layer. In this presentation, we will discuss the bond quality and the bonding interface state in details.
The authors would like to acknowledge Mitsubishi Heavy Industries Machine Tool Co., Ltd. for assisting with surface-activated bonding experiment. This study was supported in part by JSPS KAKENHI Grant Number JP17H04925.

Figure 1