4H-SiC Substrate for Lithium Niobate (LiNbO3) Wafer Bonding

Lithium niobate (LiNbO3, LN) crystals are widely used in surface acoustic wave (SAW) devices, acousto-optic modulators, and high-power laser sources due to their excellent electromechanical coupling coefficients, acousto-optic (AO) effects, and nonlinear optical effects. However, due to the poor thermal characteristics of these SAW filters, temperature compensation has recently attracted great interest, and some methods have been reported to solve this problem. Directly bonding SAW filter devices onto a supporting substrate with low coefficient of thermal expansion (CTE) and large Young’s modulus is an effective method to suppress the large thermal expansion of LiNbO3. According to reports, direct wafer bonding of LiNbO3 onto sapphire, glass, and silicon substrates can improve device performance.
However, the heat dissipation of SAW filters will become a major issue for future high-power applications, therefore, choosing a suitable substrate and considering heat dissipation is an important issue. Silicon carbide (SiC) is considered a suitable supporting substrate due to its low CTE, large Young’s modulus, high electrical resistivity, and high thermal conductivity. SiC is widely used as a heat dissipation substrate due to its higher thermal conductivity than Si. Therefore, combining SAW filters and SiC substrates into a single chip is a reasonable strategy to improve temperature compensation and heat dissipation performance for high-power applications. In addition, the LiNbO3/SiC structure is also very attractive for photonics applications. PAM-XIAMEN can offer 4H-SiC substrate to bond LiNbO3, building photonic integrated circuits. More substrate parameters please see table below:

1. 4H-SiC Substrate for Wafer Bonding Applications

Product	4H-N SiC, Research Grade
Diameter	4”
Type/ Dopant	N / Nitrogen
Orientation	<0001>4deg.off+/-0.5 degree
Thickness	350±25 um
MPD	＜10 cm-2
Resistivity	0.015 ~ 0.028 Ω·cm
TTV	<15um
BOW	<25um
WARP	<35um
Primary flat	(10-10)+/-5deg.
Primary flat length	32mm +/-2mm
Secondary flat	Si face up: 90deg. CW. From prime flat +/-5deg.
Secondary flat length	18mm+/-2mm
Surface Finish	DSP/Si face epi-ready with CMP
Surface Roughness	<0.5 nm

 

2. Wafer Bonding Technology – Surface Activated Bonding (SAB)

Wafer bonding is a powerful tool for manufacturing heterogeneous substrates. Traditional wafer bonding techniques include plasma activated bonding and organic adhesive bonding. The former method usually requires an annealing process to achieve the strong bonding required for practical applications. However, LiNbO3 (CTE (a, b axis): 15.4×10-6 K-1) and LiTaO3 (CTE: 16.1×10-6 K-1) crystals do not match the CTE of most semiconductors, such as Si (CTE: 2.6×10-6 K-1). Therefore, the annealing process makes it difficult to directly bond these materials as cracks or wafer warping may occur. Although the organic adhesive bonding method can achieve low-temperature bonding of dissimilar materials, the intermediate organic adhesive layer introduces a large thermal resistance due to its low thermal conductivity. Therefore, direct bonding is more desirable for achieving efficient heat dissipation in device applications.

The surface activated direct bonding (SAB) is a promising candidate for direct room temperature bonding of heterogeneous materials with significant CTE mismatch. Due to the attractive force between atomically clean smooth surfaces, surface activation processes have been used to achieve strong room temperature bonding. The SAB method has been successfully applied to room temperature bonding of LiNbO3/Si, SiC/SiC, SiC/Si, GaN/diamond for power device applications, and GaAs/SiC for optical applications. Therefore, it is believed that the SAB method is suitable for LiNbO3/SiC film wafer bonding.

3. 4H-SiC for LN Wafer Bonding by SAB

Researchers have studied the application of surface activated bonding in direct bonding of LiNbO3 and SiC. Using SAB method for negative bonding of LiNbO3 wafer to the Si surface of SiC wafer at room temperature. After placing two wafers into the bonding chamber, evacuate the chamber and activate the two wafers through Ar FAB bombardment. When the background pressure drops below 1×10-5 Pa, Ar FAB bombardment begins on the two wafers. Subsequently, the activated surfaces of LiNbO3 and SiC were in direct contact under pressure loading at room temperature. The Ar FAB bombardment time is 60 seconds, which is sufficient to remove organic pollutants on the bonding surface during the wafer bonding process flow.

Fig. 1 LiNbO3/SiC wafer bonded at room temperature

SAB successfully formed room temperature bonded LiNbO3/SiC wafer for the first time using argon fast atomic beam bombardment. Cross sectional transmission electron microscopy (TEM) observation shows a gapless bonding interface at the atomic level. The adhesive interface has a quasi amorphous layer several nanometers thick, which is believed to have been formed during the bombardment process. The resulting LiNbO3 on SiC structure can be used to achieve new configurations for various devices, including surface acoustic wave filters for high-power applications, which can meet temperature compensation and heat dissipation requirements.

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