The architecture of disk lasers exhibits significant advantages in terms of pump power and thermal management efficiency, representing a crucial development pathway for high-power and high-energy laser systems. However, bonding technology has long been a limiting factor in performance optimization. Traditional high-temperature soldering leads to solder splashing, secondary melting damage, and high thermal resistance, while direct bonding is difficult to implement widely due to challenges like lattice mismatch and stringent surface requirements. In comparison, the interlayer bonding offers a promising low-temperature alternative. Therefore, this study systematically investigates the influence of bonding interlayer thickness on the mechanical properties of Yb∶YAG/SiC disk laser device to optimize the bonding process for high-power disk lasers. It also explores how the interlayer thickness, precisely controlled by the spin speed, affects both the bonding strength and bonding stress. Establishing this quantitative relationship is essential for developing kilowatt-class disk lasers that achieve minimal thermal resistance along with maximum structural integrity.

Following the process flowchart illustrated in Fig. 2, bonding experiments were conducted using SiC wafers with a diameter of 2.0 cm and a thickness of 1.5 mm as heat sinks, and Yb∶YAG disks with a diameter of 1.5 cm and a thickness of 0.15 mm (doped with 8% atomic percent of Yb) as the laser gain medium. During the process, the spin speed was adjusted to control the thickness of the bonding interlayer, which was subsequently quantified using spectroscopic ellipsometry. Spin coating was carried out for 60 s at spin speeds of 2×10³, 4×10³, 6×10³, 8×10³, and 10×10³ r/min, with three replicates for each condition. After bonding, area mapping was performed on each disk device using a confocal micro-Raman spectrometer to obtain the two-dimensional (2D) stress distribution. The shift in the 261 cm^-1 T_2g peak was utilized to assess the average interface stress. Finally, tensile pull tests were performed using a universal testing machine to evaluate the bonding strength of the Yb∶YAG/SiC disk devices fabricated under each spin speed condition.

With a fixed spin-coating duration of 60 s, the average thickness of the bonding interlayer decreases monotonically from 5.54 to 1.06 μm as the spin speed increases from 2×10³ to 10×10³ r/min (Fig. 4). However, the rate of reduction significantly diminishes once the speed exceeds 6×10³ r/min. This phenomenon can be attributed to the combined effects of centrifugal force, shear-thinning behavior, and solvent evaporation: initially, centrifugal force is the dominant mechanism driving film thinning; as the interlayer becomes thinner, shear-thinning behavior and enhanced solvent evaporation become predominant phenomena. By correlating the measured interlayer thickness with mechanical performance: bonding stress (Fig.8) and bonding strength (Fig. 10), a coupling relationship of spin speed, bonding strength, and bonding stress is established. At a spin speed of 6×10³ r/min, the interlayer stabilizes at an average thickness of 1.86 μm, corresponding to the maximum bonding strength of 18.74 MPa and the bonding stress of 262.15 MPa. This moderate spin speed facilitates uniform layer formation with minimal defects. The resulting interlayer thickness is sufficiently substantial to ensure effective mechanical interlocking, while simultaneously thin enough to mitigate thermal expansion mismatch-induced stress.

This study systematically explores the influence of spin speed on interlayer thickness and its subsequent effect on the mechanical performance of Yb∶YAG/SiC disk laser gain devices. Experimental results demonstrate that a spin speed of 6×10³ r/min produces an interlayer thickness of 1.86 μm, achieving the highest bonding strength of 18.74 MPa and a bonding stress of 262.15 MPa. However, when the spin speed is increased to 8×103 r/min or higher, the interlayer thickness decreases to no more than 1.42 μm, leading to excessive stress accumulation at the interface and a heightened risk of bonding failure. These findings establish an empirical basis for optimizing spin-coating parameters for Yb∶YAG/SiC interlayer bonding and provide valuable insights for improving the mechanical reliability of high-power disk lasers.