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Abstract
To improve the thermal performance of Nd:YAG lasers, a Nd:YAG laser crystal is bonded on a SiC wafer by atomic diffusion bonding (ADB) via a Mo/Au nano-interlayer at room temperature. In addition, a two-dimensional model of the Nd:YAG-SiC with a Mo/Au nano-interlayer is developed to investigate the thermal aberration and temperature distribution inside the Nd:YAG. The result shows that the bonded Nd:YAG-SiC exhibits an extremely low voidage, along with a 106-nm-thick metal interlayer. The simulation reveals that the Nd:YAG-SiC has a maximum temperature of 393.3 K with a reduction of 28.5 K and a less thermal aberration near the axis compared to the Nd:YAG-CuW at a pump power density of 5 kW/cm2.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Nd:YAG laser has a high signal-to-noise ratio, simple structure and high operational stability, which providing prospects for application in the fields of industrial processing and scientific research [1–4]. However, the internal temperature inside Nd:YAG rises rapidly under continuous pumping and leads a large thermal aberration, which affecting the laser output quality [5]. At a high pumping power, the thermal aberration inside laser crystal becomes more serious [6]. It well known that optical path difference (OPD) is used to describe the thermal aberration. In order to reduce the thermal aberration, laser crystal is evolving from rod to thin-disk shape [7,8]. Additionally, the heat sink is commonly bonded to the bottom surface of the thin-disk laser crystal for further improving the heat dissipation. CuW is normally soldered to the thin-disk laser crystal as a heat sink by indium solder [9]. The effects of thin-disk thickness [10] and pump beam distribution [11] on the thermal aberrations of the laser system with CuW heat sink are investigated by numerical simulation and experimental measurement. Compared with CuW heat sink, SiC heat sink has better thermal stability and higher thermal conductivity, which can significantly enhance the reliability of laser system [12]. Unfortunately, it is reported that the bonding technique for the integration of heat sink and laser crystal by indium solder will lead voids at bonding interface, which results the high aberration inside laser crystal [13]. Therefore, a high-quality bonding technique for composite Nd:YAG-SiC with low thermal aberration is critical issues for the Nd:YAG laser system.
Nowadays, there are two mainstream bonding techniques including atomic diffusion bonding (ADB) and surface-activated bonding (SAB) [14–17]. SAB technique uses fast ions, atoms and plasma to activate the sample surface in an ultra-high vacuum cavity. During the activation process, a large number of dangling bonds generated on the sample surface, and then samples are bonded together by the mechanical pressure [18,19]. However, SAB technique requires a ultra-low sample surface roughness (<0.5 nm) and ultra-high vacuum cavity (<10−5 Pa) during the surface activation. In contrast, ADB technique utilizes the diffusion and ductility of metal film to bond materials together and achieve bonding with the surface roughness exceeding 1 nm [16,20]. Furthermore, ADB technique requires the deposition of a multilayer film including a buffer layer and a bonding layer on the sample surface. Due to the high self-diffusion coefficient and stable properties of gold, Au film is normally selected as the bonding layer [21,22]. Ti and Cr are previously reported to be used as buffer layers, but the bonding quality by Ti/Au or Cr/Au interlayer is degraded after high temperature treatment [16,21].
In this work, in order to reduce the thermal aberration inside Nd:YAG, Mo/Au nano-interlayer is used to bond Nd:YAG and SiC at room-temperature. The surface roughness of the samples before and after Mo/Au film deposition is characterized by atomic force microscopy (AFM), and the bonding interface quality is systematically analyzed by scanning acoustic microscope (SAM) and scanning electron microscopy (SEM). In addition, a two-dimensional numerical model of the composite Nd:YAG structure is developed for investigating the temperature and OPD distributions of the composite Nd:YAG with the heat sink of CuW and SiC.
2. Materials and methods
The 4H-SiC wafer (10 mm × 10 mm × 0.5 mm) and Nd:YAG (φ10 mm) crystal are selected as bonding samples. The samples are all cleaned by acetone, absolute ethanol and deionized water and quickly dried for removing dust particles, respectively. The bonding process flow of Nd:YAG-SiC by ADB technique via a Mo/Au nano-interlayer is shown in Fig. 1. Firstly, the sample surface is pre-treated by Ar plasma for removing the oxide layer. After that, Mo/Au nano-films are deposited on the surface of samples by RF magnetron sputtering system, respectively. Subsequently, the samples are bonded at room temperature under a pressure of 1 MPa. The surface roughness of the sample is characterized by AFM (Innova/Bruker). For investigating the quality of the bonding interface, the voids and nano-morphology at the bonding interface are characterized SAM (D9500/Sonoscan) and SEM (GeminiSEM500/Zeiss).
Fig. 1. Bonding Process flow of the Nd:YAG-SiC by ADB technique via a Mo/Au nano-interlayer (a) Ar plasma treatment; (b) Mo deposition; (c) Au deposition; (d) wafer bonding.
A numerical model for investigating the temperature and OPD distributions of composite Nd:YAG structure is shown in Fig. 2. The Nd:YAG is bonded to heat sink by the Mo/Au nano-interlayer, and the YAG cap is set to be in close contact with the Nd:YAG. The rear surface of the heat sink is cooled by high-speed water with the heat exchange coefficient of H0. The other surfaces of the composite structure are exposed to air, and they are set to be thermally isolated for simplifying the calculation. Moreover, the bottom edge C of the heat sink is mechanically fixed.
The structure parameters, pumping parameters and cooling parameters are shown in the Table 1.
The thermodynamic parameters of the material are shown in Table 2.
Table 1. Parameters Used for the composite structure
Table 2. Parameters Used for Model
3. Results and discussion
3.1 Surface morphology characterization of Nd:YAG and SiC
For ADB technique, it is reported that the surface roughness after film-deposition will affect the bonding quality [25]. Therefore, it is necessary to investigate the effect on the sample surface roughness before and after Mo/Au nano-film deposition. Figure 3 shows that the surface roughnesses of the samples before and after Mo/Au nano-film depositions. It can be seen that surface roughnesses of an original SiC, an original Nd:YAG, a Mo/Au nano-film deposited on the SiC and a Mo/Au nano-film deposited on the Nd:YAG are 1.47 nm, 0.35 nm, 1.63 nm and 0.58 nm, respectively. The surface roughnesses of the SiC wafer and the Nd:YAG crystal increase by 0.23 nm and 0.16 nm after Mo/Au nano-film depositions. Although the surface roughnesses of the Nd:YAG and the SiC have increased after Mo/Au nano-film depositions, the increases are not significant due to the low surfaces roughnesses of the samples original state. In addition, it is observed that the Mo/Au nano-film on the sample surface is flat and free of stains, due to the fact that the film deposition by magnetron sputtering is proceeded under high vacuum (5.0 × 10−3 Pa). AFM indicates that the surface of the samples still maintains the nanoscale-smooth surface after Mo/Au nano-films deposition, which are beneficial for high-quality bonding.
Fig. 3. AFM images of (a) an original SiC, (b) a Mo/Au film deposited on SiC wafer, (c) an original Nd:YAG, (d) a Mo/Au film deposited on Nd:YAG.
3.2 Bonding interface analysis by SAM and SEM
The optical image of the bonded sample is shown in Fig. 4(a). The bonded Nd:YAG-SiC exhibits no fracture at 1 MPa mechanical pressure. In addition, the bonded sample is almost opaque as seen from top to view due to the deposition of the Mo/Au nano-film, so the voids at bonding interface cannot be visualized from the optical image. The C-SAM image of the bonded sample is presented in Fig. 4(b). It can be seen that the bonded sample with extremely low bonding voidage is obtained based on the Mo/Au nano-interlayer at room temperature. The Nd:YAG and SiC are almost completely bonded except for a void at the edge, probably due to the contaminants at the edge of bonding surface.
The cross-section SEM image of the bonding interface of Nd:YAG-SiC is shown in Fig. 5. Obviously, the multilayer structure of Nd:YAG-SiC is clearly visible and the nano-interlayer of Mo/Au is continuous. In addition, the thickness of Mo/Au nano-interlayer is 106 nm and without nanoscale defects. As can be seen from Figs. 5(b)-(d), the elements of bonding interface show the layered distribution, and the C and Si elements are mainly distributed on the lower side of the bonding interface. As presented in Fig. 5(e), the Al element are distributed on both sides of the bonding interface, probably owing to interference from the sample stage. In addition, the distribution of O element is mainly in the upper side of the bonding interface. In Figs. 5(g)-(h), the distributions of Mo and Au elements are mainly in the bonding layer. However, due to a long scan mapping time, a little amount of Au and Mo are also detected on both sides of the bonding interface.
Fig. 5. (a) Cross-sectional SEM image of the Nd:YAG-SiC bonding interface. (b) EDS mapping sum of C, Si, Al, O, Au and Mo. (c), (d), (e), (f), (g) and (h) are element mapping of C, Si, Al, O, Au and Mo, respectively.
The above experimental results reveal that the Nd:YAG-SiC samples with high bonding quality can be obtained based on the ADB technique through the Mo/Au nano-interlayer at room temperature.
3.3 Temperature distribution inside the Nd:YAG on the SiC and CuW heat sinks
A two-dimensional finite element by multi-physics coupling is used to calculate the temperature distributions of the composite Nd:YAG on the SiC and CuW heat sinks. Figure 6(a) shows the radial temperature distribution inside Nd:YAG on the CuW heat sink. Since the heat is generated inside the Nd:YAG and dissipated through the heat sink, so the temperature distribution inside the laser crystal exists a high temperature on the upper surface and a low temperature on the lower surface. The radial temperature distribution inside the Nd:YAG is similar to a super-Gaussian function, which is due to the super-Gaussian distribution of the pump beam in space. The radial temperature distribution of the Nd:YAG on SiC heat sink is shown in Fig. 6(b). The maximum internal temperature of the Nd:YAG on SiC heat sink is 393.3 K, which is 28.5 K lower than that of the Nd:YAG on CuW heat sink. Compared to the CuW heat sink, the SiC heat sink with higher thermal conductivity can lead a significant temperature reduction inside the Nd:YAG.
Fig. 6. Radial-temperature distribution in different positions inside the Nd:YAG for different heat sink of (a) CuW heat sink, (b) SiC heat sink
3.4 Thermal aberration inside the Nd:YAG on the SiC and CuW heat sinks
The existence of a temperature gradient within the thin-disk laser crystal, resulting in thermal stress inside the disk. Therefore, the thin disk is bent like a convex mirror due to the stress, which is the major contribution to thermal lensing. The distribution of the first principal stress inside the indium-soldered Nd:YAG-CuW and the Mo/Au/Mo bonded Nd:YAG-SiC is shown in Fig. 7. It can be seen that the maximum tensile stress inside the Nd:YAG-CuW and Nd:YAG-SiC is 58.2 MPa and 35.7 MPa, respectively. Notably, at the bonding interface, the first principal stress of the Nd:YAG-SiC structure is significantly higher than that of the Nd:YAG-CuW structure, due to the softness of indium.
Fig. 7. Distribution of the first principal stress inside (a) indium-soldered Nd:YAG-CuW and (b) Mo/Au/Mo bonded Nd:YAG-SiC.
Thermal aberration can be described by the OPD, which is divided into four parts [9]:
4. Conclution
In this work, the Nd:YAG laser crystal and SiC wafer are bonded by ADB technique based on Mo/Au nano-interlayer. It has been demonstrated that there is no significant fracture on the surface of the bonded sample under mechanical bonding pressure of 1 MPa. A bonding interface with ultralow voidage is obtained, along with 106 nm Mo/Au nano-interlayer without any nano-defect. In addition, a two-dimensional numerical model is developed to investigate the thermal aberration and temperature distribution inside composite Nd:YAG structure. The simulations show that the maximum temperature inside the Nd:YAG on SiC heat sink is 393.3 K, which is 28.5 K lower than Nd:YAG on CuW heat sink. Furthermore, the Nd:YAG-SiC has less thermal aberration near the axis compared to the Nd:YAG-CuW. Therefore, by using the ADB bonding technique and investigating the thermal distribution and aberration, we provide a new idea for the design of new composite laser crystal.
Fig. 8. Total OPD and OPD components inside the Nd:YAG for different heat sink of (a) CuW heat sink, (b) SiC heat sink.
Funding
Foundation of Science and Technology on Low-Light-Level Night Vision Laboratory (614241204021703); Fundamental Research Funds for the Central Universities (xzd022020005); National Key Research and Development Program of China (2021YFB3602100).
Acknowledgments
The authors would like to thank Ms Zhang at Instrument Analysis Center of Xi’an Jiaotong University for her assistance with TEM and EDS analysis.
Disclosures
The authors declare no conflicts of interest.
Data Availability
Data underlying the results presented in this paper are not publicly available but may be obtained from the authors upon reasonable request.
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