飞秒激光加工CVD单晶金刚石的烧蚀特征和机理研究

Ablation Characteristics and Material Removal Mechanism of CVD Single Crystal Diamond Under Femtosecond Laser Irradiation

为探究飞秒激光烧蚀金刚石的烧蚀特征及机理,进行了飞秒激光加工CVD单晶金刚石实验及温度场仿真模拟研究。研究了飞秒激光能量密度、扫描速度、扫描次数对金刚石烧蚀区内纳米结构的影响。研究表明,金刚石被加工区域出现了沿着<110>晶向的微裂纹,在微槽边缘区域形成了周期为100~230 nm的纳米条纹,微槽中心区域形成了周期为460~640 nm的纳米条纹,且纳米条纹的形貌与激光加工参数密切相关。通过实验获得了金刚石的烧蚀阈值为3.20 J/cm^(2),且当激光能量密度为24.34 J/cm^(2)时,金刚石的烧蚀速率为44.8 nm/pulse,材料去除率为4.34×10^(−10)g/pulse。拉曼检测发现,微槽底部的金刚石发生了石墨化,理论计算的石墨层厚度为11.1 nm。根据不同激光能量密度下拉曼峰的频移计算飞秒激光辐照后金刚石微槽内的残余应力,当激光能量密度增加至24.34 J/cm^(2)时,残余拉应力增大至1389 MPa。温度场仿真结果表明,飞秒激光加工金刚石的材料去除主要是以金刚石升华为主,且飞秒激光辐照能量大多集中在金刚石的表层,几乎不会通过热传导扩散到金刚石内部。

Diamond has a wide range of applications in the field of precision and ultra-precision machining of hard materials,high frequency and high voltage electronic devices,chip thermal management,and precision optics due to its extremely high hardness,excellent thermal conductivity,extremely high electron mobility,and wide-band optical transparency.Furthermore,it has been demonstrated that the preparation of microstructures on diamonds can significantly enhance the performance of diamond-based devices.However,it is difficult to efficiently prepare microstructures on diamond surfaces using conventional mechanical or chemical methods.Laser processing has become an advanced method to fabricate diamond microstructures due to its low cost,simple process,non-contact machining,high flexibility and high efficiency.Currently,most studies are focused on the optimization of processing parameters and the preparation of diamond microstructures.However,little attention has been paid to the effect of laser parameters on the micro/nano-structures in the laser-ablated area,and the material removal mechanism of the femtosecond laser processing diamond is not clear.Here,in this work,the effects of laser processing parameters such as laser fluence,scanning speed,and number of scans on the microscopic morphology of diamond microgrooves were investigated.The ablation threshold,ablation rate,and material removal rate of diamond processed by femtosecond laser were further obtained.Then the phase transformation of the diamond under femtosecond laser irradiation was analyzed through Raman detection. The effect of laserfluence on the Raman spectra which was detected at the central region of the diamond microgroove wasfurther investigated. The residual stresses in the center of the diamond microgrooves under different laserfluences were calculated based on the wave number shift of the diamond peak. Finally, the temperaturefield of femtosecond laser-irradiated diamond was simulated using ANSYS finite element software, andthe removal mechanism of diamond material was analyzed. It was found that femtosecond laser ablation ofdiamond microgrooves resulted in a clean surface and debris-free edges, but micro-cracks along the 110crystal orientation appeared in the laser-machined area. Periodic nano-ripples were formed within themicrogrooves. The topographies of nano-ripples were closely dependent on the laser processingparameters. As the laser fluence increases, the order and uniformity of the nano-ripples in the microgroovefirst become better and then gradually become worse. With the increasing laser scanning speed, theperiodicity of nano-ripples in the center of the microgroove becomes larger, and the nano-ripples at theedge of the microgroove become discontinuous and uneven. As the number of laser scans increases, nanorippleswith a periodicity close to half-wavelength appear in the center of the microgroove. However, withthe continuous increase in the number of scans, a large amount of heat accumulation is imposed, leading toirregularities in the nano-ripples at the center of the microgrooves and poor uniformity of the nanoripples atthe edges of the microgrooves. The ablation threshold of CVD single-crystal diamond was experimentallycalculated to be 3.20 J/cm^(2). When the laser fluence was increased to 24.34 J/cm^(2), the ablation rate ofdiamond was 44.8 nm/pulse, the material removal rate was 4.34×10^(−10) g/pulse, and the residual tensilestress was increased to 1 389 MPa. The Raman detection revealed a very thin graphite layer at the bottomof the microgroove, and the thickness of the graphite layer was theoretically calculated to be 11.1 nm.Finally, Simulation results illustrate that the temperature at the center of the femtosecond laser beam hasfar exceeded the temperature of diamond sublimation. In addition, the femtosecond laser energy irradiatedon the diamond is mainly distributed on the diamond surface, while it conducted to the inside of thediamond is very little. The thickness of the graphite layer obtained from the simulation is very close to thetheoretical calculation.