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激光烧蚀是一个复杂的多物理场耦合过程,烧蚀过程中同时存在多相态物质且材料属性发生突变,为简化模型和计算,提出如下假设[10]:(a)假设液体为层流状态的不可压缩流体;(b)假设流体之间不可相互渗透;(c)假设材料各向同性,且熔沸点固定;(d)忽略等离子体的影响。
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采用仿真软件内置两相流模块建立2维有限元模型,如图 1所示。模型由两部分组成,上半部分代表加工环境条件,下半部分代表单晶硅材料。模型用于模拟单晶硅在不同加工环境中的温度场、速度场分布。
激光以边界热源方式加载单晶硅上表面位置,激光能量呈高斯分布[11],如下式所示:
$ Q_1=\frac{3 E \eta}{\pi {R_0}^2 t} \exp \left(-\frac{3 x}{{R_0}^2}\right) $
(1) 式中,E是激光单脉冲能量,η是材料对激光的吸收率,R0是激光聚焦半径,t是脉冲宽度,x是激光热源位置。
激光烧蚀过程中材料的相态变化主要通过温度来反映,纳秒激光烧蚀单晶硅材料的过程是在一个标准大气压下,且环境温度为T0=300 K的条件下进行的,激光未作用材料表面时材料各处温度均匀一致。因此在仿真软件中将单晶硅材料底面和侧面设置为绝热面,绝热边界条件为狄利克雷边界条件。材料上表面的冷却方式分别为:表面对环境辐射、自然对流和热传导。自然对流换热过程处理为“外部自然对流的向上冷却”。另一方面是因为材料蒸发造成的能量损失Q2。因此,激光烧蚀过程中能量守恒方程如下式所示[12-13]:
$ \rho c_p\left[\frac{\partial T}{\partial t}+u \cdot \nabla T\right]=\nabla \cdot(\kappa \nabla T)+Q_1-Q_2 $
(2) $ Q_2=h\left(T-T_0\right)+\varepsilon k_{\mathrm{B}}\left(T^4-T_0^4\right) $
(3) 式中,ρ是材料密度,cp是材料比定压热容,u是材料的粘度,T是不同时刻下材料的温度,κ是材料的导热系数,Q1是激光热源,Q2是激光热量损耗,h是材料的对流换热传递系数,ε是材料表面辐射系数,kB是玻尔兹曼常数。
模型采用Level-Set法对固-液-气界面进行追踪。建模过程主要为传热部分和流体流动部分,如图 2所示。模型分两部分完成,在步骤1中根据材料的熔点、沸点等参数对烧蚀过程中材料的物理状态进行判别,通过后处理去除气态物质获得烧蚀几何形状。在步骤2中,依据步骤1中材料相态变化对几何模型进行实时更新,各个网格内材料属性也根据材料的相态变化进行重新定义。
由于网格单元的密集程度会间接影响仿真精度和仿真时间,采用自由剖分三角形网格,并开启自适应网格对网格不断加密直至计算结束。为了降低在计算过程中发生雅格比矩阵翻转等计算错误,上下计算域均采取等分网格的方式。网格单元大小主要参数如下: 最大单元尺寸0.5 μm,最小单元尺寸0.1 μm,曲率因子为0.3,狭窄区域分辨区域为1,求解自由度个数为786149,网格划分结果如图 3所示。
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实验中激光器为抽运Nd∶YAG固体激光器,通过非线性光学晶体倍频,经棱镜分束获得266 nm波长激光,调Q脉冲宽度30 ns,重复频率50 Hz,单脉冲最大能量1 mJ[14]。静水环境及真空环境光路系统分别如图 4和图 5所示。
激光加工系统中,首先由激光器产生激光,通过调整电流获得所需能量,激光束经过反射镜与透镜(焦距50 mm)聚焦到工件表面。静水环境中,激光能量主要作用于水层而不利于水下材料加工,一般将激光聚焦点设置在固液结合面或以下位置[15],因此采取将工件浸没在水体下方1 mm处加工,为防止激光烧蚀引起水滴飞溅到透镜上,在水容器与透镜之间设置高透薄膜以提高实验的准确性。真空环境中,工件垂直放置于真空腔中利用前级泵和分子泵抽取真空,真空度设置为2.2 kPa。利用数字显微镜(Olympus DSX1000)和原子力显微镜(NT-MDT)对微孔形貌和表面喷溅进行检测[16]。实验材料通过商业购买的Si(P型100),材料参数如表 1所示。
表 1 单晶硅材料属性[17]
Table 1. Main properties of mono-crystalline silicon materials
material melting point/℃ density/
(g·cm-3)conductivity of heat/
(W·cm-1·K-1)boiling point /℃ Si(P-100) 1410 2.33 1.5 2355
不同环境中纳秒激光烧蚀单晶硅的物质抛出机理
Investigation on mechanism of material ejection by nanosecond laser ablation of mono-crystalline silicon under different environments
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摘要: 为了探究纳秒激光烧蚀单晶硅过程中熔融物质的喷溅过程, 对不同加工环境中纳秒激光烧蚀单晶硅的过程进行了模拟仿真及实验验证。采用Level-Set界面追踪法, 通过仿真软件建立有限元模型, 对空气、静水和真空加工环境中纳秒激光烧蚀单晶硅时物质抛出过程进行模拟仿真, 研究了不同加工环境中温度场、速度场对表面喷溅的影响; 采用波长266 nm、脉宽30 ns和频率50 Hz的单脉冲激光烧蚀单晶硅的工艺实验对仿真结果进行验证, 通过原子力显微镜和数字显微镜对烧蚀结果进行了表征。结果表明, 空气环境中, t=30 ns时熔融物质的喷溅速率达到14.1 m/s, 在微孔内部蒸汽压力的作用下, 烧蚀区域熔融物质向外喷出; 静水环境中, t=30 ns时熔融物质的喷溅速率为1.68 m/s, 远低于空气中的喷溅速率, 熔融物质快速冷却; 真空环境中, 材料在短时间内汽化, t=30 ns时熔融物质的喷溅速率最大可达18.4 m/s, 较高的喷溅速率有利于物质的抛出。加工环境对纳秒激光烧蚀单晶硅的物质抛出影响较大, 这为提高纳秒激光加工单晶硅的加工质量提供了参考。Abstract: In order to investigate the process of molten material ejection during nanosecond laser ablation of mono-crystalline silicon, simulation and experimental investigation on nanosecond laser ablation of silicon under different environments were carried out. Simulation on the material ejection during nanosecond laser ablation of silicon under air, water, and vacuum environments with simulation software was carried out based on the Level-Set method. The effects of the temperature field and velocity field on surface sputtering under different environments were investigated. Experiments on laser ablation of mono-crystalline silicon using a wavelength of 266 nm, a pulse width of 30 ns, and a frequency of 50 Hz were done to verify the simulation results. The ablation results were observed by atomic force microscopy and digital microscopy. The results show that the spatter velocity under the air environment is 14.1 m/s at t=30 ns, and the molten material ejectes the ablated holes outwards due to the action of vapor pressure in the ablated area. Under the water environment, the spatter velocity is 1.68 m/s at t=30 ns, much lower than that in air conditions. Under the vacuum environment, the material is vaporized for a short time, and the spatter velocity of the molten material is 18.4 m/s at t=30 ns. The molten materials eject productively due to the high spatter velocity. The adhesion of molten material on the surface is improved. The processing environment has a strong influence on the material ejection during the nanosecond laser ablation of silicon. The investigation provides a reference for improving the quality during nanosecond laser micro-machining.
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表 1 单晶硅材料属性[17]
Table 1. Main properties of mono-crystalline silicon materials
material melting point/℃ density/
(g·cm-3)conductivity of heat/
(W·cm-1·K-1)boiling point /℃ Si(P-100) 1410 2.33 1.5 2355 -
[1] 王佳敏, 季艳慧, 梁志勇, 等. 532 nm皮秒脉冲激光对单晶硅的损伤特性研究[J]. 中国光学, 2022, 15(2): 242-250. WANG J M, JI Y H, LIANG Zh Y, et al. Damage characteristics of a 532 nm picosecond pulse laser on monocrystalline silicon[J]. Chin-ese Optics, 2022, 15(2): 242-250(in Chinese). [2] 聂世琳, 管迎春. 紫外激光器及其在微加工中的应用[J]. 光电工程, 2017, 44(12): 1169 -1179. NIE Sh L, GUAN Y Ch. Review of UV laser and its applications in micromachining[J]. Opto-Electronic Engineering, 2017, 44(12): 1169-1179(in Chinese). [3] 卢红. 不同环境下激光与材料的相互作用[D]. 天津: 天津大学, 2017: 10-17. LU H. Interaction between laser and materials under different environmental conditions[D]. Tianjin: Tianjin University, 2017: 10-17 (in Chinese). [4] 张晋平. 飞秒激光加工高深径比微孔的研究[D]. 上海: 上海交通大学, 2015: 20-26. ZHANG J P. Research of high-aspect ratio holes drilled by femtose-cond laser[D]. Shanghai: Shanghai Jiao Tong University, 2015: 20-26(in Chinese). [5] UENO I, SHOJI M. Thermal-fluid phenomena induced by nanosecond-pulse heating of materials in water[J]. Journal of Heat Transfer, 2001, 123(6): 1123-1132. doi: 10.1115/1.1409264 [6] 周翔, 段军, 陈航, 等. 水辅助激光无重铸层钻孔Al2O3陶瓷实验研究[J]. 激光技术, 2018, 42(2): 271-275. ZHOU X, DUAN J, CHEN H, et al. Experimental study about water-assisted laser drill on Al2O3 ceramics without recast layer[J]. Laser Technology, 2018, 42(2): 271-275(in Chinese). [7] 朱帅杰, 张朝阳, 储松林, 等. 海量微孔水辅助法皮秒激光加工技术的研究及应用[J]. 中国激光, 2020, 47(3): 03020028. ZHU Sh J, ZHANG Ch Y, CHU S L, et al. Research and application of massive micropores water-assisted picosecond laser processing technology[J]. Chinese Journal of Lasers, 2020, 47(3): 03020028 (in Chinese). [8] DOU H Q, YAO C Z, LIU H, et al. Femtosecond laser ablation of Al-Mg alloy in vacuum and air[J]. Applied Surface Science, 2018, 447(31): 388-392. [9] WANG Q W, YE Y C, PENG Y, et al. Friction and cutting characteristics of micro-textured diamond tools fabricated with femtosecond laser[J]. Tribology International, 2020, 154: 106720. [10] 王卫江, 贾凯, 房瑞娜, 等. 激光诱导等离子体辐射特性的研究综述[J]. 激光技术, 2022, 46(4): 499-510. WANG W J, JIA K, FANG R N, et al. Review on radiation features of laser-induced plasma[J]. Laser Technology, 2022, 46(4): 499-510(in Chinese). [11] WANG X, QIN Y, WANG B, et al. Numerical and experimental study of the thermal stress of silicon induced by a millisecond laser[J]. Applied Optics, 2011, 50 (21) : 3725-3732. doi: 10.1364/AO.50.003725 [12] 张廷忠. 毫秒激光打孔过程熔融喷溅、重铸层和微裂纹形成机理研究[D]. 南京: 南京理工大学, 2017: 31-33. ZHANG T Zh. Study on the mechanism of melt ejection recast, layer and micro crack formation in millisecond laser drilling[D]. Nanjing: Nanjing University of Science and Technology, 2017: 31-33(in Chinese). [13] 钟发成, 吕雪明, 李佳桂, 等. 组合激光对单晶硅热作用的数值分析[J]. 激光技术, 2017, 41(5): 637-643. ZHONG F Ch, LV X M, LI J G, et al. Numerical analysis of thermal effect of the combined laser on single crystal silicon polymers irradiated by high power laser[J]. Laser Technology, 2017, 41(5): 637-643(in Chinese). [14] 齐立涛, 李雪, 刘亚升. 单脉冲266 nm固体激光烧蚀蓝宝石的工艺和机理[J]. 黑龙江科技大学学报, 2020, 30(4): 416-421. QI L T, LI X, LIU Y Sh. Process and mechanism for sapphire ablation by singlepulse 266 nm solid-state laser[J]. Journal of Heilongjiang University of Science and Technology, 2020, 30(4): 416-421 (in Chinese). [15] 曹州权, 谢小柱, 陈蔚芳, 等. 液体辅助激光加工过程中压力检测及应用研究进展[J]. 光电工程, 2017, 44(4): 381-392. CAO Zh Q, XIE X Zh, CHEN W F, et al. Research progress of pressure detection and applications in liquid-assisted laser machining[J]. Opto-Electronic Engineer, 2017, 44(4): 381-392(in Chin-ese). [16] 齐立涛, 刘凤聪. 266 nm纳秒固体激光在CH薄膜上打孔的工艺实验研究[J]. 激光技术, 2022, 46(6): 767-772. QI L T, LIU F C. Experimental investigation on 266nm nanosecond solid-state laser drilling on CH film[J]. Laser Technology, 2022, 46(6): 767-772(in Chinese). [17] 鲍家定. 水层特性对水下激光划切单晶硅质量的影响研究[D]. 桂林: 桂林电子科技大学, 2021: 28-34. BAO J D. Study on the influence of water layer characteristics on quality of under water laser cutting monocrystalline silicon[D]. Gui-lin: Guilin University of Electronic Technology, 2021: 28-34(in Ch-inese). [18] 彭根琛. 铝合金真空激光焊接特性及能量传输机理研究[D]. 哈尔滨: 哈尔滨工业大学, 2020: 20-25. PENG G Ch. Research on characteristics and energy transfer mechanism of laser welding under vacuum for aluminum alloys[D]. Harbin: Harbin Institute of Technology, 2020: 20-25 (in Chinese). [19] 张潇允, 张巍, 夏盛强, 等. 高功率激光辐照CFRP的温度场和应力场的数值分析[J]. 激光技术, 2021, 45(5): 636-641. ZHANG X Y, ZHANG W, XIA Sh Q, et al. Study on numerical analysis of temperature field and stress field of carbon fiber reinforced polymers irradiated by high power laser[J]. Laser Technology, 2021, 45(5): 636-641(in Chinese). [20] LI G L, SHI D, H H S, et al. Research on the mechanism of laser drilling alumina ceramics in shallow water[J]. The International Journal of Advanced Manufacturing Technology, 2022, 118(11): 3631-3639.