【作者】 张恒；

【导师】 陈林森；

【作者基本信息】 苏州大学 ， 光学工程， 2008， 博士

【摘要】 自激光出现以来,利用激光进行微细加工一直是该领域的研究热点。纳秒激光具有脉宽窄、功率大和高稳定性等优点,在微加工领域得到了广泛应用。本文主要开展了纳秒级脉冲激光在若干金属表面诱导和制作微结构的研究,在理论上探讨了诱导的物理机理,完成了相关的实验和测试,并将该项技术应用于背光模组导光板模仁和数字衍射光变图像的制作,取得了产业化成果。本论文研究内容及主要成果归纳如下:1.激光与材料的作用机理与过程非常复杂,可能会引发局部加热、烧蚀、光化学反应、等离子激发等物理化学过程,非常有必要在理论上对纳秒激光对材料的作用机理进行研究。基于热力学理论的热效应模型,本文对激光烧蚀金属表面过程进行了数值模拟,发现纳秒激光与金属材料相互作用过程中的温度场呈高斯型分布,并由此计算得到铜,镍,钛金属材料的热损伤阈值,进而研究了热损伤阈值、激光脉冲数目和刻蚀深度三个物理量的关系,损伤阈值与脉冲数目的关系符合反比关系,最后趋于常数;金属的烧蚀深度与激光脉冲数目近似成平方根关系。2.纳秒激光聚焦在金属表面,能够在金属表面形成烧蚀痕迹,这是纳秒激光微加工的直接原因。本文首先设计了紫外波段(波长为351nm)单光束在金属表面烧蚀微结构的实验,得到了不同激光参数下金属铜、镍、钛材料的损伤阈值。实验结果表明,高激光能量可以烧蚀出深的凹坑结构,凹坑深度的可由激光脉冲能量和脉冲数目控制。单光束实验在微机械加工,精密打孔,激光打标等方面具有很大的应用前景。3.单光束激光在金属表面烧蚀微结构的精细程度受瑞利判据的限制,无法达到亚微米级的加工精度。双光束干涉法能在一定程度上提高加工精度,得到更加精细的亚微米光栅结构。本文开展了双光束干涉方法在金属铜、镍和钛表面刻蚀光栅的实验研究,得到了亚微米量级线宽的光栅;对激光能量、脉冲数目、光束口径等工艺参数对光栅形貌和衍射效率等的影响做了实验研究。光栅槽深达到200~400nm、衍射效率可以达到14%,双光束干涉法直接作用于金属特别适合于制作数字衍射光变图像。4.导光板是平板显示背光模组里的重要组成元件,将点或线光源转化成高亮度且均匀的面光源。传统上制作导光板方法光能利用率较低,本文利用纳秒激光在金属镍表面刻蚀的微锥形凹坑结构,通过改变其直径和深度,制作导光板模仁并结合微纳米压印技术,直接在PMMA或PC材质上进行压印,制作出高均匀度和亮度的导光板。此制作工艺已用到手机按键导光膜制作。5.在金属表面双光束干涉刻蚀光栅结构,特别适合于制作数码激光光变图像模版。这种新方法可控制光栅的槽深在200~400 nm之间,提高光栅的衍射效率。本文制作出了良好的数字衍射光变图像模版,相比于传统的LIGA工艺,减少制作流程,图像的衍射效果接近。为数字衍射光变图像存储与制版提供了一种新方法。本论文的工作为微锥形结构在激光打标、模仁制作、微机械制作等相关光学领域的应用发展提供了必要的理论基础和技术储备,同时,微光栅结构在立体光刻、数字衍射光变图像的制作及视读防伪器件加工具有很高的研究价值和广泛的应用前景。更多还原

【Abstract】 Since the first appearance in 1960s, the laser fabrication has been a hot topic in the field of microfabrication. With the characteristics of high stability, high energy and narrow band, nanosecond laser has found wide applications in microfabrication processing. In this thesis, fabrication of microstructures on metals induced by nanosecond laser is presented. The physical mechanism of these phenomena and the related experimental results are demonstrated. As cases of application, Ni-based imprinting template for light guide plate used in backlight system and the digital diffraction optical variable imaging device are fabricated, which have been in mass production for market.Generally following items are included in this thesis:1. Due to local heating, ablation and excitation of plasma etc, the inter-reaction between the incident laser beam and the matters is very complicated. It is necessary to investigate the physical mechanism of the physical and chemical procedure. Based on the thermal effect model of thermodynamic theory, numerical simulation on the ablating of metal using nanosecond laser is carried out. It can be found that the special distribution of temperature field is Guassian-like and the damage threshold of metal Cu, Ni and Ti is obtained theorecitcaly. Futhermore, the relationship of damage threshold, number of laser pulse and etching depth is discussed. It can be shown that the damage threshold is inverse to and the etching depth is proportional to the number of laser pulse.2. Nanosecond pulsed laser ablation is especially appropriate for metal ablation as high power level for short period of time enabling the creation of deep trenches. A single beam experimental setup has been built to ablate metal with operating wavelength of 351nm in the UV range. The damage threshold of Cu, Ni and Ti are achieved experimentally. The experimental results also show that the trenches can be created by high laser energy and the trench’s depth is decided by the incident energy and the number of pulses. Submicron focal spots in this setup enable the creation of very small feature which is useful in micromachining, holes drilling, laser marking, etc..3. The resolution of single-beam pulsed laser ablation is determined by the Rayleigh criterion. By interfering two pulsed laser beams to produce grating, the resolution of the proposed approach can be enhanced by a factor of more than 2 times. To implement two beams interference, it is important to find how the incident energy, the number of laser pulse, the aperture of lens, etc can have influence on surface profile and diffraction efficiency of microstructures. In our experiment, the grating depth is about 200~400nm and the diffraction efficiency measured is 14%. This approach is suitable for fabricating digital diffraction optical variable imaging device (DOVID).4. As an indispensable component in backlight system, the light guide plate has the function of changing the irradiation pattern of point/line source to uniform lighting. The conventional method of fabricating LGP suffers from the deficiency of low efficiency. In our study, nanosecond laser is used as an ablation tool to drill substrate (Ni) to creat the template for the following nanoimprinting lithography process. The fabricated LGP has the advantage of good uniformity and high brightness, whose substrate is PC or PMMA. This novel type of LGP has already been in mass production for mobile telephone.5. The method of two beam interference on metal is suitable for fabricating digital diffraction optical variable imaging device (DOVID). This new and novel approach can fabricate gratings with high diffraction efficiency and the grating depth can vary from 200 to 400nm. Compared with the conventional LIGA process, the proposed method is simple and provides a new fabrication method of making DOVID and optical devices used in optical storage.In summary, the work presented will lay solid foundation for the laser marking, fabrication of imprint template, micromachine, theoretically and experimentally. It can be predicted that the method will find wide applications in fabrication of DOVID and readable security authentication solutions.更多还原