【作者】 王文君；

【导师】 梅雪松；

【作者基本信息】 西安交通大学 ， 机械工程， 2008， 博士

【摘要】 二十多年来,飞秒超短脉冲激光技术迅速发展起来,凭借其超快时间分辨和超高峰值功率的特征,在微/纳加工、微电子学、无线电通讯、测量系统、医学和材料科学等各个领域开拓出了广泛的应用范围。飞秒激光金属微细加工技术出现于上世纪九十年代中末期,以其极小的热影响区、极高的加工分辨率等显著优势,逐渐取代了传统的机械加工技术,在微细加工方面占据着越来越重要的地位。但是,在将飞秒激光金属微加工技术真正应用到MEMS加工领域的过程中,还存在一些问题尚未得到很好地解决。其中较为常见的是加工形状单一以及如何在保证加工效率的前提下提高加工形貌两方面的问题。本论文从理论和实验出发,针对上述两类问题进行了研究。具体研究内容如下:1、利用一维双温方程数值模拟了飞秒激光与金属材料的相互作用,计算了能量传递随材料自身特性、激光参数以及深度的变化。模拟计算了由金铬膜构成的掩模母版与飞秒激光的相互作用,比较了前向烧蚀和后向烧蚀的优劣,认为飞秒激光直写加工掩膜版时应选用前向烧蚀,实现高加工效率和高加工精度。建立二维双温方程,计算了能量沿材料径向的传递和径向热影响区,发现径向的热扩散不能忽略。2、在双温方程理论计算的基础上,建立飞秒激光加工实验系统,实验研究了基本加工参数(能量密度、脉冲数、扫描速度)对加工尺寸的影响及其局限性,提出一种多特征尺寸微结构图形高效加工的工艺方法,改变加工表面和焦平面之间的相对距离(即相对焦点位置)和能量密度实现从纳米到微米不同特征尺寸图形的加工。该方法避免了加工过程中频繁更换数值孔径和不同特征图形之间的多次缝合和叠层,提高了飞秒激光加工的效率和整体图形的加工精度,有利于实现自动和高效加工。最后,在优化参数精确控制尺寸的基础上,加工出了纳米级微光栅。3、提出一种飞秒激光多脉冲烧蚀的理论模型,以双温方程和对数烧蚀为基础,利用激光空间传播特性确定烧蚀过程中的能量密度分布和烧蚀方向,实现飞秒激光加工形状的准确预测。该模型的优点在于考虑了烧蚀过程中的束斑变化特性,克服了传统预测方法(简单累加)的缺陷,可准确预测任意加工参数下的烧蚀形状。在孔形仿真的基础上,提出扫描加工模型,针对微槽结构的扫描加工进行仿真,能够通过预测获得更准确的加工深度,拓展了模型的应用范围。该模型能够为飞秒激光微纳制造过程中的参数选择提供依据。4、全面揭示了飞秒激光金属加工中微结构的形状特征。在讨论了基本加工参数对加工形状的影响的基础上,从实验和理论上证实了激光光束特性是决定微结构形状的重要参数。研究发现,采用会聚光束、发散光束和平行光束进行烧蚀,能够分别形成V型、U型和抛物线型微结构;提出一种采用双层材料结构和两步倾斜烧蚀实现微结构形状精确控制的加工工艺方法,充分利用飞秒激光烧蚀的材料选择性,采用烧蚀阈值不同的双层材料结构,避免了激光加工中宽度和深度的相互关联,使不同宽度的微结构都具有相同深度和平整底面;再结合能够在更大范围内改变微结构侧壁角的两步倾斜烧蚀工艺,突破了改变激光参数调整形状的局限。该研究结果将有效扩展飞秒激光烧蚀技术在微纳制造领域的应用。另外,通过对工件与光束间三维运动轨迹的优化,也能够有效提高加工深度以及控制微结构形状。为满足多种复杂三维形状微结构的加工要求,可以将各种形状控制工艺相结合,制定综合加工工艺。5、系统讨论了飞秒激光金属烧蚀的表面形貌特征及其控制方法。首先,研究加工参数和激光非线性传播对加工形貌的影响,发现能量密度是决定加工形貌质量的主要因素,而扫描速度和扫描次数的合理选择,也能够显著提高加工质量;光丝效应会导致激光束的锥角辐射,产生波浪形底面。在此基础上,全面分析了焦点位置对形貌的影响,提出了利用焦点位置提升加工形貌的工艺方法。另外,利用线偏振激光产生纳米级周期结构的特点,提出了采用飞秒激光制备超疏水表面的工艺方法,产生周期性“微米结构(槽宽)+纳米结构(波纹宽度)”的复合结构,满足疏水表面的要求。这些结果为研究项目的顺利完成和该领域研究工作的深入发展打下了良好的基础。此外,本文还对该技术的发展方向进行了展望。更多还原

【Abstract】 In recent years, femtosecond pulsed laser technique has developed rapidly. For its characteristics of ultrafast time resolution and ultrahigh peak power, femtosecond laser has a very wide application in the fields of micro/nano fabrication, micro electronics, wireless communication, measurement system, medicine and material science, etc. Femtosecond laser micro-fabrication of metals occurs in the 90’s of the last century. Based on the little thermal-affected zone, higher resolution and other obvious advantages, femtosecond laser fabrication technique has replaced gradually some traditional mechanical fabrication techniques and played a more and more important role in the field of micro fabrication. However, in the process of applying femtosecond laser micro fabrication technique to MEMS field, some questions have not been solved entirely yet. In these questions, the simple ablated shape and how to improve the ablation morphology in the base of high effciency are familiar. Aiming at these two questions, this dissertation does a lot of researches using theoretical and experimental methods. The detailed contents are as belows:1. The interaction between femtosecond laser and metals is simulated by one-dimension two-temperature equation, and the changes of energy transfer with inherent quality of material, laser parameters and depth are calculated. The interaction between the photomask composed of gold and chromium films and femtosecond laser is calculated, front side ablation and rear side ablation are compared, then it is found that the front side ablation should be selected to achieve high efficiency and high precision in femtosecond laser direct-writing photomask. A two-dimensions two-temperature equation is established to compute the energy tranfer and the heat affected zone along the radial direction of material. It is found that the heat transfer along the radial direction of material cannot be ignored.2. On the base of the theoretic calculation of two-temperature equations, the femtosecond laser fabrication experiment systems are established, and the effects of the elemental process parameters such as fluence, number of pulses and scan speed on the ablation size and their limitations are studied. A new process fabricating micro-structure patterns with many feature sizes is proposed, in which changing the relative distance between the sample surface and the focal plane (relative focus position) and fluence to fabricate patterns with the feature sizes from nanometer to micrometer. This process avoids the frequent replacement of focusing lenses and the oversewing and superposition among patterns and improves the fabricating efficiency of femtosecond laser and precision of the whole patterns. At last, based on the optimization of parameters and the precise control of size, the micro-gratings with the feature size of nanometers are fabricated.3. A theory modeling of femtosecond laser multi-pulse ablation is proposed on the bases of two-temperature equations and logarithmic ablation, in which the fluence distribution and the ablation direction are determined by the spatial propagation characteristic of laser beam in the ablation, this modeling can forecast the micro-structure shapes ablated by femtosecond laser exactly. This modeling involves changing characteristic of spot in the ablation, overcomes the disadvantage of simple accumulation in the traditional forecasting method, and can forecast accurately micro-structure shapes ablated with random process parameters. In the basis of the shape simulation of micro-holes, the scan fabrication modeling is proposed to forecast the shapes of micro-grooves obtained by scan fabrication. This modeling can forecast ablation depth more accurately, and extend the applied range of the modeling. This modeling can provide the evidence for the selection of parameters in the micro/nano fabrication of femtosecond laser.4. The shape characteristics of metallic micro-structures ablated by femtosecond laser are investigated completely. The effects of some elementary process parameters on the ablation shape are discussed. On this basis, it is proved from experimental and theoretical results that the laser beam characteristic is a crucial parameter determining the micro-structure shape. It is found that micro-structures with‘V’style,‘U’style and parabola style can be formed by convergent beam, divergent beam and parallel beam. A process for controlling precisely micro-structure shape is proposed, in which a structure with two-layers materials and two-steps inclination ablation are adopted. This method uses adequately the material selection characteristic of femtosecond laser, adopts two-layers materials with different ablation thresholds, cuts off the connection between the ablation width and ablation depth in laser fabrication, and makes micro-structures with different widths have the same depth and flat bottom; then combined with two-steps inclination ablation method which can adjust sidewall angles of micro-structures in a large range, this process overcomes the limitation of the adjusting shape method by changing laser parameters. This conclusion will extend availably the application of femtosecond laser ablation technique in the micro/nano manufacture field. In addition, through the optimization of 3D motion track between sample and beam, the ablation depth can be increased and the ablation shape can be controlled. In order to meet the fabrication demand of micro-structures with some kinds of complex 3D shape, above several shape control processes should be combined to establish the integrated process. 5. The surface morphology characteristic of metals ablated by femtosecond laser and its control method is discussed detailedly. Firstly, the effects of process parameters and non-linear propagation characteristic of laser on the fabrication morphology are studied, it is found that the fluence is the main factor affecting the ablation morphology, and selecting the suitable scan speed and number of overscans also can improve the fabricating quality remarkably; the filamentation occurred in or before the focus will cause conical emission of femtosecond pulses and produce the wavy bottom. On this base, the effect of focus position on morphology is analyzed fully, and the process using the focus position to improve the morphology is proposed. In addition, using the ripples with the period of nanometerthe formed by the linear polarization laser, a method for preparing the hydrophobic surface using femtosecond laser is proposed, which can produce the periodical compound structures with micron structure (the groove width) and nanometer structure (the ripple width), and meet the demand of hydrophobic surface.All these results can do help to our projects and can boost the development of this research area. We also prospect the trend of this technology in the end.更多还原