Study Of Shape And Morphology Control In Femtosecond Laser Fabrication Of Metals

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.