Research On Fabrication Of Microstructures On Metals Induced By Nanosecond Laser And Its Applications

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.