355 nm diode-pumped solid-state (DPSS) ultraviolet (UV) laser-induced surface texturing on a Si substrate is reported in this paper. Surface patterns consisting of miro-size pits were produced by direct laser ablation. By fine-tuning key process variables such as laser beam fluence, pulse number, and beam spot separation, uniform and repeatable micropatterns with nano features have been generated on the Si surface. Under appropriate optimum laser conditions, the surface contact angle of the Si substrate was increased from 64.2 • to 97.5 • . Furthermore, it was demonstrated that the laser textured Si substrate could be used as a template for imprinting. After imprinting the laser-induced pattern onto the sol-gel film, the sol-gel film surface contact angle was increased by nearly 30 • up to 138 • due to two-tier rough surfaces.
A single step direct picosecond laser texturing process was demonstrated to be able to obtain a superhydrophobic surface on a nickel substrate, a key material for mold fabrication in the manufacture of various devices, including polymeric microfluidic devices. A two-scale hierarchical surface structure of regular 2D array micro-bumps with nano-ripples was produced on a nickel surface. The laser textured surface initially showed superhydrophilicity with almost complete wetting of the structured surface just after laser treatment, then quickly changed to nearly superhydrophobic with a water contact angle (WCA) of 140° in less than 1 d, and finally became superhydrophobic with a WCA of more than 150° and a contact angle hysteresis (CAH) of less than 5°. The mechanism involved in the process is discussed in terms of surface morphology and surface chemistry. The ultra-fast laser induced NiO catalytic effect was thought to play a key role in modifying the surface chemistry so as to lower the surface energy. The developed process has the potential to improve the performance of nickel mold in the fabrication of microfluidic devices.
Some recent research in laser-induced surface ripple structures, surface ablation, surface colouration, and their potential industrial applications are discussed in this article. Both wavelength and sub-wavelength periodic surface structures were observed on semiconductors such as InP and GaN/sapphire surfaces after irradiation of femtosecond laser pulses. The orientation of the periodic structures was dependant on the laser beam polarization, and the period was dependent on the incident laser fluence. Such surface periodic structures may find applications in controlling surface hydrophobic and hydrophilic properties. Excimer laser ablation of epoxy compounds from wafer level chip size packages was found to be a feasible method to expose the micron-size Au bumps for solder reflow, which is a critical process in manufacturing portable electronic products. Studies on controlled surface oxidation by laser pulses showed that a range of colours can be achieved on a stainless steel surface. Selective Cr oxidation and iron oxides were detected. The appearance of colour is the result of the light constructive interference of the transparent oxide layer and is determined by the oxide thickness and the refractive index. The potential industrial applications of the techniques are discussed. This paper discusses three areas of laser surface processing: laser-induced surface ripples, laser removal of the mould compound in microelectronics, and oxidation and analysis in laser colour marking. Potential industrial applications are discussed.
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