Laser Micro-machining of Three-Dimensional Microstructures in Optical Materials
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Laser Micro-machining of Three-Dimensional Microstructures in Optical Materials X.H. Wang, P.T. Lai and H.W. Choi Department of Electrical and Electronics Engineering, The University of Hong Kong ABSTRACT We demostrate an advanced precision cutting tool using a 349 nm nanosecondpulsed UV laser micromachining setup. After expansion and collimation, the laser beam is directed vertically and focused with a high performance triplet lens. With an Al mirror inserted in the path of the convergent beam, the beam can be focused on a horizontal machining plane at any desired tilting angles. Microstructures of a wide range of geometries on hard materials can be formed using this custom machining method. Conventional linear and rotary machining on sapphire materials have been demonstrated. INTRODUCTION A variety of laser processes, including laser lithography, laser reinforcement and selective laser sintering are considered as emerging technologies for rapid prototyping [1]. However, the development and application of finely focused laser beams has stagnated since the widespread adoption of wafer dicing in the semiconductor industry [2,3]. The rapid progress and advancement of photonics devices, such as light-emitting diodes (LEDs) and laser diodes (LDs) calls for the rapid development of 3-dimensional fabrication technologies to enable the formation and assembly of micro-scale solid components. Focused laser beam micromachining is a promising candidate for this purpose. EXPERIMENTAL DETAILS Focused laser beam has widely been recognized as a tool suitable for in-depth fabrication or precise cutting and has been widely applied in wafer dicing. The physical process is that of ablation, due to the extremely high power density incident on the target material. This is realized by passing the laser beam through a high optical quality UV objective lens as shown in figure 1. In our experiments, a third harmonic ND:YLF diodepumped solid state (DPSS) laser at 349 nm was used as the source, operating at 1000 Hz pulse repetition rate. After beam expansion and collimation, the laser beam is turned 90° by a laser line mirror and focused on the horizontal machining plane to a very tiny spot several micrometers in diameter. The added feature of the present set-up, as illustrated in the schematic diagram of figure 1, is the placement of a tilting mirror within the optical path, which serves to deflect the convergent beam to strike the sample at an oblique angle to the horizontal working plane. The size of the beam waist at the focusing point is not only limited by the capability of UV objective lens but also sensitive to the coaxiality of the optics. Using this modified set-up, it is relatively easy to optimize and monitor the location of the beam from the upper optical pathway through a tube lens together with a CCD camera. Once the optical setup is optimized before insertion of the tilting mirror,
the mirror can be inserted without affecting the coaxiality of the laser beam, so that the dimension of the beam spot remains unaff
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