R2R (Roll-to-roll) production is a powerful process for patterning owing to its cost effectiveness and high productivity. Nanopatterned cylindrical molds for continuous fabrication of nanopatterned polymer film have been generally fabricated by various methods such as e-beam lithography, anodic oxidation, photolithography, or wrapping of nanopatterned sheets. However, these fabrication processes exhibit problems such as high economic and time costs, and creation of seam and stitch. In this paper, to overcome these problems, we suggest the cost-effective laser interference lithography (LIL) for fabrication of a nanopatterned cylindrical molds. LIL is a suitable process for fabricating high-resolution periodic nanometer patterns in large areas without the creation of seams or stitches. Periodic line patterns with hundred-nanometer periodicity were fabricated by LIL and pattern distortions were observed. Period and width of the line patterns on the cylindrical mold were measured along the circumference to experimentally confirm the distortion as related to the projection of light. Further, it was observed that the degree of distortion was dependent on the diameter of the cylindrical mold and the position along the circumference. However, it was theoretically expected that this distortion can be minimized below 0.2% by using a cylindrical mold with a sufficiently large diameter and control of the exposure area.
Polymethyl methacrylate (PMMA) microstructures were fabricated by a polymeric microreplication technology using ultrasonic vibration energy. A commercial ultrasonic welder system was used to apply ultrasonic vibration energy for micromolding. Two different types of nickel micromolds, which were equipped with pillar-type and pore-type microstructures, were fabricated. PMMA was used as the polymer microreplication material, and the optimal molding times were determined to be 2 s and 2.5 s for the pillar-type and pore-type micromolds, respectively. Compared with conventional polymer microreplication technologies, the proposed ultrasonic microreplication technology showed an extremely short processing time. Heat energy generated by ultrasonic vibration locally affected the vicinity of the contact area between the micromold and the polymer substrate. Consequently, only that very limited area was melted so that the bulk material was not seriously affected by the thermal effect and thermal shrinkage could be minimized. Furthermore, although the replication process was not performed in vacuum conditions, the ultrasonic micromolding showed high fidelity in polymer microreplication using the pore-type micromold.
A layer-by-layer laser ablation process of polymers with a diode-pumped solid state (DPSS) third harmonic Nd:YVO4 laser with wavelength of 355 nm is developed to fabricate a 3-D microshape. Polymer fabrication using DPSS lasers has some advantages compared with the conventional polymer ablation process using KrF and ArF lasers with 248 and 193 nm wavelengths, such as pumping efficiency and low maintenance fees. This method also makes it possible to fabricate 2-D patterns or 3-D shapes rapidly and cheaply because CAD/CAM software and precision stages only are used without complex projection mask techniques. The photomachinability of polymers is highly influenced by laser wavelength and its own chemical structure. So the optical characteristics of polymers for a 355-nm laser source are investigated experimentally and theoretically. The photophysical and photochemical parameters such as laser fluence, focusing position, and ambient gas are considered to reduce the plume effect, which induces the redeposited debris on the surface of a substrate. Thus, the process parameters are tuned to optimize gaining precision surface shape and quality. This direct photomachining technology using DPSS lasers could be expected to manufacture the prototype of microdevices and be the mold for the laser-LIGA process.
Multiple pulse laser ablation of polymer is performed with DPSS (Diode Pumped Solid State) 3rd harmonic Nd:YVO4 laser (355 nm) in order to fabricate three-dimensional micro components. Here we considered mechanistic aspects of the interaction between UV laser and polymer to obtain optimum process conditions for maskless photomachining using DPSSL. The photo-physical and photochemical parameters such as laser wavelength and optical characteristics of polymers are investigated by experiments to reduce plume effect, which induce the re-deposited debris on the surface of substrate. In this study, LDST (laser direct sculpting technique) are developed to gain various three-dimensional shape with size less than 500 micrometer. Main process sequences are from rapid prototyping technology such as CAD/CAM modeling of products, machining path generation, layer-by-layer machining, and so on. This method can be applied to manufacture the prototype of micro device and the polymer mould for mass production without expensive mask fabrication.
Pulsed laser ablation is important in a variety of engineering applications involving precise removal of materials in laser micromachining and laser treatment of bio-materials. Particularly, detailed numerical simulation of complex laser ablation phenomena in air, taking the interaction between ablation plume and air into account, is required for many practical applications. In this paper, high-power pulsed laser ablation under atmospheric pressure is studied with emphasis on the vaporization model, especially recondensation ratio over the Knudsen layer. Furthermore, parametric studies are carried out to analyze the effect of laser fluence and background pressure on surface ablation and the dynamics of ablation plume. In the numerical calculation, the temperature, pressure, density, and vaporization flux on a solid substrate are obtained by a heat-transfer computation code based on the enthalpy method. The plume dynamics is calculated considering the effect of mass diffusion into the ambient air and plasma shielding. To verify the computation results, experiments for measuring the propagation of a laser induced shock wave are conducted as well.
Micromachining using the DPSS 3rd Harmonic Laser (355nm) has outstanding advantages as a UV source in comparison with Excimer lasers in various aspects such as maintenance cost, maskless machining, high repetition rate and so on. It also has the greater absorptivity of many materials in contrast to other IR sources.
In this paper, the process for micro-drilling of through and blind hope in Cu/PI/Cu substrate with the UV DPSSL and a scanning device is investigated by both experimental and numerical methods. It is known that there is a large gap between the ablation threshold of copper and that of PI. We use the multi path for through hole with high energy density and we use Archimedes spiral path for blind hole with different energy densities to ablate different material. Furthermore, Matlab simulations considering the energy threshold of material is performed to anticipate the ablation shape according to the duplication of pulse, and FEM thermal analysis is used to predict the ablation depth of copper.
This study would be widely applicable to various laser micromachining applications including through and blind hole micro-drilling of PCB, and micromachining of semiconductor components, medical parts and printer nozzles amongst others.
The feasibility of laser ablation in micro-machining of 3D structure of MEMS (Micro Electro Mechanical Systems) parts, specifically micro optics was studied in this paper. The micro-machining characteristics of polymer such as etching rate vs. energy fluence, number of pulse are investigated experimentally. The threshold energy density of polyurethane is about 30 mJ/cm2 and ablated depth per pulse can be precisely controlled in the range of 0.1-0.8μm by the attenuation of energy fluence. By mask moving technique, the micro prism, cylindrical lens and inclined surface were fabricated. These 3D structures can be used as master in electro-plating mold. This paper also summarized the work on the development of a simulation program for modeling the process of machining quasi-three dimensional shapes with the excimer laser beam on a constant moving polymer. Relatively simple masks of rectangle, triangle and half circle shape are considered. The etching depth is calculated by considering the number of laser pulses and the wavelength of laser beam irradiated on the various specimen surface such as PMMA, polyurethane and PI. It was found that similar shapes as experimental results, mask shape was designed to gain-lens surface which we want. As another method to manufacture micro lens the mask is made circular type and rotated during laser beam illumination. Opened mask area and scanning speed determine the surface shape of lens. Precise control of various parameters is admitted to fabricate micro optics.
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