Won-Seok Chang

A study on the prediction of the laser weld shape with varying heat source equations and the thermal distortion of a small structure in micro-joining ☆

Abstract In the area of laser welding, numerous studies have been performed the past decades using either analytical or numerical approaches, or both combined. However, most of previous studies were process oriented and modeled differently in conduction and keyhole welding. In this research, the results of calculation using various heat source equations that have been proposed in previous studies were compared to the predictions of a new model. This model treats the problem of predicting, by numerical means, the thermo-mechanical behavior of laser spot welding for thin stainless steel plates. A finite element code, ABAQUS, is used for the heat-transfer analysis with a 3D model. Experimental studies of the laser spot welding have also been conducted to validate the numerical models presented. The results suggest that the predicted temperature profiles and weld dimensions vary according to the heat source equation of the laser beam. For this reason, it is essential to incorporate an accurate description of the heat source. Thermal and mechanical analyses of the laser micro-welding of a small structure are performed from these results.

LED Light Coupler Design for a Ultra Thin Light Guide

A LED light coupler is proposed for coupling light from a mini side LED of 0.4 mm height to a very thin (

Numerical and experimental study on the thermal damage of thin Cr films induced by excimer laser irradiation

{\sim}0.3 mm

A study on a vision sensor based laser welding system for bellows

) light guide. Due to the ultra thinness of the light guide, conventional light couplings between LEDs and light guides do not provide adequate coupling efficiency. The designed LED coupler is a compound lens of 3.5 mm length, 1.2 mm height, and 3.3 mm width. The courier first collimates the light from the LED and then concentrates the light in a manner similar to a compound parabolic concentrate, (CPC) into the light guide. The exit surface of the coupler is additionally textured to have micro patterns in order to control the radiance profile.

Self-fabricated single mode waveguide in fluoride glass excited by self-channeled plasma filaments

Single-shot laser damage of thin Cr films on glass substrates has been studied to understand the cracking and peeling-off mechanism. A mathematical model is developed for the calculation of transient heat transfer and thermal stresses in Cr films during excimer laser irradiation and cooling, the transient temperature, and the stress–strain fields are analyzed by using a three-dimensional finite-element model of heat flow. The finite-element program ABAQUS, with user subroutines, is adopted to perform the numerical analysis. A KrF excimer laser is used in experiments as a source of UV radiation. Morphological inspection of damaged Cr films is carried out by using scanning electron microscopy and the threshold fluences for visible damages are investigated for various film thicknesses. According to the numerical analysis for the experimentally determined cracking and peeling-off conditions, cracking is found to be the result of the tensile brittle fracture due to the excessive thermal stresses formed during ...

Resolution enhancing using cantilevered tip-on-aperture silicon probe in scanning near-field optical microscopy

Abstract Welded metal bellows are commonly manufactured by welding pairs of washer-shaped diaphragms of thin sheet metal. Two types of welding operations are required: inner edge welding to make pairs of diaphragms, and outer edge welding to form a bellows by welding the number of pairs. In this study, the inner and outer edges of bellows were welded by using a CW Nd:YAG laser. For the inner edge welding, the Nd:YAG laser beam was directed at an angle of 45° to the edge because of the limited accessibility. In the outer edge welding, welded pairs were fixed on a jig and compressed axially. The difference between the center of the bellows and that of a jig shaft causes an eccentricity, while the tolerance between a motor shaft and a jig shaft causes a wobble-type motion. A vision sensor based on the optical triangulation was used for seam tracking. Two image processing algorithms that can find a bellows edge were developed for inner and outer edge welding. A geometrical modeling that describes the difference between the position of the Nd:YAG laser spot on a bellows edge and that of the laser diode stripe of the vision sensor was established. The seam tracking by a two-axis Cartesian robot was performed after the vision sensor determined the positional deviation in each sampling. Experiments were performed with 0.1 mm thick stainless steel bellows and 0.5 mm outer edges pitch. The seam tracking by the sensed positional deviation and geometrical modeling was performed successfully with an acceptable error.

Femtosecond-Laser-Coupled Near-Field Scanning Optical Microscopy Patterning Using Self-Assembled Monolayers

Self-fabricated permanent structure of single mode waveguide in optical fluoride glasses was demonstrated using the self-channeled plasma filament excited by a femtosecond (130fs) Ti:sapphire laser (λp=790nm). The photoinduced refractive index modification in ZrF4–BaF2–LaF3–AlF3–NaF glasses reached a length of approximately 10–15mm from the input surface of the optical glass with the diameters ranging from 5to8μm at the input intensities of more than 1.0×1012W∕cm2. The graded refractive index profiles were fabricated to be a symmetric form from the center of optical fluoride glass, and a maximum value of refractive index change (Δn) was measured to be 1.3×10−2. The beam profile of the output beam transmitted through the modified region showed that the photoinduced refractive index modification produced a permanent structure of single mode waveguide.

Fabrication of Nano Dot and Line Arrays Using NSOM Lithography

Scanning near-field optical microscopy (SNOM) achieves a resolution beyond the diffraction limit of conventional optical microscopy systems by utilizing subwavelength aperture probe scanning. A problem associated with SNOM is that the light throughput decreases markedly as the aperture diameter decreases. Apertureless scanning near-field optical microscopes obtain a much better resolution by concentrating the light field near the tip apex. However, a far-field illumination by a focused laser beam generates a large background scattering signal. Both disadvantages are overcome using the tip-on-aperture (TOA) approach, as presented in previous works. In this study, a finite difference time domain analysis of the degree of electromagnetic field enhancement is performed to verify the efficiency of TOA probes. For plasmon enhancement, silver is deposited on commercially available cantilevered SNOM tips with 20 nm thicknesses. To form the aperture and TOA in the probes, electron beam-induced deposition and focused ion beam machining were applied at the end of the sharpened tip. The results show that cantilevered TOA probes were highly efficient for improvements of the resolution of optical and topological measurement of nanostructures.

Fabrication of Cantilevered Tip-on-Aperture Probe for Enhancing Resolution of Scanning Near-Field Optical Microscopy System

Investigations into the fabrication of nanoscale patterns on gold substrates via direct scanning near-field lithography were carried out. A laser beam is scanned over the workpiece where the pattern formation is desired by near-field scanning optical microscopy (NSOM). A pipette-type nanoprobe having a 100 nm aperture at its apex is used with the NSOM and a frequency-doubled Ti:sapphire femtosecond laser at a wavelength of 395 nm as the illumination source to prevent pulse dispersion of the femtosecond laser pulse. Self-assembled monolayers (SAMs) formed by the adsorption of alkanethiols onto the gold substrate are employed as very thin photoresists. The process underlying photopatterning of SAMs on gold is well-known at the phenomenological level. Alkanethiolates formed by the adsorption of alkanethiols are oxidized to alkylsulfonates upon exposure to UV light in the presence of air. Specifically, it is known that deep-UV light of wavelength less than 200 nm is necessary for oxidation to occur. The results show that an ultrafast laser can replace a deep-UV laser source for the photopatterning of thin organic films. Femtosecond-laser photolithography may be applied to create the patterning of a surface chemical structure or a three-dimensional nanostructure by combination with suitable etching methods.

Modification of Carbon Nanotube Templates Using Femtosecond Laser Pulses

Using a cantilever type nanoprobe having a 100㎚m aperture at the apex of the pyramidal tip of a near-field scanning optical microscope (NSOM), nanopatterning of polymer films are conducted. Two different types of polymer, namely a positive photoresist (DPR-i5500) and an azopolymer (Poly disperse orange-3), spincoated on a silicon wafer are used as the substrate. A He-Cd laser with a wavelength of 442㎚ is employed as the illumination source. The optical near-field produced at the tip of the nanoprobe induces a photochemical reaction on the irradiated region, leading to the fabrication of nanostructures below the diffraction limit of the laser light. By controlling the process parameters properly, nanopatterns as small as 100㎚ are produced on both the photoresist and azopolymer samples. The shape and size variations of the nanopatterns are examined with respect to the key process parameters such as laser beam power, irradiation time or scanning speed of the probe, operation modes of the NSOM (DC and AC modes), etc. The characteristic features during the fabrication of ordered structures such as dot or line arrays using NSOM lithography are investigated. Not only the direct writing of nano array structures on the polymer films but also the fabrication of NSOM-written patterns on the silicon substrate were investigated by introducing a passivation layer over the silicon surface. Possible application of thereby developed NSOM lithography technology to the fabrication of data storage is discussed.