Hongyu Zheng

Laser-effected darkening in TPEs with TiO2 additives

Abstract Investigations were carried out for laser-effected darkening and damage-free laser marking on thermoplastic elastomers (TPEs). Titanium dioxide (TiO 2 ) was studied as the laser-sensitive additive to the TPEs for its role in enhancing laser marking contrast and its effect on the TPE properties. Laser beam characteristics, processing variables and percentage loading of TiO 2 in the TPEs were found to have significant effect on the marking contrast. Surface damage-free and high contrast marking have been achieved with short pulse UV lasers. The laser-effected darkening was found to penetrate into the material in the order of a few tens of microns. XPS analysis was carried out to understand the laser-effected darkening mechanism. Potential applications of the technique are highlighted.

Process study for laser-induced surface coloration

A KrF excimer laser beam is used to induce controllable and high-contrast colors on a stainless-steel surface under different processing conditions. The laser beam acts as a localized heating source that promotes the formation of semitransparent oxide films of varying thicknesses and morphologies on the sample surface. Ellipsometry and atomic force microscopy were employed to understand the laser-induced colors by studying the surface morphology and optical properties of the oxide films and their thickness. It is found that the laser-induced colors vary with the oxide layer thickness. The laser-treated areas have granular structures, which affect oxide growth and light scattering.

Excimer-Laser-Induced Oxidation of GaN Epilayer

The 248 nm excimer-laser-induced oxidation of GaN epilayers with assisting oxygen was investigated. The mechanism of laser-induced oxide formation was discussed. It was revealed that the laser-induced oxidation reaction of GaN occurred at 250 mJ/cm2 which was much lower than the threshold fluence for GaN decomposition or damage (above 600 mJ/cm2). It was postulated that laser light excitation of electron–hole pairs was the main mechanism responsible for the enhancement of the oxidation reaction of GaN. Using glancing-angle X-ray diffraction, the induced oxide was determined to be monoclinic β-Ga2O3. It was also found that with an increase in laser pulse number, the oxygen concentration in the oxide film increased to a value corresponding to Ga2O3. The morphology of the laser-induced oxide surface was smooth and uniform. The process has the potential for being applied in the fabrication of GaN-based electronic and optoelectronic devices.

Selectively deposited copper on laser-treated polyimide using electroless plating

Many reviews about the interconnection line fabrication by laser processing method were reported recently. UV laser process polyimide has been studied thoroughly during the past decade. In this report, we discussed the utilization of surface potential changing on polyimide film irradiated by excimer KrF laser and metallized the UV laser treated polyimide surface by electroless copper deposition. A new negatively charged polymer stabilized Pd solution was applied as catalyst in this experiment. We also produced pattern-wised fine line on KrF laser induced PI surface using this method.

Thermal effects of diode-end-pumped Nd:YVO4 solid state laser

Thermal effects of diode end-pumped Nd:YVO4 laser are analyzed in this paper. A laser interferometer is set up to determine the optical path difference resulting from thermal effects in crystal under lasing condition. The beam distortions in Nd:YVO4 rod are measured with a He-Ne laser at wavelength of 632.8nm. The changes of interference patterns record the deformation of the rod with increasing pump power. The experimental and theoretical curves of effective thermal focal lengths versus pump power are obtained, and it is found that the theoretical analysis agrees well with experimental measurement. With the analytic results of thermal effects, an optimized resonator is designed to insure that the laser operates in stable mode with an output power of 3.2W, slope efficiency of 52 percent and M2 factor of 1.19.

Laser-induced conductivity in aluminum nitride

The electrical resistance of AlN, an insulator with a typical electrical resistance greater than 1013 (Omega) - cm, was significantly reduce dafter irradiation from UV lasers. A typical value of a few hundreds (Omega) -cm was consistently achieved. The possible changes in the surface microstructure were investigated using a number of techniques, which included analysis of surface roughness, Raman spectroscopy and XPS. A thin aluminium layer is believed to have formed on the top surface that accounted for the shining metallic appearance after laser irradiation. The relationship between various laser parameters and the electrical conductivity were investigated. The etch rates of AlN with respective to laser power density are presented. The potential industrial application of this technology is discussed.

Laser sintering process of Ag nanoparticle based ink on polyethylene terephthalate for printed electronics

The sintering of nanoparticle based inks is a key process in the manufacture of functional printed electronics on flexible and thermally sensitive substrates. The large area format in a roll to roll process has attracted the interest of both the industry and research communities in recent years. As a potential option, this paper studies the laser sintering process of printed Ag nanoparticle based inks on Polyethylene terephthalate (PET) polymer to produce conductive circuits. Among the laser systems investigated are pico and nanosecond Nd:YAG laser (1064 nm) as well as Ytterbium fibre laser (1070 nm). The sintering process of the Ag nanoparticles was evaluated in terms of the attained resistivity and the speed of the process. A phenomenological explanation of the sintering process is provided. This research work also identifies the issues encountered and the challenges for process scalability.The sintering of nanoparticle based inks is a key process in the manufacture of functional printed electronics on flexible and thermally sensitive substrates. The large area format in a roll to roll process has attracted the interest of both the industry and research communities in recent years. As a potential option, this paper studies the laser sintering process of printed Ag nanoparticle based inks on Polyethylene terephthalate (PET) polymer to produce conductive circuits. Among the laser systems investigated are pico and nanosecond Nd:YAG laser (1064 nm) as well as Ytterbium fibre laser (1070 nm). The sintering process of the Ag nanoparticles was evaluated in terms of the attained resistivity and the speed of the process. A phenomenological explanation of the sintering process is provided. This research work also identifies the issues encountered and the challenges for process scalability.

Laser cleaning of carbonaceous deposits on combustion engine components

Stubborn carbonaceous deposits on combustion engine components are known to reduce engine durability and performance. The deposits are extremely hard and difficult to remove. Laser cleaning technique may potentially be used to remove these deposits during engine remanufacturing but is rarely reported. In the present work, we demon-strated the feasibility of using a nano-second pulse width and high peak power laser with homogenised beam profile at 1064 nm wavelength to completely remove carbon deposits from steel and aluminium pistons. This one-step process without use of chemicals or any other additives is environmentally friendly and may be carried out with high efficiency in open air. Surfaces before and after laser cleaning were examined using FTIR, XPS, and SEM/EDS to identify changes in chemical analysis, morphology as well as microstructure. Cross-sectional examination of the laser cleaned regions was further investigated to reveal any heat affected zone. Some thermal melting and removal of base materials were found to occur but could be minimized with optimized process parameters. Numerical analysis of thermal effect involved in laser cleaning was further carried out by finite element analysis (FEA) using ANSYS.Stubborn carbonaceous deposits on combustion engine components are known to reduce engine durability and performance. The deposits are extremely hard and difficult to remove. Laser cleaning technique may potentially be used to remove these deposits during engine remanufacturing but is rarely reported. In the present work, we demon-strated the feasibility of using a nano-second pulse width and high peak power laser with homogenised beam profile at 1064 nm wavelength to completely remove carbon deposits from steel and aluminium pistons. This one-step process without use of chemicals or any other additives is environmentally friendly and may be carried out with high efficiency in open air. Surfaces before and after laser cleaning were examined using FTIR, XPS, and SEM/EDS to identify changes in chemical analysis, morphology as well as microstructure. Cross-sectional examination of the laser cleaned regions was further investigated to reveal any heat affected zone. Some thermal melting and removal of base mat...

Laser direct writing of copper on polyimide, FR4, and Al203 substrates from solid-metalorganic film

Laser induced copper deposition from solid copper formate precursor films has been studied on polyimide, FR4 and Al2O3 substrates. Unlike most work reported in the literatures, we used 532 nm Nd:YAG laser beam instead of CW Ar+ laser for the process. A writing speed of 10 mm/s was achieved for deposition of micron-thick copper lines with a typical electrical resistivity of around 85 (mu) ohm-cm. To further increase the electrical conductivity and copper thickness, selective electroless copper plating was performed on the laser processed sample. This has reduced the electrical resistivity of the copper line to below 5 (mu) ohm-cm that is about 3 times the value of bulk copper (1.673 (mu) ohm-cm). A typical copper thickness exceeding 10 micrometers has been achieved after the electroless plating process. The surface morphology and chemical composition were analyzed by using SEM and EDS. The copper line was found to adhere well to the substrate. Besides circuit repair and customization, the reported technique is potentially useful for rapid prototyping of PCB circuits.