Hiroyuki Niino

Micromachining by laser ablation of liquid: superheated liquid and phase explosion

Transparent materials such as quartz, calcium fluoride and fluoropolymer, are difficult to fabricate by conventional laser processing. We fabricated them successfully with a micro-size by laser ablation of liquid layer attached on the backside of the target. The threshold fluence for etching depends on absorption coefficient of liquid, and thermal properties of target materials. In the further study, we observed the dependency of the etch rate on etch size. The results suggest that the thermal energy diffuses more efficiently when the irradiation area is too small. However, when increase the etch depth to high aspect ratio, in the strong laser fluence, the etch rate becomes faster than that with large irradiation area and low aspect ratio. We discussed that the mechanism is due to the combination of two processes in the interface of a target material and a liquid: one is a heating process by the superheated liquid and the other is an attacking process by high temperature and pressure. We suggested that the temperature of the superheated liquid can exceed the thermodynamic critical temperature Tc upon irradiation at a high laser fluence, and the liquid corresponded to transient supercritical fluid. The generation of super-heated liquid is based on a enormous heat release form organic molecule by a cyclic multiphoton absorption mechanism.

Micromachining of transparent materials by laser ablation of organic solution

Transparent materials such as fused silica, quartz, calcium fluoride, and fluorocarbon polymer were etched upon irradiation of organic solution containing pyrene with a conventional KrF excimer laser. Threshold fluence for etching was 240 mJ/cm2 for fused silica. Etch rate remarkably depended on a concentration of pyrene: higher etch rate with the increase of pyrene concentration. It means that pyrene molecules play an important role in this process. The etch rate can be easily controlled through changing a laser pulse number, a laser fluence and a concentration of solution. The mechanism for this process is discussed by cyclic multiphotonic absorption of pyrene in the excited states, thermal relaxation, and formation of super-heated solution. As the results, it is suggested that the process is based on the combination of two processes in the interface between the transparent materials and the liquid: one is a heating process by a super-heated liquid and the other is an attacking process by a high temperature and pressure vapor.

Micromachining of optically transparent materials by laser ablation of a solution containing pyrene

Optically transparent materials such as fused silica, quartz crystal, calcium fluoride, and fluorocarbon polymer were etched upon irradiation of organic solution containing pyrene with a conventional KrF or XeCl excimer laser. Threshold fluences for etching were 240 mJ/cm2 for fused silica, 330 mJ/cm2 for quartz crystal, 740 mJ/cm2 for calcium fluoride, and 45 mJ/cm2 for fluorocarbon polymer. These threshold values were remarkably low compared with those of direct ablation by using conventional lasers. Their etch rates remarkably depended on a concentration of pyrene: the etch rate became higher as the pyrene concentration increased. It means that pyrene molecules play an important role in this process. The mechanisms for this process is discussed by cyclic multiphotonic absorption of pyrene in the excited states, thermal relaxation, and formation of super-heated solution. As the results suggest, the process is based on the combination of two physical processes in the interface between the transparent materials and the liquid: one is a heating process by a super-heated liquid and the other is an attacking process by a high temperature and pressure vapor. The mechanism is also referred to thermal properties of materials.

Morphological surface change of elastomers by excimer laser ablation

Laser ablation of composite materials consisted of elastic polymer and carbon black was carried out for surface modification of composite sheets. The ablated surface was studied by scanning electron microscopy, XPS, and Raman spectroscopy. High intensity irradiation produced cylindrical protuberances on the surface. With the increase of laser pulses the protuberances became large in 20-30 micrometers by aggregation of small ones. Low intensity irradiation produced dome-like Voronoi polyhedrons as the results of development of dispersed carbon particles. The microstructure formation is of interest from the viewpoints of tribological applications.

Progress in laser-induced backside wet etching

We have investigated a one-step method to fabricate a microstructure on a silica glass plate by using laser- induced backside wet etching (LIBWE) that consists of excimer laser mask projection system and diode- pumped solid state (DPSS) laser beam scanning system. Well-defined deep microtrenches without crack formations on a fused silica glass plate were fabricated by LIBWE method with the UV lasers. We have demonstrated the micro- fabrication of pits-array and patterned grating on the surface of silica glass plates by LIBWE method at 248u2005nm and 266u2005nm with dye solutions. The two new systems allow us to use rapid prototyping of high precision surface microfabrication of silica glass as laser direct-write processing.We have investigated a one-step method to fabricate a microstructure on a silica glass plate by using laser- induced backside wet etching (LIBWE) that consists of excimer laser mask projection system and diode- pumped solid state (DPSS) laser beam scanning system. Well-defined deep microtrenches without crack formations on a fused silica glass plate were fabricated by LIBWE method with the UV lasers. We have demonstrated the micro- fabrication of pits-array and patterned grating on the surface of silica glass plates by LIBWE method at 248u2005nm and 266u2005nm with dye solutions. The two new systems allow us to use rapid prototyping of high precision surface microfabrication of silica glass as laser direct-write processing.

Formation of radicals on the surface of polymer films by excimer laser ablation: Application to surface reaction of ablated polymer films with a styrene derivative

Excimer laser ablation of poly(ethylene naphthalate) (PEN) and poly(ethylene terephthalate) (PET) films has been investigated from the viewpoint of chemical reactions. The formation of radicals was confirmed from the observation of a broad singlet‐like peak in the electron spin resonance (ESR) of PEN film in a sealed capillary tube after KrF excimer laser ablation at 77 K. On the surface of ablated polymer films, the chemical reaction of the radicals with a styrene derivative occurred under a cryogenic atmosphere.

Multi-oxides thin film formation on iron-base metal by YVO4 pulsed laser color marking

Multi-oxidized thin films were fabricated by applying laser color marking technique on a stainless steel surface with laser irradiation using an YVO4 pulsed-laser at 30u2005kHz in atmospheric conditions. These multi-oxidized thin films emitted several colors including purple, violet, ultramarine, orange, and green. Using this technique, these mono-colors were produced on the surface according to the scanning repetition number and laser power. Results of XRD analyses for the colored areas showed several types of oxides consisting of FeO, Fe3O4, CrO, Cr2O3, Cr3O4, NiO, Ni2O3, NiCr2O4, and their mixed oxides, which consisted of oxygen vacancy structures. Each of these oxides has its original color for each composition and structure if the mono-phase area producing it shows clear mono-color corresponding to the composition and structure.Multi-oxidized thin films were fabricated by applying laser color marking technique on a stainless steel surface with laser irradiation using an YVO4 pulsed-laser at 30u2005kHz in atmospheric conditions. These multi-oxidized thin films emitted several colors including purple, violet, ultramarine, orange, and green. Using this technique, these mono-colors were produced on the surface according to the scanning repetition number and laser power. Results of XRD analyses for the colored areas showed several types of oxides consisting of FeO, Fe3O4, CrO, Cr2O3, Cr3O4, NiO, Ni2O3, NiCr2O4, and their mixed oxides, which consisted of oxygen vacancy structures. Each of these oxides has its original color for each composition and structure if the mono-phase area producing it shows clear mono-color corresponding to the composition and structure.

A microfluidic bioarray system using laser-machined surface microstructures

Laser-induced backside wet etching (LIBWE) method has been developed as a technique for micromachining of transparent materials. Such technique can be applied for fabricating microfluidic devices used as Lab on a Chip or total microanalysis system (μTAS). In such devices, various functions are integrated onto one chip. Microstructure with 1μm resolution fabricated within microfluidic channels can afford additional functions to the chip. Color-encoded microbeads with surface functional groups randomly arranged in the microstructure can be used for bioarray analyses. We have fabricated a novel microfluidic device incorporating two-dimensional array of microbeads with 10 μm diameter. The performance of the microfluidic bead array was confirmed by a capturing experiment of DNA.Laser-induced backside wet etching (LIBWE) method has been developed as a technique for micromachining of transparent materials. Such technique can be applied for fabricating microfluidic devices used as Lab on a Chip or total microanalysis system (μTAS). In such devices, various functions are integrated onto one chip. Microstructure with 1μm resolution fabricated within microfluidic channels can afford additional functions to the chip. Color-encoded microbeads with surface functional groups randomly arranged in the microstructure can be used for bioarray analyses. We have fabricated a novel microfluidic device incorporating two-dimensional array of microbeads with 10 μm diameter. The performance of the microfluidic bead array was confirmed by a capturing experiment of DNA.

Laser ablation of solid nitrogen films at a cryogenic temperature

Photo-dissociation and laser ablation of solid nitrogen film at 10 K was carried out upon irradiation with a picosecond UV laser (FHG of Nd:YLF laser; 263 nm, 8 ps, 10 Hz) in vacuum. The optical emission lines, attributed to molecular and atomic nitrogen of the film, were monitored by a time-resolved spectroscopic technique. The mechanism of these processes was discussed on the basis of multi-photon absorption of molecular nitrogen.

Laser ablation of solid films at a cryogenic temperature

In order to create new possibilities of laser ablation technique, the laser ablation of nitrogen solid films deposited on a copper plate at 10 K was performed upon irradiation with a picosecond UV laser in vacuum. UV and visible emissions, which were ascribed to the transition form excited molecular and atomic nitrogen, were detected on the film during the laser irradiation at the fluence of 5 J/cm2/pulse. These excited species would be produced in a multi-photon absorption process of the nitrogen by the ps- laser irradiation. At the fluence of ca. 10 J/cm2/pulse, ablation of the film was observed. Plume of the ablation reacted with the surface of graphite. XPS analysis indicated that nitrides formed don the graphite surface by the exposure to the plume. A novel technique for surface modification of materials is made possible by the ps-laser ablation of nitrogen solid film.