Akira Yabe

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.

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.

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.

Preparation of metal oxide thin films using coating photolysis process with ArF excimer laser

The preparation of metal oxide thin films have been developed using the metalorganic (MO) compounds coating photolysis process with ArF excimer laser irradiation at room temperature. The effect of the starting materials and irradiation method on the product films was investigated by FT-IR, UV, XRD and SEM. It was found that metal acetylacetonates or metal 2-ethylhexanoate was effective as the starting materials. When using metal acetylacetonates as the starting materials, crystallized TiO2, In2O3 and ZrO2 were obtained with ArF laser irradiation at 50 mJ/cm2 at a repetition rate of 5 Hz for 5 min. When using An-acac, Fe, Sn, or In 2-ethylhexanoate as the starting material, a two-step process consisting of both preliminary weak (10mJ/cm2) and sufficiently strong irradiation (50mJ/cm2) was found to be effective for obtaining crystallized ZnO, Fe2O3, SnO2 and In2O3 films. In addition, crystallized complex oxide thin films such as ITO, PbTo3 and PbZrO3 were successfully obtained from the metal acetylacetonates or metal 2-ethylhexanoate using MO coating photolysis process. Patterned metal oxide thin films were also obtained by the ArF laser irradiation through the photomask, followed by leaching with solvents. The crystallization mechanism was discussed from the point of view of the photochemical reaction and photothermal reaction.

Photochemical Processing of Polymer Films with Excimer Laser.

紫外レーザーであるエキシマレーザーを使用して,高分子材料の超精密エッチングや新たな表面改質技術が生まれている.フッ素樹脂の親水化による微細なパターンの無電解めっき法も見い出された.ここでは,高分子の爆発的な分解反応であるアブレーションの利用や高分子膜表面での光化学反応の活用による高分子材料の高機能化を紹介する.