Kotaro Obata

Advanced materials processing based on interaction of laser beam and a medium

Abstract Hybrid laser processing for precision microfabrication of hard materials, in which the interaction of a conventional pulsed laser beam and a medium on the material surface leads to effective ablation and modification, is reviewed. The main role of the medium is to produce strong absorption of the nanosecond laser beam by the materials. Simultaneous irradiation with the vacuum ultraviolet (VUV) laser beam which possesses extremely small laser fluence greatly improves the ablation quality and modification efficiency for hard materials such as fused silica, crystal quartz, sapphire, GaN, and SiC by the ultraviolet (UV) laser irradiation (VUV–UV multiwavelength excitation process). Metal plasma generated by the laser beam effectively assists high-quality ablation of transparent materials, resulting in microstructuring, cutting, color marking, printing and selective metallization of glass materials (laser-induced plasma-assisted ablation (LIPAA)). The detailed discussion presented here includes the ablation mechanism of hybrid laser processing.

GaN ablation etching by simultaneous irradiation with F2 laser and KrF excimer laser

GaN(0001)/c-Al2O3 is etched by simultaneous irradiation with an F2 laser and a KrF excimer laser. The use of an F2 laser in addition to a KrF excimer laser reduces the roughness of the GaN surface compared with the case of KrF irradiation etching. An F2 laser, below the etching threshold, simultaneously irradiated with a KrF excimer laser, decreases the etching rate due to an increase in the absorption coefficient of the sample surface against the KrF excimer laser. A very sharp etching sidewall and flat surface are obtained when the etching depth reaches the Al2O3 substrate.GaN(0001)/c-Al2O3 is etched by simultaneous irradiation with an F2 laser and a KrF excimer laser. The use of an F2 laser in addition to a KrF excimer laser reduces the roughness of the GaN surface compared with the case of KrF irradiation etching. An F2 laser, below the etching threshold, simultaneously irradiated with a KrF excimer laser, decreases the etching rate due to an increase in the absorption coefficient of the sample surface against the KrF excimer laser. A very sharp etching sidewall and flat surface are obtained when the etching depth reaches the Al2O3 substrate.

Efficient refractive-index modification of fused silica by a resonance-photoionization-like process using F(2) and KrF excimer lasers.

Novel materials processing by a multiwavelength excitation process using F(2) and KrF excimer lasers for high-efficiency and high-speed refractive-index modification of fused silica is demonstrated. We find this process to be essentially superior to single-wavelength F(2) -laser processing: The multiwavelength excitation process achieves more than twice the diffraction efficiency of fused silica modified by a F(2) laser at the same total number of photons in each irradiated laser beam supplied to the fused-silica substrate. This superiority is attributed to a resonance-photoionization-like process based on excited-state absorption.

Transient electron excitation in laser-induced plasma-assisted ablation of transparent materials

We investigate the mechanism of laser-induced plasma-assisted ablation (LIPAA), by which high-quality and high-efficiency ablation of transparent materials, such as glass, can be performed with a single conventional pulsed laser. The laser-induced plasma induces transient absorption of the laser beam (532nm) by the glass substrate. The origin of the transient absorption is electron excitation by ions with kinetic energy more than approximately 10eV in the plasma, which is observed by measuring transient polarization change in the glass substrate applied with a high external pulsed electric field during the plasma-assisted electron excitation (plasma-conductivity measurement). A possible mechanism of LIPAA is proposed based on the results obtained.

Micromachining of transparent materials by laser-induced plasma-assisted ablation (LIPAA)

Laser-induced plasma-assisted ablation (LIPAA) process developed for glass materials has been applied for micromachining of a variety of transparent hard and soft materials. We have developed the proto-type LIPAA system using a second harmonic of diode pumped Q-switched Nd:YAG laser for the practical use. In this paper, micromachining and scribing of glass and sapphire is demonstrated using the developed system. Additionally, another application such as selective metallization of glass and polyimide with successive metal plating process is investigated. However, mechanism of this process is complex and still remains unknown. To have a better understanding of this process, double-pulse irradiation of a near-IR femtosecond (fs) laser with a delay time is also investigated. A possible mechanism is discussed based on the obtained results.

Microprocessing of glass by hybrid laser processing

Hybrid laser processing for precision microfabrication of glass materials, in which the interaction of a conventional pulsed laser beam and another medium on the material surface leads to effective ablation and modification, is reviewed. The main role of the medium is to produce strong absorption of the nanosecond laser beam by the materials. Simultaneous irradiation of the vacuum ultraviolet (VUV)laser beam, which possesses extremely small laser fluence, with the ultraviolet (UV) laser greatly improves the ablation quality and modification efficiency for fused (VUV-UV multiwavelength excitation processing). Metal plasma generated by the laser beam effectively for assists high- quality ablation of transparent materials, resulting in microstructuring, cutting, color marking, printing and selective metallization of glass materials (laser-induced plasma-assisted ablation (LIPAA)). The detailed discussion described in this paper includes the ablation mechanism of hybrid laser processing.

TiN growth by hybrid radical beam-PLD for Si barrier metal

Combination of PLD and nitrogen radical beam has grown high quality TiN films on Si substrate without silicidation at the interface between TiN thin film and Si substrate even at growth temperature more than 700 degrees C. Additionally, X- ray photoelectron spectroscopy revealed that this method achieved synthesis of almost stoichiometric TiN films. Diffusion barrier characteristics of the grown film were examined by deposition of Al thin films of about 400 nm thick on the TiN grown films, followed by post-thermal treatment at 500 degrees C for 30 minutes. Scanning electron microscopy (SEM) observation and Rutherford backscattering spectroscopy analysis revealed that sharp interfaces between Al and TiN were maintained after the thermal treatment, indicating excellent property of the TiN films as Si barrier metal.

Advanced laser processing of glass materials

Three kinds of advanced technologies using lasers for glass microprocessing are reviewed. Simultaneous irradiation of vacuum ultraviolet (VUV) laser beam, which possesses extremely small laser fluence, with ultraviolet (UV) laser achieves enhanced high surface and edge quality ablation in fused silica and other hard materials with little debris deposition as well as high-speed and high-efficiency refractive index modification of fused silica (VUV-UV multiwavelength excitation processing). Metal plasma generated by the laser beam effectively assists high-quality ablation of transparent materials, resulting in surface microstructuring, high-speed holes drilling, crack-free marking, color marking, painting and metal interconnection for the various kinds of glass materials (laser-induced plasma-assisted ablation (LIPAA)). In the meanwhile, a nature of multiphoton absorption of femtosecond laser by transparent materials realizes fabrication of true three-dimensional microstructures embedded in photosensitive glass.

Si barrier metal grown by hybrid radical beam pulsed laser deposition of TiN

Titanium nitride (TiN) thin films are of great use for electronic and mechanical industry due to their excellent properties of good electric conductivity and diffusion barrier. Especially, they are expected to be used as a diffusion barrier metal in Si VLSI. The pulsed laser deposition (PLD) using an excimer laser is very attractive for growth of TiN thin films as well as other nitride films. The most simple way for TiN growth by PLD is use of TiN target. However, the TiN film grown by conventional PLD using the TiN target becomes Ti-rich. In addition, high purity TiN target can not be commercially available (typically 99.5%). To overcome these problems, we report growth of high quality TiN thin film epitaxially grown on Si(100) substrate by hybrid nitrogen radical beam PLD using a high purity titanium (99.99%) metal target and nitrogen radical beam gun.

F2 laser ablation of GaN

F2 laser ablation etching of GaN has been demonstrated. The etching geometry, etching rate and microroughness were investigated, and compared to the case of KrF excimer laser ablation etching. The etching process is consisted of the ablation and hydrochloric acid treatment. Very sharp edge was found along the etched area. The microroughness of etched surface is reduced as the laser intensity increases. The f2 laser ablation of GaN is thought to be initiated by direct photoionization by single-7.9 eV photon absorption.