伊東 翔

Abstract Wheel scribing on glass generates a vertical crack with a periodic stripe pattern beneath the wheel (hereafter referred to as the first crack). After the passage of the scribing wheel, sometimes seconds later, the first crack is repropagated with a smooth surface (the second crack). The second crack propagates to 90% or more of the glass thickness under suitable scribing conditions, facilitating the breaking process. The mechanism of secondary crack propagation has not been sufficiently explained in previous studies. Therefore, this study used analytical and experimental methods to examine stress distribution and crack propagation behavior during wheel scribing. Finite element analysis suggests that the increase in the stress intensity factor contributing to the propagation of the second crack was due to not only the crack opening force but also the bottom deformation of the glass specimen. An analytical model accounting for the bottom deformation can simulate the characteristic behavior, such as rapid deepening when the scribing load exceeds a specific threshold value, of the second crack. This study indicates that the elastoplastic deformation caused by wheel contact induces the deformation of the entire specimen, and the state of the bottom constraints is important for controlling the second crack.

This work reports a selective median crack propagation phenomenon in glass, leading to a novel glass cutting process. We found that by scribing a glass sample to the extent of plastic deformation with a deformation depth of 100–400 nm, followed by inducing an initial crack, a subsurface crack with a depth of ~ 10 μm was propagated backward along the centerline of the scribed region with a speed of 1 μm/s order. The crack depth and propagation speed were increased by increasing the scribing load. We conclude that the propagation direction was determined by the effect of the shear stress caused by a scribing tip sliding motion.

The effects of the ambient temperature and relative humidity on crack propagation behavior during wheel scribing were investigated. A chamber was built to allow dynamic observation of crack propagation behavior in a controlled atmosphere. A developed miniature scriber was installed in the chamber, and the crack propagation behavior was observed from lateral and back sides during wheel scribing under various atmospheric conditions. As a result, the median crack propagation rate increased with relative humidity. We speculated that this was caused by the stress corrosion of glass. Although stress corrosion is considered to be more reactive at higher temperatures, the results of scribing at different temperatures showed that higher temperatures did not necessarily increase median crack propagation. This is due to the formation of lateral cracks before the median cracks have fully propagated. These results suggest that the interaction between multiple cracks should be considered when discussing the effects of temperature and humidity in wheel scribing.

We report a new phenomenon of generating glass nanofibers from substrates. A non-alkaline glass substrate was irradiated with a laser beam of 355 nm wavelength and a pulse width of nanosecond order while scanning the laser beam. This resulted in the generation of nanofibers from the back surface of the substrate. Scanning electron microscope images showed that the diameters of the nanofibers were of 100 nm order. The images also revealed that microparticles existed at the tips of the nanofibers. According to the investigation of the generation conditions, when the laser beam was focused below the top surface, particularly below the back surface, nanofibers were easily generated. By in situ monitoring, it was found that a region wider than the beam diameter (1/e (2)) on the surface was modified. Then, nanofibers were generated in the vicinity of the beam spot along with the ejection of microparticles. Compared with previously reported phenomena, in our process, it is more important for sufficient heat to have accumulated for nanofibers to form.

The modification of transparent materials using a femtosecond laser has been studied. We previously proposed a new method of modifying glass using a continuous-wave laser, named continuous-wave laser backside irradiation (CW-LBI), in which CW laser illumination induces a change in the refractive index without cracking the glass. In this study, we investigated the process of glass modification by CW-LBI using time-lapsed imaging and by calculation of the temperature. First, the modification of glass was observed by the shadowgraph method, and it was found that thermal radiation was included in the phenomena that occurred when the glass was modified. Second, the time-lapsed temperature distribution was calculated. The temperature distribution was roughly in accordance with the shape of the modified area observed in the shadowgraphs. We concluded that the radius of the modified area is dependent on the temperature reached in the glass.

High-aspect-ratio microdrilling demonstrated by the forth harmonic of nanosecond Nd:YVO₄ laser. In this paper, we investigated the depths of the holes drilled in alumina, aluminum alloy, cemented carbide, high speed steel, silicon and stainless steel. Firstly, the depths of the holes drilled with the repetition rate of 1 kHz, 5 kHz, 10 kHz and 15 kHz, were measured. As a result, the deepest holes were drilled at the repetition rate of 10 kHz in all the materials. At the tip of the holes drilled in cemented carbide, high speed steel, silicon and stainless steel were blanched. The depths of the holes were 600 μm - 1400 μm. The range of the holes with the diameters less than 10 μm were measured, and aspect ratios were calculated. As a result aspect ratios of alumina, aluminum alloy, cemented carbide, high speed steel, silicon, and stainless steel were 104, 26, 17, 56, 64 and 55, respectively. No clear redepositon layer formed on the inner surface of silicon. Scanning electron microscope observation revealed that inner surface was not flat.

It has been reported that heat accumulation enables the ablation threshold of the material lower and ablation rate higher in laser drilling. In this paper, the influence of the heat accumulation on high-aspect-ratio hole drilling with the fourth harmonic wave of Nd : YVO₄ laser was investigated. Repetition rate caused the much difference of the depth, for example, the depths of the holes were changed from 1200 μm to 3000 μm, by changing the repetition rate from 1 kHz to 10 kHz, when the laser energy was set at 100 μJ/pulse. The repetition rates which the deepest holes were obtained at were dependent on the pulse energies. Temperature rise with 10 kHz pulses was calculated to be 20 times higher than that with 1 kHz pulses, when the successive pulse illuminated. The reasons that the repetition rate caused the different of the depths, were the diameters at the entrance of the holes, the heat accumulation effect and shielding effect by ablated particles.

High-aspect-ratio microdrilling of borosilicate glass has been demonstrated by the forth harmonic of nanosecond Nd:YVO<sub>4</sub> laser. In this paper, we investigated about the influence of the pulse width on the hole profile. The pulse width was expanded by splitting the laser beam and mixing after passing through optical passes with different length. Firstly, the holes drilled with various polarized beam. As a result, depths of the holes drilled with different polarization of the beam were same. Then depths of the drilled holes with different pulse width were measured. Deeper holes were obtained with faster pulses when the deep holes were drilled. However, no clear difference was observed when the drilled holes were shallow. The profiles of the holes drilled with different pulse width were measured. As a result, the profiles were almost same around the entrance. On the other hand, the diameters of the holes drilled with slower pulses were a few μm smaller than that with faster pulses around the bottom. It was shown that pulse width influenced more when deep holes were drilled.

High-aspect-ratio microdrilling of borosilicate glass has been demonstrated by the forth harmonic of nanosecond Nd : YVO<sub>4</sub> laser. In this paper, we investigated about the influence of the beam profile on the hole profile. Beam profile was changed by using focal lens with the focal length of 10 mm, 20 mm, 30 mm, 50 mm and 100 mm. As a result the glass was drilled deeper than 2200 μm using the focusing lenses with the focal length of 20 mm, 30 mm and 50 mm. On the other hand, the depths of the holes were less than 800 μm using the focusing lenses with the focal length of 10 mm and 100 mm. The profiles of the holes drilled with the focusing lens with the focal length of 20 mm, 30 mm and 50 mm were almost same. The diameters of the holes were smaller than 15 μm in the range deeper than 500 μm, therefore the aspect ratio was higher than 110. Drilled depths and profiles were measured with changing focus positions. No clear were difference were observed when the defocus distances were less than ∼70 % of Rayleigh range. Transmitted laser energy passing through the holes were measured by using the glass plates with different thickness and compared with calculated energy supposing the laser beam propagates as Gaussian beam and no reflection on the inner surface of the drilled hole. As a result the energy of the transmitted light was almost same as that of the calculated energy when the plate was thinner than 1 mm. On contrast, when the plate thickness was 2 mm, the transmitted light was larger than that of the calculated energy. This result conducts the laser beam was reflected and the reflected laser was dominant when the hole was deeper than 1mm.

High-aspect-ratio microdrilling of borosilicate glass has been demonstrated by the forth harmonic of nanosecond Nd : YVO<sub>4</sub> laser. Borosilicate glass plate with a thickness of 2 mm was drilled through. Firstly, the profile of the drilled hole was measured. As a result, diameters were φ8.2 ± 3.1 μm andφ6.3 ± 1.0 μm at the depth from 480 μm to 2040 μm and from 1360 μm to 2040 μm, resulting in an aspect ratio of higher than 190 and 100, respectively. Drilling process was observed by ceasing laser illumination every 500 pulses. Depth of the drilled hole increased ∼200 μm by very 500 pulses. Re-deposition layer with a thickness of a few micrometer was observed in the range from the top surface to a depth of 1600 μm. Inner surface of the drilled hole was observed by scanning electron microscope (SEM). Melted surface was observed at around the surface (in the range ∼100 μm from the surface) area. Debris with diameters of less than 500 nm was observed in the range of 100 μm - 600 μm from the surface. At around 650 μm, debris with diameters of 500 nm - 1 μm were recognized. In the area deeper than 700 μm, no clear debris was observed and flat surface was obtained. Cracks were observed in the same area where the re-deposition occurred.

We propose a new technique of manipulating a metal particle in borosilicate glass. A metal particle that is heated by laser illumination heats the surrounding glass by radiation and conduction. A softened glass enabled metal particle migration. A 1-mu m-thick platinum film was deposited on the back surface of a glass plate and irradiated with a green CW laser beam through the glass. As a result, the platinum film was melted and implanted into the glass as a particle. Platinum particles with diameters of 3 to 50 mu m migrated at speeds up to 10 mm/s. In addition to platinum particles, nickel and austenitic stainless steel (SUS304) particles can be implanted. (C) 2010 Optical Society of America

We propose a new technique of manipulating a metal particle in borosilicate glass. A platinum film was deposited on the back surface of a glass plate and irradiated with a green CW laser beam through the glass. As a result, the platinum film was melted and implanted into the glass as a particle. The glass around the platinum particle heated by the laser illumination was softened, which enabled the platinum particle migration in the glass. Platinum particles with diameters of 3 to 50 μm migrated at the speeds up to 10 mm/s. In addition to platinum, nickel and austenitic stainless steel (SUS304) particles was able to be implanted.