The quenching of the ε phase of iron, which has not been observed under a conventional shock compression, was attained using a femtosecond laser. The crystalline structure in a recovered iron sample was determined using an electron backscatter diffraction pattern, an electron diffraction pattern, and a synchrotron X-ray diffraction methods. A small quantity of the γ phase of iron also existed. Thermodynamic state inside the shock front has to be known because the shock induced phase transition occurs inside the shock front. Therefore, the temperature inside the shock front was calculated using thermodynamic equations. It was found that the ε phase was induced by the shock itself but not the γ phase. The γ phase was suggested to be induced as an intermediate structure between the α-ε transition. The femtosecond laser driven shock may have the potential to quench high-pressure phases which has not been attained using conventional methods.
Microspike-arrays were fabricated by irradiating a femtosecond laser on a tungsten surface through a mask opening in air. Natural logarithm of the calculated intensity distributions diffracted at the edge of the mask opening was qualitatively consistent with the experimental results of the shape and arrays of microspikes fabricated. The shape and the array of microspikes depend on the intensity distribution diffracted at the edge of the mask opening. This microspike-array has a potential to be micro emitter tips.
Waveguide filters with extremely thermally stabilized KrF laser-induced gratings were fabricated in the highly photosensitive Ge-B-SiO<sub>2</sub> thin films. It was discovered that a completely new-type grating with high diffraction efficiency and thermal stability could be formed by annealing a conventional laser-induced grating at 600°C. Such thermally induced gratings couldn't be erased after repeated heat treatment alternating between room temperature and 600°C. We printed a grating in slab waveguide by irradiation with a KrF excimer laser followed by the annealing at 600°C, and then formed the channel in the region of the grating using standard photolithography process. The diffraction peak of 17 dB in depth at 1535.04 nm wavelength was observed after repeated heat treatment alternating between room temperature and 400°C. These thermally stabilized waveguide filters are promising candidate for the highly reliable optical and sensing devices.
The investigation of laser induced forward transfer (LIFT) process using femtosecond pulsed laser comparing with that using excimer laser is reported. Ni thin film of several hundreds of nanometer thickness, which is deposited on fused silica substrate, was irradiated by single pulse of KrF excimer laser (wavelength: 248 nm, pulse width: 30 ns) or femtosecond pulsed laser (wavelength: 800 nm, pulse width: 120 fs), and transferred to a Si acceptor substrate. It is shown that laser beam profile affected the removal of thin film. It is revealed that adhesion of particles was inhibited using femtosecond pulsed laser in comparison with the case of excimer LIFT process.
Femtosecond pulsed laser induced phase transition in iron was investigated using electron backscatter diffraction pattern (EBSP) analyzing system in this study. Mirror polished surface of single crystalline iron (purity: 99.99%) was irradiated by femtosecond pulsed laser (wavelength: 800 nm, pulse width: 120 fs, fluence: 2.5 J/cm<sup>2</sup>, intensity: 1.6x10<sup>13</sup> W/cm<sup>2</sup>, number of pulses: 2000 pulses) in argon atmosphere. Electron beam irradiated the mirror polished vertical section by using colloidal silica under the bottom of the laser irradiated part, and the electron backscatter diffraction pattern was analyzed to determine the crystalline structure. ε phase of hcp structure found to exist around 4 μm deeper from the bottom. γ phase of fcc structure was not detected. This result shows the shock induced by femtosecond pulsed laser irradiation causes the α ↔ ε phase transition. It is suggested that this experimental method has a potential to investigate the existence and its crystalline structure of high pressure and high temperature phase of iron (β phase).
Electron emission from the surface of copper target irradiated with femtsecond pulsed laser (wavelength 800nm, pulse width 100fs) was measured at the atmospheric pressure. A tiny metal probe was used to detect the electric potential made by the charged particles. It was found that the energy of emitted electrons became higher gradually when laser irradiation was repeated on the same spot.
In recent years, the microfabrication technology has splendidly been developing in the various industrial applications. It is effective that laser wavelength and pulse duration in laser microfabrication are shorter on the viewpoints of enhancement of spatial resolution and improvements of microfabrication quality. In this paper, we report on the ablation characteristics of the chromium thin film on quartz substrate (what is called Cr binary photomask) by using femtosecond laser (130fs Ti:sapphire laser). The diffraction pattern of laser intense distribution is observed in image printing with the optical rectangular slit, as the result of effects of minimized thermal diffusion with femtosecond laser pulses. We have obtained the Cr ablation without substrate damage with sufficiently wider ablation laser power range for multiple laser pulse irradiation, though ablation of the large band gap materials like quartz is easily caused due to the multi-photon absorption process in femtosecond laser irradiation. Further we indicated that ablation region does not depend on the diffraction limit with the femtosecond laser pulses.
Micro patterns of some μm size were fabricated by transferring metal thin films using Laser-Induced Forward Transfer (LIFT) technique. The oxygen composition ratio of deposited patterns fabricated by varying laser irradiation conditions was measured by using XPS. Then we investigated the dependence of the oxygen composition ratio of deposited patterns on the thin film-acceptor substrate distance and laser fluence. LIFT was performed using a single shot of KrF excimer laser (wavelength: 248nm, pulse width: 30ns). Sn thin film, with a few hundreds of nanometer thickness deposited on quartz substrate using electron beam evaporation method, were removed by laser irradiation, and deposited on acceptor substrate (Si wafer) after transfer in air under room temperature. As a result of XPS analysis of deposited patterns, it was revealed that the oxygen composition ratio depended on laser fluence and the distance from a thin film to an acceptor substrate and tended to increase and then fall with increase of laser fluence when the film-acceptor substrate distance was fixed. In order to investigate this tendency, we photographed the shadowgraph of the transferring thin film. From this investigation, it was revealed that higher fluence causes higher velocity. As the velocity becomes higher, the time from the beginning of removal to attachment on the acceptor substrate becomes shorter. So the higher laser fluence is, the lower the oxygen composition ratio is.
The purpose of this study is to investigate the correlation between Laser Induced Forward Transfer (LIFT) process observed experimentally and the deposited structure, especially the size accuracy. Ablated plume and shadowgraph of transferring materials were observed using image intensified CCD camera. The intensity of reflected He-Ne laser from the front and rear side of thin films, respectively, were measured using photodiode to investigate the behavior of thin film during laser pulse. Metal thin films (Au and Ni), with several tens - hundreds of nanometer in thickness deposited on quartz substrate using ion sputtering deposition method, were irradiated by KrF excimer laser (wavelength 248 nm, pulse width 30 ns). The measurement of the reflected He-ne laser shows that the film removal finishes during incident laser pulse. Ablated plume images and shadowgraphs of transferring materials show that the velocity of both the top of ablated plume and the transferring materials become faster in increase fo fluence, and that the transferring materials precede the plume. Optimum fluence exists at each film thickness to achieve high size accuracy of deposited structure. At lower fluence, the deposited structure shows bad feature due to incomplete removal from the support substrate. At higher fluence, the big shock causes the wide range of spread of deposited structure when the transferring particles have a collision with the acceptor substrate. At optimum fluence, the high size accuracy of deposited structure is achieved as the film-substrate distance is made as short as possible.
Drilling rate of thin silicon wafer of 50(mu) thickness was determined as a function of beam diameter and laser fluence of KrF excimer laser with a pulse width of approximately 30ns FMHW. Analysis of drilling process indicated that decreasing beam diameter and laser fluence enhanced the drilling rate with improved quality of the drilled hole. The extent of debris and molten particles ejected from the hole was also reduced as the laser fluence was decreased. The drilling rate, approximately 0.6(mu) per pulse at beam diameters larger than 100(mu) , increased significantly as the beam diameter decreased especially below 20(mu) , reaching approximately as large as 4(mu) per pulse at 10(mu) in diameter under constant laser fluence. On the other hand, only very small increase in drilling rate was observed as the laser fluence was increased. A simple formula was derived where the drilling rate is proportional to the fourth root of the laser fluence and inversely proportional to the square root of the beam diameter, assuming that the silicon is removed in a liquid state out of the hole.
In laser rear patterning, a thin film deposited on the supporting substrate is irradiated by a laser beam from the rear side of the thin film through the substrate to remove the irradiated area of the film. In this study, a KrF excimer laser with a pulse width of 30 ns and different values of fluences was focused onto metal such as a gold and a copper thin films, which were deposited on a fused silica substrate using the ion sputter deposition method. The intensities of the incident and the transmitted laser beams were measured simultaneously using photodiodes during the laser rear patterning. The results show that the film removal started after approximately 10 ns of laser irradiation under optimized deposition condition. During the laser rear patterning, it was found that the recoil force of the evaporation generated between the film and the substrate pressed the film. As a result, the molten part at the edge of the unirradiated part was peeled and ejected away by the momentum from the recoil force.
In this paper, CO<sub>2</sub> laser drilling process for printed wiring board and the application to the in-process monitoring
of the via hole quality are described. The process of CO<sub>2</sub> laser drilling was investigated on the basis of high speed
photograph, the light emission and thermal conduction. It was found that the temperature of the decomposed epoxy resin suddenly increased when the smear thickness become less than 2μm. Based on this analysis, a simple in-process monitoring technique was developed to estimate the smear feature by detecting the light emission using a photo sensor. The removing process of the smear by KrF excimer laser was also investigated on the basis of the spectrum of the light
emission and the reflected excimer laser. The electric contact was accomplished by excimer laser removal the smear.
The front and rear patterning process of metal thin film irradiated by the KrF excimer laser is analyzed in this study. In the font patterning of Cu thin film with a thickness of 0.1-0.6 micrometers on polymer substrate, high speed shadowgraphs were taken by irradiating a SHG YAG laser beam co-axially with the excimer laser beam. At the optimal fluences, the molten film separated along the outer edge of the laser-irradiated region was driven toward the center of the irradiated region by the surface tension force, and the was detached from the substrate as small droplets. No explosive removal was observed in our laser patterning experiment contrary to the case reported in the literature. Under this condition, little change in the reflectivity, approximately 25 percent, was observed during laser irradiation. At the excess fluences, the recoil force of evaporation provided the outward radial flow to extend the patterning zone into the unirradiated zone so that the edge of the unirradiated region was peeled off by the momentum of the metal driven by the recoil force. As a result, the reflectivity decreased drastically in the latter half of laser pulse. In the rear patterning, the feature of the deposited metal on opposite substrate removed from the thin film was observed by an optical microscope. Intensities of incident and transmitted beam were measured simultaneously using PIN-photodiodes. Film removal started after approximately 1/100 of that of the incident beam. In the rear patterning process it was found that the recoil force of the evaporation and plasma expansion generated between the film and the substrate pressed the film subsequently, the molten part at the edge of the unirradiated part was peeled and flied away by the momentum for the recoil force.