Laser processing of polymeric materials by means of 100-mW class QCL lasers with emission wavelength of 7.728 and 4.329 μm were examined. Polymeric materials show absorption bands based on transitions between vibronic levels in mid-infrared (MIR) wavelength. Since such absorption bands are very sharp, resonant conditions with laser wavelength are critical. Quantum cascade laser (QCL) is a promising laser source for mid-infrared laser processing; emission wavelength can be customized by changing the heterostructure. In this work, we have employed 100-mW class QCLs and applied for focused irradiation at several polymeric materials, polypropylene (PP), polycarbonate (PC), polyacetal (POM).
For high quality film printing, we have newly developed a laser-induced forward transfer with optical stamp (shortly, LIFTop), that consists of a two-step transfer process. In the first step, functional films were transferred onto a transparent polymer having high adhesiveness and elasticity like PDMS, which we call “optical stamp”. This stamp helps to avoid the transferred film from pattern spread and fracture. Then, the transferred micro-pattern on the optical stamp was further transferred onto a final target. In this talk, metal and transparent conductive oxide film transfer was demonstrated using our new LIFT process.
For higher cell-to-module efficiency in Cu(In,Ga)Se2 (CIGS) thin-film solar cells, it is important to reduce the loss of active area due to integrated connection. The integrated connection contains three scribing processes: P1 (back contact insulation), P2 (electrical connection) and P3 (transparent conductive oxide, shortly TCO front contact insulation). In this work, we focused on ultrashort-pulse laser scribing (λ=1034 nm, Δτ=300 fs) of TCO via lift-off process as damage-less P3 scribing of CIGS thin-film solar cells. The lift-off of TCO was caused by laser ablation of only an upper part of CIGS light-absorbing layer. The dependence of lift-off behavior on the laser pulse energy and TCO film thickness has been investigated. It was observed that the lift-off of TCO formed a heat-affected zone (HAZ) with a thickness up to 250 nm beneath the trench bottom, where the CIGS experienced to melt. Further, thinner TCO film required lower laser energy threshold for the TCO lift-off, which is favorable to higher solar cell efficiency due to smaller HAZ. Using the TCO liftoff as P3, a submodule with an active area of approximately 3.5 cm2 made by all laser scribing exhibited the conversion efficiency of 11.6 %. After post-annealing at 85 °C for 15 h in vacuum for recovering laser-induced damages, the efficiency was successfully improved to 15.0 %, which is comparable to mechanically-scribed one.
For high-resolution printing, we have developed a novel way of functional microdots deposition based on laser-induced forward transfer, which is referred to laser-induced dot transfer (LIDT). LIDT is one of promising additive manufacturing techniques because it can realize flexible patterning of micron and submicron-sized dots at atmospheric room-temperature conditions. Recently we have achieved printing of functional oxide microdots by a double-pulse LIDT with the first pulse for preheat and the second pulse for transfer, resulting in more precise control of laser-induced hightemperature and thermal-stress in a source film. In this paper, temporal temperature distributions during the transfer process have been investigated using a finite element method approach. High-resolution printing of functional microdots is promising for future optoelectronic integrations.
We report on the laser cutting of carbon fiber reinforced thermo-plastics (CFRTP) with a cw IR fiber laser (single-mode
fiber laser, average power: 350 W). CFRTP is a high strength composite material with a lightweight, and is increasingly
being used various applications. A well-defined cutting of CFRTP which were free of debris and thermal-damages
around the grooves, were performed by the laser irradiation with a fast beam galvanometer scanning on a multiple-scanpass
method.
We report on the laser cutting of carbon fiber reinforced plastics (CFRP) with a cw IR fiber laser (average power: 1kW). CFRP is a high strength composite material with a lightweight, and is increasingly being used various applications. A well-defined cutting of CFRP which were free of debris and thermal-damages around the grooves, were performed by the laser irradiation with a fast beam galvanometer scanning on a multiple-scan-pass method.
By laser-induced backside wet etching (LIBWE), we can fabricate deep microstructures with high aspect ratios. To fabricate such
deep microstructures by means of a mask-projection system, appropriate optimization of the sample positions is indispensable to
maintain the imaging conditions of the mask-projection system at the etch front. By applying the appropriate optimization, the
trenches with homogeneous widths and high aspect ratios could be fabricated. At present, a trench with an aspect ratio of 102 (width:
9.7 μm, depth: 986 μm) was successfully prepared. When the irradiated area was gradually shifted in a direction perpendicular to the
incidence of the laser beam, inclined features could be fabricated in the deep microtrenches. The tilting angles of the features could be
can be flexibly controlled between -30° and +30° by changing the shifting speed. Moreover, the angles could be varied within a
single deep microtrench. Fabrication of deep microhole array was also demonstrated. Fabricated microholes were converted into
through holes on grinding.
By laser-induced backside wet etching (LIBWE), we can fabricate microfluidic channels on silica glasses. These channels have
smooth bottom surfaces that can be combined with various optical detection techniques. Antiresonant guided optical wave (ARGOW)
is a concept for guiding light in the system composed of a low refractive index medium surrounded by a medium with higher
refractive index. The ARGOW was examined by using the wide channels (length: 17 mm, width: 1 mm, depth: 45 μm) prepared by
LIBWE, whose bottom surface showed root-mean-square (RMS) roughness less than 100 nm. The LIBWE can fabricate deep
trenches with high aspect ratio (~57 (length: 1mm, width: 10 μm, depth: 566 μm)). By using such deep channels, ARGOW system
within a plane parallel to the top surface of glass can be fabricated. In this work, surface roughness of the sidewall of deep trenches
prepared with LIBWE was evaluated, and the possibility of utilization for guiding light is discussed.
We have investigated a one-step method to fabricate a microstructure on a silica glass plate using laser-induced backside
wet etching (LIBWE) upon irradiation with DPSS (diode-pumped solid state) lasers. Well-defined deep microtrenches
without crack formations on a fused silica glass plate were fabricated by LIBWE method. As the laser beam of DPSS
UV laser at a high repetition rate up to 5 - 100 kHz is scanned on the sample surface with the galvanometer controlled by
a computer for flexible operations, galvanometer-based point scanning system is suitable for a rapid prototyping process
according to electronic design data in the computer. The behavior of liquid ablation (explosive vaporization) was
monitored by impulse pressure detection with a fast-response piezoelectric pressure gauge. LIBWE method is suitable
for rapid prototyping and rapid manufacturing of surface microstructuing of silica glass as mask-less exposure system in
a conventional atmospheric environment.
We printed FeSi2 micro-dot array on various kinds of substrates utilizing laser-induced forward transfer (LIFT). An
amorphous FeSi2 was deposited by sputtering on a transparent plate as a source film. A single KrF excimer laser pulse
through a mask-projection system was imaged with a small micrometer-sized grid pattern onto a film/plate interface,
resulting in the deposition of FeSi2 micro-dot array on a facing substrate with a high number density of 104 mm-2. FeSi2
in the &bgr; crystalline phase is a promising eco-friendly semiconductor because of NIR electroluminescence used for optical
networking as well as abundant components reserve on the earth and non-toxicity. However, the &bgr;-FeSi2 film fabrication
generally required high-temperature multi-processes which hamper its integration and performance reproducibility.
Using the LIFT of micro-dot array, we succeeded in room-temperature preparation of &bgr;-FeSi2. Micro-Raman
spectroscopy confirmed the &bgr; crystalline phase in the micro-dots deposited on an unheated silica glass substrate. Thus,
the LIFT is useful for integrating functional micro-dot array accompanied by the crystallization at lower temperatures.
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.
Surface micro-structuring of silica glass plates was performed by using laser-induced backside wet etching (LIBWE) upon irradiation with a single-mode laser beam from a diode-pumped solid-state UV laser at 266 nm. We have succeeded in a well-defined micro-pattern formation without debris and microcrack formations around the etched area on the basis of galvanometer-based point scanning system with the laser beam. The behavior of liquid ablation (explosive vaporization) was monitored by impulse pressure detection with a fast-response piezoelectric pressure gauge. LIBWE method is suitable for rapid prototyping and rapid manufacturing of surface microstructuing of silica glass as mask-less exposure system in a conventional atmospheric environment.
Surface micro-structuring of silica glass plates was performed by using laser-induced backside wet etching (LIBWE) upon irradiation with a single-mode laser beam from a diode-pumped solid state UV laser at 266 nm. We have succeeded in a well-defined micro-pattern formation without debris and microcrack formations around the etched area on the basis of Galvanometer-based point scanning system with the laser beam. This method is suitable for rapid prototyping and rapid manufacturing of surface microstructuing of silica glass in a convetional atmospheric environment.
For the application of F2 lasers to micro-/nano-fabrication including photolithography, one of the important research subjects is to eliminate the chromatic aberration, and combination of lenses made of two optical materials with different refractive indices is the effective way. However, only CaF2 is widely applied to lenses for a vacuum ultraviolet (VUV) beam, and a "second material" to be paired with CaF2 is strongly requested. Here, we examined two fluoride crystals which are transparent in the VUV region, BaF2 and LiCaAlF6 grown by the Czochralski technique, as candidates for VUV optics. We investigated the change of the VUV transmittance and the onset of optical damage of these fluoride crystals against F2 laser irradiation. These crystals showed good optical tolerance against cumulative F2 laser irradiation, and the damage threshold of LiCaAlF6 was similar to that of VUV grade CaF2 while that of BaF2 was about half of the other two fluoride crystals.
Silica glass is an important material in optics and optoelectronic devices because of its outstanding properties, such as transparence in a wide wavelength range, strong damage resistance for laser irradiation, and high chemical stability. In order to develop simpler processes of micro-fabricating silica glass using a pulsed laser, we have investigated a one-step method to microfabricate a silica glass plate using laser-induced backside wet etching (LIBWE) upon irradiation with ns-pulsed UV lasers. Two laser irradiation systems were employed for the fabrication; (1) mask projection system with a KrF excimer laser and (2) Galvanometer-based point scanning system with a single mode laser beam from a diode-pumped solid state laser at 266 nm. We have succeeded in a fine micro-fabrication of silica glass surface. In addition, a novel approach in the fabrication of microarrays of dye and protein on fused silica plates using LIBWE technique was achieved. The surace of fused silica plates was initially pre-coated using trimethoxysilane self-assembled monolayers (SAMs), then etched using the LIBWE method to obtain the desired microstructures on the plate surface. Using this technique, the SAMs on the non-irradiated areas were able to survive the LIBWE process, and were used as templates for the subsequent deposition of dye molecules, proteins, or polystyrene microbeads via chemical bonding or physical adsorption.
We have developed a room-temperature fabrication process for β-FeSi2 microprecipitates through the active use of droplets generated by pulsed laser deposition. The droplets generated by KrF excimer laser ablation of an α-FeSi2 metallic target were characterized by micro-Raman spectroscopy with a spatial resolution of 1 μm and X-ray diffraction measurements. As a result, it was confirmed that the micron-sized droplets precipitated as the β-FeSi2 semiconducting phase on silicon and silica glass substrates maintained even at room temperature, whereas the rest of the deposited film was amorphous. It was also found that films containing a high density of β-FeSi2 microprecipitates exhibited 1.55 μm photoluminescence at low temperature after annealing at 800°C for 6 h in an argon atmosphere.
Surface micro-structuring of fused silica glass plates was performed by single-shot irradiation with a single-mode laser beam from a diode-pumped solid state UV laser at 355 nm. Well-defined micropattern without debris and microcrack formations around the etched area was fabricated by laser ablation with a focused laser-beam in the ambient air. The time-resolved optical emission spectra of plume were measured to elucidate the ablation behavior of silica glass induced by nanosecond-pulsed laser irradiation at 355 nm where absorption of silica glass is negligibly small. This method is suitable for rapid prototyping of surface microstructuing without a clean room environment.
Silica glass is an important material in optics and optoelectronics because of its outstanding properties, such as transparence in a wide wavelength range, strong damage resistance for laser irradiation, and high chemical stability. In order to develop simpler processes of micro-fabricating silica glass using a pulsed laser, we have investigated a one-step method to microfabricate a silica glass plate using laser-induced backside wet etching (LIBWE) upon irradiation with a ns-pulsed excimer laser. Our idea of LIBWE is based on the deposition of laser energy on the surface of silica glass using ablation of a dye solution. When the dye solution was ablated upon the laser irradiation, the etching of a surface layer was performed on the silica glass. We have succeeded in the micro-fabrication of such transparent materials as silica glass, quartz, calcium fluoride, sapphire and fluorocarbon resin. The advantages of our LIBWE method are as follows, (1) a lwo laser fluence and constant etch rate, (2) microfabrication without debris and cracks formation, (3) large area irradiation with an excimer laser beam through a mask projection, (4) simple pre/post-treatment on target substrates. This is a one-step process simpler method at ambient pressure, which would be used for mass production.
A novel approach for the fabrication of micropatterns where dye molecules can be site-selectively desposited is described. The micropatterns were fabricated on the surface of fused silica plates using the technique of laser-induced backside wet etching (LIBWE). Prior to the LIBWE process, the surface properties of the fused silica plates were modified with self-assembling trimethoxysilanes bearing functional moieties. On the non-irradiated areas, the self-assembled monolayers (SAMs) survived the LIBWE process, and the remaining SAM acted as a template for the subsequent dye deposition either with chemical bonding or physical adsorption. Site-selective dye deposition was visualized with fluorescence microscopic observation. These results are applicable for micro-fluidic reactors and chemical sensors.
The early stage of optical damae caused by F2 laser irradiation on the wide bandgap fluoride crystal, CaF2, is investigated and compared with the case of ArF laser irradiation. Besides a blue emission band due to self-trapped exciton, sharp emission lines appear and grow at a fluence of about 2 J cm-2, showing the initiation of the optical damage and growth of plume from the F2-laser-irradiated surface of CaF2. There exist cracks and melted structures on the laser-damaged surface, which are caused by thermal stress and vaporization due to laser absorption and following local heating.
Laser-induced backside wet etching of silica glass plates was performed by the excitation of a pure toluene solution with a ns-pulsed KrF excimer laser at 248 nm. Well-defined grid micropattern was fabricated without debris and microcrack around the etched area. To understand the etching mechanism, the formation and propagation of shockwave and bubble were monitored by time-resolved optical microscopy at the interface between the silica glass and the toluene solution after laser irradiation. Transient high-pressure as well as high-temperature generated by UV laser irradiation plays a key role in the etching process.
Time-of-flight (TOF) mass and optical emission spectroscopies have been performed on the ablation plume from α-FeSi2 alloy target under KrF excimer laser irradiation at a fluence of 0.35-2.5J/cm2 to characterize the mass, kinetic energies and excited states of the ejected species. According to the TOF mass measurements in vacuum, the most prominent species were Si and Fe atoms and ions over the entire fluence range, in addition to Si dimer. At 0.4-0.7 J/cm2, only neutrals of Si, Fe and Si2 with the kinetic energy of around 0.2eV were observed. At the fluences above 0.7J/cm2, doubly and singly charged Si and Fe ions appeared abruptly increased their number density and kinetic energies from 6 eV at 0.7 J/cm2 to over 100 eV at 2.5 J/cm2. Consistent with the TOF mass spectra, the optical emission lines stemmed from the monatomic Si and Fe as well as Si dimer in the wavelength range of 240-800 nm in vacuum. On the other hand, we confirmed some luminescent lines appeared only in helium atmosphere of 10 Torr, suggesting the cluster formation such as FeSi.
We have investigated dynamics of ablated species in ZnO plume under ArF excimer laser irradiation with emission imaging and spectroscopy, and discussed the effect of a surrounding inert gas. Surrounding He or Ar gas strongly cools down the plume and encourages aggregation of ablated species, which is visualized by second laser irradiation to the plume.
Laser ablation of nitrogen solid film deposited on a copper plate at 10 K was investigated by the irradiation of a picosecond UV laser at 263 nm in vacuum. Photo-dissociation of nitrogen molecule in the solid film was confirmed by the optical emissions, which were ascribed to atomic nitrogen, during the laser irradiation at the fluence of 5 J cm-2 pulse-1. This photolysis was discussed by the comparison with laser-induced breakdown of nitrogen gas. At the fluence over ca. 10 J cm-2 pulse-1, the ablation of the frozen nitrogen film was observed. Employing the ablation plume including a reactive species such as nitrogen atoms, the surface reaction of a graphite oriented pyrolytic graphite plate and silicon wafer was studied. XPS analysis indicated that nitrides were formed on the surfaces by the treatment. The ps-laser ablation of nitrogen solid film provides a novel technique for surface modification of materials.
With the widespread application of excimer lasers for micro- processing, optically transparent materials in the UV region have become more important as optical components. The transparent materials currently available commercially are silica glass and fluoride crystals, CaF2 and MgF2. The resistance of these materials against cumulative irradiation of excimer lasers is required from the viewpoint of application, and it is important to clarify the mechanisms of the optical damage on these materials. In this paper, we report the onset of laser ablation, that is, the initiation of optical breakdown and plume formation, in CaF2 crystal under cumulative irradiation of an ArF excimer laser. When the laser fluence is below the ablation threshold, a blue luminescence due to self-trapped exciton is observed from the whole laser-irradiated region. When the fluence ins increased near the threshold, successive irradiation finally cause a bright, localized luminescence due to the initiation of laser ablation. SEM images of the laser-damaged region show two features: (1) a small bump with pits of the order of 0.1 micrometers formed by UV laser absorption and following local heating, (2) small cracks with triangular fragments caused by mechanisms stress under local heating.
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.
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.
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