ArF Excimer Laser Irradiation Research Articles

歯牙のエキシマレーザーアブレーション機構

The mechanism associated with excimer laser ablation of tooth enamel had been studied using fast photography and electron probe micro analyzer (EPMA). A human tooth enamel was adopted as a sample. The ArF and KrF excimer lasers were used in this study. Time-resolved photographs of ablation plume were obtained using a microscope-mounted camera together with pulsed illumination from a XeCl excimer laser pumped dye solution. Possible mechanisms to explain laser ablation were examined. The results indicated that the ArF and KrF excimer laser promoted primarily photothermal ablation. Photochemical ablation had occurred only in the ArF excimer laser irradiation. Photomechanical ablation appeared to be negligible during the ArF and KrF excimer laser ablation of tooth enamel.

ArFエキシマレーザーを用いた角膜切除中の圧力波モニター

ArFエキシマレーザーを用いた角膜切除中の圧力波モニター

Pulsed laser processings of polymer and ceramic surfaces.

We present here our study on pulsed laser processings, particularly on molecular alignment, the change of microporosity and wettability of polymer and ceramic surfaces. Polarized pulsed laser-induced periodic surface structures (LIPSS) of sub-0.25μm size in a large area have been reported with polyimides, poly(ethylene terephthalate) and other polymer films using 4th harmonic Nd:YAG laser[1] and Ar-F excimer laser irradiation.[2] These LIPSS patterns were formed by repeated heating and cooling in polymer surfaces following intensity distribution caused by interference of reflected surface light and incoming polarized laser beam. Because of directional electric field of the polarized laser pulses, polymer functional groups align along the electric field in LIPSS image areas, as reported here for the first time using polarized reflected FT-IR. We also report the change of microporosity and wettability caused by pulsed laser using the fundamental, 2nd and 3rd harmonic irradiation of Nd:YAG laser. We extended our study to ceramic surfaces where electroless metal deposition induced by pulsed laser irradiation were carried out.

Morphological effects of ArF excimer laser irradiation on enamel and dentin.

The aim of this investigation to determine the range of morphological and ablative effects that can be achieved on dental enamel and dentin using ArF excimer laser irradiation at a wavelength of 193nm. Caries-free coronal enamel and dentin surfaces of 20 extracted human teeth were subjected to irradiation at 193nm using a Lamda-Physik model EMG 103 MSC and ArF fill. Morphology of cavity floors and walls were assessed by light microscopy and SEM. Morphological surface effects and ablation could be controlled effectively and reliably by choice of parameter combination, allowing the operator to achieve either a smooth, flat, or increasingly rough surface with differing degrees of selective ablation. No signs of thermal damage were apparent. Excimer laser irradiation at 193nm provided clinically useful cavity preparations and surface morphological effects.

ArF excimer laser irradiation of human dentin

ArF excimer laser irradiation of human dentin

ArF excimer laser irradiation of human dentin.

The use of excimer lasers for treatment of dental hard tissues has considerable potential because the combined characteristics of low wavelength and short pulse result in limited heat diffusion and, therefore, tissue ablation without the problems of collateral damage. To date, there are relatively few published studies concerning the effects of excimer laser irradiation on dental hard tissues. Thus the present study was conducted to examine the morphological changes in tooth dentin subsequent to ArF excimer laser irradiation. The morphologic changes induced in normal, nondiseased human dentin following irradiation by an ArF excimer laser at fluences ranging from 1 to 4 J/cm2 and the number of laser pulses ranging from 50 to 1,000 were evaluated by scanning electron microscopy. Two modes of ablation, photochemical at low fluences and thermal at high fluences, were observed. A fluence of 1 J/ cm2 when combined with 50 or 100 pulses produced a uniform ablation of the dentin surface without signs of tissue melting. At fluences > 1.5 J/cm2, the thermal mode of ablation was more efficient at removing intertubular dentin than peritubular dentin. Further, when compared to the lower fluences, the higher settings produced a rougher ablation crater surface. Additionally, the higher fluences produced surface melting with each pulse and sealing of exposed dentinal tubules after irradiation with 100-300 laser pulses. The photochemical and thermal mechanisms of tooth dentin ablation were identified based on significant differences in tissue morphology following laser irradiation. The rates of tissue ablation and the observed morphologic changes indicate that the ArF excimer laser could be useful for caries removal and sealing of exposed dentinal tubules.

Direct Patterning of Spin-on-Glass Materials by ArF Excimer Laser Irradiation and Their New Application to Hard-mask Processes

Several spin-on-glass (SOG) materials were examined as single layer resists for ArF excimer laser lithography, with the goal of directly forming a hard mask from these materials for dry-etching underlying metal films. Perhydro-silazane (PHSN) was found to be photo-reactive at 193 nm wavelength as well as polyphenylmethyl-silsesquioxane (PMSQ) and polyhydroxybenzyl-silsesquioxane (HSQ), which we have reported previously. These materials showed a sufficient resolution performance and sensitivity at 193 nm. The Fourier-transform infrared (FTIR) and X-ray photoelectron spectrometry (XPS) analyses showed that the basic reaction is photo-oxidation, though the imaging mechanism in each material is quite different. The etching resistance of these materials was significantly improved by special treatment after patterning, whereas those without the treatment were insufficient. For example, etching rate of PHSN after baking in steam ambient was comparable to that for CVD SiO2 in RIE using SF6 gas. 0.2 µ m patterns were transferred into poly-Si films by dry-etching using these materials as hard masks.

Whiskerlike structure growth on silicon exposed to ArF excimer laser irradiation

The effects of ArF excimer laser irradiation on silicon single crystals in air have been studied. The etch rate versus fluence curve shows three well defined zones, with very different etch rates and dependences. In the intermediate zone (from 1.5 to 2.5 J/cm2), narrow (1–2 μm diameter) and tall columns (3–30 μm) start to grow after irradiation with some hundreds of laser pulses. These whiskerlike columns, with height between one and two orders of magnitude higher than the depth of the crater, have not been formed by preferential etching of the surrounding material, but through hydrodynamical processes.

All laser-assisted heteroepitaxial growth of Si0.8Ge0.2 on Si(100): Pulsed laser deposition and laser induced melting solidification

The growth of heteroepitaxial Si0.8Ge0.2 films on Si(100) by a novel all laser-assisted technique using only ArF excimer laser radiation is demonstrated. Amorphous 30 nm thick films are grown by pulsed laser deposition from alternating pure Si and Ge targets on clean Si substrates. Melting and rapid solidification is then induced by pulsed irradiation (0.54 J/cm2), promoting epitaxial growth.

Intracluster reactions: The formation of hydrazine complexes from ammonia clusters following ArF excimer laser excitation

We have found a novel intracluster reaction proceeding in large ammonia or ammonia/argon clusters upon excitation with ArF excimer laser radiation which leads to the formation of unprotonated and prototaned hydrazine clusters, (N2H4)+n and (N2H4)nH+ (n=1–4). To our knowledge it is the first time that the production of bare clusters of a different chemical species has been observed as a result of an intracluster reaction. Besides neat hydrazine complexes also mixed cluster ions containing hydrazine and ammonia molecules were detected, both as unprotonated and protonated species. However, the bare hydrazine clusters were found to be more abundant. It is assumed that the wavelength of the exciting laser radiation is of great importance for inducing the observed reaction.

Modification of silicon nitride films to oxynitrides by ArF excimer laser irradiation

An intensive study of the effects induced by the ArF excimer laser irradiation of silicon nitride films has been carried out. Amorphous silicon nitride films were deposited on silicon substrates by CO 2 laser-induced chemical vapour deposition (LCVD) in parallel configuration using a SiH 4/NH 3/Ar gas mixture. Post-deposition UV photon irradiation of the silicon nitride films was performed at room temperature and in an inert gas atmosphere. Changes in composition and refractive index were systematically followed by Fourier transform infrared spectroscopy (FTIR), and single-wavelength ellipsometry. The FTIR studies show that the initial ArF laser pulses provoke the elimination of bonded hydrogen (Si-H, N-H) incorporated in the as-deposited silicon nitride films. By increasing the UV irradiation time, incorporation of oxygen in the film is observed allowing the film composition to be tailored from silicon nitride to silicon oxide. This tendency is confirmed by a gradual decrease in the film refractive index. The proposed film modification mechanism is basically the breaking of Si-H, N-H and Si-N bonds, followed by a reaction with the adsorbed water in the films.

The mechanism of two-photon photochemical laser-induced deposition of silicon thin films from silane

We have investigated the mechanism of silicon thin film deposition by ArF excimer laser irradiation of silane gas diluted with argon. The Si films were deposited by a focused laser beam irradiating in parallel to silicon and silicon dioxide substrates at a gas flow rate of 20 SCCM, total pressure of 60 Torr and repetition rate of 15 Hz. At laser energy fluences higher than 160 mJ/cm2 the deposition rate was almost independent of the incident laser energy, while at a lower energy the deposition rate depended strongly on the laser energy. A 3/2 power law was found for absorption measurements carried out at the same pressure under flow conditions and for several repetition rates at average laser power above 300 mW, regardless of the laser repetition rate. This kind of behavior is typical of a multiphoton absorption process involving saturation effects caused by focusing of the laser beam. Below 300 mW the power dependence indicated a two-photon absorption process. From the observed photochemical yield we found the value 5.7×10-44 cm4 s molec-1 for the two-photon absorption cross section.

Hydrophilic Surface Modification and Metallization of Poly(tetrafluoroethylene) Film by Excimer Laser Processing

Abstract The chemical surface modification of poly(tetrafluoroethylene) (PTFE) film was carried out in hydrazine gas photolyzed with ArF excimer laser irradiation. Hydrophilic surface appeared on PTFE film after the laser treatment. The surface was analyzed with a measurement of water contact angle, X-ray photoelectron spectroscopy, and FTIR spectroscopy (attenuated total reflection). In addition, on the basis of the hydrophilic behavior, we succeeded in the area-selective electroless plating of nickel and copper metal on the chemically modified surface.

Effects of synthesis conditions on existence and nonexistence of the ArF excimer laser and x-ray induced B2 alpha band in type-III fused silicas.

ArF excimer laser and x-ray induced absorption bands in type-III fused silica synthesized under reducing (sample DR) and oxidizing (sample DO) conditions were investigated. Under ArF excimer laser and x-ray irradiation, an absorption band at 5.0 eV called the ${\mathit{B}}_{2}$\ensuremath{\alpha} band and ascribed to an unrelaxed oxygen-deficient center was observed only in the sample DR and was not observed in the sample DO. Emission bands at 4.4 and 2.7 eV induced by exciting the ${\mathit{B}}_{2}$\ensuremath{\alpha} band only appeared in the sample DR. The creation mechanism of the ${\mathit{B}}_{2}$\ensuremath{\alpha} band in the sample DR and the reason for the nonexistence of the ${\mathit{B}}_{2}$\ensuremath{\alpha} band in the sample DO were discussed.

Photochemical surface modification of polypropylene for adhesion enhancement by using an excimer laser

Polypropylene (PP) surface in water was photochemically modified to render it hydrophilic using ArF excimer laser radiation. The chemical stability of PP is attributed to the C-H and C-H3 bonds present. Thus, it is considered that H atoms are selectively pulled out from the area irradiated with ArF excimer laser light and are replaced with OH functional groups in the presence of water. In this treatment, the irradiated sample becomes hydrophilic with enhanced adhesion properties. The experimental conditions for this surface treatment were ArF laser fluence of 12.5 mJ/cm2 and a shot number of 10000. The treated PP and stainless steel were bonded with epoxy adhesive and the tensile shear strength was 46 kg/cm2.

Excimer laser-induced photochemical modification and adhesion improvement of a fluororesin surface

Modification of a selective area of a fluororesin surface was accomplished by using ArF excimer laser radiation and a boron complex with oleophilic or hydrophilic functional groups. The chemical stability of fluororesin is attributed to the presence of C-F bonds. The F atoms were abstracted by B atoms selectively from the area irradiated with excimer laser radiation and were replaced with the desired functional groups. In this modification, B(CH3)3 and B(OH)3 were used: a boron compound with methyl groups to generate an oleophilic surface, and one with hydroxyl groups to generate a hydrophilic surface. As a result, the resin surface exposed to ArF laser radiation becomes oleophilic or hydrophilic. Both samples were bonded to stainless steel plates with an epoxy bonding agent and the tensile shear strength was 1.2 x 107 Pa in both cases.

Study on excimer laser induced defects in SiO2 films on Si by variable-energy positron annihilation spectroscopy

Using variable-energy positron annihilation spectroscopy, we have investigated the defects induced by ArF excimer laser (6.4 eV) irradiation of SiO2 films (500 nm thick) on Si(100). Films of both thermally grown oxide, and plasma enhanced chemical vapor deposited oxide were studied. It is proposed that E′ centers (≡Si⋅), nonbridging oxygen hole centers (NBOHC, ≡Si-O⋅) and their charge transfer states may trap positrons and affect the positronium annihilation modes. For the thermally grown oxide, the content of E′ centers, and hence original structural defects such as oxygen vacancies (≡Si-Si≡) at the SiO2/Si interface, is much higher than that in the SiO2 film itself. For SiO2 films deposited on Si by plasma-enhanced chemical vapor deposition, the impurity of ≡Si-OH (1.2 wt %) throughout the oxide layer acts as a precursor to the radiation-induced defects. The NBOHC formed strongly trap positrons and decrease the positronium formation fraction. The annealing behavior of the radiation-induced defects in both samples is characteristic of the original structural defects.

Laser-induced hydrogen desorption from polysilane and its application to silicon pattern generation

Polysilane thin films which are mainly composed of (SiH 2) n chains terminated with SiH 3 have been synthesized from a silane plasma at a substrate temperature of — 110°C and investigated as an ultraviolet (UV) sensitive material. ArF excimer laser irradiation at room temperature causes the desorption of bonded hydrogen from the film. Simultaneously, the Si network formation and crystallization occur in the irradiated region. Since the nonirradiated part of polysilane is easily oxidized in air, the laser irradiation onto polysilane through a Cr mask and subsequent air exposure of the film result in the selective oxidation of the nonirradiated region. A fine polycrystalline Si pattern has been obtained by dilute HF etching of the oxidized region.

Effects of excimer-laser annealing on low-temperature-deposited silicon-nitride film

We have found that an undesirable SiNH component is reduced drastically from the low-temperature-deposited SiN surface by intense ArF excimer-laser irradiation on the low-temperature-deposited SiN film. This pre-annealing of the SiN film is very effective in forming an abrupt Si/SiN interface, and thus plays a very important role in high-performance excimer-laser-crystallized poly-Si/SiN thin-film transistors aiming at high-quality liquid-crystal displays. Annealing characteristics are also presented for the various SiN film thickness and for both the ArF and KrF excimer-laser lights.

Fine SiO2 Pattern Generation by Excimer Laser-Induced Modification of Polysiloxene-Based Thin Films

Polysiloxene-based thin films have been grown by the glow discharge of an Si2H6+O2 gas mixture at a substrate temperature of -110° C. Infrared absorption spectra have shown that the matrix is mainly composed of (–SiH2O–)n chains which are terminated with SiH3 and partly have Si–Si branches. The bonded hydrogen in the network is effused by ArF excimer laser irradiation, resulting in three-dimensional SiO2 network formation. The nonirradiated region is easily etched off by a 0.16% HF solution, while the etch rate of the irradiated region is extremely lowered. The fine SiO2 pattern is generated by the excimer laser irradiation through a metal stencil mask and subsequent etching of the nonirradiated region.