ArF Excimer Laser Irradiation Research Articles

Characterization of Polycrystalline Si Films Produced by ArF Excimer Laser Irradiation

ArF excimer laser irradiation of amorphous silicon (a-Si) films has great potential on the production of poly-crystalline silicon (poly-Si) thin film transistor liquid crystal displays (TFT-LCDs). The main advantage of using poly-Si films instead of a-Si films to fabricate high performance electronic devices is due to its ability to obtain higher field effect mobility. However, the surface morphology, grain size and defect density of the poly-Si film have profound effect on whether this is to be achieved. The specimens used in this study contain a 100 nm-thick a-Si thin film grown on glass substrate by plasma enhanced chemical vapor deposition (PECVD) technique. The influence of processing parameters such as laser fluence, shot number and repetition rate on the surface morphology and recrystallization behaviour of laser annealed a-Si films was investigated in this study. The results showed that the threshold fluence for partially melting a-Si thin films at 1Hz and single shot was around 150 mJ/cm 2 . Further increasing laser fluence and shot number resulted in the formation of microvoids trapped inside poly-Si film due to the evolution of hydrogen gas during the laser annealing process. The surface morphology induced by laser annealing of a-Si thin films is found dependent upon the laser fluence and shot number. The super-lateral grown (SLG) poly-Si grains can be obtained at the fluence around 190 mJ/cm 2 .

Electrical Properties of La0.7Ca0.3MnO3 Thin Films Obtained by Metal-Organic Deposition (MOD) using Excimer Laser and Thermal Annealing

Epitaxial La0.7Ca0.3MnO3 (LCMO) thin films were prepared by thermal and excimer laser (EL) metal-organic deposition (MOD). Using simple thermal annealing (STA) at 800–1000°C, the LCMO films were well epitaxially grown on single-crystalline SrTiO3 substrates. When, using a KrF or an ArF excimer laser irradiation, the LCMO films were also epitaxially grown at a substrate temperature of 500°C. The films produced by STA exhibited metal–insulator transition temperatures Tp's ranging from 282 to 290 K while those obtained by EL irradiation had a Tp of approximately 205 K. For bolometric applications, the temperature coefficient of resistance (TCR) of the films was calculated. The LCMO films obtained by STA have a TCR of 8.5%/K at 228 K. On the other hand, using the EL irradiation, the TCR values are 5.8%/K at 165 K and 6.5%/K at 128 K for the KrF and ArF excimer lasers, respectively.

Electrowetting–induced photochemical surface modification onto fluorocarbon

ABSTRACTThe PTFE was modified into hydrophilic with 1/100 of the shots number required to obtain the same contact angle with water by the laser irradiation alone, when irradiating an ArF excimer laser on the sample surface at the moment of applying a 6 kV to the water placed on the PTFE surface to decrease the contact angle with water.A plasma treatment method is widely used for plastic surface modification, but the hydrophilic property generated by this method fades away soon. On the other hand, we have previously reported that the ArF excimer laser light was applied on a sample surface in the presence of water to substitute hydrophilic groups, which was modified to have a long–lasting hydrophilic property. This method, however, needed 3000 to 10000 shots of the laser irradiation, and it is less economical compared with the plasma processing that requires only one–minute irradiation. There is an electro–wetting method, in which the contact angle with water decreases temporarily when a high voltage is applied between the water and the sample, but the contact angle is restored to its original position when stopping the voltage application.Thus, we demonstrated the surface modification of PTFE maintaining the hydrophilic property for a long period with only 100 shots, by irradiating the ArF excimer laser on the sample at the moment when the wettability became high by the electro–wetting method. Water was placed in the gap between the silica glass and the PTFE to create a thin liquid layer with capillary phenomenon. A high voltage (6 kV) of direct current (DC) or alternating current (AC) was applied on the gap, and the ArF excimer laser was vertically irradiated on the sample surface. The water was photo–dissociated to produce H and OH. At the same time, the C—F bond of the PTFE was also photo–dissociated, and the F atom bonded to the H atom to produce HF. The OH group united with the dangling bond of C, which resulted in modifying the PTFE surface to be hydrophilic.To evaluate the wettability of the modified sample, the contact angle with water was measured. Improving the contact angle with water from 110 degrees for the untreated sample to 50 degrees for the treated sample had needed 10000 shots at the laser fluence of 5 mJ/cm2. By combined high voltage application and ArF excimer laser irradiation treatments, however, the 50–degree contact angle was yielded with 500 shots, 1/20 of 10000, when applying the DC of 6 kV, and with 100 shots only, 1/100 of 10000, when applying the AC of 6 kV. Moreover, the modified sample was observed for a change in contact angle with passage of time. The contact angle was 60 degrees after applying the high voltage, and 110 degrees when stopped. On the other hand, the sample modified by combined the high voltage application and ArF excimer laser irradiation maintained the 50–degree contact angle for one month after stopping the voltage application.

Modification of expanded polytetrafluoroethylene by UV irradiation in reactive and inert atmosphere

The surface properties of expanded polytetrafluoroethylene (ePTFE) can be modified by 172 nm excimer lamp irradiation in ammonia (NH3) or acetylene (C2H2) atmosphere. This surface modification is either related to the substitution of fluorine by nitrogen- and/or hydrogen-containing groups or to the deposition of material from the gas phase. Additional oxygen-containing functionalities are incorporated by reactions with the residual gas and/or oxidation in air after treatment. The changes in surface properties are discussed on the basis of water contact-angle measurements, X-ray photoelectron spectroscopy (XPS) and IR spectroscopy on dense PTFE samples. Surface modification yields a reduction of the tear strength and break-down elongation in tensile tests. Field-emission scanning electron microscopy (FE-SEM) revealed that the tensile failure occurred at the contact of fibrils and nodes in the material. With 193 nm ArF excimer-laser irradiation in NH3 atmosphere, both the modification efficiency and material degradation are considerably lower.

High birefringence control and polarization insensitive Bragg grating fabricated in PECVD planar waveguide with UV polarized irradiation

A method for rapid compensation of high intrinsic birefringence in plasma-enhanced chemical vapor deposition (PECVD) based planar waveguides is presented. The intrinsic waveguide birefringence is compensated by a large UV induced opposite birefringence generated with polarized UV ArF excimer laser irradiation (oriented normal to the waveguide axis) and enhanced further through hydrogen loading. The fabrication of polarization insensitive Bragg gratings in PECVD planar waveguides with a large intrinsic birefringence of 6.3×10−4 is demonstrated by trimming it to zero birefringence with a low fluence blanket UV polarized exposure. After annealing for removal of the hydrogen, the polarization insensitive nature of the Bragg grating is unchanged.

Ablation and compaction of amorphous SiO 2 irradiated with ArF excimer laser

The structure of amorphous SiO 2 exposed to ArF excimer laser irradiation was examined. Threshold fluence for causing ablation with a single pulse depended on sample preparation: more specifically, 1 J/cm 2 for thermally grown SiO 2 films on silicon and 2.5 J/cm 2 for bulk SiO 2. It was found that the bond angle of Si–O–Si was reduced by irradiation near the interface of thermally grown SiO 2 films. In contrast, evolution of the bond angle by irradiation was absent in both the bulk SiO 2 and SiO 2 film-near the top surface, even though the concentration of puckered four-membered rings deduced from Raman spectra dramatically increased. It is assumed that planar three-membered rings were generated in the SiO 2 thin layer near the interface, and puckered four-membered rings were generated in the bulk SiO 2. The concentration of both the Si 3+ and Si 2+ structure was increased at a fluence of 800 mJ/cm 2 with an increasing number of pulses, although generation of both was absent at higher fluence for a single pulse. The author proposes that the structure of SiO 2 is created by flash heating and quenching by pulse laser irradiation. Structural similarities were found between the irradiated SiO 2 and SiO 2 at high temperatures.

Protein adsorption on PTFE surface modified by ArF excimer laser treatment

Poly (tetrafluoroethylene) (PTFE) is a material that causes only a few rejections in a living body but has low tissue affinity. A soft tissue implant material that not only has high biocompatibility but also excellent bondability has been developed by photo-chemically incorporating OH functional groups on the PTFE surface with ArF excimer laser irradiation. Protein adsorption on untreated and treated samples was evaluated by scanning electron microscopy (SEM) and attenuated total reflection Fourier-transform infrared (ATR–FT-IR) spectroscopy, with bovine serum albumin (ALB) and fibrin (FIB) solutions. It has been found that protein adsorption increases with the increase in the OH group density on the PTFE surface. The maximum adsorption of both ALB and FIB was found on the PTFE sample treated with a laser fluence of 20 mJ/cm2 and a shot number of 2000, whose water contact angle was 28 degrees; the quantities of both ALB and FIB adsorbed increased by a factor 2 as compared with untreated sample.

Adhesive and morphological characteristics of surface chemically modified polytetrafluoroethylene films

In the present paper, we report an experimental determination of adhesive and topographic characteristics of chemically modified surface of polytetrafluoroethylene (PTFE) films. The surface chemistry was modified by ArF excimer laser irradiation in presence of triethylene-tetramine photoreagent. The applied laser fluence was varied in the range of 0.4–9 mJ/cm 2, and the number of laser pulses incident on the same area was 1500. To detect the changes in the adhesive features of the treated Teflon samples, we measured receding contact angle for distilled water and adhesion strength, respectively. It was found that the receding contact angle decreased from 96° to 30–37° and the adhesion strength of two-component epoxy glue to the treated sample surface increased from 0.03 to 9 MPa in the applied laser fluence range. Additionally, it was demonstrated that the adhesion of human cells to the modified Teflon samples is far better than to the untreated ones. The contact mode and pulsed force mode atomic force microscopic investigations of the treated samples demonstrated that the measured effective contact area of the irradiated films does not differ significantly from that of the original films, but the derived adhesion force is stronger on the modified samples than on the untreated ones. Hence, the increased adhesion of the treated Teflon films is caused by the higher surface energy.

Structuring of poly ether ether ketone by ArF excimer laser radiation in different atmospheres

Structuring of poly ether ether ketone (PEEK) by 193 nm ArF excimer laser radiation has been investigated. Experiments were carried out in different atmospheres (air, vacuum, Ar, O 2) in order to study its influence on the quality of the structures and the formation of the debris. Repetition rate makes little effect on the ablation rate and roughness of the structure in presence of any kind of atmosphere, indicating for the structuring of PEEK by ArF laser radiation a large window of processing. The roughness at the bottom of the structures and the morphology of the side walls are strongly affected by the properties of the atmosphere. The smallest roughness is achieved at 0.6 J/cm 2 for all kinds of processing gases. Debris around the structures can be diminished by structuring in vacuum. Plasma expansion speed has been measured by using high speed photography.

Laser ablation rate of hydroxylapatite in different atmospheres

Hydroxylapatite (Ca 10(PO 4) 6(OH) 2) is a calcium phosphate used as coating for dental and orthopaedic implants, because its composition and structure are similar to the mineral part of bone. Pulsed laser deposition has been applied as an alternative to the commercial technique for the production of hydroxylapatite coatings: plasma spraying. Hydroxylapatite targets were ablated at 0.9 J cm −2 using an ArF excimer laser (193 nm) at 20 Hz in order to investigate the ablation rate of hydroxylapatite in different atmospheres: water vapour, Ar and O 2. The ablation rate was measured by profilometry for different pressures in a range of 15–80 Pa. The ablation rate depends on the backscattering of the ablated particles by the molecules of the gas, which produces different amounts of re-deposited material on the target surface for each gas. The ablation rate in a water vapour atmosphere presents a particular behaviour due to the formation of different calcium phosphate phases from the original hydroxylapatite under ArF excimer laser irradiation as compared those formed in other ambient gases.

Deep UV laser induced fluorescence in fluoride thin films

Fluorescence experiments have been performed to study the interaction of 193-nm laser radiation with dielectric thin films of LaF3, AlF3, and MgF2. Spectral- and time-resolved measurements reveal the presence of cerium in LaF3 and the influence of hydrocarbons in MgF2 and LaF3. Virtually no fluorescence response is observable in the case of AlF3. Supplementary measurements on multilayer stacks confirm the contribution of hydrocarbon and cerium emission in high-reflective UV mirrors upon ArF excimer laser irradiation. Energy density dependent measurements indicate a linear absorption process as the origin of UV laser induced fluorescence in LaF3. Luminescence calculations are applied as a helpful tool in order to account for interference effects that are inherently to be found in the multilayer emission spectra.

Ignition of Rapid Compressed CH4-Air Pre-Mixture by ArF Excimer Laser.

The purpose of this study is to investigate the effect of supplying 0 radicals generated by ArF excimer laser irradiation on the ignition of a CH4-Air mixture. The laser-induced ignition experiment was performed in CH4-air mixtures using a Rapid Compression Expansion Machine (RCEM). To study the ignition process, the pressure in the combustion chamber was monitored and OH* chemiluminescence images were taken using a high-speed camera. The laser-induced ignition was classified 3 patterns ; point ignition, line ignition and ignition near the window. These ignition patterns were decided by the magnitude of the absorption cofficient of oxygen and the ignition types and locations were predictable if the absorption coefficient was known. Then, we evaluated the advantages of applying the laser bulk ignition technique by comparing the pressure profiles with high laser power and with minimum laser ignition power. It was considered that 0 radical generation along the laser beam path caused to reduce the ignition delay and enhance the combustion rate after the ignition.

Deep UV laser induced luminescence in oxide thin films

Time-resolved luminescence experiments have been set up in order to study the interaction of 193-nm laser radiation with dielectric thin films. At room temperature, Al2O3 coatings show photoluminescence upon ArF excimer laser irradiation, with significant intensity contributions besides the known substrate emission. Time- and energy-resolved measurements indicate the presence of oxygen-defect centers in Al2O3 coatings, which suggests a strong single-photon interaction at 193 nm by F+ and F center absorption. Measurements on highly reflective thin-film stacks, consisting of quarter-wave Al2O3 and SiO2 layers, indicate similar UV excitations, mainly from color centers of Al2O3.

Effects of H2 impregnation on excimer-laser-induced oxygen-deficient center formation in synthetic SiO2 glass

We examined the effect of H2 impregnation on defect formation upon F2 laser (7.9 eV) and ArF excimer laser (6.4 eV) irradiation. It was revealed that H2 impregnation enhanced the formation of oxygen-deficient center (Si–Si bond) as well as suppressed the formation of E′ center and nonbridging oxygen hole center. A Si–Si bond gives an intense absorption band peaking at 7.6 eV, which contributes the absorption at the wavelength of F2 laser light. These results indicate that H2-free SiO2 glass, which is clearly inappropriate for KrF and ArF excimer laser optics, is more suitable for F2 laser optics than H2-impregnated glass.

Preparation of Pt/TiO 2 nanocomposite films by 2-beam pulsed laser deposition

Pt/TiO 2 nanocomposite films were prepared using 2-beam pulsed laser deposition (2B-PLD) technique, where sequential irradiation of ArF excimer laser for ionization after Nd:YAG laser for ablation of the target was utilized. The effect of the time delay of the ArF laser after the ablation laser irradiation on the structures of Pt/TiO 2 nanocomposite films was studied. It was inferred from X-ray photoelectron spectroscopy (XPS) of the as-deposited films that the coordination structures around Ti atoms in the films were affected by the time delays. Single phase TiO 2 in anatase form was obtained in the annealed Pt/TiO 2 nanocomposite films prepared at 200–250 ns of the time delays, which suggests that the effective photoionization of the ablated species could take place by the ArF laser irradiation in this time range and affect the structure of the deposits.

Pulsed laser deposition of semiconductor-ITO composite films on electric-field-applied substrates

The DC electric-field effect on the crystallinity of II–VI semiconductor in composite systems has been investigated for CdS–ITO films fabricated via alternative pulsed laser deposition (PLD) of CdS and indium tin oxide (ITO) on electric-field-applied substrates. The alternative laser ablation was performed under irradiation of ArF excimer laser in mixture gas of helium and oxygen. The application of electric-field facilitated the preferential crystal-growth of CdS in nanometer scale at low pressure, whereas all the films grown without the field were amorphous. There is a large difference in the crystallization between the films grown on field-applied and heated substrates; the latter showed the crystal-growth with random orientations. This difference indicates that the existence of electric-field has an influence on the transformation from amorphous to crystalline phase of CdS. The driving force for the field-induced crystallization is also discussed in the light of the Joule heat.

Laser-induced optical emission of carbon plume by excimer and Nd:YAG laser irradiation

We investigated the effect of additional laser irradiation on the energy state of a laser-ablated plume from a graphite target. Optical emissions from the plume were observed by a spectrometer equipped with an intensified charged couple device (ICCD) immediately after additional laser irradiation at various time delays relative to the ablation. The emission intensity from atomic carbon, ionic carbon, C 2 and C 3 increased when an ArF excimer laser irradiated the carbon plume. The atomic carbon absorbed the ArF excimer laser resonantly and was ionized. Using the fourth harmonic output of an Nd:YAG laser or a KrF excimer laser, the emission intensity from C 2 and C 3 only increased. However, no emission spectra change was observed with additional irradiation fundamental output of an Nd:YAG laser. The emission intensity from the atomic and ionic carbon and carbon clusters (C 2 and C 3) following ArF excimer laser irradiation of the plume was stronger than that using any other lasers.

Photochemistry in phosphorus-doped silica glass by ArF excimer laser irradiation: Crucial effect of H2 loading

A striking difference in photochemistry induced by ArF excimer laser irradiation was found between H2-unloaded and H2-loaded P-doped SiO2 glasses. Although a pair of phosphorus-electron center and phosphorus-oxygen-hole center (POHC) were produced in the former, PHO2 (pyramidal shape) and PO2 (planar) centers were created instead of the POHC in the latter. The present finding strongly suggests that H2 loading treatment of P-doped SiO2 at ambient temperature induces a drastic structural alternation around phosphorus, leading to new precursor structures which account to the photosensitivity of the materials.

Measurement of the surface temperature of the cornea during ArF excimer laser ablation by thermal radiometry with a 15-nanosecond time response.

The purpose of this work was to develop a temperature measurement system with a nanosecond time response to monitor the transient temperature of the corneal surface during laser refractive surgery. Thermal radiation from the surface of the porcine cornea during ArF excimer laser irradiation was measured using a photovoltaic HgCdTe detector with a response bandwidth of 150 MHz. Maximum thermal radiation occurred at 31 +/- 4 nanoseconds, which was longer than the time response of the measurement system. The temperature derived from the detected signal reached over 100 degrees C at a fluence of 80 mJ/cm(2), which was the ablation threshold, and reached 240 degrees C at a fluence of 180 mJ/cm(2). The present system of temperature measurement with a time response of 15.7 nanoseconds revealed that the transient surface temperature of the cornea during ablation is much higher than that previously reported.

レーザ照射による水中へのダイヤモンドライクカーボン微粒子の合成

Chemical vapor deposition (CVD) is one of the most popular film deposition methods on solid substrates. We propose a new powder synthesis method utilizing CVD process, that uses a liquid instead of a solid as a substrate. The gas close to the liquid surface and/or dissolved in the liquid was decomposed by laser irradiation and particles were synthesized. Water was used as a liquid, because of its large solubility. Diamond like carbon particles were synthesized by ArF excimer laser irradiation to the methane dissolved in water. Grain size of the particle synthesized from the gas close to the liquid surface was 50-200nm, and that from the gas dissolved in the water was 200-700nm. Grain size could be varied with laser power. Energy dispense spectroscopy (EDS) clarified that the powder mostly consisted of carbon, and oxygen and nitrogen could not detected. High resolution transmission electron microscope (HR-TEM) and electron diffraction revealed that the powder was composed of diamond like carbon (DLC) and multiwall carbon nanotube was grown on the DLC