Excimer Laser Wavelength Research Articles

Dissociative and nondissociative photoionization ofH2from theE,FΣg+1excited state

We present a theoretical study of dissociative and nondissociative photoionization of ${\mathrm{H}}_{2}$ from the $E,F^{1}\ensuremath{\Sigma}_{g}^{+}(\ensuremath{\upsilon},J=0)$ excited state for $\ensuremath{\upsilon}=0--9$ in the photon energy range $3\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}14\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. We have found that, for most initial $\ensuremath{\upsilon}$'s, dissociative ionization is the dominant process for photon energies well above the dissociative ionization threshold. In this photon energy range, resonance structures arising from autoionization of the ${Q}_{1}$ doubly excited states of ${\mathrm{H}}_{2}$ are observed. Cross sections differential in the energy of the remaining ${\mathrm{H}}_{2}^{+}$ ion or in the proton kinetic energy are analyzed in detail for a photon energy of $6.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ corresponding to the wavelength of an ArF excimer laser. Comparison of our results with the available experimental measurements is good. We show, however, that contribution of the ${Q}_{1}$ doubly excited states to the measured dissociative ionization cross section is more important than originally believed.

Photoemission studies from metal by UV lasers

Studies of electron emission by photoelectric process from pure Zn cathodes by UV laser beams were performed. As UV sources, two different wavelength excimer lasers were utilized in order to investigate the photoemission as pure photoelectric process and multiphoton process. The Schottky effect was also considered. The utilized laser sources were a KrF operating at 248nm wavelength (5eV), 23ns full width at half maximum (FWHM), and a XeCl operating at 308nm wavelength (4.02eV), 10ns FWHM. The cathode work function was 4.33eV, a value lower and higher than the photon energies of KrF and XeCl lasers, respectively. The photocathodes were tested in a vacuum photodiode cell at 10−7mbar. The cathodes were irradiated at normal incidence and the anode-cathode distance was set to 3.7mm. Due to the electrical breakdown into the photodiode gap, the maximum applied accelerating voltage was 20kV. Under the above experimental conditions a maximum of 5.4MV∕m electric field resulted. Under the space charge effect, the electron emission was higher than the one expected by the Child-Langmuir law. In saturation regime the electron emission increased with the accelerating voltage owing to the Schottky effect and plasma formation. The highest output current was achieved with the KrF laser at 14mJ laser energy. Its value was about 12A, corresponding to a global quantum efficiency of approximately 1×10−4, while the temporal quantum efficiency presented a maximum value of 1.1×10−4 located at the tail of the laser pulse. The estimated efficiencies with the XeCl laser were lower than the KrF ones as well as the output current and the plasma formation, although higher energy values than the KrF ones were utilized. By the results obtained, we conclude that the plasma generation is strongly due to the extracted current but weakly to the used laser intensity.

Excimer laser annealing of perovskite thin films: morphology and gas-sensor properties

Pulsed laser annealing was used to modify surface morphology and to enhance crystallization of amorphous films of the p-type perovskite SrFeyCo1-yO2.5+x (y=0.5). The films were prepared by the pulsed laser deposition technique on sapphire substrates. Both film deposition and film annealing was done using a KrF excimer laser (wavelength = 248 nm). The effects of laser energy and pulse number on film morphology, structure and gas-sensing properties were investigated. An amorphous film did not show any sensor response to oxygen composition changes, while the same film after 80 pulses of annealing at 100mJ/cm2 showed a fast response at 300°C and 400°C. In comparison to a dense crystalline film deposited at 700°C, the annealed film showed a faster response to oxygen composition changes at 300°C.

Spatial light modulator for maskless optical projection lithography

Spatial light modulators (SLMs) designed to replace photomasks for optical lithography have been designed, fabricated, and tested. These microelectromechanical devices are fabricated with alternating polycrystalline Si and sacrificial SiO2 layers that are patterned by a 193nm wavelength scanner to dimensions as small as 150nm. Aerial image simulations were used to define the mechanical requirements of the devices. Piston motion of electrically actuated devices was measured with an optical profilometer. The measurements were fit to a simple equation to within 1nm precision, which is adequate for defining 50nm features lithographically. Transient response measurements show that one version of the SLM responds to actuation as quickly as 20μs, fast enough for current 193nm wavelength excimer laser sources.

Laser-based temperature imaging close to surfaces with toluene and NO-LIF

Two novel techniques based on Laser-Induced Fluorescence (LIF) were applied to measure gas-phase temperature distributions in boundary layers close to wall surfaces. Singleline toluene-LIF thermometry was used to image temperature in a nitrogen gas flow above a heated wall. The nitrogen gas flow was doped with evaporated toluene. When excited at 266 nm, the toluene LIF-signal shows an exponential dependence on temperature. This behavior was used to calculate absolute temperatures from LIF images after calibration at known conditions. The second technique, multi-line NO-LIF thermometry was applied to image temperature in the quenching boundary layer close to a metal wall located on a flat flame burner. A small amount of nitric oxide was mixed into the air/methane mixture. NO molecules were excited in the A-X (0, 0)-band at 225 nm. NO-LIF excitation spectra were acquired by tuning the excimer laser wavelength and recording the NO LIF-signal with an ICCD camera. Absolute temperatures were calculated for every pixel by fitting simulated excitation spectra to the experimental data. Temperature distributions close to the wall surface were measured at two different flow-rate conditions. A high nominal spatial resolution of 0.016 mm/pixel in direction perpendicular to the wall was reached. Wall surface temperatures were recorded simultaneously by embedded thermocouples and compared with gas-phase temperature near the wall surface.

Laser pulse impact characterization using ultrasonic transducers

Experiments were conducted with an excimer laser (wavelength 248 nm) to cause damage to gelatinous materials, which have optical properties similar to skin tissue. The laser pulses, when they impact the specimen, generate an elastic wave that propagates through the material. A 150-MHz broadband ultrasonic transducer was placed underneath the specimen to observe the material response and the pressure generated by the laser pulse. Different incident laser energies and laser beam spot sizes were utilized to obtain a range of input parameters, and the corresponding transducer responses were recorded. Fourier analysis of the signals was performed to identify the frequency response from the laser pulse. Additional tests were carried out to observe the effects due to a train of laser pulses. Earlier tests were performed with 200-kHz hydrophone-calibrated transducer but due to the shortness of the laser pulses (∼20 ns), a higher frequency transducer was used to accurately characterize the laser impact. The initial results indicate that there is a dominant frequency response around the 25–30-MHz regime.

Laser-induced epitaxial recrystallization after alkali-ion implantion into α-quartz

Ion implantation and recrystallization of quartz may be a very important process for photonic applications. We have investigated how laser irradiation can be used to epitaxially recrystallize layers of α-quartz, which were amorphized by 175 keV Rb + or 250 keV Cs + ion implantation at a fluence of 2.5 × 10 16 cm −2. The samples were irradiated with pulses of a XeCl excimer laser (wavelength 308 nm, pulse length 55 ns, energy density 3.2–5 J/cm 2). The thickness of the amorphous layer and the quality of the recrystallized layer were analyzed by Rutherford Backscattering Channeling Spectroscopy. Partial epitaxial recrystallization was found for all laser-irradiated samples that proceeded faster for higher laser energy density, however no full epitaxy was achieved up to the maximum laser energy of 5 J/cm 2. The degree of epitaxy was more or less the same for 20 or 200 laser shots.

Phase Transitions in Thin Titanium Oxide Films under the Action of Excimer Laser Radiation

We have studied the action of pulsed vacuum ultraviolet (VUV) radiation of an ArF excimer laser (wavelength, 193 nm; pulse duration, 20 ns; pulse energy density, 40–50 J/cm2) on thin amorphous films of titanium oxide (TiO2) grown by light-assisted molecular beam epitaxy. A comparison of the Raman spectra of samples measured before and after exposure shows that VUV irradiation at increasing fluence induces the crystallization of amorphous TiO2 to anatase, which is further transformed into rutile.

Physico-chemical changes of human enamel irradiated with ArF excimer laser

Irradiation of tooth enamel by lasers can alter its structure and chemical composition. The purpose of this work was to determine the compositional changes and to elucidate the mechanisms of laser-enamel interaction using ArF excimer laser. Human enamel irradiated with ArF excimer laser (wavelength, 193 nm) at different fluences, was examined using X-ray diffraction, infrared (IR) spectroscopy and microprobe analysis. Ablation without significant compositional changes in irradiated enamel was evident when low fluences (approximately 200-380 mJ/cm2) were used. However, fluences between 640 and 2,300 mJ/cm2 resulted in an increased Ca/P ratio, decreased amount of carbonate and protein, and the formation of tricalcium phosphate and tetracalcium phosphate, suggesting the involvement of a photothermal mechanism. The results show that ArF excimer laser can alter the chemical composition and morphology of the highly mineralized (96%) dental enamel, depending on the fluence used.

Properties of gas sensor using CNTs thin film prepared by PLD/CVD method

ABSTRACTCarbon nanotube (CNT) is a promising material which has potential for applications to various nanotechnology devices owing to its unique features like high electrical conductivity, mechanical strength and large specific surface area. Recently, gas sensors using CNTs or carbon nano-fiber, which have extremely high sensitivity at a room temperature with fast response, have been reported. Being exposed to oxidizing gas like NO, NO2 or O3, the conductance of the single-walled carbon nanotubes (SWNTs) changes due to charge transfer between the SWNT surface and gas molecules adsorbed. Therefore CNTs will be applicable to O2 and O3 gas sensors in various fields.CNTs thin film sensor was prepared and its performance was investigated. CNTs thin film was prepared on a SiN substrate using PLD/CVD method. To prepare a sensor device an Al2O3 substrate with Pt interdigital electrodes (sensor substrate) was used. In this method, Fe catalytic thin film was deposited by pulsed laser deposition (PLD) method using KrF excimer laser of wavelength 248 nm, repetition rate 10 Hz, energy fluence 3 J/cm2. During PLD process the substrate temperature and the ambient gas pressure were kept at room temperature of 25 °C and 3.5×10−5 Torr, respectively. The thickness and roughness of the films were modified by changing a number of ablation laser pulses from 300 to 3,000. A small number of laser pulses deposited Fe nano-particles of less than 10 nm in diameter on the substrate. We used 1000 pulses for a catalytic Fe film preparation as small-sized catalyst is necessary to grow SWNTs. CNTs were grown from Fe thin film on Si or sensor substrates by thermal CVD method. Ethylene gas was used as carbon source. The substrate was set in the quartz reaction tube heated to 1000 °C in an electric furnace. CNTs were grown for 20 ∼ 40 minutes. In our previous studies, it was found that SWNTs can grow under this process. The prepared CNTs were characterized using SEM, TEM and Raman spectroscopy. From SEM observation, randomly oriented CNTs were found on both the Si substrate and the sensor substrate. A diameter of CNTs was found 20 ∼ 50 nm which tended to be proportional to the size of catalyst particle.The Sensitivity of CNT gas sensor was evaluated by measuring the electrical characteristic of the sensor. The sensor was exposed to NO gas of different concentration in a chamber. Resistance of the sensor was measured by two-terminal method, while the sensor was heated from room temperature to a high temperature on a block heater. The Sensitivity of CNT gas sensors, response time and reproducibility was measured. Initial resistance of the film was about 450 and it decreases with temperature increase. This shows that a prepared CNTs sensor film has semiconductor characteristics. Measured maximum sensitivity of CNTs gas sensor was 6.9 % at sensor temperature 290 deg. Detail studies and the latest data will be presented at the symposium.

Preparation of superhard amorphous carbon films with low internal stress

Tetrahedral amorphous carbon (ta-C) films were prepared by pulsed laser deposition using a 248-nm excimer laser wavelength and up to 45 J/cm 2 laser pulse energy fluence. Fluences above 6 J/cm 2 and mean kinetic energies of the ablated species above 30 eV, respectively, as well as substrate temperatures below 100 °C were found to be necessary for the formation of ta-C films with high sp 3 percentage. Such films are completely amorphous and have up to 85% sp 3 bonds. As-deposited films show high compressive stresses in the range of 8–10 GPa. The possibilities to reduce those stresses by means of thermal and pulsed laser annealing were investigated. We found that both methods allow the preparation of nearly stress-free diamond-like carbon films with several micrometers of thickness and good adherence to Si and WC hard metal substrates. The Vickers microhardness of those films was measured to be 55–65 GPa and the Young's modulus was measured to be 800–900 GPa by using a dynamic indentation method.

Simultaneous determinations of U-Pb age and REE abundances for zircons using ArF excimer laser ablation-ICPMS

Using a laser ablation-inductively coupled plasma mass spectrometer (LA-ICPMS), U-Pb age and rare earth element (REE) abundances have been determined simultaneously from a single 20 μm ablation pit of zircon. The laser ablation system utilizing 193 nm wave-length ArF excimer laser produces stable and reproducible signal intensities resulted in good precisions on measurements of element concentrations and isotopic ratios. Because of the higher energy density of the deep ultra-violet laser beam, ablation fractionation between Pb and U were reduced even with the prolonged ablation, and thus accuracy of Pb-U age was improved significantly. A chicane-type ion lens system was applied to a quadrupole-based ICPMS instrument. With the chicane ion lens, higher elemental sensitivity (4 times for light mass range and 3 times for mid to heavy mass range) and lower white background (<5 cps for light to mid mass range and <2 cps for heavy mass range) were achieved. This further improved analytical precisions for low concentration elements in samples. The 238U-206Pb ages for Nancy standard zircon (Nancy 91500), SHRIMP calibration standard zircon (SL13) and Antarctic zircon (PMA7) obtained in this study were 1064 ± 24 Ma, 569 ± 78 Ma and 2438 ± 101 Ma (2-sigma), respectively. Relative age differences from previous reports were 0.2%, 0.4% and 3.2% respectively, demonstrative of high reliability of the method. The REE abundances in zircon samples were calibrated using a NIST 610 glass standard reference material. The resulting REE abundance data for zircons (Nancy 91500 and SL13) show good agreement with those for literature values within the analytical precision of ∼20%. The matrix effect that may occur between the synthetic glass standard and zircon crystals is obviously smaller than the precision and thus negligible for this precision levels. The data presented here demonstrate clearly that the combination of ArF excimer laser and ICPMS equipped with the chicane ion lens has a potential to become a significant tool for zircon geochemistry.

Influence of substrate temperature and ion-beam energy on the syntheses of aluminium nitride thin films by nitrogen-ion-assisted pulsed-laser deposition

Aluminium nitride (AlN) thin films have been grown on Si(100), Si(111) and Sapphire Al2O3(001) substrates by pulsed KrF excimer laser (wavelength 248 nm, duration 30 ns) ablation of an AlN target with the assistance of nitrogen-ion-beam bombardment. The influence of process parameters such as substrate temperature and ion-beam energy has been investigated in order to obtain high-quality AlN films. The AlN films deposited by pulsed-laser deposition (PLD) have been characterized by X-ray diffraction (XRD) to determine the crystalline quality, grain size and growth orientation with respect to the substrate. The XRD spectra of AlN films on Si(100), Si(111) and Sapphire substrates yield full-width-half-maximum (FWHM) values of approximately 1.6. The bonding characteristics in the films have been evaluated by Raman spectroscopy. The chemical composition of the films has been characterized by X-ray photoelectron spectroscopy (XPS). The surface morphology of the films has been measured by atomic force microscopy (AFM). At a substrate temperature of at least 600 °C, polycrystalline AlN films with orientations of AlN(100) and AlN(101) have been synthesized.

Reactive pulsed laser deposition of silica and doped silica thin films

Silicon oxide and Si-, GeO 2-, B 2O 3- and P 2O 5-doped SiO 2 films were deposited under various conditions by pulsed laser deposition using a KrF (248 nm wavelength) excimer laser, and were characterized by profilometry, ellipsometry and Rutherford backscattering. Relationships between laser energy, repetition rate, substrate temperature and oxygen pressure, and the composition, optical properties and roughness of the films were examined. SiO 2 deposited from a Si target at a substrate temperature of 400 °C, oxygen pressure of approximately 6 Pa, laser energy of 550 mJ/pulse, and laser repetition rate of 25 Hz had a composition and refractive index close to the bulk values. Doped silica was made by ablation from sintered mixed oxide targets. Germanium and phosphorus doping increased the refractive index as compared to pure SiO 2, but boron doping decreased the refractive index.

Gold-black coatings for freestanding pyroelectric detectors

We describe the process of depositing gold-black on thin, freestandingpyroelectric detector substrates and compare this with previous workdocumented in the literature. We have evaluated gold-black coatings onthin, freestanding pyroelectric detector substrates by means of scanningelectron microscope, Fourier transform infrared spectrophotometerreflectance, and spectral responsivity measurements. Spectrophotometricmeasurements indicate that reflectance at normal incidence varies by lessthan 1% at wavelengths shorter than 2.5 µmand by less than 10% at 10 µm.These results are correlated with the spectral responsivity of the detector anddemonstrate that radiation not reflected by the gold-black is absorbed by the detectorelement. We have evaluated gold-black coatings as a function of position at twowavelengths and found variations of less than 1% at 1.25 µmand less than 5% at 10.3 µm,which demonstrates that spatial uniformity can be coating dependent.Gold-black coatings exposed to a 193 nm wavelength excimer laser wereevaluated by visual inspection for damage and determined to have adamage threshold of approximately 38 mJ cm−2.

Ablative removal of codeposits on JT-60 carbon tiles by an excimer laser

The codeposits on JT-60 tiles experienced hydrogen plasma burning were irradiated by focused beams of an excimer laser. The removal rate of the JT-60 codeposits was low when the laser energy density was smaller than the ablation threshold (1.0 J/cm 2), but reached to 1.1 μm/pulse at the laser energy density of 7.6 J/cm 2. The effective absorption coefficient k in the JT-60 codeposits at ArF excimer laser wavelength was determined to be 1.9 μm −1, which is almost one order smaller than the optical absorption coefficient at the same wavelength in graphite (16.4 μm −1). In the process of ablative removal of the codeposits, hydrogen was released predominantly in the form of hydrogen molecule and water formation could be ruled out. The temperature rise on the surface was measured on the basis of Planck’s law of radiation, and the temperature during the irradiation at the laser energy density of 0.5 J/cm 2 decreased from 3570 K at the beginning of the irradiation to 2550 K at 1000th pulse of the irradiation.

Absolute quantum yield measurements for the formation of oxygen atoms after UV laser excitation of SO2 at 222-4 nm

The dynamics of formation of oxygen atoms after UV photoexcitation of SO2 in the gas-phase was studied by pulsed laser photolysis-laser-inducedfluorescence ‘pump-and-probe’ technique in a flow reactor. SO2 at room-temperature was excited at the KrCl excimer laser wavelength (222.4 nm) and O(3Pj) photofragments were detected under collision-free conditions by vacuum ultraviolet laser-induced fluorescence. The use of narrow-band probe laser radiation, generated viaresonant third-order sum-difference frequency conversion of dye laser radiation in Krypton, allowed the measurement of the nascent O(3Pj=2,1,0) fine-structure state distribution:n j=2/nj=1/nj=0 = (0.88 ± 0.02)/(0.10 ± 0.01)/(0.02 ± 0.01). Employing NO2photolysis as a reference, a value of Φ0(3P) = 0.13 ± 0.05 for the absolute O(3P) atom quantum yield was determined. The measured O(3P) quantum yield is compared with the results of earlier fluorescence quantum yield measurements. A suitable mechanism is suggested in which the dissociation proceeds via internal conversion from high rotational states of the initially excited SO2(~C1B2 (1, 2, 2) vibronic level to nearby continuum states of the electronic ground state.

Ultraviolet optical and microstructural properties of MgF2 and LaF3 coatings deposited by ion-beam sputtering and boat and electron-beam evaporation.

Single layers of MgF2 and LaF3 were deposited upon superpolished fused-silica and CaF2 substrates by ion-beam sputtering (IBS) as well as by boat and electron beam (e-beam) evaporation and were characterized by a variety of complementary analytical techniques. Besides undergoing photometric and ellipsometric inspection, the samples were investigated at 193 and 633 nm by an optical scatter measurement facility. The structural properties were assessed with atomic-force microscopy, x-ray diffraction, TEM techniques that involved conventional thinning methods for the layers. For measurement of mechanical stress in the coatings, special silicon substrates were coated and analyzed. The dispersion behavior of both deposition materials, which was determined on the basis of various independent photometric measurements and data reduction techniques, is in good agreement with that published in the literature and with the bulk properties of the materials. The refractive indices of the MgF2 coatings ranged from 1.415 to 1.440 for the wavelength of the ArF excimer laser (193 nm) and from 1.435 to 1.465 for the wavelength of the F2 excimer laser (157 nm). For single layers of LaF3 the refractive indices extended from 1.67 to 1.70 at 193 nm to approximately 1.80 at 157 nm. The IBS process achieves the best homogeneity and the lowest surface roughness values (close to 1 nm(rms)) of the processes compared in the joint experiment. In contrast to MgF2 boat and e-beam evaporated coatings, which exhibit tensile mechanical stress ranging from 300 to 400 MPa, IBS coatings exhibit high compressive stress of as much as 910 MPa. A similar tendency was found for coating stress in LaF3 single layers. Experimental results are discussed with respect to the microstructural and compositional properties as well as to the surface topography of the coatings.

Optimization of laser-induced forward transfer process of metal thin films

The optimization of laser-induced forward transfer (LIFT) process using laser ablation of thin film and the evaluation of the dependence in laser spot size on the resolution of deposited material are reported. Ni thin film of several hundred 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), and transferred to a Si acceptor substrate. Images of the plume, which were photographed using image intensified CCD camera, indicated the cause of transfer of particles. The deposited material with clear contour was obtained under the condition that thin film was in contact with an acceptor substrate to inhibit the transfer of particles which were generated by laser irradiation. The resolution of a deposited material tended to be improved as the size of laser spot became smaller.

Highly cross-linked polysilane as antireflective coating for deep ultraviolet lithography to improve durability during SiO2 etching

Highly cross-linked polysilanes were recently investigated in an attempt to improve drawbacks of bottom antireflective coatings (BARCs) composed of loosely cross-linked polysilanes that are used for deep ultraviolet (UV) lithography. A highly cross-linked structure was prepared by thermally crosslinking poly(phenylmethylsilane-co-methylhydrosilane-co-methysilyne) with m-diethynylbenzene during baking after coating. Resist profiles are achieved on it without producing a foot or leaving residue at the bottom of the resist. The refractive indices at the KrF excimer laser wavelength (248 nm) are n=1.93 and k=0.32. The reflection is reduced 0.9% regardless of variation in the thickness of the substrate. Highly cross-linked polysilanes improve the melting of loosely cross-linked polysilanes during BARC etching. They also improve the surface roughness of loosely cross-linked polysilanes after substrate (-SiO2) etching. The etch selectivity of the highly cross-linked polysilane BARC/resist during BARC etching is 2.1, which is higher than that of organic BARC/resist (∼1). The etch resistance of the highly cross-linked polysilane during substrate etching is 1.1 times greater than that of the resist. Highly cross-linked polysilanes can not only be etched with high selectivity against resist but can also be superior etch mask for substrate etching.