XeF Laser Research Articles

P‐77: Novel and Highly Reliable XeF and KrF Excimer Laser Annealing for Reducing the Cost of Flat Panel Display Equipment

One of the most important goals of flat panel display (FPD) manufacturing is to reduce the cost of excimer laser annealing (ELA) processes of low‐temperature poly‐Si (LTPS) technology. We developed novel XeF (351 nm) and KrF (248 nm) excimer lasers specially designed for ELA. The semiconductor‐field‐proven excimer laser technologies are applied to stabilize the operation and to reduce the cost.

A narrow linewidth and tunable XeF(C–A) laser

With a comparatively simple design, a wavelength tunable optically pumped XeF(C–A) laser with narrow linewidth is set up. Experiments are performed under different conditions to narrow the linewidth of the XeF(C–A) laser to the range of 0.2–1 nm, with an output energy of higher than 2 J in the spectral range of 460–500 nm. The wavelength of the XeF(C–A) laser can be tuned in the spectral range of 448–518 nm.

An optically pumped XeF(C-A) laser with repetitive rate of 10 Hz

A novel XeF(C-A) laser which can be operated in repetition mode has been developed based on surface discharge optical pumping technique. Its maximum repetitive rate is up to 10 Hz. The influence of repetitive rate and gas flow rate on the stability of output energy is studied and the main factor which influences the stability of output energy is analyzed. The experimental results show that increasing the gas flow rate into laser chamber can improve the stability of the output energy. The ideal output energy results of 20 laser pulses under different repetitive rates and their optimal experimental conditions are presented. Output energies of more than 4 J and better stability can be obtained when the laser device operates at 1, 2, and 5 Hz, respectively. When the gas feed rate is larger than 53 l/s, the stability of output energy is improved obviously at the repetitive rate of 10 Hz, and the average energy of 20 laser pulses is up to 3.2 J.

ChemInform Abstract: Determination of the Photochemical Efficiency of o-Quinodimethane Ring Closure in Room-Temperature Solutions by Using Time-Delayed, Two-Color Photolysis Technique.

Photochemical efficiency of o-quinodimethane (3) ring closure at room temperature was determined by using a time-delayed, two-color photolysis technique. o-Quinodimethane (3) was generated by the photolysis of 1,2-bis[(phenylseleno)methyl]benzene (1) by a KrF (248 nm) laser pulse and thus-generated 3 was photolyzed by a subsequent XeCl (308 nm)/XeF (351 nm) laser pulse with varying delay time of 0 to 3 s. The time profile of 3 was monitored by the chemical analyses of benzocyclobutene (5) (a photochemical product of 3), which was formed by a one-photon process, and the spiro dimer of 3 (4) (a thermal product of 3) in the two-color photolysis experiments. The time profile of 3 followed a second-order decay kinetics. The photochemical efficiency was obtained by the analysis of the delay-time dependence of the product yields; those of the consumption of 3 and the conversion 3 → 5 by a single pulse of the excimer laser were 81% and 5.7% for the XeCl laser, and 73% and 2.3% for the XeF laser. This difference w...

Effect of XeF laser treatment on structure of nanocrystalline diamond films

Nanocrystalline diamond (NCD) films were deposited on Si substrates by microwave plasma-enhanced chemical vapor deposition (MPECVD) using methane/hydrogen/oxygen (30/169/0.2 sccm) as process gases. Subsequently a thin (0.33 μm) and a thick (1.01 μm) NCD films were irradiated with XeF excimer laser ( λ = 351 nm) with 300 and 600 mJ cm − 2 of energy densities in air. The NCD films became rougher after laser irradiations. Fraction of graphitic clusters decreased but oxygen content increased in the thin NCD film after laser irradiation. Opposite phenomena were observed for the thick NCD films. Effect of laser irradiation to oxygenation and graphitization of NCD films was correlated with structural properties of free surface and grain boundaries of the thin and thick NCD films.

Formation of color centers in a soda-lime silicate glass by excimer laser irradiation

We have investigated defect generation in soda-lime silicate and iron-doped soda-lime silicateglasses by excimer laser irradiation in order to apply coloration due to radiation-induceddefects as a coloring technique for practical glass products. The laser irradiationgenerated various kinds of defects, i.e., non-bridging oxygen hole centers (NBOHCs),E′ centers, and trapped electron centers, as does x-ray andγ-ray irradiation. The amounts of generated NBOHCs, monitored by the absorptionintensity, increased at first with the irradiation time for both the ArF and XeF lasers, andeventually became saturated. The saturated values for the ArF laser irradiation werealmost the same regardless of the laser intensity, whereas those for the XeF laserirradiation were dependent on the intensity; a higher intensity generated a largeramount of NBOHCs. From the comparison of the energies of the photon and theabsorption edge of the soda-lime silicate glasses, the defect generation reactions wereexpected to be one-photon and two-photon processes for the ArF and XeF lasers,respectively. In order to explain the defect generation behavior, we used a simple kineticmodel in which the NBOHCs are reversibly generated and annihilated through thephoto-reaction. The model includes a stretched exponential function, which is oftenobserved for reactions occurring in amorphous materials. The dependences of theamounts of the generated NBOHCs on the irradiation time and intensity of the laserpulses derived from the model were consistent with the experimental results.

Pulsed laser deposition of molybdenum oxide thin films

Thin films of molybdenum oxide were deposited in vacuum by pulsed laser ablation using a xenon fluoride (351 nm) and a krypton fluoride (248 nm) excimer lasers. The films were deposited on unheated substrates and were post-annealed in air in the temperature range 300–500°C. The structural, morphological, chemical, and optical properties of the films were studied. As-deposited films were found to be dark. The transparency of the films was improved with annealing in air. The films were polycrystalline with diffraction peaks that belong to the orthorhombic phase of MoO3. The surface morphology of the films showed a layered structure. Both the grain size and surface roughness increased with annealing temperature. The stoichiometry of the films improved upon annealing in air, with the best stoichiometry of MoO2.95 obtained for films deposited by the XeF laser and annealed at 400°C. Similarly, the best transparency, with a transmittance exceeding 80%, was obtained with the films annealed in the temperature range 400–450°C.

Discharge instability induced by external laser preionization of a XeF discharge laser

External-laser-induced preionization of excimer lasers was investigated. A discharge XeF laser was preionized by two different UV lasers [a KrF laser (λ=249 nm) and an ArF laser (λ=193 nm)], and the improvements in performance of the XeF laser were compared. The XeF laser beam profiles were measured by an intensified CCD (ICCD) camera with temporal resolution of 10 ns. Striated XeF laser profiles were obtained with 249 nm laser preionization, whereas there was no striation in the profiles for 193 nm laser preionization. These striations originated from discharge in the XeF laser induced by laser preionization. The influence of excited rare-gas atoms on the discharge instability was examined.

Nonlinear and electron-beam-induced absorption in pure quartz glasses at the wavelengths of excimer lasers

The two-photon absorption coefficients β at wavelengths 248 and 193nm with pulse widths 80 and 60ns, respectively, have been measured in KS-4V, KU-1, and Corning 7980 glasses. The value of β at 248nm in KS-4V was 0.16cm∕GW, which was about 30% higher than in the other glasses. At a wavelength of 193nm, the value of β virtually coincided at a level of 1.1cm∕GW in all the samples. The coupling factors between the electron-beam (EB)-induced quasi-steady-state optical density at wavelengths 353, 248, and 193nm and the EB power density on the samples were measured for the same glasses, using the radiations of XeF, KrF, and ArF lasers. These factors in KS-4V were 1, 4, and 6cm2∕GW, respectively. The measurement error did not exceed 50%.

High-pulse-repetition-rate UV lasers with the inductance-capacitance discharge stabilisation

Compact high-pulse-repetition-rate XeF and KrF excimer lasers and an N2 laser with plate electrodes and the inductive-capacitance discharge stabilisation are studied. The composition and pressure of the active medium of lasers are optimised for obtaining the maximum output energy and maximum pulse repetition rate at comparatively low (no more than 19 m s-1) active-medium flow rates in the interelectrode gap. The pulse repetition rate achieved 4–5 kHz for the relative root-mean-square deviation of the laser pulse energy less than 2%. It is found that the energy of the N2-laser pulses changes periodically under the action of acoustic perturbations appearing at high pulse repetition rates. It is shown that the use of the inductance-capacitance stabilisation of the discharge provides the increase in the maximum pulse repetition rate by 0.5–1.5 kHz (depending on the active medium type). It is found that the stability of the output energy and maximum pulse repetition rate depend on the location of preionisation sparks with respect to the gas flow direction. Some ways for the development of the technology of plate electrodes and inductance-capacitance discharge stabilisation are proposed.

Photolysis and desorption of adsorbed halomethane molecules under long-wavelength radiation of excimer lasers

The photolysis and desorption of CH2I2, CH3I, CCl4, CHCl3, CH2Cl2, CF2Cl2, and CHF2Cl molecules adsorbed on fused silica under the action of XeF and XeCl laser radiation absorbed by these molecules have been studied. The desorption of molecules that occurs due to expansion to the long-wavelength region of the absorption spectrum of molecules in the adsorbed state, compared to the gas phase, predominates. The laser desorption is characterized by a strong nonlinear dependence on the density of radiation energy. Depending on the relationship between the laser radiation wavelength and the spectrum of electronic states of molecules, photolysis is observed upon absorption of either one or two photons. At an increased fluence of laser radiation energy, the one-photon detachment of the primary CH2I fragment from the CH2I2 molecule changes into the three-photon process. A similar behavior is revealed for the desorption of CH3I molecules from clusters formed on the surface in multilayer adsorption coverages.

Photolytical XeF(C–A) laser amplifier of femtosecond optical pulses: gain measurements and pump efficiency

Gain characteristics of a photolytically driven XeF(C–A) laser amplifier are studied experimentally in the unsaturated amplification regime. The gaseous active medium is optically pumped by vacuum-ultraviolet (VUV) radiation from two large-area multichannel surface discharges initiated along opposite walls of the amplifier chamber. A total gain factor of 102 is obtained for the ultrashort optical pulses under multipass amplification in the active volume of 40 × 18 × 4 cm3 dimensions with a spatially homogeneous gain distribution. Spectral measurements reveal a good conservation of the seed pulse spectrum. Small-signal gain reaching 2×10-3 cm-1 is observed for the blue-green seed pulses of 150 fs duration, as well as for the continuous seed radiation at 488 nm. The obtained gain values, being compared with the gain calculated for the measured pumping radiation power, indicate that the quantum yield of the XeF(B) formation as a result of the XeF2 photodissociation is high and approaches unity within the spectral band of the XeF2 VUV photodissociation continuum.

10 J energy-level optically pumped XeF(C-A) laser with repetition mode

An optically pumped XeF(C-A) laser with 0.1-1 Hz pulse-repetition mode is demonstrated. The maximum output energy is up to 18.7 J, which corresponds to the conversion efficiency of the stored electrical energy to laser output energy of 0.25%. The laser active medium is optically excited by vacuum-ultraviolet radiation of two single-channel surface discharges initiated along opposite walls of the laser cell. The laser resonator output coupling is 10%, and the initial XeF2 concentration in the active gas mixture is ~1 x 10(17) cm(-3). The duration of the observed laser pulses is 0.8 to 1.2 micros, and the spectral band is 470-500 nm.

Primary Processes During the Photodeposition of Ag Clusters on TiO2 Nanoparticles

In-situ transient laser flash photolysis absorption studies were carried out on transparent colloidal TiO2 suspensions in the presence of Ag+ ions and polyvinyl alcohol irradiated by an XeF laser (351nm wavelength) to investigate the early stages of Ag photodeposition on the TiO2 nanoparticles in the nanosecond and millisecond timescales. Photon fluxes of 5 × 10-5 and 1 × 10-6 mol photon/L were used; these were equivalent to approximately 60 and 2 photons/TiO2 particle, respectively. Metallic silver clusters were detected immediately after the laser pulse using both photon fluxes, with calculated quantum yields of 0.3 and 0.5, respectively. In both cases, the metallic silver clusters formed had the same characteristic absorption maxima, assigned to silver clusters containing ≥12 silver atoms. The formation of these larger than expected silver clusters, even at low photon fluxes, is believed to be a result of an electron transfer within the TiO2 aggregates, similar to an antenna effect. It is proposed that the silver deposits form an Ohmic contact on the TiO2 particle thus allowing the continuous electron transfer through the metallic silver nuclei and hence the further reduction of adsorbed Ag+ ions. This study reports the first observation of an antenna type effect for a photocatalytic reduction reaction.

Excimer laser ablation of sintered hydroxyapatite

The ablation treatment of a sintered hydroxyapatite (Ca 10(PO 4) 6(OH) 2) sample has been studied by irradiating the excimer laser in air at one atmosphere for the micro machining treatment. Three different excimer lasers of ArF, KrF and XeF with wavelengths of 193 nm, 248 nm and 351 nm, respectively were used to examine the effect of the wavelength on the ablation rate and the surface morphology of the ablated area. Photochemical ablation of sintered hydroxyapatite is possible with ArF and KrF excimer lasers and the flat bottom shape of an ablated hole with sharp edge can be obtained. On the contrary, thermal ablation occurred in the case of XeF laser due to a long wavelength, and the ablation rate was much larger than those in the case of ArF and KrF lasers, though the ablated surface was rough and not flat. Threshold values of laser fluence for ablation were about 0.8, 1.5 and 6 J/cm 2 for ArF, KrF and XeF, respectively.

Efficient electric-discharge XeF laser pumped by a generator with an inductive energy storage

The parameters of discharge and laser radiation in an Ne—Xe—NF3 mixture excited by a double discharge from a generator with an inductive energy storage and a current interrupter based on semiconductor SOS diodes are studied. It is shown that a high-voltage prepulse formed by a generator with inductive energy storage increases considerably the stability and duration of volume discharge in mixtures with NF3, and also increases the emission energy and pulse duration of laser radiation at the B—X transition of XeF* molecules. In the case of spark preionisation, radiation pulses with a total duration of up to 200 ns and the full width at half-maximum up to 100 ns are obtained. The maximum output energy of an XeF laser (λ = 348, 351 and 353 nm) was 0.5 J for the electric efficiency up to 1.6%.

Photoinduced non-linear optical effects in lanthanum calcium borate single crystals

We have revealed that lanthanum calcium borate (La2CaB10O19) crystals show two-photon absorption (TPA) favored by UV laser field. UV-induced TPA measurements were performed under illumination of a Xe-F laser (λ = 217nm) as a photoinducing (pumping) beam. This pumping laser beam created a thin surface layer (about 85 nm) that was a source of the observed photoinduced TPA. Use of the longer photoinducing wavelength leads to a substantial decrease of the photoinduced TPA. The performed DFT calculations of the band energy dispersion clearly show that the effect observed is due to specific band energy dispersion between the sub-bands. The Raman shifted Nd-YAG laser radiation (λ = 1.9 μm) as well as its second and fourth harmonics (λ = 950 and λ = 475 nm, respectively) were used as fundamental (probing) beams of the TPA. The highest values of the TPA β coefficient were achieved for polarization of the pumping light directed along the second-order crystallographic axis. The obtained values of TPA coefficients are higher than for BiBO crystals that were also investigated by us.

The development of a joule level of XeF(C-A) laser by optical pumping

A joule level of XeF(C-A) laser optically pumped by a sectioned surface discharge was developed. The irradiative intensity of pumping source was diagnosed by calculating XeF2 photo-dissociation wave evolvement which was photographed by a framing camera. The photon flux in the wavelength region of 140 to 170nm is about 5 × 1023 photon s−1cm−2, that corresponds to the irradiative brightness temperature of more than 25000 K. The laser experiments were carried out in different conditions. The maximum laser output energy of 2.5 J was obtained with the total conversion efficiency of 0.1%.

Etching microscopic defects in polycarbonate due to high dose ArF or KrF laser exposure

The ArF or KrF excimer laser exposure on the polycarbonate (PC) with corresponding doses higher than ϕ th 5.2 J / cm 2 , at 32 mJ / cm 2 fluence per pulse and 5 Hz pulse repetition rate (PRR), induces regular defects leading to self assembled defect structure following electrochemical etching (ECE). We have observed the conical-like structure for ϕ > ϕ th , whereas the polymer experiences hardening effect due to crosslinking when ϕ < ϕ th . Subsequently, conical-like, structure turns into track-like pits developing under ECE multiple treeing. Self assembled defect structure may be seen by naked eye as white spots, despite SEM illustrates a type of periodic pit formation-morphology. The exact explanation of the effect is not well understood yet. It looks like alpha tracks in the polymer surface, however the PC pieces were simply treated by excimer lasers at high doses, and they have not been exposed to the nuclear particles afterwards. We could not observe those effects at 308 nm (XeCl laser) or longer wavelengths at 351 nm (XeF laser) where UV photoablation does not occur. It indicates that UV ablation establishes surface degradation at shorter wavelengths, leading to laser micro etching. The mean track (defect) density is about one order of magnitude greater than the normal alpha tracks. Increasing UV doses, polymer undergoes a plateau, corresponding to etched defect saturation on PC.

Compact 4-kHz XeF laser with a multisectional discharge gap

A XeF electric-discharge laser with a pulse repetition rate f of up to 4 kHz is developed. The laser electrode unit is based on plate electrodes with inductive—capacitive discharge stabilisation. He and Ne are used as buffer gases, and NF3 serves as a fluorine donor. A narrow (∼1 mm) discharge is achieved; the specific energy deposition per unit length of the active volume is as high as 2 J m-1. The maximum energy in a laser pulse is ∼3 mJ for NF3—Xe—He and NF3—Xe—Ne mixtures at total pressures of 0.8 and 1.2 atm, respectively, and the maximum lasing efficiency is ∼0.73%. The maximum gas velocity in the working gap is 19 m s-1. The laser-pulse energy at a high pulse repetition rate (4 kHz) virtually coincides with that obtained at a low repetition rate. The mean output pulse power at f = 4 kHz reaches 12 W, and the rms deviation of the laser-pulse energy is ∼2.5%.