Excimer Laser Fluence Research Articles

Effect of excimer laser fluence on the surface structuring of Ti under vacuum condition

The effect of variation of excimer laser fluences on the surface structuring of titanium (Ti) targets has been investigated. The KrF excimer laser (λ = 248 nm, t = 20 ns, repetition rate 20 Hz) has been employed for this purpose. The targets were irradiated for various laser fluences ranging from 0.86 J cm−2 to 1.27 J cm−2 under vacuum condition. Various diagnostic techniques like scanning electron microscope (SEM), atomic force microscope (AFM), and X-ray diffraction (XRD) have been utilized to investigate the surface topography and structural changes of laser ablated targets. SEM analysis reveals the formation of laser-induced periodic surface structures (LIPSS) at the central ablated region. The periodicity of LIPSS increases from 5 μm to 88 μm with the increase of fluence from 0.86 J cm−2 to 1.27 J cm−2. The formation of grains is observed at the peripheral ablated region for all laser fluences. Reduction in grain size from 7.7 μm to 3.8 μm is observed with increasing fluence from minimum to maximum value. AFM analysis exhibits the growth of several numbers of bumps and protrusions on irradiated targets. The height of bumps is reduced from 100 nm to 40 nm with increasing fluence. XRD analysis shows that no new phases were formed under vacuum condition for all fluence values. However, an alteration in the peak intensity, crystallinity, and d-spacing for various diffraction planes of Ti has been observed after irradiation.

Role of dual-laser ablation in controlling the Pb depletion in epitaxial growth of Pb(Zr0.52Ti0.48)O3 thin films with enhanced surface quality and ferroelectric properties

Pb depletion in Pb(Zr0.52Ti0.48)O3 (PZT) thin films has remained as a major setback in the growth of defect-free PZT thin films by pulsed laser ablation techniques. At low excimer (KrF) laser fluences, the high volatility of Pb in PZT leads to non-congruent target ablation and, consequently, non-stoichiometric films, whereas, at high laser fluences, the inherent ejection of molten droplets from the target leads to particulate laden films, which is undesirable in heterostructure growth. To overcome these issues, a dual-laser ablation (PLDDL) process that combines an excimer (KrF) laser and CO2 laser pulses was used to grow epitaxial PZT films on SrTiO3 (100) and MgO (100) substrates. Intensified-charge-coupled-detector (ICCD) images and optical emission spectroscopy of the laser-ablated plumes in PLDDL revealed a broader angular expansion and enhanced excitation of the ablated species as compared to those for single-laser ablation (PLDSL). This led to the growth of particulate-free PZT films with higher Pb content, better crystallinity, and lower surface roughness as compared to those deposited using PLDSL. For FE measurements, PZT capacitors were fabricated in situ using the latticed-matched metallic oxide, La0.7Sr0.3MnO3, as the top and bottom electrodes. PZT films deposited using PLDDL exhibited enhanced polarization for all driving voltages as compared to those deposited using PLDSL. A highest remanent polarization (Pr) of ∼91 μC/cm2 and low coercive field of ∼40 kV/cm was recorded at 9 V driving voltage. Fatigue characterization revealed that PZT films deposited using PLDDL showed unchanging polarization, even after 109 switching cycles.

Optimally enhanced optical emission in laser-induced breakdown spectroscopy by combining spatial confinement and dual-pulse irradiation

In laser-induced breakdown spectroscopy (LIBS), a pair of aluminum-plate walls were used to spatially confine the plasmas produced in air by a first laser pulse (KrF excimer laser) from chromium (Cr) targets with a second laser pulse (Nd:YAG laser at 532 nm, 360 mJ/pulse) introduced parallel to the sample surface to re-excite the plasmas. Optical emission enhancement was achieved by combing the spatial confinement and dual-pulse LIBS (DP-LIBS), and then optimized by adjusting the distance between the two walls and the interpulse delay time between both laser pulses. A significant enhancement factor of 168.6 for the emission intensity of the Cr lines was obtained at an excimer laser fluence of 5.6 J/cm(2) using the combined spatial confinement and DP-LIBS, as compared with an enhancement factor of 106.1 was obtained with DP-LIBS only. The enhancement mechanisms based on shock wave theory and reheating in DP-LIBS are discussed.

Patterning of ITO on Glass with Two-Step Using Excimer Laser and Wet Chemical Re-Etching

ITO electrode patterns were fabricated using excimer laser direct-writing technique. The influence of the process parameters (the excimer laser fluence and the stage velocity) on the micromachining quality (the etching depth and ridge height of the edge ) of ITO patterns were experimentally studied. In this paper, the effect of laser fluence and the speed of working platform on the fabrication quality were discussed. The lower laser fluence and suitable platform speed will be very helpful to improve the edge quality of patterning of ITO. However, the recast ridge at the edge also was shown. Use of 39% solution of hydrochloric acid made it possible to further minimize the recast ridge at the edges.

Spectroscopic Study of the Ã1A" - X~1A' System of CHBr

We report fluorescence excitation and emission spectra of the CHBr molecule generated via pyrolysis of <TEX>$CH_3Br$</TEX> in a molecular beam experiment. The 193 nm attenuation cross sections were estimated from the reduction of the CHBr signal as a function of the excimer laser fluence. The derived 193 nm absorption cross section for CHBr [<TEX>$(3.24{\pm}0.59){\times}10^{-17}\;cm^2$</TEX>] is slightly higher than the absorption cross section previously determined for CHCl [<TEX>$(2.6{\pm}0.8){\times}10^{-17}\;cm^2$</TEX>], but the difference is within the estimated uncertainties in the measured cross section.

Effects of angle of probe laser and pinhole diameter on the time-resolved optical inspection system

A real-time in-situ time-resolved optical inspection system comprising two He–Ne probe lasers, a digital oscilloscope, and three fast photodetectors is developed. The effects of angle of probe laser and pinhole diameter on the time-resolved optical inspection system are investigated during excimer laser crystallization. The longest melt duration and ablation excimer laser fluence are independent of pinhole size. The pinhole with a diameter of 0.3 mm is recommended to use in the time-resolved optical inspection system. Determination of melt duration and observation of explosive crystallization oscillation are independent of angle of probe laser, but four criteria for the relationship between probe lasers and photodetectors should be followed.

Investigation of the Pb Depletion in Single and Dual Pulsed Laser Deposited Epitaxial PZT Thin Films and Their Structural Characterization

AbstractWe have investigated the Pb depletion in laser ablated PZT (PbZr0.52Ti0.48O3) films through a systematic study of PZT target-laser interactions for both single laser and dual-laser ablation methods. The study includes films deposited from a stoichiometric and an excess PbO PZT targets. The films were deposited on single crystal SrTiO3[100] substrates at 550˚C with a background oxygen pressure of 500 mT. Single laser deposited films at a laser fluence of 5J/cm2 produced the highest Pb content while dual-laser ablated films where an excimer and a CO2 pulsed lasers were synchronized for ablation produced high Pb content for an excimer laser fluence of less than 2J/cm2. This enabled the growth of particulate-free PZT films with high Pb content. ICCD imaging of the plasma plumes showed variations in the expansion profiles at different laser fluencies that correlated well with the Pb content observed in the deposited films.

Observation of explosive crystallization during excimer laser annealing using in situ time-resolved optical reflection and transmission measurements

XeF excimer laser-induced melting and recrystallization of amorphous silicon was investigated using in situ on-line time-resolved reflection and transmission measurements with a nanosecond time resolution. Explosive crystallization was observed for 900 Å thick amorphous silicon on SiO 2 deposited on non-alkali glass substrate upon 25 ns pulse duration of excimer laser by continuous-wave He-Ne probe laser. Three distinct regrowth regimes were investigated on the basis of various excimer laser fluences. Scanning electron microscopy, Raman spectroscopy and atomic force microscopy were used to evaluate the excimer laser-irradiated region of the sample. Grain size, surface roughness and melt duration as a function of various laser fluences are also determined.

Characterization of polycrystalline Ge thin films fabricated by short-pulse XeF excimer laser crystallization

XeF excimer laser-induced melting and recrystallization dynamics of amorphous germanium are investigated using time-resolved optical reflection and transmission measurements with a nanosecond time resolution, field-emission scanning electron microscopy, and micro-Raman spectroscopy. It is found that the disc-shaped grain with a diameter of approximately 0.8 µm is located in the complete melting regime with a melt phase duration of approximately 141–200 ns. The significant change of transmissivity is a key phenomenon revealing the excessive excimer laser fluence during excimer laser crystallization by in-situ optical measurements. Differences between the melting and recrystallization phenomenon for Si and Ge thin films are also discussed.

Characterization of polycrystalline silicon thin films fabricated by excimer laser crystallization

Phase transformation mechanisms during excimer laser crystallization and analysis of the resulting microstructures of polycrystalline silicon have been investigated in detail using in-situ time-resolved optical reflection and transmission measurements, field-emission scanning electron microscopy, high-resolution transmission electron microscopy, atomic force microscopy, and micro-Raman spectroscopy. Grain size, surface roughness, crystallinity, and melt-phase duration as functions of various excimer laser fluences are determined. A detailed microstructure development model of Si thin films upon complete melting using excimer laser irradiation is proposed. The results of this work can help in the development and fabrication of singular-grain Si thin film transistors.

Do Environmental Factors Influence Excimer Laser Pulse Fluence and Efficacy?

To evaluate excimer laser fluence after experimentally induced changes in room temperature and relative air humidity. Excimer laser fluence was analyzed with a specific power meter under three different gradients of room temperature and relative air humidity as well as the reproducibility of myopic correction of -3.00 diopters in a polymethylmethacrylate plate test. Under the simulated environmental conditions, no significant difference in laser energy fluence and reproduction of myopic correction was observed. In a setting with controlled temperature and relative air humidity, subtle changes in environmental factors do not appear to influence laser fluence and efficacy.

In-Situ and Ex-Situ Measurements on Silicon Thin Films Fabricated by Excimer Laser Annealing

The phase transitions occurring at the surface of amorphous silicon and a-Si/SiO2interface of the sample during XeF excimer laser annealing has been investigated by nonintrusive in-situ real-time optical reflectivity and transmissivity measurements with nanosecond time resolution. Five distinct regimes were demonstrated on the basis of various excimer laser fluences. The structural transformation scenario of silicon films based on the recrystallization mechanism is proposed to interpret the formation of the grain microstructure. The ex-situ microstructural characterizations of excimer laser-irradiated region as a function of various excimer laser energy densities were characterized by scanning electron microscopy and micro-Raman scattering measurements. A super-lateral growth length of approximately 1μm was obtained in a 90-nm-thick a-Si film by a single excimer laser pulse without any substrate heating. Micro-Raman scattering measurements as a function of various excimer laser energy densities were also carried out.

Surface roughness analysis and improvement of micro-fluidic channel with excimer laser

Micro-fluidic channel was fabricated on a PMMA sheet (thickness of 1.5 mm) using 248 nm excimer laser direct-writing technique. The influence of excimer laser fluence on the micromachining quality (channel’s depth and surface roughness) was analyzed. Higher laser fluence results in the augmentation of channel’s depth and surface roughness. The excimer laser polishing processing is favorable for the improvement of surface roughness. However, over-abundant polishing will generate tenuous stripe on the channel’s edge and worsen the surface roughness.

Nanosecond Time Resolution In Situ Optical Reflection and Transmission Measurements during XeF Excimer Laser Interaction with Amorphous Silicon Thin Films

XeF excimer laser-induced melting and recrystallization of amorphous silicon was studied using in-situ online time-resolved reflection and transmission measurements with a nanosecond time resolution. The explosive crystallization was observed for 50nm thick amorphous silicon on SiO2 deposited on non-alkali glass substrate upon 25ns pulse duration of excimer laser. Three distinct regrowth regimes were found using various excimer laser fluences. Scanning electron microscopy, Raman spectroscopy and atomic force microscopy were used to evaluate the excimer laser- irradiated region of the sample. Grain size, surface roughness and melt duration as a function of different laser fluences are also determined.

Metallic thin films heated by pulsed lasers. Numerical simulation of the thermal field and the melting kinetics

This modeling is especially applied to the pulsed laser induced heating and melting of a metallic film deposited on a substrate. Study of the thermal field over a surface is usually performed by considering the assumption of ‘semi-infinite medium’. However, a thin film deposited on a rough substrate surface induces bad thermal contacts commonly known as ‘thermal contact resistance’. This interfacial thermal resistance affects the melting kinetics mainly when the film thickness (Z) is small comparatively to the heat diffusion length (ZT). In this work the heat conduction equation and related boundary conditions are resolved by using the implicit finite differences method. The heat source (i.e. the laser intensity) is treated as a surface boundary layer. The thermal contact resistance is introduced in the computation procedure when the heat wave propagation reaches the thin film/substrate interface. It is then possible to calculate the critical temperatures and the melting threshold fluence for high and low contact resistance values. Under these conditions, the temperature profile and melting depth are plotted considering different thickness.. Finally, for 750 mJ/cm² excimer laser fluence and 0.1 cm²/s thin film apparent diffusivity results show that for Z/ZT higher than 0.5, there is no sensitive effect of the thermal contact resistance on the melting kinetics.

Electron and excited particle densities in a carbon ablation plume

In a plume produced by excimer ArF laser, the number densities of excited atoms and electrons, and the expansion velocity of ablated carbon atoms in an Ar gas were measured and compared with those obtained in vacuum. A saturation in C( 3 1 P 1 0 ) density was observed as the laser fluence increased. This saturation was shown to be closely related to the electron density saturation. The excitation of Ar(4s′[1/2] 1 0) and C( 3 1 P 1 0 ) was interpreted by electron collisions and was dependent on gas pressure and excimer laser fluence.

Study on dopant activation of phosphorous implanted polycrystalline silicon thin films by KrF excimer laser annealing

The KrF excimer laser annealing of phosphorous implanted polycrystalline silicon (poly-Si) films had been investigated completely. Resistors were fabricated to measure sheet resistance of poly-Si film. The interference effect, heat absorption of capping oxide as well as transformation of poly-Si grain size during laser annealing were reported. Depending on the carrier concentrations, poly-Si exhibits different sheet resistance behavior when the excimer laser fluence is higher than the full-melt threshold fluence. The sheet resistance of poly-Si film has an abnormal increase from 5×10 4 to 4×10 6 Ω/□ when the excimer laser energy is higher than full-melt threshold energy.

Influence of laser fluence and irradiation timing of F 2 laser on ablation properties of fused silica in F 2 -KrF excimer laser multi-wavelength excitation process

A collinear irradiation system of F2 and KrF excimer lasers for high-quality and high-efficiency ablation of hard materials by the F2 and KrF excimer lasers’ multi-wavelength excitation process has been developed. This system achieves well-defined micropatterning of fused silica with little thermal influence and little debris deposition. In addition, the dependence of ablation rate on various conditions such as laser fluence, irradiation timing of each laser beam, and pulse number is examined to investigate the role of the F2 laser in this process. The multi-wavelength excitation effect is strongly affected by the irradiation timing, and an extremely high ablation rate of over 30 nm/pulse is obtained between -10 ns and 10 ns of the delay time of F2 laser irradiation. The KrF excimer laser ablation threshold decreases and its effective absorption coefficient increases with increasing F2 laser fluence. Moreover, the ablation rate shows a linear increase with the logarithm of KrF excimer laser fluence when the F2 laser is simultaneously irradiated, while single KrF excimer laser ablation shows a nonlinear increase. The ablation mechanism is discussed based on these results.

Ultra-large lateral grain growth by double laser recrystallization of a-Si films

A new double laser recrystallization technique that can produce ultra-large direction- and location-controlled lateral grains is presented. An excimer laser is combined with a pulse-modulated Ar+ laser to yield grains of tens of micrometers in size. The effect of different parameters on lateral grain growth is investigated. These parameters include the time delay between the two lasers, the excimer-laser fluence, the Ar+-laser power and the pulse duration. The process has a wide process window and is insensitive to both the excimer-laser fluence and the Ar+-laser power fluctuations. Preheating of the a-Si film with the Ar+ laser before firing the excimer laser is necessary for inducing lateral grain growth. The transient excimer-laser irradiation is believed to generate nucleation sites for initiating the subsequent lateral grain growth. The surface roughness of the recrystallized poly-Si is measured by atomic force microscopy.

Excimer laser destruction of micrometer size particles in a gas volume

ArF excimer laser ablation of micrometer size smoke grains was investigated. It was demonstrated that this procedure is suitable for destruction of these particulates in consequence of very fast heating, oxidation, burning, and evaporation. The time-resolved investigations revealed that the destruction process takes place between 0 and 7 ns after the maximum of the excimer pulse. The efficiency of breaking into small pieces was found to be dependent on the excimer laser fluence. Atomic force microscopic investigations of the sedimented smoke particles proved that the size of the grains significantly decreased by the excimer laser radiation.