The ablation threshold in amorphous diamondlike carbon films exposed to an ArF excimer laser radiation

An experimental study of the laser-induced ablation threshold in amorphous diamondlike carbon films is reported. The aim is to assess the possibility of using the material as a photoresist in vacuum-ultraviolet laser lithography. Grown on silicon substrates, 10-nm films were irradiated by 20-ns pulses of a 193-nm ArF excimer laser with variable pulse energy per unit area E _p. It is found that the etch rate is very low if E _p < 20 mJ/cm², whereas a single pulse suffices to remove the film completely if E _p=60 mJ/cm². This is attributed to an increase in the thermal ablation component.     

Depth-profiled characterization of complex refractive index of ion implanted optically transparent polymers using multilayer calculations and reflectance data

The material in the ion-modified surface layer formed in polymethylmethacrylate (PMMA) is optically characterized by calculations based on multilayer model and optical reflectance data. PMMA was subjected to a low energy (50 keV) silicon ion implantation at the fluences of 3.2 × 10¹⁵ cm⁻² and 3.2 × 10¹⁶ cm⁻². Both real and imaginary components of the complex refractive index of this optically transparent polymer are modeled in a geometry that includes a gradient of their in-depth spatial distribution.     

Influence of laser wavelength and beam profile on Nd:YAG laser assisted formation of polycrystalline-Si films

Influence of wavelengths and beam profiles of a pulsed Nd³⁺:YAG laser on the formation of a polycrystalline-silicon (poly-Si) on a-Si thin film is investigated. Two sets of samples of amorphous-Silicon (a-Si) thin films deposited on glass (a-Si/glass) and crystalline Si (a-Si/c-Si) substrates were treated with different laser-fluence values. After the laser treatment, the films were analyzed by a scanning electron microscope, the Raman spectroscopy technique and the resistance-measurement technique. In the case of the third harmonics (355 nm) of the Nd³⁺:YAG laser, poly-Si films were obtained with laser-fluence values ranging from 260 mJ/cm² to 560 mJ/cm², where as in the case of the second harmonics (532 nm), the process window for the formation of poly-Si films, in terms of the laser fluence, was ranging from 300 mJ/cm² to 480 mJ/cm². On the other hand, in the case of samples treated with the fundamental wavelength (1064 nm), a narrow process window with higher laser-fluence values around 1100 mJ/cm² was observed. Further, the substrate was also affected because of the higher laser-fluence value. It has also been observed that the crystallization characteristics of poly-Si films improved with the flat-top intensity distribution as compared to the Gaussian intensity distribution of the Nd³⁺:YAG laser beam. A theoretical simulation based on thermal modeling was performed to understand the mechanism of crystallization.     

Synthesis and characterization of cross-linked silicone thin films by pulsed laser ablation deposition (PLAD)

Stable, uniform and cross-linked silicone films have been synthesized by pulsed laser ablation deposition (PLAD) for the first time. A KrF excimer (248 nm) laser was used in the synthesis. The effect of incident energy density on the deposited film chemistry was examined in depth. The surface analysis showed that at low energy densities (100–150 mJ/cm²), smooth, hydrophobic films similar in structure to the starting cross-linked silicone elastomer were obtained. Beyond 200 mJ/cm², hydrophilic films with oxygen contents much higher than the starting polymer were obtained. These films also exhibited a more particulate texture suggesting ablation of particles and/or polymeric clusters from the silicone target. The results demonstrate that the PLAD process may prove valuable for the preparation of cross-linked silicone thin films with tailored properties. Received: 27 June 2001 / Accepted: 17 July 2001     

Photoablation of thin indium oxide films under the action of a pulsed arf excimer laser

The laser-induced ablation threshold of indium oxide (In₂O₃) films was studied in order to evaluate the possibility of using this material as a photoresist for vacuum ultraviolet lithography. In₂O₃ films with a thickness of about 30 nm were prepared by electron beam cathode sputtering with deposition onto quartz substrates in a rarefied oxygen-containing atmosphere. Then the films were irradiated by 20-ns pulses of an ArF excimer laser operating at a wavelength of λ=193 nm and a variable pulse intensity E _p. For a laser intensity below 30 mJ/cm², the oxide etching rate is negligibly small. As the laser radiation intensity increases above this threshold, the etching becomes more effective due to the development of a thermal ablation component.     

Effect of crystallization parameters and presence of aqueous extract of Nigella Sativa on growth inhibition of calcium oxalate monohydrate particles

For the first time, nucleation kinetics, crystal growth and agglomeration of calcium oxalate monohydrate (COM) are studied as a crystal growth inhibitor with and without Nigella Sativa(NS) extract. The induction period was determined under different supersaturation ratios ranging from 1.63 to 4.51 at 37°C using the conductivity method. Based on the classical homogeneous nucleation theory; the critical nucleation parameters were evaluated at higher supersaturation ratio. The calculated surface energy was increased from 7.97?mJ/m² without additive to 10.31?mJ/m² with NS extract. However, the nucleation rate at a supersaturation ratio of 3.26 corresponding to 5.44 with the NS extract was decreased from 3.9?×?10²⁹ nuclei/cm³.s (without additive) to 1.3?×?10²⁹ nuclei/cm³.s with NS extract addition. The number of molecules required for the formation of a stable nucleus was calculated with and without NS extract at different supersaturation ratios. The crystallite sizes of the formed crystals without and with the additive are 93 and 51?nm, respectively. Scanning electron microscopy (SEM) photomicrographs show the formation of small crystals and less aggregated with the NS inhibitor compared with the baseline (without the inhibitor). This study can help us with a clear understanding of the inhibition mechanism of an aqueous NS extract on COM crystals.     

Investigation of laser ablation loading of polymers

For polyethylene and polytetrafluoroethylene targets irradiated by laser pulses with energy densities within 100–1000 J/cm², the ablation loading coefficients amount to 8–11% and 8–10%, respectively. These values satisfactorily agree with theoretical estimates of the ablation loading, which are about 6–8%.     

Highly transparent and conducting In doped CdO synthesized by sol-gel solution processing

Cadmium oxide (CdO) is a much-studied wide gap semiconductor with an inherent high mobility of?>?100 cm²/Vs, high electron concentration of?>?10²¹ cm^?3 and a wide optical transparency window of?>?1800 nm. These unique properties make CdO a potential transparent conductor for full spectrum photovoltaics. However, in order to achieve optimum material properties for optoelectronic applications, CdO was grown by vacuum-based physical or chemical vapor deposition methods. In this work, we explored the application of a low-cost sol-gel spin coating method to achieve highly conducting and transparent CdO thin films doped with 0–10% In (CdO:In). We find that while as-grown CdO:In films are nanocrystalline/amorphous with a high resistivity of?~?1 Ω-cm, polycrystalline and highly conducting films can be obtained after optimized annealing at?≥?400 °C. However, the electron concentration n saturates at?~?5?×?10²⁰ cm^?3 for In concentration?>?5% (or N_In?~?1.9?×?10²¹ cm^?3). This low activation of In may be attributed to the high density of native defects and/or impurities incorporated in the sol-gel process. With 5% In doping, we obtained a low resistivity of ρ?~?2.5?×?10^–4 Ω-cm and a high mobility μ?~?50 cm²/Vs. These values of σ and µ are better than those reported for other TCOs synthesized by solution processes and comparable to conventional commercial TCOs grown by physical vapor deposition methods. Benefiting from their high mobility, these sol-gel CdO:In films are optically transparent over a wide spectral range up to λ?>?1800 nm, making them promising as transparent conductors for optoelectronic devices utilizing the infrared photons.

     

Combined effect of substrate temperature and laser energy density on laser ablated BiFeO3 thin films

Bismuth ferrite (BiFeO₃) thin films were prepared on Pt(111)/Ti/SiO₂/Si substrates by laser ablation deposition at various substrate temperature (T _sub) and laser energy density (D _L). The combined effect of T _sub and D _L on crystalline phase, preferential orientation and surface morphology of the films were investigated. The microstructure and morphology of the films showed a strong dependence on T _sub and D _L simultaneously. BiFeO₃ thin films with single phase could be prepared at T _sub-D _L of 893 K?1 J/cm², 943 K?2 J/cm² and 963 K?3 J/cm², respectively. Film deposited at T _sub = 963 K and D _L = 3 J/cm² exhibited better ferroelectric property due to the combination of high (111) preferential orientation, dense surface morphology and less Fe²⁺ content.     

Hafnium Diboride as a saturable absorber for Q-switched lasers

Hafnium diboride (HfB₂) is used in hypervelocity re-entry vehicles (such as intercontinental ballistic missile (ICBM) or ICBM heat shields), nuclear reactors and aerodynamic leading edges because of its strong tensile strength and high thermal resistance. HfB₂ is an ultra-high-temperature ceramic (UHTC) with a melting point of 3250°C. In this paper, we study the performance of HfB₂ as a saturable absorber in Q-switched lasers. HfB₂ has a non-saturable absorptance of 2.5?dB/µm, a saturable absorptance of 0.8?dB/µm and a saturation fluence of 12?µJ/cm². In addition, the saturation lifetime is estimated as 989.3?ps, while the recovery lifetimes are estimated as 254.9 and 22.1?ps. As a saturable absorber in our Q-switched laser, we observed pulses with durations between 880 and 2000?ns. With the addition of an acousto-optic modulator, we have observed Q-switching mode-locking, with pulses as low as 250?ns. HfB₂ can potentially work at very high power, since its damage fluence is no less than 361?mJ/cm².     

Optimal composition of an acrylamide and N,N′-methylenebisacrylamide holographic recording material

Abstract

We study the effect of the addition of a cross-linking agent (N,N′-methylenebisacrylamide) to a photopolymerizable matrix for real-time holography. Optimization of the concentration of this component has been achieved, paying attention to holographic parameters such as energy sensitivity and diffraction efficiency. Diffraction efficiencies of around 88% have been obtained with energy exposures of 12 mJ cm^?2. At the same time considering a nonlinear response of the material we have carried out a theoretical study of the experimental results. We observed a nonlinear response of the material with regard to the storage intensity. This is a very interesting point in reflection holographic optical elements.     

Insight into excimer laser crystallization exploiting ellipsometry: Effect of silicon film precursor

The optical diagnostic of spectroscopic ellipsometry is shown to be an effective tool to investigate the mechanism of excimer laser crystallization (ELC) of silicon thin films. A detailed spectroscopic ellipsometric investigation of the microstructures of polycrystalline Si films obtained on SiO₂/Si wafers by ELC of a-Si:H and nc-Si films deposited, respectively, by SiH₄ plasma enhanced chemical vapor deposition (PECVD) and SiF₄-PECVD is presented. It is shown that ellipsometric spectra of the pseudodielectric function of polysilicon thin films allows to discern the three different ELC regimes of partial melting, super lateral growth and complete melting. Exploiting ellipsometry and atomic force microscopy, it is shown that ELC of nc-Si has very low energy density threshold of 95 mJ/cm² for complete melting, and that re-crystallization to large grains of ∼ 2 μm can be achieved by multi-shot irradiation at an energy density as low as 260 mJ/cm² when using nc-Si when compared to 340 mJ/cm² for the ELC of a-Si films.     

Development of an algorithm for corneal reshaping with a scanning laser beam

The corneal-ablation rate, the beam-intensity distribution, and the initial and the desired corneal topographies are used to calculate a spatial distribution map of laser pulses. The optimal values of the parameters are determined with a computer model, for a system that produces 213-nm radiation with a Gaussian beam-intensity distribution and a peak radiant exposure of 400 mJ/cm(2). The model shows that with a beam diameter of 0.5 mm, an overlap of 80%, and a 5-mm treatment zone, the roughness is less than 6% of the central ablation depth, the refractive error after correction is less than 0.1 D for corrections of myopia of 1, 3, and 6 D and less than 0.4 D for a correction of myopia of 10 D, and the number of pulses per diopter of correction is 2500 when the beam-intensity distribution is Gaussian and 580 when it is flat.     

Thin epitaxial GaN films ablated by a pulsed KrF excimer laser

We have studied the laser-induced ablation thresholds in heteroepitaxial gallium nitride (GaN) layers grown on sapphire substrates. The relationship between the laser-ablated GaN film thickness and the flux density of VUV laser radiation with a wavelength of 248 nm has been established. The photo-and thermoablation thresholds have been observed at a radiation intensity of 252 and 520 mJ/cm², respectively. The enthalpy of GaN dissociation estimated from an analysis of thermoablation data is 150 kJ/mol, in agreement with the published reference values. The experimental results were implemented in the technology of blue light-emitting diodes (LEDs), which allowed the LED output yield to be significantly increased.     

Ni implantation-induced enhancement of the crystallisation of amorphous Si

We report on the effect of Ni implantation on the crystallisation process of amorphous Si formed by ion implantation during excimer laser annealing. Scanning transmission electron microscopy and X-ray photoemission spectroscopy results show that NiSi₂ precipitates are formed in the Si–Ni-implanted samples. It is shown that the Ni implantation results in the enhanced crystallisation of amorphous Si during laser annealing. The Si–Ni-implanted samples become epitaxial to the Si substrate at 600 mJ/cm², while the Si-implanted samples produce epitaxial relationship at 800 mJ/cm². Possible mechanisms by which implanted Ni atoms play a role are given to describe the enhanced crystallisation process of amorphous Si.     

Densification behavior,doping profile and planar waveguide laser performance of the tape casting YAG/Nd:YAG/YAG ceramics

The sintering behavior and doping concentration profile of the planar waveguide YAG/Nd:YAG/YAG ceramics by the tape casting and solid-state reaction method were investigated on the basis of densification trajectory, microstructure evolution, and Nd³⁺ ions diffusion. The porosity of the green body by tape casting and cold isostatic pressing is about 38.6%. And the green bodies were consolidated from 1100 °C to 1800 °C for 0.5–20 h to study the densification and the doping diffusion behaviors. At the temperature higher than 1500 °C, pure YAG phase is formed, followed by the densification and grain growth process. With the increase of temperature, two sintering stages occur, corresponding to remarkable densification and significant grain growth, respectively. The mechanism controlling densification at 1550 °C is grain boundary diffusion. The diffusion of Nd³⁺ ions is more sensitive to temperature than the sintering time, and the minimum temperature required for the obvious diffusion of Nd³⁺ ions is higher than 1700 °C. Finally, planar waveguide YAG/1.5 at.%Nd:YAG/YAG transparent ceramics with in-line transmittance of 84.8% at 1064 nm were obtained by vacuum-sintering at 1780 °C for 30 h. The fluorescence lifetime of ⁴F_3/2 state of Nd³⁺ in the specimen is about 259 μs. The prepared ceramic waveguide was tested in a laser amplifier and the laser pulse was amplificated from 87 mJ to 238 mJ, with the pump energy of 680 mJ.     

Formation of In nanoparticles on InP wafers by laser-assisted etching

Indium nanoparticles were formed by laser etching an InP (100) wafer in a 10% chlorine–helium atmosphere maintained at ~5–8 × 10⁻⁵ Torr. The wafer was irradiated by a homogenized ultraviolet beam with a series of 50–4500 pulses at a fluence of 230 mJ/cm². The surface was also irradiated using fluences from 50 to 340 mJ/cm² with 600 pulses. The irradiated surfaces were studied using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and Raman spectroscopy. Raman spectroscopy confirmed that the irradiated surface layer remains crystalline. According to EDS analysis, the surface particles are composed primarily of indium. SEM images show that the number of pulses and the pulse intensity can control the size distribution of the particles.     

Evanescent wave holographic recording in a photopolymer

We present a study of holographic recording with an evanescent reference and a homogeneous (plane) object wave. The grating step was 324 nm. The dependence of the diffraction efficiency on exposure was investigated. The maximum value obtained was 0.01% at 5 mJ cm^?2 exposure. The optimal pre-exposure, needed for grating adhesion to the glass substrate, was 1 μJ cm^?2.     

Development,optimization, and characterization of a novel tea tree oil nanogel using response surface methodology

Purpose: To develop and optimize nanoemulsion (NE)-based emulgel (EG) formulation as a potential vehicle for topical delivery of tea tree oil (TTO).

Methodology: Central composite design was adopted for optimizing the processing conditions for NE preparation by high energy emulsification method viz. surfactant concentration, co-surfactant concentration, and stirring speed. The optimized NE was developed into emulgel (EG) using pH sensitive polymer Carbopol 940 and triethanolamine as alkalizer. The prepared EG was evaluated for its pH, viscosity, and texture parameters, ex vivo permeation at 37?°C and stability. Antimicrobial evaluation of EG in comparison to conventional gel and pure TTO was also carried out against selected microbial strains.

Results and discussion: Optimized NE had particle size and zeta potential of 16.23?±?0.411?nm and 36.11?±?1.234?mV, respectively. TEM analysis revealed the spherical shape of droplets. The pH of EG (5.57?±?0.05?) was found to be in accordance with the range of human skin pH. EG also illustrated efficient permeation (79.58?μL/cm²) and flux value (J_SS) of 7.96?μL cm²/h through skin in 10?h. Viscosity and texture parameters, firmness (9.3?±?0.08?g), spreadability (2.26?±?0.06?mJ), extrudability (61.6?±?0.05?mJ), and adhesiveness (8.66?±?0.08?g) depict its suitability for topical application. Antimicrobial evaluation of EG with same amount of TTO as conventional gel revealed broader zones of growth inhibitions against all the selected microbial strains. Moreover, EG was also found to be nonirritant (PII 0.0833). These parameters were consistent over 90 d.

Conclusion: TTO EG turned out to be a promising vehicle for the topical delivery of TTO with enhanced therapeutic efficacy.     

The detection sensitivity of heavy impurities in Si using 280 keV He2+ and C2+ back-scattering

Background limitations, especially those involving pulse pile-up, on the sensitivity to detection of heavy elements near the surface of Si have been investigated with back-scattered 280 keV He²⁺ and C²⁺. Using gated electronics, pulse pile-up was reduced by as much as a factor of 1/60, which permitted a detection sensitivity, i.e. equal signal and background, of approximately 2×10¹¹ Tl atoms/cm² for He²⁺ back-scattering, while C²⁺ back-scattering data showed a detection sensitivity of about 2×10¹⁰ Tl atoms/cm². Back-scattering from Si implanted with 20 keV Tl⁺ to doses of 1×10¹³ and 1×10¹² ions/cm² was used for experimental confirmation of the He²⁺ and C²⁺ back-scattering sensitivities.