Sapphire surface polariton splitting due to resonance with magnesium oxide film phonon

Surface polariton spectra of magnesium oxide films (of thickness ranging from 10 to 300 nm) prepared by chemical vapor deposition technique on sapphire substrates have been studied. The splitting of the dispersion curve of the sapphire surface polariton appears near 700 cm⁻¹ due to the resonance interaction of the sapphire substrate surface polariton with the film optical phonon. This splitting is proportional to the square root of the film thickness. Optical constants of ultrathin magnesium oxide films have been obtained from surface polariton spectra measurements. For the thinnest film (10 nm) they appear to be close to the bulk crystal values.     

Patterning of PLZT and PSZT thin films by excimer laser

The patterning of lanthanum-doped lead zirconate titanate (PLZT) and strontium-doped lead zirconate titanate (PSZT) thin films has been examined using a 5-ns pulsed excimer laser. Both types of film were deposited by rf magnetron sputtering with in situ heating and a controlled cooling rate in order to obtain the perovskite-structured films. The depth of laser ablation in both PSZT and PLZT films showed a logarithmic dependence on fluence. The ablation rate of PLZT films was slightly higher than that of PSZT films over the range of fluence (10–150 J/cm²) and increased linearly with number of pulses. The threshold fluence required to initiate ablation was ∼ 1.25 J/cm² for PLZT and ∼ 1.87 J/cm² for PSZT films. Individual squares were patterned with areas ranging from 10×10 μm² up to 30×30 μm² using single and multiple pulses. The morphology of the etched surfaces comprised globules which had diameters of 200–250 nm in PLZT and 1400 nm in PSZT films. The diameter of the globules has been shown to increase with fluence until reaching an approximately constant size at ≤ 20 J/cm² in both types of film. The composition of the films following ablation has been compared using X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy. PACS 79.20.Ds; 82.80.Pv; 82.80.Ej     

Ablation of silicon suboxide thin layers

We investigate the ablation of SiO _x thin films on fused silica substrates using single-pulse exposures at 193 nm and 248 nm. Two ablation modes are considered: front side (the surface of a film is irradiated from above) and rear side (a film is irradiated through its supporting substrate). Fluence is varied from below 200 mJ/cm² to above 3 J/cm². SiO _x films of thickness 200 nm, 400 nm, and 600 nm are ablated. In the case of rear-side illumination, at moderate fluences (around 0.5 mJ/cm²) the ablation depth corresponds roughly to the film thickness, above 1 J/cm² part of the substrate is ablated as well. In the case of front-side ablation the single-pulse ablation depth is limited for all film thicknesses to less than 200 nm even at fluences up to 4 J/cm². Experimental results are discussed in relation to film thickness, fluence, and ablation mode. Simple numerical calculations are performed to clarify the influence of heat transport on the ablation process.     

Structural, optical and electrical properties of tin oxide thin film deposited by APCVD method

Tin oxide (SnO₂) thin films have been grown on glass substrates using atmospheric pressure chemical vapour deposition (APCVD) method. During the deposition, the substrate temperature was kept at 400°C–500°C. The structural properties, surface morphology and chemical composition of the deposited film were studied by X-ray diffraction (XRD), scanning electron microscope (SEM) and Rutherford back scattering (RBS) spectrum. XRD pattern showed that the preferred orientation was (110) having tetragonal structure. The optical properties of the films were studied by measuring the transmittance, absorbance and reflectance spectra between λ = 254 nm to 1400 nm and the optical constants were calculated. Typical SnO₂ film transmits ∼ 94% of visible light. The electrical properties of the films were studied using four-probe method and Hall-voltage measurement experiment. The films showed room temperature conductivity in the range 1.08 × 10² to 1.69 × 10² Ω⁻¹cm⁻¹.     

Irradiation of amorphous Ta<Subscript>42</Subscript>Si<Subscript>13</Subscript>N<Subscript>45</Subscript> film with a femtosecond laser pulse

Films of 260 nm thickness, with atomic composition Ta₄₂Si₁₃N₄₅, on 4″ silicon wafers, have been irradiated in air with single laser pulses of 200 femtoseconds duration and 800 nm wave length. As sputter-deposited, the films are structurally amorphous. A laterally truncated Gaussian beam with a near-uniform fluence of ∼0.6 J/cm² incident normally on such a film ablates 23 nm of the film. Cross-sectional transmission electron micrographs show that the surface of the remaining film is smooth and flat on a long-range scale, but contains densely distributed sharp nanoprotrusions that sometimes surpass the height of the original surface. Dark field micrographs of the remaining material show no nanograins. Neither does glancing angle X-ray diffraction with a beam illuminating many diffraction spots. By all evidence, the remaining film remains amorphous after the pulsed femtosecond irradiation.     

Effects of laser fluence on the structural properties of pulsed laser deposited ruthenium thin films

Ruthenium (Ru) has received great interest in recent years for applications in microelectronics. Pulsed laser deposition (PLD) enables the growth of Ru thin films at low temperatures. In this paper, we report for the first time the characterization of pulsed laser deposited Ru thin films. The deposition processes were carried out at room temperature in vacuum environment for different durations with a pulsed Nd:YAG laser of 355-nm laser wavelength, employing various laser fluences ranging from 2 J/cm² to 8 J/cm². The effect of the laser fluence on the structural properties of the deposited Ru films was investigated using surface profilometry, scanning electron microscopy (SEM), and X-ray diffraction (XRD). Ru droplets, some spherical in shape and some flattened into round discs were found on the deposited Ru. The droplets were correlated to ripple formations on the target during the laser-induced ejection from the target. In addition, crystalline Ru with orientations of (100), (101), and (002) was observed in the XRD spectra and their intensities were found to increase with increasing laser fluence and film thickness. Grain sizes ranging from 20 nm to 35 nm were deduced using the Scherrer formula. Optical emission spectroscopy (OES) and energy-dispersive X-ray spectroscopy (EDS) show that the composition of the plume and the deposited Ru film was of high purity.     

Femtosecond-laser-induced-crystallization and simultaneous formation of light trapping microstructures in thin a-Si:H films

We report the observation of crystallization and simultaneous formation of surface microstructures in hydrogenated amorphous silicon (a-Si:H) thin films as one step laser processing. Light trapping microstructures of around 300 nm in height were formed on a-Si:H films of thickness in the range of 1.5 μm to 2 μm deposited on soda lime glass after exposure to femtosecond laser pulses. Scanning electron microscope (SEM) images show the formation of spikes that are around 1 μm part and their heights could be controlled by the laser fluences. Atomic force microscope (AFM) images were taken to study the roughness created on the surface. The mean roughness of the textured surface increases with laser fluence at smaller power densities, and for power densities beyond 0.5 J/cm² the film removal deteriorates the texturing. X-ray diffraction results indicate the formation of a nano-crystalline structure with (111) and (311) crystal orientation after the laser treatment. The observed black color and enhanced optical absorption in the near infrared region in laser treated films may be due to a combined effect of light trapping in the micro-structured silicon surface because of multiple total internal reflections, phase change in the film, possible defect sites induced after laser treatment and formation of SiO_x. Demonstration of light trapping microstructures in thin a-Si:H films and simultaneous crystallization could provide new opportunities for optoelectronic devices. PACS 42.55.Px; 42.62.Cf; 81.05.Ge     

Excimer-laser-assisted RF glow-discharge deposition of amorphous and microcrystalline silicon thin films

We combine the deposition of Hydrogenated amorphous Silicon (a-Si:H) by rf glow discharge with XeCl-excimer laser irradiation of the growing surface in order to obtain different kinds of silicon films in the same deposition system. In-situ UV-visible ellipsometry allows us to measure the optical properties of the films as the laser fluence is increased from 0 up to 180 mJ/cm² in separate depositions. For fixed glow-discharge conditions and a substrate temperature of 250° C we observe dramatic changes in the film structure as the laser fluence is increased. With respect to a reference a-Si:H film (no laser irradiation) we observe at low laser fluences (15–60 mJ/cm²) that the film remains amorphous but demonstrates enchanced surface roughness and bulk porosity. At intermediate fluences (80–165 m/Jcm²), we obtain an amorphous film with an enhanced density with respect to the reference film. Finally, at high fluences (165–180 mJ/cm²), we obtain microcrystalline films. The in-situ ellipsometry measurements are complemented by ex-situ measurements of the dark conductivity, X-ray diffraction, and Elastic Recoil Detection Analysis (ERDA). Simulation of the temperature profiles for different film thicknesses and for three laser fluences indicates that crystallization occurs if the surface temperature reaches the melting point of a-Si:H ( 1420 K). The effects of laser treatment on the film properties are discussed by taking into account the photonic and thermal effects of laser irradiation.Presented at LASERION 93, Munich, June 21–23, 1993     

Opto-chemical response of Makrofol-KG to swift heavy ion irradiation

In the present study, the effects of swift heavy ion beam irradiation on the structural, chemical and optical properties of Makrofol solid-state nuclear track detector (SSNTD) were investigated. Makrofol-KG films of 40 μm thickness were irradiated with oxygen beam (O^8 +) with fluences ranging between 10¹⁰ ion/cm² and 10¹² ion/cm². Structural, chemical and optical properties were investigated using X-ray diffraction, FTIR spectroscopy and UV–visible spectroscopy methods. It is observed that the direct and indirect band gaps of Makrofol-KG decrease after the irradiation. The XRD study shows that the crystalline size in the films decreases at higher fluences. The intensity plots of FTIR measurements indicate the degradation of Makrofol at higher fluences. Roughness of the surface increases at higher fluence.     

Synthesis of ZnO thin films by 40 ps @ 532 nm laser pulses

The synthesis by pulsed laser deposition of ZnO thin films with a Nd:YAG laser system delivering pulses of 40 ps @ 532 nm is reported. The laser beam irradiated the target placed inside a vacuum chamber evacuated down to 1.33×10⁻¹ Pa. The incident laser fluence was of 28 J/cm² in a spot of 0.1 mm². The ablated material was collected onto double face polished (111) Si or quartz wafers placed parallel at a separation distance of 7 mm. The AFM, SEM, UV-Vis, FT-IR and absorption ellipsometry results indicated that we obtained pure ZnO films with a rather uniform surface, having an average roughness of 37 nm. We observed by SEM that particulates are present on ZnO film surface or embedded into bulk. Their density and dimension were intermediary between particulates observed on similar structures deposited with fs or ns laser pulses. We noticed that the density of the particulates is increasing while their average size is decreasing when passing from ns to ps and fs laser pulses. The average transmission in the UV-Vis spectral region was found to be higher than 85%.     

Growth of thin films of low molecular weight proteins by matrix assisted pulsed laser evaporation (MAPLE)

Thin films of lysozyme and myoglobin grown by matrix assisted pulsed laser evaporation (MAPLE) from a water ice matrix have been investigated. The deposition rate of these two low molecular weight proteins (lysozyme: 14307 amu and myoglobin: 17083 amu) exhibits a maximum of about 1–2 ng/cm² per pulse at a fluence of 1–2 J/cm² and decreases slowly with increasing fluence. This rate is presumably determined by the matrix rather by the proteins. A significant fraction of the proteins are intact in the film as determined by MALDI (Matrix assisted laser desorption ionization) spectrometry. The results for lysozyme demonstrate that the fragmentation rate of the proteins during the MAPLE process is not influenced by the pH of the water solution prior to freezing.     

Mechanism of backside etching of transparent materials with nanosecond UV-lasers

In consequence of high interest in micro- and nanomachining of transparent materials by laser irradiation, studies on the mechanism of laser-induced backside wet etching (LIBWE) are presented. To reveal the role of the surface modification due to LIBWE the backside ablation (BSA) of LIBWE-modified fused silica (mFS) surfaces at 248 nm was investigated. The threshold fluence and the etch rate of BSA are similar to that of LIBWE and amount ∼250 mJ/cm² and 30 nm for 1 J/cm², respectively. The sample transmission after backside ablation of mFS increases and proves the decreasing thickness of the absorbing layer. Time-resolved reflection studies at LIBWE and BSA of mFS show similar patterns in the backside reflection that can be assigned an ablation process as the comparison to thin polymer films demonstrates. By fitting the BSA data to an exponential decay absorption model a modification depth and a surface absorption of about 38 nm and α ^S ∼1.3×10⁷ m⁻¹ were calculated, respectively. In conclusion of the results a new model for LIBWE is proposed.     

Femtosecond laser ablation characteristics of nickel-based superalloy C263

Femtosecond laser (180 fs, 775 nm, 1 kHz) ablation characteristics of the nickel-based superalloy C263 are investigated. The single pulse ablation threshold is measured to be 0.26±0.03 J/cm² and the incubation parameter ξ=0.72±0.03 by also measuring the dependence of ablation threshold on the number of laser pulses. The ablation rate exhibits two logarithmic dependencies on fluence corresponding to ablation determined by the optical penetration depth at fluences below ∼5 J/cm² (for single pulse) and by the electron thermal diffusion length above that fluence. The central surface morphology of ablated craters (dimples) with laser fluence and number of laser pulses shows the development of several kinds of periodic structures (ripples) with different periodicities as well as the formation of resolidified material and holes at the centre of the ablated crater at high fluences. The debris produced during ablation consists of crystalline C263 oxidized nanoparticles with diameters of ∼2–20 nm (for F=9.6 J/cm²). The mechanisms involved in femtosecond laser microprocessing of the superalloy C263 as well as in the synthesis of C263 nanoparticles are elucidated and discussed in terms of the properties of the material.     

Annealing of sputtered gold nano-structures

Abstract The effects of annealing on gold structures sputtered onto glass substrate were studied using AFM, UV-Vis methods and electrical measurements. The colour of the as-deposited films changes from blue to green with increasing deposition time. After 1 hour annealing at 300°C the structures acquire red colour regardless of the film thickness. The annealing results in dramatic changes of surface morphology and roughness and creation of relatively large “spherolytic and hummock-like” structures in the gold layer. For deposited structures a non-zero optical band gap E_g^optE_{\mathrm{g}}^{\mathrm{opt}} was determined from UV-Vis spectra using Tauc’s model and it indicates a semi-conducting character of the structures. The annealing leads to an increase of the band gap. Electrical resistance of the deposited unannealed structures decreases dramatically for deposition times above 50 s. For annealed structures the resistance fall comes until after 250 s deposition time.     

A point source analytical model of inverse pulsed laser deposition

A simple analytical model for inverse pulsed laser deposition is proposed. In the model the motion of the evaporated material is assumed to emerge as from a point source located above the surface of evaporation at some distance. The obtained thickness profiles of inverse deposited films agree well with those calculated by the test particle Monte Carlo method. The proposed approach has been applied for analysis of experimental data on inverse pulsed laser deposition of graphite in nitrogen atmosphere with nanosecond pulses of laser fluences between 1 and 7 J/cm². The model describes well the thickness profiles and pressure dependence of film growth rate for inverse deposition.     

Effect of laser fluence on the deposition and hardnessof boron carbide thin films

We deposited amorphous thin films of boron carbide by pulsed laser deposition using a B₄C target at room temperature. As the laser fluence increased from 1 to 3 J/cm², the number of 0.25–5 μm particulates embedded in the films decreased, and the B/C atomic ratio of the films increased from 1.8 to 3.2. The arrival of melt droplets, atoms, and small molecular species depending on laser fluence appeared to be involved in the film formation. In addition, with increasing fluence the nanoindentation hardness of the films increased from 14 to 32 GPa. We believe that the dominant factor in the observed increase in the films’ hardness is the arrival of highly energetic ions and atoms that results in the formation of denser films. Received: 23 March 2001 / Accepted: 1 July 2001 / Published online: 2 October 2001     

Pulsed electron-beam melting of a copper — steel 316 system: Evolution of the chemical composition,microstructure, and properties

The surface topography, chemical composition, microstructure, nanohardness, and tribological characteristics of a Cu (film, 512 nm)-stainless steel 316 (substrate) system subjected to pulsed melting by a low-energy (20–30 keV), high-current electron beam (2–3 μs, 2–10 J/cm²) were investigated. The film was deposited by sputtering a Cu target in the plasma of a microwave discharge in argon. To prevent local exfoliation of the film due to cratering, the substrate was multiply pre-irradiated with 8–10 J/cm². On single irradiation, the bulk of the film survived, and a diffusion layer containing the film and substrate components was formed at the interface. The thickness of this layer was 120–170 nm irrespective of the energy density. The diffusion layer consisted of subgrains of γ-Fe solid solution and nanosized particles of copper. In the surface layer of thickness 0.5–1 μm, which included the copper film quenched from melt and the diffusion layer, the nanohardness and the wear resistance nonmonotonicly varied with energy density, reaching, respectively, a maximum and a minimum in the range 4.3–6.3 J/cm². As the number of pulsed melting cycles was increased to five in the same energy density range, there occurred mixing of the film-substrate system and a surface layer of thickness ∼2 μm was formed which contained ∼20 at. % copper. Displacement of the excess copper during crystallization resulted in the formation of two-phase nanocrystal interlayers separating the γ-phase grains. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 6–13, December, 2005.     

Effect of the laser fluence on the surface characterization of β-FeSi2 films prepared by pulsed laser deposition

Single-phase β-FeSi₂ films on silicon (1 0 0) were fabricated by pulse laser deposition. The structure and crystal quality of the samples were characterized by X-ray diffraction and Fourier transform infrared spectroscopy. The field scanning electron microscopy showed that the film thickness increases with the increasing of the laser fluence. Moreover, atomic force microscopy observations revealed the changes of surface properties with different laser fluence. Based upon all experimental results, it is found that 7 J/cm² is the most favorable for the formation of β-FeSi₂ thin films.     

Acoustic superscatterer and its multilayer realization

Picosecond (40 ps) pulsed Nd:YAG laser irradiation of a WTi thin film on silicon with a wavelength of 532 nm and a fluence 2.1 J/cm² was performed in air. This led to significant changes of the chemical composition and morphology on the surface of the WTi thin film. The results show an increase in surface roughness, due to formation of conical structures, about 50 nm wide in the base, and a very thin oxide layer composed of WO₃ and TiO₂, with a dominant TiO₂ phase at the top, within the depth of about 20 nm. The thickness of the oxide layer was dependent on the number of laser pulses. The samples were analyzed by scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy.     

Pulsed laser treatment of gold and black gold thin films fabricated by thermal evaporation

The effect of pulsed laser treatment of metal, and metal blacks, was studied. Gold and black gold thin films were fabricated by thermal evaporation of gold in a vacuum and nitrogen atmosphere respectively. Black gold films were grown in a nitrogen atmosphere at pressures of 200 Pa and 300 Pa. UV pulsed laser radiation (λ = 266 nm, τ = 4 ns), with fluence ranging from 1 mJ·cm⁻² to 250 mJ·cm⁻² was used for the film treatment in a vacuum and nitrogen atmosphere. The nitrogen pressure was varied up to 100 kPa. Surface structure modifications were analyzed by optical microscopy, atomic force microscopy (AFM) and scanning electron microscopy (SEM). Energy dispersive X-ray spectroscopy (EDX) was used for chemical characterization of the samples. A significant dependence of the film optical and structural properties on laser treatment conditions (laser fluence, ambient pressure and number of applied pulses) was found. The threshold for observable damage and initiation of changes of morphology for gold and black gold surfaces was determined. Distinct modifications were observed for fluences greater than 106 mJ·cm⁻² and 3.5 mJ·cm⁻² for the gold and black gold films respectively. Absorbtivity of the black gold film is found to decrease with an increase in the number of laser pulses. Microstructural and nanostructural modifications after laser treatment of the black gold film were observed. EDX analysis revealed that no impurities were introduced into the samples during both the deposition and laser treatment.