Large Range Of Fluences Research Articles

Amorphization and Ablation of Crystalline Silicon Using Ultrafast Lasers: Dependencies on the Pulse Duration and Irradiation Wavelength

AbstractUsing lasers to achieve controlled crystallographic phase changes in silicon with high spatial precision promises new manufacturing solutions in semiconductor technologies, including silicon photonics. Recent demonstrations of improved amorphization thicknesses position ultrafast lasers as an optimum tool to meet current challenges. Here, the literature on silicon transformations is reviewed and complemented with new experimental data. This includes amorphization and ablation response as a function of pulse duration (τ = 13.9 to 134 fs at λ = 800 nm) and laser wavelength (λ = 258 to 4000 nm with τ = 200 fs pulses). For pulse duration‐dependent studies on Si(111), the amorphization fluence threshold decreases with shorter durations, emphasizing the significance of non‐linear absorption in the range of considered conditions. For wavelength‐dependent studies, the amorphization threshold increases sharply from λ = 258 to 1030 nm, followed by near‐constant behavior up to λ = 3000 nm. Conversely, the ablation threshold fluence increases in these specified ranges. Differences in the obtained amorphization thicknesses on Si(111) and Si(100) are also discussed, identifying an anomalously large fluence range for amorphization at λ = 258 nm. Finally, the question of the lateral resolution, shown as independent of the interaction nonlinearity is addressed.

Nanosecond laser damage initiation at 0.35 μm in fused silica.

In nanosecond regime, the laser-induced damage density at the exit surface of fused silica optics at the wavelength of 0.35μm shows a characteristic behavior: in a specific fluence range, the surface damage density begins to grow exponentially as a function of fluence and then tends to saturate at high fluences. Up to now, no satisfactory explanation of these peculiarities could be provided. We herein detail a statistical model based on laser-matter interaction, where two types of absorbing precursors are involved in the energy deposit: subsurface micro-cracks and surface impurities. We show that the reported model predicts this characteristic damage density for a large range of fluences and different polishing processes.

Multidiagnostic analysis of ion dynamics in ultrafast laser ablation of metals over a large fluence range

The dynamics of ions in ultrafast laser ablation of metals is studied over fluences ranging from the ablation threshold up to ≈75 J/cm2 by means of three well-established diagnostic techniques. Langmuir probe, Faraday cup, and spectrally resolved intensified charge coupled device imaging simultaneously monitored the ions produced during ultrafast laser ablation of a pure copper target with 800 nm, ≈50 fs, Ti: Sapphire laser pulses. The fluence dependence of ion yield is analyzed, resulting in the observance of three different regimes. The specific ion yield shows a maximum at about 4–5 J/cm2, followed by a gradual reduction and a transition to a high-fluence regime above ≈50 J/cm2. The fluence dependence of the copper ions angular distribution is also analyzed, observing a gradual increase in forward-peaking of Cu ions for fluences up to ≈10 J/cm2. A broader ion component is observed at larger angles for fluences larger than ≈10 J/cm2. Finally, an experimental characterization of the ionic angular distribution for several metallic targets (Mg, Al, Cr, Fe, Cu, and W) is carried out at a relatively high fluence of ≈66 J/cm2. Interestingly, the ion emission from the volatile metals shows a narrow, forward-peaked distribution, and a high peak ion yield compared to the refractory metals. Moreover, the width of ionic angular distributions presents a striking correlation with the peak ion yield.

High fluence laser damage precursors and their mitigation in fused silica

The use of any optical material is limited at high fluences by laser-induced damage to optical surfaces. In many optical materials, the damage results from a series of sources which initiate at a large range of fluences and intensities. Much progress has been made recently eliminating silica surface damage due to fracture-related precursors at relatively low fluences (i.e., less than 10 J/cm(2), when damaged by 355 nm, 5 ns pulses). At higher fluence, most materials are limited by other classes of damage precursors which exhibit a strong threshold behavior and high areal density (>10(5) cm(-2)); we refer to these collectively as high fluence precursors. Here, we show that a variety of nominally transparent materials in trace quantities can act as surface damage precursors. We show that by minimizing the presence of precipitates during chemical processing, we can reduce damage density in silica at high fluence by more than 100 times while shifting the fluence onset of observable damage by about 7 J/cm(2). A better understanding of the complex chemistry and physics of cleaning, rinsing, and drying will likely lead to even further improvements in the damage performance of silica and potentially other optical materials.

Strain build-up in SiC implanted at different temperatures

Single crystals of 4H-SiC were implanted with helium ions at temperatures of 400 and 700°C in a large range of fluences. The damage accumulation versus fluence was studied through the tensile elastic strain determined by using X-ray diffraction measurements. Results were analyzed via the multi-step damage accumulation model. At low dose (step 1) the strain can be described assuming a thermally activated process with low activation energy. Damage cross-sections, independent of implantation temperature, for interstitial-type defects were determined. With increasing dose, the contribution of other defects arises leading to an accelerated strain build-up namely the second step of the disordering process. However, in this regime, the strain cannot be fully described due to others operative mechanisms such as the formation of tiny bubbles under severe conditions of implantation. The formation of bubbles accelerates the development of the elastic strain. The values of damage cross-sections show that only small clusters contribute to the tensile elastic strain.

Neutron flux and annealing effects on irradiation hardening of RPV materials

This paper aims to examine an eventual effect of neutron flux, sometimes referred to as dose rate effect, on irradiation hardening of a typical A533B reactor pressure vessel steel. Tensile tests on both low flux (reactor surveillance data) and high flux (BR2 reactor) were performed in a large fluence range. The obtained results indicate two features. First, the surveillance data exhibit a constant (∼90 MPa) higher yield strength than the high flux data. However, this difference cannot be explained from a flux effect but most probably from differences in the initial tensile properties. The hardening kinetic of both low and high flux is the same. Annealing at low temperature, 345 °C/40 h, to eventually reveal unstable matrix damage did not affect both BR2 and surveillance specimens. This is confirmed by other annealing experimental data including both tensile and hardness measurements and tensile data on A508 forging and weld. It is suggested that the absence of flux effect on the tensile properties while different radiation-induced microstructures can be attributed to thermal ageing effects.

Study of Ion Beam Mixing by x ray reflectometry

ABSTRACTWe present in this text a new experimental tool to study the mixing of atoms under irradiation. Based on physics of x ray diffraction, the specular reflectivy of x ray was used to estimate the Auto Correlation Function associated with the electron density gradient. The accuracy of the ACF is around 1 nanometer and does not evolve with the thickness of the probed layer. Thus, this point allows accurately measuring the broadening of the electron density gradient spreading induced by irradiation. Such an accurate profile extracted over a large range of fluences (about 3 decades) would lead to the determination of the functional dependence of this spreading with the fluence. This could allow pointing out the main mechanisms triggering the atomic mixing over large distances when atomic mixing occurring in thermal spikes is washed out.

Multilevel RTS in Proton Irradiated CMOS Image Sensors Manufactured in a Deep Submicron Technology

A new automated method able to detect multilevel random telegraph signals (RTS) in pixel arrays and to extract their main characteristics is presented. The proposed method is applied to several proton irradiated pixel arrays manufactured using a 0.18 mum CMOS process dedicated to imaging. Despite the large proton energy range and the large fluence range used, similar exponential RTS amplitude distributions are observed. A mean maximum amplitude independent of displacement damage dose is extracted from these distributions and the number of RTS defects appears to scale well with total nonionizing energy loss. These conclusions allow the prediction of RTS amplitude distributions. The effect of electric field on RTS amplitude is also studied and no significant relation between applied bias and RTS amplitude is observed.

Defects in High Energy Ion Irradiated 4H-SiC

The defects produced by 7.0 MeV C+ irradiation in 4H-SiC epitaxial layer were followed by Deep Level Transient Spectroscopy, current-voltage measurements and Transmission Electron Microscopy in a large fluence range (109-51013 ions/cm2). At low fluence (109 -1010 ions/cm2), the formation of three main level defects located at 0.68 eV, 0.98 eV and 1.4 eV below the conduction band edge is detected. The trap concentration increases with ion fluence suggesting that these levels are associated to the point defects generated by ion irradiation. In this fluence range the leakage current of the diodes does not change. At higher fluence an evolution of defects occurs, as the concentration of traps at 0.68 eV and 1.4 eV decreases, while the intensity of the level at 0.98 eV remains constant. In this fluence range complex defects are formed and an increase of a factor five in the leakage current is measured.

Characterization of organic material micro-structures transferred by laser in nanosecond and picosecond regimes

The laser-induced forward transfer technique has been performed on thin layers of conducting organic materials for applications in plastic micro-electronics. This process is a promising alternative for fabrication of organic electronic components on flexible supports when usual techniques, such like ink-jet printing, cannot be considered. For example, when the organic material has no solubility properties or when complex architectures are needed. Experiments on the influence of pulse duration (nanosecond and picosecond) and wavelength on a large range of fluences have been proceeded using different lasers. An optimization of the process has been carried out by inserting a thin layer of absorbing metallic material between the substrate and the organic film. The advantage of this technique is to preserve organic layers from being damaged by thermal and photochemical effects during the interaction. The morphology and thickness of the deposit have been investigated by optical and scanning electronic microscopy. This experimental study is supplemented by electrical characterization of the deposits.

Damage formation and annealing at low temperatures in ion implanted ZnO

N, Ar, and Er ions were implanted into ZnO at 15 K within a large fluence range. The Rutherford backscattering technique in the channeling mode was used to study in situ the damage built-up in the Zn sublattice at 15 K. Several stages in the damage formation were observed. From the linear increase of the damage for low implantation fluences, an upper limit of the Zn displacement energy of 65 eV could be estimated for [0001] oriented ZnO. Annealing measurements below room temperature show a significant recovery of the lattice starting at temperatures between 80 and 130 K for a sample implanted with low Er fluence. Samples with higher damage levels do not reveal any damage recovery up to room temperature, pointing to the formation of stable defect complexes.

Effects of crystalline quality on the phase stability of cubic boron nitride thin films under medium-energy ion irradiation

To investigate the effect of radiation damage on the stability and the compressive stress of cubic boron nitride (c-BN) thin films, c-BN films with various crystalline qualities prepared by dual beam ion assisted deposition were irradiated at room temperature with 300 keV Ar+ ions over a large fluence range up to 2×1016 cm−2. Fourier transform infrared spectroscopy (FTIR) data were taken before and after each irradiation step. The results show that the c-BN films with high crystallinity are significantly more resistant against medium-energy bombardment than those of lower crystalline quality. However, even for pure c-BN films without any sp2-bonded BN, there is a mechanism present, which causes the transformation from pure c-BN to h-BN or to an amorphous BN phase. Additional high resolution transmission electron microscopy (HRTEM) results support the conclusion from the FTIR data. For c-BN films with thickness smaller than the projected range of the bombarding Ar ions, complete stress relaxation was found for ion fluences approaching 4×1015 cm−2. This relaxation is accompanied, however, by a significant increase of the width of c-BN FTIR TO-line. This observation points to a build-up of disorder and/or a decreasing average grain size due to the bombardment.

Direct write of resistive lines on SiC

The optimal parameters of a nanosecond pulsed laser for writing of tracks of controlled resistance onto high resistivity silicon carbide were investigated. A controlled energy deposition approach was carried out to minimize changes to the surface topography. The effect of fluence and irradiance on the ability to write narrow lines with controlled resistance were the explored parameters. Two focal length lenses and a large range of fluences were chosen to determine the best parameters to achieve controlled resistance. The experimental results indicate that both the beam fluence and irradiance have a significant effect on achieving an optimal resistance range.

Pulsed laser-target effects and high-resolution process diagnostics

In pulsed laser-target interaction at fluences and fluxes in a range of interest close to the air-breakdown threshold limits, the processes of heating, boiling, melting, chemical decomposition or vaporization occur quasi-explosively. Nonlinear effects due to plasma formation then largely determine the response of the materials. Investigations were carried out at various IR wavelengths (at 2.7 μm, 3.8 μm and 10.6 μm) using a multigas laser (at more than 10 J/pulse) which could be operated as a chemical HF or DF laser and as CO2 laser in the single-pulse mode. Studies in the repetitively pulsed mode, providing both high peak power densities and rather high average power densities simultaneously, were carried out at λ=10.6 μm with a specially designed multi-kW CO2 laser system. Both thermal and mechanical effects were studied on dielectric, semiconducting and metallic targets in a large fluence range from several up to hundreds of joules per cm2.

Vacuum Ultraviolet Laser Ablation of Teflon (PTFE)

ABSTRACTVacuum-ultraviolet laser ablation of Teflon is reviewed. The 157 nm irradiation of Teflon produces clean ablation sites well suited to micromachining applications in the electronics and medical fields. At 193 nm, etching profiles are poorly defined, showing swelling characteristics commonly produced by longer wavelength lasers. Comprehensive new 193 nm ablation data are presented showing the first evidence of incubation effects for Teflon. A computer model was developed to include ablation, swelling and incubation processes. The computer results satisfactorily model the experimental data over a large fluence range of 0.6 to 13 J/cm2 with three adjustable parameters.

Indigenous development of fatigue and creep resistant steels: Ramaswamy, V. and Mediratta, S.R. Trans. Indian Inst. Met. 1989 42, (Suppl.). S83–S92

This paper aims to examine an eventual effect of neutron flux, sometimes referred to as dose rate effect, on irradiation hardening of a typical A533B reactor pressure vessel steel.Tensile tests on both low flux (reactor surveillance data) and high flux (BR2 reactor) were performed in a large fluence range. The obtained results indicate two features. First, the surveillance data exhibit a constant (∼90 MPa) higher yield strength than the high flux data. However, this difference cannot be explained from a flux effect but most probably from differences in the initial tensile properties. The hardening kinetic of both low and high flux is the same. Annealing at low temperature, 345 °C/40 h, to eventually reveal unstable matrix damage did not affect both BR2 and surveillance specimens. This is confirmed by other annealing experimental data including both tensile and hardness measurements and tensile data on A508 forging and weld. It is suggested that the absence of flux effect on the tensile properties while different radiation-induced microstructures can be attributed to thermal ageing effects.

Magnetic properties of spinel oxides after high electronic excitation induced by xenon ions

Magnetic oxides MFe 2O 4, exhibiting spinel structure, with M = Mg 2+, Ni 2+, Fe 2+ and Zn 2+ have been irradiated at GANIL by 3.1 GeV xenon ions in a large range of fluences (10 11−5 × 10 12 cm −2). The effect of the high electronic excitation induced in the spinel structure has been investigated by Mössbauer spectroscopy. For MgFe 2O 4, a computer simulation of the spectra has been achieved by four contributions: a paramagnetic fraction, an oriented fraction as in the garnet Y 3Fe 5O 12, a wide distribution of the hyperfine field and a nonirradiated volume. The orientation of the hyperfine field under the strain induced by the burst-out atoms around the tracks seems to be confirmed since the sense of the field rotation with respect to the track direction depends on the sign of magneto-striction coefficient. In a first approximation, a good correlation has been found between the paramagnetic fractions, function of the track radii, and the electrical conductivity of all the oxides irradiated at GANIL by xenon ions.