H. L. Spindler

ANALYSIS OF LASER ABSORPTION ON A ROUGH METAL SURFACE

We have developed a simple model to estimate the cumulative absorption coefficient of an ultraviolet laser pulse impinging on a pure metal, including the effects of surface roughness whose scale is much larger than the laser wavelength λ. The multiple reflections from the rough surface may increase the absorption coefficient over a pristine, flat surface by an order of magnitude. Thus, as much as 16% (at room temperature) of the power of a 248 nm KrF excimer laser pulse may be absorbed by an aluminum target. A comparison with experimental data is given.

Surface instability of multipulse laser ablation on a metallic target

Large scale wavelike patterns are observed on an aluminum surface after it is ablated by a series of KrF laser pulses (248 nm, 40 ns, 5 J/cm2). These surface structures have a wavelength on the order of 30 μm, much longer than the laser wavelength. We postulate that these wave patterns are caused by the Kelvin–Helmholtz instability at the interface between the molten aluminum and the plasma plume. A parametric study is given in terms of the molten layer’s thickness and of the spatial extent and kinetic energy density in the laser-produced plasma plume. Also included is an estimate of the cumulative growth in a multipulse laser ablation experiment. These estimates indicate that the Kelvin–Helmholtz instability is a viable mechanism for the formation of the large scale structures.

Effects of laser‐ablation target damage on particulate production investigated by laser scattering with deposited thin film and target analysis

Experiments have been carried out to correlate ablated particulate density and size to the number of KrF excimer laser (248 nm, 40 ns, <1.2 J) pulses incident on a single location of a pure solid aluminum target and to relate particulate production to target surface damage. An analysis of laser ablation deposited aluminum films on silicon substrates was used to determine the density of ablated particulate greater than 0.5 μm in diameter. For an undamaged target, the laser deposited particulate density was on the order of 8.6×105 cm−2 per 1000 shots. A damaged target (following 1000 laser pulses) produced a density on the order of 1.6×106 cm−2 per 1000 shots on the substrate. Dye laser optical scattering was also used to measure, in real time, the velocity of the particulate and the relative particulate density in the laser‐ablation plume versus target damage. Results indicated a rapid rise in the production of particulate as target damage was increased up to 3000 laser pulses; after this number of shots t...

Ionization dynamics of iron plumes generated by laser ablation versus a laser‐ablation‐assisted‐plasma discharge ion source

The ionization dynamics (iron ion and neutral atom absolute line densities) produced in the KrF excimer laser ablation of iron and a laser‐ablation‐assisted plasma discharge (LAAPD) ion source have been characterized by a new dye‐laser‐based resonant ultraviolet interferometry diagnostic. The ablated material is produced by focusing a KrF excimer laser (248 nm,<1 J, 40 ns) onto a solid iron target. The LAAPD ion source configuration employs an annular electrode in front of the grounded target. Simultaneous to the excimer laser striking the target, a three‐element, inductor–capacitor, pulse‐forming network is discharged across the electrode–target gap. Peak discharge parameters of 3600 V and 680 A yield a peak discharge power of 1.3 MW through the laser ablation plume. Iron neutral atom line densities are measured by tuning the dye laser near the 271.903 nm (a 5D–y 5P0) ground‐state and 273.358 nm (a 5F–w 5D0) excited‐state transitions while iron singly ionized line densities are measured using the 263.105 nm (a 6D–z 6D0) and 273.955 nm (a 4D–z 4D0) excited‐state transitions. The line density, expansion velocity, temperature, and number of each species have been characterized as a function of time for laser ablation and the LAAPD. Data analysis assuming a Boltzmann distribution yields the ionization ratio (ni/nn) and indicates that the laser ablation plume is substantially ionized. With application of the discharge, neutral iron atoms are depleted from the plume, while iron ions are created, resulting in a factor of ∼5 increase in the plume ionization ratio. Species temperatures range from 0.5 to 1.0 eV while ion line densities in excess of 1×1015 cm−2 have been measured, implying peak ion densities of ∼1×1015 cm−3.The ionization dynamics (iron ion and neutral atom absolute line densities) produced in the KrF excimer laser ablation of iron and a laser‐ablation‐assisted plasma discharge (LAAPD) ion source have been characterized by a new dye‐laser‐based resonant ultraviolet interferometry diagnostic. The ablated material is produced by focusing a KrF excimer laser (248 nm,<1 J, 40 ns) onto a solid iron target. The LAAPD ion source configuration employs an annular electrode in front of the grounded target. Simultaneous to the excimer laser striking the target, a three‐element, inductor–capacitor, pulse‐forming network is discharged across the electrode–target gap. Peak discharge parameters of 3600 V and 680 A yield a peak discharge power of 1.3 MW through the laser ablation plume. Iron neutral atom line densities are measured by tuning the dye laser near the 271.903 nm (a 5D–y 5P0) ground‐state and 273.358 nm (a 5F–w 5D0) excited‐state transitions while iron singly ionized line densities are measured using the 263.105...

Characterization of a laser-ablation-assisted-plasma-discharge-metallic ion source

Experiments have been carried out to characterize further the properties of a new laser-ablation-assisted-plasma-discharge source of metallic aluminium ions. Laser ablation is accomplished by focusing a KrF excimer laser (<1.2 J, 40 ns, 248 nm) onto a solid aluminium target with a fluence of approximately 10 J cm-2. Through gated optical emission spectroscopy, the laser ablation plume optical emission is observed to contain only aluminium neutral atom transitions after approximately 100 ns. With the application of a 3.6 kV, 760 A discharge, the neutral atom plume is transformed into a plasma with the emission dominated by Al+ and Al2+ ion transitions. Through time-resolved spectroscopy, emission intensity from the Al neutral species and the Al2+ ion species is observed to coincide with current peaks through the plasma. Spectroscopic measurements indicate an Al2+ electronic temperature of 3 eV (and an Al+ electronic temperature of 1 eV) which, since local thermodynamic equilibrium (LTE) is applicable for the observed emission, provide a free electron temperature of 1 to 3 eV. A simple LTE model suggests an electron temperature of 1.2 eV for equal Al+ and Al2+ ion fractions. A floating double Langmuir probe measurement 1 mm in front of the laser ablation spot indicates an electron temperature of roughly 1 eV and an ion density of approximately 5*1014 cm-3 during the second current lobe.

Surface instability on a metal target from multi-pulse KrF laser ablation

Summary form only given, as follows. Aluminum targets were ablated by focusing a KrF excimer laser (248 nm, 40 ns, <1.2 J) down to a spot size of 0.05 cm/sup 2/ with a fluence of approximately 4.9 J/cm/sup 2/. After a few tens of pulses, surface irregularities (corrugations and pits) progressively emerge, with size 1-100 /spl mu/m which is much larger than the laser wavelength. After hundreds of laser pulses, large scale wavelike patterns, on the order of 30 /spl mu/m, are observed on the aluminum surface. We propose that these wave patterns are caused by the Kelvin-Helmholtz instability at the interface of the molten aluminum and the plasma plume. A parametric study is given in terms of the molten layers thickness and of the spatial extent and kinetic energy density in the laser-produced plasma plume. Also included is an estimate of the cumulative growth in a multi-pulse laser ablation experiment. These estimates indicate that the Kelvin-Helmholtz instability is a viable mechanism for the formation of the large scale structures. Once formed, these large scale surface roughness causes multiple reflections of the laser light, and may increase the absorption coefficient over a pristine, flat surface by an order of magnitude.

Analysis of surface structures of a metal target from multi-pulse KrF laser ablation

Summary form only given, as follows. Aluminum targets were ablated by focusing a KrF excimer laser (248 nm, 40 ns, <1.2 J) down to a spot size of 0.05 cm/sup 2/ with a fluence of approximately 4.9 J/cm/sup 2/. After a few tens of pulses, surface irregularities (corrugations and pits) progressively emerge, with size 1-100 /spl mu/m which is much larger than the laser wavelength. Such large scale surface roughness causes multiple reflections of the laser light, and may increase the absorption coefficient over a pristine, flat surface by an order of magnitude. Thus, as much as 16% (at room temperature) of the power of the KrF laser may be absorbed by the aluminum target. Scaling laws on the enhanced absorption due to surface roughness are derived. We have also examined various physical mechanisms that lead to these large scale surface structures. The most promising candidate appears to be hydrodynamic instabilities of intense plasma formation near to the surface. A model is developed which yields the growth rate as a function of wave number, thickness of molten layer, energy density and spatial extent of the surface plasma, and the thermophysical properties of the irradiated material. We found that there is a threshold of plasma energy density for the occurrence of the instability.

Ablation of metals by a channelspark electron beam

Summary form only given. A new experiment has been constructed for ablation of materials by a channelspark electron beam. This experiment has the goal of electron beam ablative deposition of thin films. The channelspark (pseudospark) generator obtained from Forschungszentrum Karlsruhe has operating parameters: V=15-20 kV, 1/spl les/1.5 kA, and pulselength of /spl sim/100 ns in a beam diameter of /spl les/2 mm. Initial experiments are concentrating on characterization of the channelspark electron beam generation at different pressures. Electron beam ablation of metals (Al and Fe) is being investigated by collected e-beam current measurements and optical diagnostics. Target analysis is compared between e-beam ablation and KrF laser ablation to elucidate the differences in the physical mechanisms. Electron beam energy deposition has been modeled by the ITS-TIGER code of Sandia National Labs. The electron beam energy deposition profile can be utilized to predict the temperature profile induced in the metal target.

Characteristics of metal thin film deposition by laser ablation and laser-ablation-assisted-plasma-discharge

Experiments are under way to deposit thin films of Al on Si substrates by two laser-ablative deposition processes: KrF excimer laser ablation and laser-ablation-assisted-plasma-discharges. Laser ablation is accomplished by focusing a KrF excimer laser (<1.2 J, 40 ns, 248 nm) onto a pure (99.999%) solid aluminum target. Typical ablation fluences range from 4-10 J/cm/sup 2/. Through gated optical emission spectroscopy, the laser ablation plume optical emission is observed to contain only aluminum neutral transitions after approximately 100 ns. With the application of a 3.6 kV, 760 A discharge, the neutral atom plume is transformed into a plasma with the emission dominated by Al/sup +/ and Al/sup 2+/ transitions. Spectroscopic measurements indicate an Al/sup +/ electronic temperature of 1 eV and an Al/sup 2+/ electronic temperature of 3 eV. Since LTE is applicable for the observed emission, the free electron temperature of the discharge plasma is between 1 and 3 eV. A floating double-Langmuir probe measurement indicates a discharge electron temperature of 1 eV and an ion density of approximately 5/spl times/10/sup 14/ cm/sup -3/. Film characteristics under investigation include the material deposition rate and the particulate density and size. Initial studies find particulate diameters up to 20 microns with laser alone. Optical diagnostics have also been applied to monitor the ablation plasma plume and the discharge plasma during the deposition.