J. S. Lash

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.

LASER DIAGNOSTIC EXPERIMENTS ON KRF LASER ABLATION PLASMA-PLUME DYNAMICS RELEVANT TO MANUFACTURING APPLICATIONS

A brief review is given of the potential applications of laser ablation in the automotive and electronics manufacturing industries. Experiments are presented on KrF laser ablation of three materials relevant to manufacturing applications: aluminum metal vs aluminum–nitride (AlN) and alumina (Al2O3) ceramics. Plasma and neutral‐atom diagnostic data are presented from resonant‐holographic‐interferometry, dye‐laser‐resonance‐absorption photography, and HeNe laser deflection. Data show that plasma electron densities in excess of 1018 cm−3 exist in the ablation of AlN, with lower densities in Al and Al2O3. Aluminum neutral and ion expansion velocities are in the range of cm/μs. Ambipolar electric fields are estimated to be 5–50 V/cm.

Resonant holographic interferometry measurements of laser ablation plumes in vacuum, gas, and plasma environments

Resonant holographic interferometry and dye‐laser‐resonance‐absorption photography have been utilized to investigate the expansion of the laser ablation plumes produced by a KrF excimer laser beam (248 nm) focused onto an aluminum target (≊0.1 cm2, 2–6 J/cm2). Plume expansion was studied in vacuum and in background argon gas pressures of 14 mTorr, 52 mTorr, 210 mTorr, 1 Torr, and 35 Torr. The existing theory for the interpretation of resonant interferograms has been extended to account for Doppler shift effects, the diagnostic laser bandwidth, and the selective absorption of the laser beam. Absolute line densities in the range 4.3×1013–1.0×1015 cm−2 have been measured in the ablation plumes, which imply measured Al neutral densities of up to 1×1015 cm−3. The total number of Al neutral atoms in a plume has been measured to be ≊3×1014, which corresponds to a surface etch rate of ≊1 nm/pulse. Expansion velocities in the range 1.1–1.4 cm/μs were measured for the pressures ≤210 mTorr, while ≊0.3 cm/μs was meas...

Electron beam ablation versus laser ablation : plasma plume diagnostic studies

Experiments have been performed to compare XeCl laser ablation plume characteristics to those produced by electron . beam ablation. Potential advantages of electron beams include higher electrical efficiency ; 30% , and the ability to process materials with high optical reflectivity or transparency. The electron beam is generated by a channelspark with parameters: peak voltage of 15-20 kV, current of 1.5-1.7 kA, and pulse length of about 200 ns. The electron beam is ion focused to about 2 mm diameter by an argon background gas. Initial diagnostic experiments have utilized optical emission spectroscopy to characterize the ionization dynamics of the ablation plumes of Fe targets. Spectra taken during electron beam ablation are composed of singly ionized iron, with negligible emission from neutral iron. This is in sharp contrast with XeCl excimer laser ablation, which is composed of both neutral and ion species, the neutrals persisting strongly after the laser pulse. In addition to Fe ion emission, the channelspark emission spectrum also exhibits a high degree of excitation and ionization of the Ar background gas. Strong emission from Ar q ,A r 2q , and Ar 3q has been measured. q 1998 Elsevier

Laser‐ablation‐assisted‐plasma discharges of aluminum in a transverse‐magnetic field

Laser‐ablation‐assisted‐plasma discharges (LAAPD) have been used to enhance the ionization of laser ablated aluminum metal. Ablation is accomplished by focusing a KrF excimer laser (248 nm, 40 ns, ≤0.4 J) on a solid aluminum target with a fluence of 4 J/cm2. Peak plasma discharge voltage is 1–4 kV and peak plasma current is 0.2–1 kA, while peak power is 0.1–1 MW. Gated emission spectroscopy is used to determine the charge states and the electronic temperatures within the plasma discharge. With unmagnetized discharge parameters of 3 kV and 760 A, the observed light emission is dominated by transitions from Al2+ ions indicating nearly complete ionization of the plume. From the emission spectra intensities, an Al2+ electronic temperature of 3.3 eV is determined. Emission spectra from unmagnetized LAAPD of 1.2 kV and 280 A show no visible Al2+ ion transitions indicating cooler plasma and a lower ionization state. Introducing a 620 G transverse magnetic field (at 1.2 kV, 280 A) enhances the ionization due to t...

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...

DETECTION OF ALO MOLECULES PRODUCED BY KRF LASER-ABLATED AL ATOMS IN OXYGEN GAS AND PLASMA ENVIRONMENTS

Experiments have been performed to measure, in real time, the formation of AlO molecules from laser‐ablated Al atoms in oxygen gas and plasma environments. The Al atom plume is generated by focusing a KrF laser (4 J/cm2) on Al metal targets or polycrystalline Al2O3 (alumina) ceramic. AlO molecule formation has been characterized by emission spectroscopy at 464.82 and 484.22 nm molecular bandheads. Time‐integrated and time‐resolved optical emissions have been measured of laser‐ablated Al atoms interacting with oxygen or argon neutral‐gas versus plasma backgrounds generated by a high‐voltage capacitive discharge. Results indicate that gas/plasma‐phase reactions occur between laser‐ablated Al atoms and oxygen. Optimal enhancement of AlO optical emission is measured in oxygen plasmas at about 200 mTorr fill pressure.

Rectangular interaction structures in high power gyrotron devices

Summary form only given, as follows. Gyrotron devices utilizing rectangular interaction waveguide are currently being investigated. Some current issues under investigation include the control of polarization, and power versus pulselength of microwave emission. Annular long-pulse electron beams are generated by the Michigan Electron Long Beam Accelerator (MELBA) at parameters: electron beam voltage of 750 kV, injected current of 1-3 kA, and /spl tau/=0.5-1.0 /spl mu/s. Initial experimental results of polarized microwave radiation from a rectangular cross-section (RCS) gyrotron will be presented for small-orbit electron beams. Frequency measurements and annular e-beam generation and transport experimental results will be presented. Simulations using the E-gun code and MAGIC code will be presented.

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.