C. H. Ching

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.

Schlieren and dye laser resonance absorption photographic investigations of KrF excimer laser-ablated atoms and molecules from polyimide, polyethyleneterephthalate, and aluminum

Hydrodynamic phenomena from KrF excimer laser ablation (10−3–20 J/cm2) of polyimide, polyethyleneterephthalate, and aluminum are diagnosed by schlieren photography, shadowgraphy, and dye laser resonance absorption photography (DLRAP). Experiments were performed both in vacuum and gaseous environments (10−5–760 Torr air, nitrogen, and argon). In vacuum, ablation plumes are observed to expand like a reflected rarefaction wave. As the background gas pressure is increased, shock waves and reduced‐density ablation plumes become visible. Below 10 Torr, the ablation plume follows closely behind the shock wave. Between 20 and 100 Torr, the plume recedes behind the shock wave. Below 10 Torr and above about 200 Torr, both the plume and the shock expand with the same temporal power law dependence. Agreement is found between these power law dependences and those predicted by ideal blast wave theory. The DLRAP diagnostic clearly shows that the ablated material (CN molecule from polyimide and ground state neutral alumi...

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...

Resonant holographic interferometry of laser‐ablation plumes

Two‐dimensional species‐resolved, holographic interferometry has been used to measure absolute‐line‐density profiles of KrF laser ablation plumes in vacuum and gas. Laser ablation plumes are generated by focusing a KrF excimer laser (40 ns, 248 nm, ≤0.8 J) on a solid aluminum target at a fluence of 2–5 J/cm2. Aluminum neutral absolute‐line‐density profiles are measured to characterize the interaction of ablated material with background gases versus vacuum. The interferograms are made using a 20 ns pulsed dye laser tuned near (≤±0.020 nm) the 394.401 nm aluminum neutral transition from the ground state. Calculations have been performed to obtain absolute‐line‐density profiles from the resonant fringe shift data. Peak aluminum neutral line densities of up to 1×1015 cm−2 have been measured for plumes in backgrounds of 14 mTorr and 1 Torr argon and in vacuum.

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...

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.

Measurement of long‐pulse relativistic electron beam perpendicular‐ to‐parallel velocity ratio by Cerenkov emission and radiation darkening on a glass plate

We report measurements of the ratio of the perpendicular velocity to the parallel velocity, α= v⊥ /v∥, of a relativistic electron beam gyrating in a magnetic field by the use of (1) Cerenkov emission from a glass plate, detected by a gated microchannel plate image intensifier camera, and (2) electron‐beam‐induced radiation darkening pattern on the same glass plate. The measurements are based on a direct determination of the Larmor radius of an electron beam of known energy. Experiments were performed on a long‐pulse electron beam accelerator with e‐beam diode parameters: VD = 0.6–0.9 MV, pulse length=0.5–1 μs, ID = 1–10 kA. The experimental value of α agrees with simulation results from particle trajectory codes as well as theoretical predictions from Busch’s theorem and adiabatic theory.

Copper vapor laser machining of polyimide and polymethylmethacrylate in atmospheric pressure air

A repetitively pulsed copper vapor laser (510 and 578 nm) is used to machine an opaque polymer (polyimide‐Vespel) and a transparent polymer (polymethylmethacrylate‐Lucite). Lucite is machinable by coating the surface with an ink which is semi‐opaque to the green and yellow laser light. The repetition rate of the laser was 10 kHz with approximately 0.35 mJ/pulse and 3.5 W average power at the copper vapor laser wavelengths for a pulse width of 40 ns. The copper vapor laser thermally loads the target, generating thermal waves and sound waves in the gas which are investigated using HeNe laser beam deflection. The gas adjacent to the target is heated to steady state on the order of 100–400 s. Above the etching threshold, at approximately 10 mJ/cm2/pulse, the target is rapidly machined: 2‐mm‐diam, 2‐mm‐deep holes are drilled in 300 s in Vespel. At higher fluences of 100–150 mJ/cm2/pulse in 760 Torr of air it takes 180 s to bore through a 2‐mm‐thick disk of Vespel. The machined surfaces of the two polymers are ...

Species-resolved laser-probing investigations of the hydrodynamics of KrF excimer and copper vapor laser ablation processing of materials

Hydrodynamic phenomena from KrF excimer laser ablation (10-3-20 J/cm2) of polyimide, polyethyleneterephthalate, and aluminum are diagnosed by laser beam deflection, schlieren photography, shadowgraphy, laser-induced-fluorescence and dye-laser- resonance absorption photography (DLRAP). Experiments were performed in vacuum and gaseous environments (10-5 to 760 Torr). In vacuum, the DLRAP diagnostic shows species-resolved plume expansion which is consistent with that of a reflected rarefaction wave. Increasing the background gas pressure reveals the formation of sound/shock compared to CN in the laser-ablated polyimide (Vespel) plume/shock in inert (e.g. argon) and reactive (e.g. air) gases. At low pressures (less than 10 Torr) Al and CN species are in close contact with the shock front. As the pressure increases, the species front tends to recede, until at high pressures (over 200 Torr) the species are restrained to only a few mm above the target surface. After sufficient expansion, Al and CN are no longer detectable; only the shadowgraph of the hot gas plume remains. Since CN is observable in both inert and reactive environments, it can be concluded that CN is not a reaction product between the background gas and the ablated species. By way of comparison to excimer laser ablation processing of materials, copper vapor laser machined polyimide and polymethylmethacrylate (transparent to green and yellow copper vapor laser light) are also investigated. The two polymers are observed to have markedly different machined surfaces. Hydrodynamic effects for the copper vapor laser machined materials are investigated using HeNe laser beam deflection.

Beam breakup instability experiments on long-pulse electron-beam transport through rf cavities

Experiments designed to investigate the beam breakup (BBU) instability have been performed using the long-pulse MELBA electron-beam generator (0.5 - 1.5 microsecond(s) , 0.7 - 0.8 MV, ldiode equals 1 - 15 kA, lextracted equals 0.1 - 0.5 kA). The experiment consists of 10 identical pillbox cavities each containing a small microwave loop antenna designed to detect the TM110 beam breakup mode. For our cavity design the TM110 resonant frequency occurs at approximately 2.5 GHz. The cavities are connected by small diameter tubes which attenuate the RF cavity-to-cavity crosstalk. The MELBA diode and subsequent cavity system are immersed in a solenoidal magnetic field (0.8 - 3 kG). Microwaves of 2.5 GHz (1 - 4 kW), whose pulselength exceeds the beam pulse, can be injected into the initial cavity in order to prime the BBU instability. BBU instability growth is measured through the growth of 2.5 GHz RF between the first (or second) and tenth cavities. The BBU growth is compared with predictions made by beam-cavity coupled-mode theory.