R. A. Lindley

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.

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.

Resonant‐holographic‐interferometry for absolute measurements of excimer laser‐ablated neutral‐atom plume line‐density profiles

Experiments have been performed to measure Al neutral atom absolute line‐density profiles using resonant‐holographic‐interferometry. The ablation source is a KrF excimer laser with a per‐pulse energy of about 0.8 J. Targets are either pure aluminum or Al2O3. Aluminum ground‐state neutral atom line‐densities are probed by a dye laser tuned near the 394.401 nm line. A double‐pulse interferometry technique is employed in which one pulse includes the laser ablation plume and a second pulse generates reference fringes on a holographic plate by rotating a mirror. Holograms are reconstructed to give interference fringes on film by using a helium‐neon laser. Interferograms of laser ablated Al metal give maximum Al neutral plume line‐densities in the range of 4‐10×104 cm−2. Aluminum neutral line‐densities from Al2O3 targets are as much as 10‐20 times larger than from Al metal targets. The sensitivity of this resonant diagnostic is 4 to 5 orders of magnitude higher than nonresonant neutral‐particle interferometry a...

Laser Ablation Of Al/sub 2/O/sub 3/ Ceramic In Gas And Plasma Environments

A plasma-CVD reactor with parallel plate electrodes has been used to produce TLN coatings on three-dimensional substrates of simple geometry, using a mixture of Ha. N2 and Tic14 as reactive gases. The chemical and mechanical properties of the coatings have been determined as a function of selected parameters of the deposition process. The general characteristics of the plasma reactor and the obtained results are discussed. Krypton-fluoride laser ablation experiments have been performed on A I 2 0 3 (alumina) to investigate

Resonant holographic interferometry measurements of laser ablated atom absolute-line-density profiles in vacuum, gases, and plasmas

Summary form only given, as follows. Two-dimensional, species-resolved, double-pulsed holographic interferometry has been used to investigate the hydrodynamics of KrF laser ablation plumes in vacuum, gases, and RF plasmas. To produce the laser ablation plume, a KrF excimer laser (40 ns, 248 nm, /spl les/ 0.8 J) was focused onto a solid aluminum target at a fluence of 1.9 - 4.7 J/cm/sup 2/. The interferograms were made using an XeCl-excimer-laser-pumped dye laser (20 ns, /spl ap/ 5 mJ) tuned at or near (/spl plusmn/ 0.100 nm) the 394.401 nm aluminum neutral transition from ground state. Calculations have been performed to obtain aluminum-neutral absolute-line-density profiles from the resonant fringe shift data, assuming an average kinetic plume temperature of /spl ap/ 0.3 eV and a dye laser bandwidth of /spl ap/0.0031 nm. Peak aluminum neutral line-densities of up to 9 /spl times/ 10/sup 10/ cm/sup -2/ have been measured for plumes in backgrounds of 1 Torr and 35 Torr argon and 1 Torr argon RF-plasma, and in vacuum.