Photons have many advantages for vaporizing condensed systems, and laser vaporization sources have a flexibility not available with other methods. These sources are applied to making thin films in the well-known technique of pulsed laser deposition (PLD). The vaporized material may be further processed through a pulsed secondary gas, lending the source additional degrees of freedom. Such pulsed-gas sources have long been exploited for fundamental studies, and they are very promising for film deposition, as an alternative to chemical vapor deposition or molecular beam epitaxy. The authors outline the fundamental physics involved and go on to discuss recent experimental findings.

Pulsed laser vaporization and deposition

Primary sputtering mechanisms are conventionally grouped in terms of the categories collisional, thermal , and electronic . With pulsed photons one must. in addition, consider the emission of droplets and fragments in thermomechanical processes. Pulsed photons also lead to the situation that the density of emitted particles is sufficiently high for gas-dynamic effects to enter and for the system therefore to lose memory of the primary mechanism. One then distinguishes secondary mechanisms which include outflow, as when a finite reservoir expands, effusion , effusive release from the outer surface without recondensation, and recondensation . effusive release with recondensation. If the photon pulse interacts with the emitted particles then still further secondary mechanisms are relevant due to energy deposition in the plume of emitted particles as well as due to ionization. Finally. the laser-pulse sputtering of the polymer PMMA (polymethylmethacrylate) and the superconductor YBCO ( YBa 2 Cu 3 O 7− x ) is discussed on the basis of explicit photographs of the sputtered particles. In the case of PMMA there are two groups of particles. the first group having primary and secondary mechanisms which are presently unestablishable but the second group being reasonably attributed to thermal primary release and to secondary behavior of the effusion (or recondensation) type. In the case of YBCO there is only one group of particles having a primary mechanism which is almost certainly electronic and a secondary mechanism which is tentatively identified with outflow .

Primary and secondary mechanisms in laser-pulse sputtering

Laser ablation deposition was used to grow polycrystalline Cu‐Ni and Ag‐Ni thin films on amorphous substrates at room temperature X‐ray diffraction was employed to determine the phases present and the residual macrostress and to analyze the structural imperfection in terms of crystallite size and microstrain For confirmation and complementary microstructural data transmission electron microscopy was applied Analysis of the gross composition was achieved by electron probe microanalysis and x‐ray fluorescence The films contained substantially less Cu and Ag than the targets, which was caused by preferential scattering of ablated Cu and Ag species upon incidence at the growing films The Cu‐Ni films were entirely composed of a CuxNi1−x solid solution The Ag‐Ni films were composed of a AgxNi1−x solid solution and of pure Ag and pure Ni The nonequilibrium AgxNi1−x solid solution could contain up to 44 at % Ag The residual macrostress in the Cu‐Ni films was compressive, whereas it was tensile in the Ag‐

Laser ablation deposition of Cu‐Ni and Ag‐Ni films: Nonconservation of alloy composition and film microstructure

UV spectroscopy in chemical physics, combustion, plasma science, and photolithography: ultraviolet and vacuum ultraviolet source and materials for lithography Laser optogalvanic spectroscopy for discharge plasmas in the ultraviolet region Spectra of the isotopomers of CO, N2, and NO in the ultraviolet Photoabsorpton cross section measurements in the UV and VUV UV and VUV laser spectroscopy using florescence Time-of-flight mass detection VUV spectroscopy of diatomic and triatomic molecules and VUV optical photolithography Spectroscopy associated with the search for new UV and VUV solid-state materials for laser technology: tunable solid-state UV and VUV lasers VUV laser spectroscopy of trivalent rare earth ions in wide band-gap fluoride crystals Spectroscopy of broadband UV emitting materials based on trivalent rare earth ions Solid-state tunable lasers for UV applications: generation of coherent ultraviolet and vacuum ultraviolet radiation by nonlinear processes in intense optical fields All-solid-state, short-pulse, tunable, ultraviolet laser sources based on Ce3-activated by fluoride crystals Diode-pumped picosecond UV lasers by nonlinear frequency conversions UV spectroscopy in atmospheric science and astronomy: atmospheric ultraviolet spectroscopy Ultraviolet spectroscopy in astronomy.

Ultraviolet Spectroscopy and UV Lasers

When gases are released from a pulsed nozzle or when solids are sputtered with intense laser pulses, effusion-like expansions take place which terminate abruptly. The resulting gas-dynamic processes depend on γ, the heat capacity ratio, as well as on whether particles backscattered to the effusing surface are subject to recondensation or reflection. Certain aspects of these terminating expansions have already been treated but we consider it appropriate to examine the problem further. In particular the following topics are emphasized. (a) Following previous work, the expansions are shown to consist of a series of regions separated by lines of contact, i.e. abrupt changes of slope. (b) For conditions of recondensation, there are two regions separated by one line of contact, the first region lying in part behind the effusing surface. For conditions of reflection, there are three regions, the first of which begins at the surface. Both types of expansion terminate with a region which is a remanent of the release process. (c) The nearsurface region under conditions of reflection permits an analytical approximation valid for all γ in which the sound speed is invariant with distance and the flow velocity is linear with distance. (d) The surface itself under conditions of recondensation permits an analytical approximation valid for all γ for the sound speed. More generally the near-surface region can be resolved by the method of Stanyukovich. (e) The various analytical solutions and approximations are shown to compare favorably with numerical results. (f) Plots of density and flow velocity versus distance are found to be roughly independent of γ, thence of the nature of the sputtered particles. (g) Tabulated results are presented to enable a more general use of gas-dynamic ideas.

Pulsed-Laser Sputtering of Atoms and Molecules. Part I: Basic Solutions for Gas-Dynamic Effects*

We present experimental results of photoablated product and kinetic energy (KE) distributions from the ultraviolet laser ablation of a sintered YBa2Cu3O6+x wafer at the laser wavelengths 351, 248, and 193 nm. Data is presented which spans the laser fluence range beginning at the threshold for species ejection (50 mJ/cm2) to nearly that required for the formation of an above surface plasma (800 mJ/cm2). The goal of this experiment was to measure changes in the photophysical process as the incident laser fluence was increased above the threshold value. Our results show, that near the laser threshold fluence, the photoejected products consist of small atomic and oxide species. An unforeseen result for the UV laser wavelengths used, is the lack of CuO+/CuO and free oxygen (O+/O, O+2/O2) in the mass spectrum. In addition, the product distributions are dependent on the laser wavelength. Measured also, at laser threshold fluence, are the nascent photoejected cation kinetic energy distribution. Here our results s...

Laser photodecomposition of sintered YBa2Cu3O6+x: Ejected species population distributions and initial kinetic energies for the laser ablation wavelengths 351, 248, and 193 nm

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Laser photodecomposition of sintered YBa2Cu3O6+x: Ejected species population distributions and initial kinetic energies for the laser ablation wavelengths 351, 248, and 193 nm