In this article, we review experimental measurements taken in our laboratory for the low-fluence UV pulsed laser photodecomposition of the perovskite ceramic Bi2Sr2CaCu2O8. We present the measured photoejected product kinetic energy (KE) distributions, including that for the ion and the neutral, the atomic and polyatomic species. For the specific case of the neutral species desorption, we present new experimental results that elucidate the change in the photophysical desorption process as a function of the incident laser fluence. The results show the transition from the electronic-induced desorption to thermal evaporation as the laser fluence is increased. The KE distribution data suggests that for low-fluence laser excitation, the primary photodesorption process is two-hole localization at a binding site followed by coulomb repulsion. A fraction of the escaping ions acquire the full coulomb KE (e2/ro; ro internuclear distance). The neutral species acquire a partial fraction of the available KE (i.e., the KE gained prior to the neutralization). The appearance of polyatomic species desorption is explained via the spectator-stripping model.

Pulsed laser deposition of thin films is a technology whose fundamental processes are often poorly understood. Because of the difficulty of monitoring in real time either the ablation process itself (the laser-solid interaction), or thin film growth (plume-substrate interaction), studies have largely relied on diagnostic studies of the ablated plume and the resulting film to infer details about other steps in the process. Information gained from this approach has helped improve the production of high-temperature superconducting thin films.

We have studied plume dynamics during the in-situ pulsed laser deposition of YBa2Cu3O7−δ thin films. The 248 and 308 nm lines of an excimer laser were used to generate a plume from a bulk YBa2Cu3O7−δ target. Both fast intensified CCD imaging and spectral diagnostics were used to monitor plume dynamics. Variations in the plume distribution as a function of processing gas, pressure, fluence, energy, and spot size were monitored by film composition and spectral-and time-resolved imaging.

Fast intensified CCD photography and gated photon counting following KrF-laser irradiation of YBCO and BN targets reveal the first observations of very weak emission from slow-moving ejecta up to 2 cm from the target and times extending to ∼1.5 ms. Time-of-flight velocities inferred from the emission measurements indicate velocities (v ˜ (0.45−1.2) x 104 cm s−1) comparable to those measured for the large particles which often accompany the pulsed laser deposition process. Gated photon counting is employed to obtain temporally resolved spectra of this weak emission. The spectral shape is characteristic of blackbody emission, which shifts to longer wavelengths as the particles cool during flight in vacuum. Estimates of the temperature of the particles are made based on the emissivity of a perfect blackbody and range from 2200 K to 3200 K for both BN and YBCO when irradiated at Φ248 = 3.5 J cm−2 and 1.5 J cm−2, respectively. The temperature decrease of the particles in vacuum is compared to a radiative cooling model which gives estimates of the initial surface temperature and radii of the particles.

Emission spectra from 1200 Å to 3500 Å have been measured for laser-generated plasmas produced by the focused output (1−4 J/cm2)of a Kr-F excimer.laser onto solid targets of single and multicomponent materials (e.g., Al, Y2O3, BaO, Cu, and YBa2Cu3O7−δ). We have measured the spatial distribution and fluence dependence of the plasma emission. The emission spectra indicated that very large density gradients from the large opacity at the target surface were present. In general, the spectra were dominated by excited neutrals and singly and doubly ionized species. Plasma emission spectra from Al have been analyzed in more detail to derive plasma temperature and density. A plasma temperature of 2 eV and an electronic density of ∼1018/cm3 at 2 J/cm2 was estimated from the relative line intensities. Spatial variation of the total vacuum ultraviolet emission provided evidence for laser absorptionin the ablated material.

The intermediate species present in laser ablation plumes, particularly those formed during pulsed laser deposition (PLD) of thin films, have been identified for a variety of advanced materials systems. Optical multichannel analysis spectroscopy has been used to monitor the atomic and ionic species present, via their spectral emissions. Molecular beam-sampling mass spectrometry has been used to monitor the molecular species present, in addition to atoms and ions. With both monitoring approaches, temporal and spatial species distribution information has been obtained. Velocity distributions, obtained from the time-dependent mass spectral studies, show the effects of isentropic expansion to be predominant when compared with gasdynamic models of the plume evolution process. Also, the plume structure was found to be particularly sensitive to target elemental distribution. Examples of systems studied include high temperature superconductors, refractory compounds, ferroelectrics and nanostructured magnetic films.

Kinetics of photodeposition of superconducting films was studied by using real time in-situ monitoring of gas phase species with mass spectrometry and thin-film properties by Fourier Transform Infrared (FTIR) spectrometry. A mass spectrometer was used to study wavelength and power dependent desorbed atoms and molecules during laser ablation process. FTIR was used to monitor the surface properties of thin film. Real time surface temperatures can be easily obtained by FTIR. By combinging mass spectrometry and FTIR, an optimum condition of making high-quality superconducting film can possibly be found.

The ionic content of an excimer laser ablated Yga2Cu3O7−δ (YBCO) plume has been investigated by using an ion probe detection system. The time-of-flight ion signals indicate a bifurcation of the plume, where the degree of bifurcation is dependent on both the oxygen pressure and the laser spot size at the target. A significant enhancement in the ionic content is observed when the plume is passed through a high voltage discharging ring. The results describing the effect of the pressure, laser spot size and the discharge parameters on the propagating laser plume are presented in this paper.

Collision processes between oxygen molecules and laser ablated fragments from YBa2Cu3Oy superconductor target are described. The space/time resolved photo-emission measurements of Cu and CuO were carried out in order to investigate the oxidation of the laser ablated fragments in O2 atmosphere. Increase of the photo-emission yield of CuO was observed over 100 mTorr and it showed maximum at 2 Torr. Velocity of CuO decreases abruptly at 200 mTorr. The velocity distribution spreads above 500 mTorr. An optimum range of oxygen pressure to fabricate superconducting thin films of Tc = 90 K was from 250 mTorr to 500 mTorr. Process of the reactive collisions of ablated fragments with O2 molecules in this range is discussed.

We present a hydrodynamic model to simulate laser ablation. The model conisistently takes into account the main physical phenomena involved in laser ablation: heating, thermal conduction, hydrodynamics and atomic kinetics. It computes the ionization balance, plasma formation and expansion, and the attenuation of the laser light as it travels through the plasma. Preliminary results are presented.

We measure the stress transients resulting from pulsed excimer laser irradiation of polyimide at 351, 308, 248 and 193 nm, using thin (9 μm) piezoelectric PVDF (polyvinylidene fluoride) films. We find that fluences between 3·10−3 and 102 J/cm2 generate peak stresses between 104 and 109 Pa. Further, the results show three ranges of fluence where different physical mechanisms mediate the stress generation. In the lowest range of fluence, subsurface thermal decomposition (for λ = 351 and 308 nm) and photodecomposition (for λ = 248 and 193 nm) govern the generation of the observed stresses. At higher fluences we identify two regimes, independent of laser wavelength, where the gas dynamic expansion of the ablation products and plasma formation and expansion, are responsible for the generated stresses.

Molecular dynamics simulations are performed of Cu thin film growth on Cu (111). Ion-Assisted Deposition is simulated by bombarding the substrate with Cu+ ions with a kinetic energy of 80 eV, while 1 eV Cu atoms are used for the simulation of Laser Ablation Deposition. It appears that Ion-Assisted Deposition leads to sputtering, enhanced surface mobility, surface disorder, mixing and rather deep damage. This is discussed in some detail. Laser Ablation Deposition, using laser fluences just above the ablation threshold, does not lead to damage and mixing. Sharper interfaces and more perfect heterostructures and superlattices can be produced using Laser Ablation Deposition.

Laser diagnostic methods are developed and used to characterize the behavior of laser ablated zirconium carbide (ZrC). Emission from zirconium atoms dominates the total emission of the plasma plumes, which are estimated to have excitation temperatures of 9000 to 12,000 K under the conditions studied. Emission from species such as C, C2, and C3 were absent from the spectra due to the inherently low emission intensities of these species compared with that of Zr. Using a CCD camera, images of the plasma plumes are obtained from the emission and through the use of planar laser induced fluorescence of zirconium atoms.

Using the pulsed laser deposition (PLD) technique to prepare PbZrxTi(1−x)03 (PZT) films from stoichiometric targets, it has been found that the Pb content of the films is very sensitive to, among other parameters, the phase of the ablation target. In an effort to better understand PZT film growth, a time-of-flight quadrupole mass spectrometer (TOF/QMS) was used to obtain the temporal and mass profiles of material ejected from two different solid targets with a nominal composition of PbZr0.54Ti0.46O3 over a laser fluence range of 0.2 to 2.0 J/cm2. The ablation targets were made from either a compressed mixture of oxides or a fully reacted PZT solid. The composition of the plume varied significantly depending upon the phase of the target and the laser fluence. In general, two distinct ejection processes were identified and the TOF profiles observed could be characterized as either thermal vaporization (fluence ≤ 0.3 J/cm2), ablation, with a highly forward directed ejection pattern normal to the target surface, or a combination of the two processes (fluences ≥ 0.3 J/cm2). The thermal vaporization (or low energy) component of the ejected species were modeled using a Maxwell-Boltzmann distribution. The fully reacted target yielded atomic species which had higher translational temperatures (about 100–700K) than the corresponding components of the pressed oxide target. The translational temperatures ranged from about 1000–6500K, depending upon the species and the laser fluence. Both targets showed distinctly different behavior as a function of laser fluence with regard to the production of PbO: the PbO/Pb ratio from the pressed oxide target was roughly constant while the PbO/Pb ratio from the fully reacted target decreased significantly with increased laser fluence. It is postulated that PbO plays an important role in the incorporation of Pb into the film during film growth.

We have investigated the pulsed laser evaporation of elemental Cd targets, with the aim of understanding the velocity distributions in the plumes and the changes which occur under moderate electrical bias. We report detailed kinetic energy distributions of the species in the laser evaporated plumes. In these experiments, frequency doubled, Q-switched pulses of a Nd:YAG laser were used at a 10 Hz repetition rate to generate the plumes. The velocity distributions of individual atomic species were determined by time-of-flight (TOF) light emission spectroscopy, while the time resolved ion/atom currents were measured with a collector above the target. We have simultaneously measured the dependence of the time resolved optical and electrical signals on the electrical bias applied between target and collector. We find that the typical kinetic energies in the plume are on the order of 10-200 eV, while the ionized species travel two to three times faster than the neutral particles. These results provide fundamental information about the physics of the pulsed laser evaporation process, and subsequent evolution of the plume.

A fast pulsed beam of neutral metal atoms is produced by laser vaporization ofa solid metal or metal compound sample in a modified high-temperature mass spectrometer. Atomic beams of several eV kinetic energy are generated as measured using time-of-flight spectroscopy. The energy range can be controlled with the laser power, similar to studies conducted on thin metal films. The solid samples, however, overcome the problem of short sample lifetime associated with irradiating thin films. Samples have been irradiated for several hours without observing a change in beam intensity or energy, thus offering an interesting source for kinetic studies. Initial results of studies done on copper, nickel, zinc and related oxide, sulfide, bromide, and chloride salts will be discussed as to their applicability to serve as a fast atom source. Spectroscopic data from these compounds will also be presented.

Results of experiments on Al, Li, and Na atomic beams produced by laser ablation using XeCI excimer laser pulses (I∼107 to l08 W/cm2) are presented. Plume emission spectra showed that further than ∼5 mm from the targets most of the excited ablated species are in neutral, atomic form. Transient absorption measurements permitted the detection of ground state atoms with different flight times, at fixed distances from the target. The resulting time of flight (TOF) distributions were converted into velocity and kinetic energy (KE) distributions.

The measured fluxes of fast atoms always increased with ablation laser intensity. The velocity distributions could not be fitted to Maxwell-Boltzmann (MB), or MB plus stream velocity type distributions. For Al plumes, comparison of velocity distributions measured at various distances from the target surface indicates that Al atoms are produced within ∼100 nsec following the leading edge of the ablation laser pulse, implying that there is a one-to-one mapping of the initial velocities to the distances traveled by the atoms after ∼1 μsec. This result should encourage efforts to develop a method of velocity selection of laser ablated metal atoms based on the spatial separation of the various velocity components along the direction of travel.

Laser vaporization processes in Si are analyzed in vacuum and in inert gas environment by space/time resolved optical spectrometry. The results show significant increases in the intensity of luminescence emitted from laser-vaporized plume in gas pressures up to 5 Torr in Ar and 20 Torr in Xe. The intensity then decreases with further increase in the pressure. The observed results are interpreted that collisions of inert gas atoms with a vaporized Si cluster enhances its fragmentation leading to ejection of electronically excited Si atoms and ions. The decomposition process of highly charged clusters are discussed in terms of electrostatic Coulomb effect.

Photoablation of isotopically pure boron metal and hexagonal boron nitride with a 532 nm laser, followed by supersonic expansion cooling, produces a variety of Bx, and BxNy, clusters, which are detected by photoionization of the neutral clusters with a 194 nm laser, followed by time-of-flight (TOF) mass spectrometry. Under certain experimental conditions, the mass spectrum derived from boron nitride shows resolved peaks corresponding to the clusters Bx+1Ny+. Under different experimental conditions, the mass spectrum shows an unresolved region corresponding to clusters ranging from atomic mass unit 100–1000 as well as resolved peaks, corresponding to Bx+ clusters for x = 2 − 100. These resolved peaks grow with increasing photoionization fluence, while the envelope of the unresolved region changes shape and grows with a diminished dependence on fluence. These data are interpreted as evidence that the unresolved clusters are being photofragmented as well as photoionizated by the 194 nm radiation, and the products of such photofragmentation are the aforementioned Bx+ clusters.

Time resolved absorption spectroscopic investigations of the KrF laser ablation of MgO-ceramics and sapphire on atmospheric conditions were carried out with a time resolution of about 5 ns and a two-dimensional space resolution of less than 100 μm. The time delay between the ablation pulse and the broad band probe pulse was varied from 10 ns up to 10 μs.

Absolute particle densities of the species were determined in dependence on the time delay and on the distance to the surface. The spatial and temporal behaviour of the particles in the ablation plume near the surface was found to be strongly affected both by air atmosphere and the reaction dynamic in the plasma zone. In the case of sapphire the time evolution of the densities of Al, Al+ and AlO species are discussed in comparison to measurements on vacuum conditions.

Laser ablation has important applications in surface modification, materials analysis, and thin film deposition. We have been examining the details of processes that lead to the emission and formation of particles (atomic/molecular ground state neutrals, excited neutrals, tions, electrons) when wide band gap materials are irradiated with pulsed UV laser light. Etching and deposition of wide bandgap materials is of particular interest due to their excellent insulating and optical properties. Our studies bear directly on achieving control of emission intensities and particle characteristics for use in film deposition and materials analysis. In model wide bandgap materials such as single crystal alkali halides and MgO (nominally transparent materials), exposure to repeated pulses of 248 nm excimer laser radiation of a few J/cm2 results in substantial interaction including extensive biaxial deformation and cleavage. Significant surface heating also occurs, consistent with strong free-carrier/laser interactions. We present strong evidence that achieving intense emission of atomic, molecular, and ionic particles actually depends on point defect production by laser-induced deformation and fracture. Defect production via dislocation motion yields orders of magnitude increases in laser vaporization of these wide bandgap materials, including cluster ion formation. The dependence of the laser-material interaction on dislocation density and mobility, as well as point defect density, suggests several novel strategies for the enhancing the ablative response or preventing laser damage.