A novel three-dimensional model has been proposed for simulating plume behavior and 'micron-sized particles'
movement and interaction. It describes particles expansion in vacuum or into an environmental gas and particles
interaction with various obstacles. The model simulates 'standard' Pulsed Laser Deposition (PLD) system behavior, but
also some droplet reduction techniques used in PLD (e.g. shadow mask technique) having good results in plasma
expansion for the first several centimeters from target. The carried out simulations have shown that in the PLD with
plasma reflection (PLDPR) the influence of mass and particle's surface area on the droplets trajectory is significant. By
increasing of the particle mass or by decreasing particle's surface, the probability of the particle to be driven into the
deposition area by plume fine particles is considerably decreasing.
A novel three-dimensional model has been proposed for simulating pulsed laser ablation plume behvior and 'micron-sized particles' movement. It describes the plume expansion in vacuum or into an environmental gas. The model combined the advantages of Monte-Carlo and Finite Element methods, being able to give a detailed image of particles behavior, both from microscopic and macroscopic points of view. The particle interaction with obstacles is aimed to provide us a more comprehensive understanding of Pulsed Laser Deposition (PLD) techniques, with the final objective of obtaining droplets-free surfaces.
Pulsed laser deposition (PLD) is widely used to prepare various kinds of thin films. From many experimental results the film surface has been found to be strongly affected by so-called <i>droplets</i>, which are relatively large target material particles in solid or liquid state carried with the plume. In order to satisfy both purposes of high deposition rate and good quality by the PLD, we have investigated the plume reflection process from the viewpoint to avoid the big particles deposited on the substrate. In the present paper we investigate the influences of the system parameters on surface thin film quality and the deposition rate. Some optimization proposals are also included for this deposition technique.
Plume reflection has been studied as a possibility for depositing on a direction perpendicular to target, convenient for manipulation of big size substrates and for avoiding the presence of droplets on film surface. Plume behaviour during reflection on a 45 degree(s) oriented plane surface has been monitored by a high- speed camera. Thin film, deposited by reflected plume, has been analysed by scanning electron microscopy (SEM) and atomic forces microscopy (AFM). Comparing with a standard deposition a 1-2 order of magnitude improvement for film roughness (RMS) has been noticed at the expense of one order of magnitude diminuation of deposition rate. The results have shown significant dependencies of film thickness and surface quality on plume reflection angle. Some aspects of big particles movement in deposition process are also presented.
In Pulsed Laser Deposition (PLD) surface roughness of a deposited film is an important parameter for many thin films applications. Plume free expansion and propagation in a plane shadow mask experimental setup have been investigated together with film roughness for several deposition parameters. In this paper we propose a simplified formula for a RMS variation. A comparison between theoretical and experimental results is presented. The model is considered to be useful for quick roughness (RMS) estimation in PLD deposition.
Reactive pulsed laser ablation deposition of thin films is a technique which has already given good results for the formation of metal and semiconductor oxide and nitride films. To improve the quality of the deposited films it is important to understand the ablation process and the materials transport phenomena from the target to the collecting substrate. Optical emission spectroscopy of the plasma plume, formed by the interaction of the laser pulse with the target is generally used to try to understand the reaction mechanisms during the transport process. An eight speed camera was also used to determine plasma plume expansion velocity and the total duration of luminous emission of the plume. The effect of ambient pressure in the ablation chamber on the plasma composition was observed.
The laser ablation is one of the best ways to obtain smooth thin film deposited on various substrates. However, to obtain a 'droplets-free' surface some special experimental setups are necessary.ONe of them is the 'eclipse' method, using a plane shadow mask. Based on studies on the plume behavior in a 'standard' deposition and in a plane shadow mask eclipse deposition, we prose a new shadow mask having a an helicoidal shape, which permit to obtain a abetter film quality - maximum droplets size about 10 times smaller than for the plane shadow mask. The plume behavior and thin film quality are presented and discussed.
Ultrafine alumina powder was produced by aluminum target ablation with a Nd:YAG laser beam (1064 nm wavelength; 340 mJ/pulse energy; 7 ns pulse duration; 10 pps repetition rate), in a 120 Torr O<SUB>2</SUB> atmosphere. A theoretical approach for the ablation process, based on laser energy absorption and energy balance in the target, is used for comparison with the experimental production rate. Three experimental irradiation conditions were chosen: (1) energy density of 5 J/cm<SUP>2</SUP>, considered reference (RDE), (2) lower energy density of 2.5 J/cm<SUP>2</SUP> (LDE) and (3) RDE with auxiliary discharge between an auxiliary electrode and target, triggered by the ablation plume (RDA). Calculated and experimental data of target weight loss show good agreement (135 ng/pulse, calculated, and 100, 110, 120 ng/pulse, experimentally, for RDE, LDE, RDA conditions, respectively). The X-Ray Diffraction spectra of alumina show formation of (gamma) -alumina particles only, especially in LDE and RDA conditions. From Scanning Electron Microscopy size analysis, a decrease of particle size, in LDE and RDA irradiating conditions, is observed. The decrease of laser beam fluence or aiding of ablation using an auxiliary discharge is both favorable for the production of high quality powder, without diminishing the production rate. This can be a good basis for future improvement of the ultrafine powder production process.
Fast-framing photography was used to study the effect of various parameters on the expansion of the visible plasma plume produced by KrF laser irradiation of a YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7-x</SUB> target. The ambient gas pressure has a considerable effect on the evolution and structure of the plasma. In vacuum, the visible emission is strongly forward-directed and has a relatively short duration. In the presence of an oxygen atmosphere, a long-lived, highly luminous front is formed, whose expansion may be well described by a drag model or a shock model (spherical or plane shock). Ahead of the luminous front there is weak emission, which for pressures over about 700 mTorr evolves into a highly emissive `tip'. The decrease of energy density on the target by increase of the laser spot (defocusing) leads to a more forward-directed motion of the visible species. Larger energy densities obtained by increasing the energy/pulse lead to a sharper, `V' shaped front. The effect of a biased ring electrode placed 0.5 cm to 2 cm in front of the target was also studied. The emission characteristics in the electrode's presence change greatly. The effect of ring-target distance, voltage magnitude and polarity, and pressure were observed.
Nanosize powders of AlN was successfully synthesized by pulsed laser ablation. The target can be either Al or AlN, with ambient gas nitrogen. With Al targets, typical powder diameters were in the range of 2.5 divided by 25 nm. In order to study the process of powder formation, we have used a high-speed camera and a time-resolved spectrum analyzing system to investigate the behavior of the ablation plasma and the ion species in the plasma. For the diagnostic experiments, the target was Al and the ambient gas was oxygen.
Structured modifications are produced by interaction of solid state media with high intensity laser beams. The most useful beams for such purposes are generated by CO<SUB>2</SUB>-TE lasers in IR field, and excimer lasers in UV field. The paper describes laser pumping circuits for both lasers with significant temporal decays that occurs between trigger and laser pulses. Also, temporal behavior of above mentioned lasers: pulse duration, delays and jitters, in various operation conditions, i.e., gases mixture, pressure and pumping energy, is made by temporal determination of pulses and delays involved. Finally, synchronization methods, electrical or optical are recommended in relation with necessary conditions of synchronization between beams.