Aurelian Marcu

Pulsed laser deposition of YBCO thin films in a shadow mask configuration

Abstract We present an investigation on a modified pulsed laser deposition method (PLD) that employs plume geometric shadowing (Nd:YAG laser, 1.06 μm, 2.5 J/cm 2 , YBCO target). A plane shadow mask and a newly proposed helical mask were used. The aim of our approach was to improve the film surface morphology. The study investigated the influence of the main geometrical parameters of the two configurations on the plume propagation and film morphology. Using the shadow plane mask method we deposited extremely smooth YBCO films with a roughness of 4–5 nm (RMS), which is an improvement of two orders of magnitude compared with standard PLD. The helical shadow mask allows deposition of films of the same roughness as in the plane mask case, but brings further improvement by eliminating the film thickness non-uniformity, typical for plane mask configuration. In both cases the deposition rate decreased up to 7–12 times compared with standard PLD.

Plume reflection in pulsed laser deposition

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.

Simplified model for RMS variation in pulsed laser deposition

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.

Deposition parameters influences in pulsed laser deposition by plume reflection

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 droplets, 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.

Plasma analysis in the process of pulsed laser deposition of aluminium nitride and titan nitride thin films

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.

Discharge-aided reactive laser ablation for ultrafine powder production

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 O2 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/cm2, considered reference (RDE), (2) lower energy density of 2.5 J/cm2 (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.

Synthesis of nanosize powders by pulsed laser ablation and related plasma diagnostics

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.

Considerations about synchronization between IR and UV-laser beams

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 CO2-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.