Articles you may be interested inNanometric multiscale rough Zn-ZnO superhydrophobic thin films: Self-diffusion of zinc and effect of UV irradiation Evidence for the physical basis and universality of the elimination of particulates using dual-laser ablation. II. Dynamic time-resolved target reflectivity of metals and film growth of ZnThe application of a dual-laser ablation process, incorporating the addition of a synchronized CO 2 laser to the traditional excimer ͑KrF͒ laser used for the ablation of targets in thin film deposition, has been previously demonstrated to be effective in the elimination of particulates in films of Y 2 O 3 ͓J. Vac. Sci. Technol. A 13, 1171 ͑1995͔͒. It has been hypothesized that the efficacy of particulate removal is related to phase transformation from the solid to liquid phase prior to excimer laser ablation of the target material. In this series of two articles we present direct physical evidence of the dynamics of the phase transformation occurring on the target surface and its effect on the morphology of film growth. Pump-probe experiments have been conducted using the CO 2 laser to probe the dynamic reflectivity of the target surface on the nanosecond timescale. These experiments were conducted for a range of materials spanning a wide range of thermal conductivity including a low thermal conductivity insulator (Y 2 O 3 ), and a sublimating oxide ͑ZnO͒, as well as a high thermal conductivity metal ͑Zn͒ to assess the universal applicability of the results. In this article ͑Part I͒ the results of these dynamic reflectivity experiments are correlated with previously reported particulate-free deposition of thin films of Y 2 O 3 . Similar experiments are conducted for ZnO. In both cases, the reflectivity measurements yielded times for the onset of melt at a variety of CO 2 laser fluences. Synchronization of the KrF laser to coincide with the onset of melt resulted in particulate-free film growth. The effect of mistiming on the quality of the deposited film is presented for ZnO.
We have presented, in Part I of this series of two articles, the applicability of dynamic melt studies of the surface of a target under pulsed CO2 laser radiation to determine the onset of melt in these targets. Determination of this time and the accurate synchronization of an excimer (KrF) laser to coincide with the onset of melt on the target surface was shown to lead to particulate-free film deposition for Y2O3 and ZnO films. A key feature of the pump–probe reflectivity studies was the dynamic enhancement of the reflected probe signal, indicating the onset of melt. Some metallic targets, depending on the diameter of the precursor powder globules, such as the zinc target used in this study, do not yield such observable enhancements. In this article (Part II) we present the determination of the time for ablation of a variety of metallic targets under pulsed CO2 laser radiation, by monitoring the dynamic target reflectivity. The melt time is either directly determined or, in the absence of observable melt, estimated based on a simple thermal model for the absorption of the laser radiation by the target. Correlation of the calculated melt times with the morphological quality of particulate-free film growth of Zn is demonstrated. The effect of variable CO2 laser fluence on the deposited films is also demonstrated. The universality of applicability of the target reflectivity studies to a wide range of dissimilar materials, as well as the physical basis for the removal of particulates in dual-laser ablation is established.
The need for precise laser pulse synchronization in a dual-laser ablation system to optimize the quality of the deposited thin films has been previously demonstrated. We present, in this article, a novel technique for the synchronization of an excimer and a CO2 laser with synchronization having a temporal fluctuation (jitter) of less than ±14 ns. This is several times better than the best precision of temporal synchronization possible using traditional electronic techniques and is crucial for the application of dual-laser ablation in the manufacturing of thin films. Evidence for reproducibility in the ablation of targets using this system is presented by analyzing the initial stages of the ablated plasma using a time-gated charge coupled device imaging system.
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