Characterization of hydroxyapatite laser ablation plumes by fast intensified CCD-imaging
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ArF excimer laser pulses (193 nm, 20 ns, 150 mJ) have been focused on a hydroxyapatite (HA) target in similar conditions to those normally used for thin film deposition. Fast intensified CCD images of HA laser ablation plumes have been taken in vacuum and under different water vapor pressures ranging from 0.01 mbar to 1 mbar. Images of HA ablation in vacuum have shown a plume freely expanding at a constant velocity of 2.3 X 106 cm/s. HA ablation under a water vapor pressure of 0.01 mbar has revealed an expansion behavior very similar to that of ablation in vacuum. Images taken under a water vapor pressure of 0.1 mbar have shown the formation of a shock structure in the plume. Finally, HA ablation under a water vapor pressure of 1 mbar has revealed the development of some irregularities in the shape of the plume.
I. INTRODUCTION Hydroxyapatite thin films are specially adequate for bioactive coatings, mainly for Ti prostheses. Due to HA complex stoichiometry, pulsed laser deposition (PLD) has been revealed as a good technique for such a purpose. In order to obtain good crystalline films, the deposition process has been carried out under different reactive atmospheres. Diverse gas mixtures have been tried, such as O 2 , O 2 /H 2 O, or Ar/H 2 O, x ~ 3 or merely using H 2 O alone.4 The results as obtained indicate that H 2 O is necessary in order to obtain thin films with good properties. Therefore, it seems interesting to analyze the HA ablation process under different water vapor atmosphere pressures, not only from a fundamental point of view but also to optimize the deposition conditions. Similar studies have been carried out with YBa 2 Cu 3 0 7 -^ laser ablation under oxygen atmosphere at diverse pressures5"7 since oxygen pressure has been revealed as one of the most determinant technological parameters in high temperature superconductor thin film growth. These works show how the material ejected from the target by the laser pulse forms a plasma, also called "plume", whose expansion dynamics is analyzed accurately. Among the different characterization techniques available, the one that shows more clearly this plume expansion is fast intensified CCD imaging.6-7 Due to its high spatial and temporal resolution, it allows one to visualize in detail the shape of the plume and its further evolution. In this paper we present an analysis by fast intensified CCD imaging of the dynamics of the HA laser ablation process under vacuum and under diverse water J. Mater. Res., Vol. 10, No. 2, Feb 1995 http://journals.cambridge.org
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vapor pressures in the range used in HA thin film deposition. II. EXPERIMENTAL A pellet made from HA powder pressed at 6 X 108 Pa pressure at room temperature was irradiated with an incident angle of 45° by means of an ArF excimer laser beam (Lambda Physik LPX 205i, 193 nm, 20 ns), limited by a 25 mm wide per 10 mm high mask, at an energy of 150 mJ per pulse, measured downstream from the mask. The pellet was placed at the focus of a 445 mm focal length spherical lens, in a high
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