Effet de la longueur d'onde sur l'ablation laser de la céramique BN. Application à l'élaboration de films minces

¹ Laboratoire de spectrométrie de masse et de chimie laser, IPEM, université de Metz, Technopole Metz 2000, 1, bd Arago, F-57078 Metz Cedex 3, France.
² Laboratoire de chimie physique pour l'environnement, CNRS, université de Nancy I, 405, rue de Vandceuvre, F-54600 Villers-lès-Nancy, France.

Résumé

L'ablation laser de cibles de nitrure de bore BN provoque, quelle que soit la longueur d'onde d'irradiation (532, 266, 248 et 193 nm), l'apparition de deux environnements chimiques du bore : l'un correspondant à BN, l'autre à du bore élémentaire. De plus, les caractéristiques morphologiques de la surface ablatée diffèrent en fonction de la longueur d'onde. La qualité des films de BN déposés sur silicium dépend également de la longueur d'onde. Généralement, les films obtenus sont amorphes, déficitaires en azote et présentent les deux environnements chimiques du bore. Mais une ablation laser conduite à 248 nm conduit à des films de bonne qualité topographique, pour lesquels le bore se trouve uniquement dans un seul environnement local correspondant à BN.

Abstract

The laser ablation of boron nitride target induces whatever the wavelength (532, 266, 248 and 193 nm) two local chemical environments of boron: the first one corresponds to BN and the second one is related to elementary boron. Moreover the morphological characteristics of the ablated surface is dependent of the laser wavelength. The quality of the obtained amorphous films deposited on a silicium substrate also depends on the laser wavelength. Generally, the films show defective surface with a nitrogen deficiency. However when the ablation is performed at 248 nm, films can be obtained with good topographic qualities and for which the boron presents a single chemical environment close to the initial target.

In order to optimize the process of synthesis of BN thin films by laser ablation deposition, surfaces of targets and of deposited thin films of BN have been characterized by X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM) and Fourier Transform Infrared Spectroscopy (FTIR). Stoichiometric, highly resistive and chemically unreactive BN surfaces have been prepared by scraping the targets. On the one hand, laser irradiation of these targets results in a defective surface with a nitrogen substoichiometry whatever the wavelength of the laser. XPS analyses show that local environment of the irradiated BN target is not dependent of the laser wavelength. But it has also been noticed that the morphological aspect of the irradiated surface is not similar. On the other hand, the obtained amorphous films show different quality depending on the laser wavelength. Usually highly defective (contaminated with carbon and oxygen and nonstoichiometric) films are obtained, except when the deposition is performed with 5 eV photons (248 nm). Then films of good quality can be prepared, the chemical properties are approaching those of pure material. This strong dependence could be correlated to the energy of the photons which is of the same order of magnitude than the band gap in the case of ablation performed with 248 nm wavelength.