The elastic properties of thin graphitic carbon and hexagonal boron nitride films have been determined by Brillouin light scattering from thermally excited surface acoustic phonons. The carbon films have been grown by plasma beam deposition at 420 o C. The BN films have been deposited by rf-magnetron sputtering and simultaneous ion plating. In the case of amorphous carbon a layered substructure has been found with a rather compliant intermediate layer at the interface. The stiffness of the films increases with increasing ion energy. As for carbon the BN coatings exhibit a pronounced elastic anisotropy indicating a preferred orientation of the hexagonal planes perpendicularly to the surface. The comparison of the Young´s moduli determined by load indentation and by Brillouin light scattering allows the conclusion that the material is composed of two or more components of different stiffness.Films grown by magnetron sputtering or plasma beam deposition are in general nanocrystalline or amorphous and do not exhibit single crystal properties. Their elastic properties need to be determined experimentally because they depend strongly on the deposition conditions, and they can in general not be derived from theoretical considerations. Brillouin light scattering (BLS) is a well established technique to obtain information on elastic properties of thin films by measuring the frequency shift of light scattered inelastically from phonons [1,2]. The information obtained by BLS usually allows to determine several independent elements of the elastic tensor and to reveal the existence of an interlayer between film and substrate. Sample PreparationBoron nitride films have been grown on (100)-silicon by 13.56 MHz rf magnetron sputtering from a polycrystalline stoichiometric BN target in a pure argon atmosphere of about 4·10 -4 mbar. The apparatus has been described in detail earlier [3]. On account of the unbalanced magnet configuration a rather high flux of argon ions can be extracted from the plasma by applying a rf bias to the substrate. The energy of the ions was near 130 eV. At an ion current density of 1.2 mA/cm 2 at the substrate the growth rate was found to be 6.2 nm/min. The argon ion flux to the substrate was about a factor of 7 larger than the flux of boron and nitrogen atoms contributing to the film growth. The substrate temperature was held constant at 350 o C. Under these conditions boron nitride films grow in the hexagonal modification [4,5] which has been controlled by infrared spectroscopy. Carbon films with a hydrogen content of 5% have been produced by plasma beam deposition on a (001) silicon substrate heated to 420 o C. The plasma beam source, which has been described in detail previously in [6], consists of a capacitive, magnetically confined r.f. discharge. Acetylene was used as working gas because it forms almost exclusively C 2 H 2 + -ions in low pressure plasmas [6]. The fraction of this ion species was larger than 95% at a background pressure of 6·10 -5 mbar. The films presented here were grown at ion e...