Dr. Marco Giuseppe Beghi Profile

Dr. Marco Giuseppe Beghi

at Politecnico di Milano

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Proceedings Article | 10 December 2001 Paper

Measurement of the elastic constants of nanometric films

Marco Beghi, Carlo Bottani, Andrea Bassi, Brian Tanner, Andrea Ferrari, K. Teo, J. Robertson

Proceedings Volume 4449, (2001) https://doi.org/10.1117/12.450085

KEYWORDS: Lithium, Carbon, Lutetium, X-rays, Reflectivity, Lawrencium, Optical metrology, Optical semiconductors, Semiconductors, Data storage

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Carbon coatings of thickness down to 2 nanometers are needed to increase the storage density in magnetic hard disks and reach the 100 Gbit/in² target. Methods to measure the properties of these ultrathin hard films still have to be developed. We show that combining Surface Brillouin Scattering (SBS) and x-ray reflectivity measurements the elastic constants of such films are accessible. Tetrahedral amorphous carbon films of thickness down to about 2 nm were deposited on Si by an S bend filtered cathodic vacuum arc, achieving a continuous coverage on large areas free of macroparticles. Film thickness and mass density are measured by x-ray reflectivity: densities about 3 g/cm³ are found, indicating a significant sp³ content. The dispersion relations of surface acoustic waves are measured by SBS. We show that for thicknesses above approximately 4 nm these waves can be described by a continuum elastic model based on a single homogeneous equivalent film. The elastic constants can then be obtained by fitting the dispersion relations, computed for given film properties, to the measured dispersion relations. For thicknesses of 3 nm or less qualitative differences among films are well measurable, but quantitative results are less reliable. We have thus shown that we can grow and characterise nanometer size tetrahedral amorphous carbon films, which maintain their high density and peculiar mechanical properties down to around 4-nm thickness, satisfying the requirements set for the hard disk coating material.

Proceedings Article | 29 September 2000 Paper

Measurement of elastic properties of thin films by surface Brillouin scattering

Marco Beghi, Carlo Bottani, Rosanna Pastorelli

Proceedings Volume 4098, (2000) https://doi.org/10.1117/12.401637

KEYWORDS: Velocity measurements, Scattering, Acoustics, Light scattering, Silicon films, Thin films, Laser scattering, Inverse problems, Silicon, Error analysis

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Elastic properties of thin supported films can be derived from the dispersion relations of surface acoustic waves (SAWs), which depend on the properties of the films themselves. Among the techniques for the measurement of SAW velocities surface Brillouin scattering (SBS) of visible light probes SAWs at the shorter wavelengths (around 0.5 micrometers ), allowing resolution down to nanometric films. Since SAW velocities can be computed as function of elastic constants and mass density of both the film and the substrate, of film thickness and of wavevector, the elastic properties can be obtained by fitting the computed velocities of the measured ones. Namely, if film thickness and density are independently measured, e.g. by X-ray reflectivity and X-ray diffraction, the elastic constants of the film can be derived by a Generalized Least Squares estimator, with corresponding confidence intervals. Accurate derivation of elastic constants requires highly accurate SAW velocity measurements. Some examples are considered in detail: diamond-like carbon films on silicon substrate and titanium silicide films, showing that elastic constants of thin films can be determined by SBS measurements with precisions ranging from reasonable to very good.

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