Photoacoustic and photothermal research of nano-scale structures
Elastic spectroscopy of sub-micron layers by laser ultrasonic methods
Laser generated surface acoustic wave (SAW) spectroscopy makes use of pulsed laser-induced transient thermoelastic displacement responses to investigate thermal and elastic properties of a material. The spatial and temporal resolution and shape of the excited dynamic temperature patterns and acoustic waves can be tuned to the application of interest by choosing the optical pattern on the sample. The way of detection of displacements and strains (beam deflection, interferometry, vibrometry, diffraction, thermal lens, transducer…) is chosen accordingly. In particular the heterodyne diffraction scheme is a powerful technique for elastic characterization of thin films and therefore more and more frequently used on samples with microscopic structures. Thanks to the high excitation and detection bandwidth (currently 500 MHz in our lab, extendable to a few GHz), it is possible to generate acoustic wavelengths of the order of 1 micron, allowing to study geometric and elastic features down to the sub-50 nm range. Relative effective thickness mapping precision levels of the order of nanometers and less can be achieved and elastic properties of individual layers of sub-micron thickness can be determined in multilayer structures. The technique allows to verify whether the thermoelastic properties in sub-micron layers are differing from their bulk values.
Also rather rough and thin polymer layers can be characterized by laser ultrasonics. The figure shows the rough topography of a 3 micron (nominal value) polymer on steel sample surface. For this sample, a good correspondence was found between values for the Young’s modulus of the polymer determined by laser heterodyne diffraction experiments (by fitting the surface acoustic wave velocity dispersion between 20 and 300 MHz) and by a nano-indentation method. One can estimate that by slightly extending the bandwidth, polymer coatings with thickness down to 100 nm can be characterized using heterodyne diffraction laser induced SAW spectroscopy.
Typical publications
Elastic properties of nc-TiN/a-Si3N4 and nc-TiN/a-BN nanocomposite films by surface Brillouin scattering
Murli H. Manghnani, Sergey N. Tkachev, Pavel V. Zinin, Christ Glorieux,Pavla Karvankova en Stan Veprek
Journal of Applied Physics 97(5), 054308(1-4) (2005)
The hardness of nanocomposite (nc) films developed recently appears to reach the hardness of diamond. High hardness is commonly attributed to the granular structure of nanocomposites (Hall– Petch effect) [E. O. Hall, Proc. Phys. Soc. Lond. B 64, 747 (1951); N. J. Petch, J. Iron Steel Inst. 174, 25 (1953)]. However, grain size in nanocomposites is generally small (5–15 nm) and falls in the region where the Hall–Petch effect does not apply. The objective of the present study is to report the elastic properties of the superhard nanocomposites determined by means of surface Brillouin scattering (SBS), and to compare the results with those obtained by nanoindentation. Two types of nanocomposite films were studied: nc-TiN/a-Si3N4 and nc-TiN/a-BN. The SBS measurements presented yield values of Young’s modulus significantly larger than those obtained from the slope of unloading indentation curve. This discrepancy is attributed to the lack of the validity of the assumptions behind the Sneddon’s derivation of the formula used for the calculation of the Young’s modulus from the indentation data
Surface acoustic wave (SAW) depth profiling of elastically inhomogeneous materials
C.Glorieux, W.Gao, S.E.Kruger, K.Van de Rostyne, W.Lauriks en J.Thoen
Journal of Applied Physics 88(7), 4394-4400 (2000)
The potential of Rayleigh wave spectroscopy for the in-depth reconstruction of elastic properties of multilayers for materials with a continuous profile of elastic properties is explored. Two models to calculate the surface acoustic wave (SAW) dispersion spectrum from the profile of the elastic parameters are elaborated and compared. It is found that the relevant elastic parameters for Rayleigh wave dispersion in multilayers are the ‘‘effective’’ Rayleigh velocities, i.e., the Rayleigh velocities calculated for virtually semi-infinite layers. For the solution of the inverse problem, a neural network and a singular value decomposition model are proposed and tested on simulated SAW spectra. The reconstruction techniques are applied to reconstruct the elastic depth profile of shot-peened steel samples from laser-generated and laser-detected SAW data.
Investigation of titanium nitride (TiN) coating by broadband laser ultrasonic spectroscopy
W.Gao, C.Glorieux, W.Lauriks en J.Thoen
Chinese Physics 11 (2), 132-138 (2002)
In this article, we present a laser ultrasonic method to investigate a titanium nitride (TiN) coating specimen. The technique is based on the principle of surface acoustic wave (SAW) dispersion during acoustic propagation on a half-space with the presence of a thin layer. Due to the high eÆciency of laser line-source excitation, we have been able to generate and detect a SAW with an excellent signal-to-noise ratio in a wide frequency band. An inverse fitting algorithm was employed to extract simultaneously the thickness and the elastic parameters of the TiN coating from the experimental SAW velocity dispersion curve.
Surface acoustic wave characterization of a thin, rough polymer film
R. Côte1, T. Van der Donck2, J.-P. Celis2 and C. Glorieux1*
Laser generated surface acoustic waves (SAW) in a heterodyne diffraction scheme is a powerful technique for elastic characterization of thin films and it is frequently used on samples of high optical quality. We show that the method can also be effectively used in difficult conditions, on rough samples. Measurements are presented on a 3 µm thick film of polymer, spin-coated on steel, and on the same sample after addition of an aluminum coating. The experimental data are interpreted using a model assuming a stack of perfect layers. The analyses show good consistency within the SAW results for both configurations, and consistency with nano-indentation results, cross-validating both approaches.
This validation of SAW analysis evidences its feasibility for extracting important elastic information also in unfavorable situations, on samples that scatter quite some probe light. The measured non-uniformity of the order of 1 micron being large compared to the total film thickness (3 µm), and comparable with the smallest inverse wave numbers used (around 6 µm), the actual acoustic behavior may deviate from the modeled one for the highest frequencies due to acoustic scattering. Nevertheless, this study shows that this non-destructive and non-contact technique can be used in high throughput parallel design schemes where several test samples need to be characterized in early development stages, when thickness uniformity or other parameters are not yet fully under control. The results confirm that (laser-induced) SAW spectroscopy method has a realistic potential for spectrally resolved visco-elastic characterization of the mechanical relaxation properties of very thin layers (down to 100 nm for smooth layers) in general, and polymer layers and glass-forming materials going through a glass transition while scanning the sample temperature.
