Acoustic Waves Characterize Surfaces
Brent D. Johnson
Surface characterization of the mechanical parameters of thin films, which are used in applications from optical to wear coatings, is typically performed by using a diamond tip to make an indentation in the material. Indentations can be performed over a range of loads. Hardness or modulus can be plotted against the ratio of indent diameter to film thickness, and a "coating only" value can be extrapolated. However, these measurements are possible only if one doesn't indent too far.
Fraunhofer USA has developed a nondestructive surface acoustic wave technique that resolves the issue of localized damage to the substrate. The LAwave uses a nitrogen laser beam that is focused onto the surface of the film. The 500-ns laser pulse generates a sound wave that propagates along the material and that is detected by a piezoelectric foil.
The dispersion and other characteristics of wave propagation can be analyzed to infer the density, Poisson ratio and Young's modulus for both the substrate and the film.
This works particularly well for thin films such as photoresists and also for hard, carbon-based coatings, metals, ceramics, thin polymers and low dielectric-constant materials. Because the sound wave travels through a significant volume of the film, the values calculated are average values for that entire volume, rather than those for a single point, as determined by the indenter method.
There are drawbacks to the technique, however. For example, if the material doesn't absorb light at the 330-nm wavelength of the nitrogen laser, a sound wave cannot be generated. By depositing a thin layer of titanium film on transparent materials to absorb the pulse, and setting the software to remove the effect, this difficulty can be overcome.
Giles Aldrich-Smith of the National Physical Laboratory in Teddington, UK, has been working with the LAwave system for a year and a half. He works with reference materials for a modulus standard as well as with thin coatings and hard polymers.
Researchers at the National Physical Laboratory in Teddington, UK, have been using the LAwave system for surface characterization for 18 months.
He said he likes the speed and nondestructive aspect of the LAwave system. Under ideal conditions, he can perform a surface test in five to 10 minutes, whereas previously, it may have Since the commercial release of the device in 2002, Fraunhofer has developed an attachment for the LAwave machine that extends its utility from measuring thin-film characteristics to monitoring the polishing of silicon wafers.
The National Institute of Standards and Technology in Gaithersburg, Md., is working on a similar device with a company from Australia, but it is using an interferometer to collect the data, which takes longer than a piezoelectric foil. However, the interferometric technique has the advantage of being fully noncontact.
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