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Measuring the elastic properties of a thin film under compression
SOLEIL Company Contents > All the news > News 2008 > Measuring the elastic properties
G. Geandier1,2, P.-O. Renault2 , P. Goudeau2 , M. Drouet2 , D. Thiaudière1 , E. Le Bourhis2
1, Synchrotron SOLEIL - L'Orme des Merisiers Saint-Aubin - BP 48 91192 GIF-sur-YVETTE CEDEX
2, Laboratoire de Physique des Matériaux, Université de Poitiers - UFR Sciences, SP2MI, Boulevard Marie et Pierre Curie, BP 30179 - 86 962 FUTUROSCOPE CHASSENEUIL CEDEX
 
We study the elastic behaviour of thin nano-structured films. The characterisation of the mechanical behaviour of these polycrystalline structures with respect to their microstructure is an essential step towards the development of technological applications, but has not yet been explored in depth. The suggested approach is to make thin films with a controlled microstructure (e.g., multilayer), to characterise the mechanical response of these films experimentally (particularly by developing experimental methods based on diffraction), and to interpret the experimental results using relevant mechanical modelling.
 
Description of the samples
Thin films are deposited on a polyimide substrate (Kapton®, 127.5 μm thick), with dimensions of 14×6 mm2 in the test specimen plane. We began by using the PVD (Physical Vapour Deposition) process to create thin (nano-scale) gold films. This material, with its strongly anisotropic elastic properties, will allow us to develop the procedure. In order to increase the applicable deformation range for the specimen, so that more measurements can be taken whilst remaining in the elastic region of gold, the thin films are deposited on a substrate under a tensile load of 50 N. The entire assembly (tensile machine + substrate) is placed in a vacuum chamber. The changes in force during deposition show that the various stages of deposition (applying vacuum, stripping the target, spraying, venting) have a slight effect on the force exerted on the substrate. A drop of 3 N in the measured force between the beginning and the end of deposition (20 minutes apart) is observed. The thickness deposited is measured by reflectometry on the control deposits made at the same time on a silicon substrate placed in the jaws of the tensile machine on either side of the test specimen. 
Experimental procedure


Fig 1 : Test geometry for the [2 -2 0] reflection 

 

The tensile device holding the pre-loaded specimen is placed in the goniometer of DIFFABS. The force measured at the beginning of the experiment was 40 N (the reduction of the force initially applied to the substrate is related to the relaxation of Kapton® and to the transport of the tensile machine). Various diffraction peaks were analysed in a reflection configuration by θ/2θ sweeps. A geometry based in the vertical plane was used for all reflections. For reflection [2-20], a surface diffraction geometry was used (angles of incidence and emergence less than 1°). Figure 1 shows a photo of the DIFFABS goniometer in this geometry. This reflection was studied because it is representative of the planes with maximum deformation (perpendicular to the tensile force), and it was possible to examine it only because of the DIFFABS six-circle goniometer, which allows special goniometric movements.

In order to measure changes in deformation in the thin film, the force applied to the substrate is reduced in steps of 3 newtons. At each step, a diffraction diagram is recorded for each reflection (a total of 10), and the deformations are calculated with respect to a reference point with the highest force value (40 N in this case). Figure 2 shows the changes in force throughout the test. The various measurement plateaus can be seen, with smaller changes within the plateaus according to the position of the tensile machine with respect to the goniometer.


Fig 2. : History of the force applied to the substrate
+ specimen assembly during the diffraction experiments.

 
 
Results and conclusions

Fig 3. : Strain for different planes (hkl) according
to applied force for an 18 nm thin gold film.

 

 

Figure 3 shows the deformations measured in the various crystallographic directions according to the force applied. The gold film has a <111> fibre texture. Elastic deformation varying from a maximum expansion perpendicular to the film of 0.05% (planes (1 1 1)) to a maximum compression of 0.45% for planes perpendicular to the axis of compression is measured in the gold particles. There are three important points to note: (i) The complete deformation tensor in 18-nm thick polycrystalline films is measured very accurately, (ii) At this small scale, the system is sensitive to elastic anisotropy (planes (1 1 1) and (2 -2 0)), and (iii) high values of elastic deformation are reached without causing delamination of the gold film on its substrate. This last result highlights the quality of this test compared to those found in the literature for the study of the mechanical properties of nano-scale objects under compression.

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