Analysis of the Mechanical Failure in a Multilayered Thin Film System Tested by Microtensile Loading
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85 Mat. Res. Soc. Symp. Proc. Vol. 391 01995 Materials Research Society
All experiments were performed with elongation rates of 0.02 mm/min. or 0.015 mm/min. RESULTS Al-0.5%Cu on pure Al This sample was used to judge how well the film adhered to the pure Al substrate. Adhesion turned out to be excellent with continuous slip lines emerging to the surface of the film from the substrate. For all the samples tested, the film adhered well up to the rupture strength of the test bar, showing no debonding. Results suggested that elongations of greater than 10% would cause changes to the surface, introducing slip lines as well as intergranular deformation. This should be kept in mind when viewing subsequent results. TiN on AI-0.5% Cu on Al Cracking (perpendicular to the loading direction) of these films was evident from the early stages of deformation (at 0.4% strain). The cracks per unit length saturated at about 1% strain with a mean of 40 per mm. This situation remained stable until 10% strain where the cracks, already opened, interconnected creating a dense array of transversal cracks as shown in Figure 1.
Figure 1-Dense crack array in TiN film at 10% strain. The cracks are perpendicular to the pull direction. on TiN on AI-0.5% Cu on Al These samples showed very similar behavior to the TiN samples. Crack saturation of about 50-60 cracks per mm. was achieved at a 5% strain. Figure 2 shows the crack array which was regularly spaced and extended over the complete width of the sample. However, when the elongation went above 15%, very localized buckling zones, caused by the Poisson contraction of the sample, appeared on the strips of the cracked film. Figure 3 shows the area of buckling that occured at the higher elongations. As the elongation was increased to 20% the number of buckled zones increased and tearing of the Al-0.5%Cu layer beneath the TiN is evident. Cracks can deviate at the TiN/SiO2 interface, deflect along this interface, and then tear the AI-0.5% Cu film. This is shown schematically in Figure 4. This crack deflection process was not studied in depth, but provides the starting point for future analysis. This type of sample was used as a comparison to samples studied previously [2]. The crack saturation numbers were about the same, but &iQ 2
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buckling was not as localized in the previous samples, suggesting that the present samples had superior adhesion.
gure 2- Crack saturation in Si0
2
samples at 2 % strain.
Figure 3- Localized buckling of Si0 2 film due to Poisson contraction.
Figure 4-Schematic of crack deflection along TiN/top layer interface and subsequent AI-0.5% Cu tearing.
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W on TiN on Al-0.5% Cu on Al With elongations up to 10%, regular transversal cracking was seen. A second cracking system became evident at 10% elongation and formed diamond shaped strips of film as shown in Figure 5.
Figure 5- Diamond shaped crack arrays in W film samples above 10% elongation. Buckling was never observed in these films, even at elongations above 20%. Crack deflection at high elongations, similar
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