Assessment of aluminium metallisation by nanoindentation
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Assessment of aluminium metallisation by nanoindentation S.M. Soare*, S.J. Bull*, A. Horsfall**, J. Dos Santos**, A.G. O’Neill** and N.G. Wright** *School of Chemical Engineering and Advanced Materials, University of Newcastle, Newcastle upon Tyne, NE1 7RU, UK. **School of Electrical, Electronic and Computer Engineering, University of Newcastle, Newcastle upon Tyne, NE1 7RU, UK.
ABSTRACT As the trend for miniaturisation in the microelectronics field continues, metallisation connecting components has smaller and smaller dimensions, especially width and thickness. The mechanical properties of the deposited metal are very different from those of the bulk material and it is important to evaluate them accurately if the reliability of the metallisation is to be optimised. The assessment of the mechanical properties of thin aluminium metallisation is possible by nanoindentation but to extract properties useful for lifetime prediction such as yield stress or creep relaxation behaviour additional modelling is necessary using finite elements analysis (FEA). In this study evaporated aluminium layers from 50nm to 600nm thick on (100) silicon were indented to various depths. Proportional loading was used to minimise the effect of creep. The loading curves were then simulated by FEA and the results compared to identify the yield properties of the coating. Modelling data for thicker samples closely follows experimental data but for thinner coatings there is a considerable gradient in properties through the film thickness.
INTRODUCTION The continuing drive towards deep sub-micron silicon technology has been stimulated by the demand for higher operating frequencies, greater circuit complexity and lower power consumption. As gate lengths approach 50nm and below there is a demand for interconnect widths to decrease to similar dimensions. At such scales the mechanical properties of the lines are considerably different from those of larger sizes and this could affect the reliability of the metallisation, particularly under conditions of thermal cycling. The elastic modulus, yield stress and creep behaviour are all likely to be important and these can be determined by nanoindentation. However, the deformation mechanisms are significantly more complex for a thin layer on a harder, stiffer substrate when compared to a thicker coating. In particular, the effect of pile-up on measured mechanical properties becomes more significant as the coating thickness is reduced. In bulk materials the pileup volume is generally proportional to the indent volume (and hence indenter load) but for thin films it has been reported that pile-up is suppressed due to the constraint of the coating material by the substrate during indentation [1]. In addition, in some cases a very thin soft film can be extruded from underneath the contact adding to the pile-up volume. For this reason it is necessary to measure the area of contact when determining the mechanical properties of very thin, soft coatings. In this study a combination of nanoindentation testin
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