Thin solid films are extensively used as structure materials in the fields of nanotechnology, bioengineering and microelectronics. For structural uses, accurate measurement of mechanical properties of the films, such as elastic modulus and hardness, is essential. Nanoindentation is the most commonly used tool to measure the mechanical properties of thin films. However, such measurement often includes the effect of the substrate on which the film is deposited. Thus, a great research effort has been directed towards developing simple and accurate methods to measure the film’s mechanical properties in the past two decades. Nevertheless, the previous methods are based on either empirical prediction or theoretical simplification, which could lead to inaccuracies. This thesis project aimed at characterising the mechanical properties of thin films from a film/substrate bilayer system using the both experimental and numerical methods.
A nanoindentation deconvolution method that combines finite element method (FEM) analysis with nanoindentation measurement was developed. A FEM model was established to simulate an indenting process and was then validated on standard materials. Of particular significance is that this FEM method utilized the load-displacement curve from nanoindentation testing as a target for optimization, rather than using individual variables e.g. elastic modulus, contact depth, etc. as in the conventional approach. In the optimization algorithm, a multi-loop iteration approach was developed, which allowed the updating of optimized parameters independently, thus accurately and efficiently. The results obtained from this method were found to be in excellent agreement with those from the empirical method. The obvious benefit for using this method is that the determination of the coefficients (which is often materials system specific and needs complicated experimental arrangement) in the empirical method is no longer required.
A nanoscratch method was developed to measure hardness of thin films because nanoscratch produces a less deep stress field than nanoindentation for the same normal load used, thus including a less substrate effect. Our FEM analysis showed that the reduction in substrate effect could be as large as up to 22%. This is of particular importance for measuring hardness of thin films. Nevertheless, the previous studies showed that the nanoscratch hardness was somewhat different from the indentation hardness. In our method, we developed new approaches to characterise the key parameters, such as scratch width and depth, and remove the elastic recovery in the determination of hardness. The nanoscratch hardness was thus found to be in excellent agreement with the nanoindentation hardness. In this method, the deconvolution approach developed for the nanoindentation was also integrated to remove the substrate effect, further improving the accuracy of measurement.
The nanoindentation and nanoscratch methods have been used to measure the elastic modulus and hardness of different bilayer systems, including PECVD silicon nitride films on silicon, gallium arsenide substrate and nickel films on silicon substrate. The comparison of the results demonstrated that both methods are in excellent agreement in the measurement of hardness; and the FEM deconvolution approach has effectively removed the substrate effect and produced accurate measurements of the mechanical properties of thin films.
In addition to the deconvolution methods, our experimental effort was also directed towards improving the measuring accuracy in nanoindentation for thin film’s properties. Our experimental results showed that the conventional tip area function could lead to a significant error when the contact depth was below 40 nm, due to the existence of singularity in the conventional area function. In this work, a new area function to calculate the contact area for the indentations where the contact depths varied from 10 nm to 40 nm was developed. The new area function has produced much better results than the conventional function for the nanoindentation tests with small penetration depths. The application on thin film materials has confirmed that the new area function has great potential to be used for mechanical property characterisation on thin films.