This article highlights a typical example of results which can be obtained with the integrated scanning force microscope (SFM) mounted on the Nano Hardness Tester (NHT) from CSM Instruments. The work expands upon previous use of the NHT for quality control of IC bonding pads which are typically composed of a thin aluminium film sputtered onto a Si substrate.
Apart from simply characterising the hardness and modulus of the material, the use of the SFM to measure the surface profile of the residual imprint can provide useful additional information concerning the response of the material to indentation (e.g., pile-up, sink-in effects), surface roughness around the imprint, and an idea of the surface structure. This can all be achieved at the touch of a button with the easy-to-operate SFM acquisition software.
A typical set of results is shown in Fig. 1 for a low load (3 mN) indentation into a bonding pad Al film. The SFM image clearly shows the residual imprint and the extent of piled-up material around the impression.
The grain structure of the film can also be seen as well as radial relaxation at the edges of the Vickers imprint. The cross-sectional profile (Fig. 1 (b)) confirms the residual depth as measured with the NHT (258 nm) and allows any pile-up to be precisely quantified. The load-depth curve (Fig. 1 (c)) enabled the Vickers hardness and elastic modulus to be calculated, respectively 1.157 GPa and 112 GPa.
Previous work has already shown the advantages of SFM profiling for the characterisation of bonding pads and future work is envisaged to investigate the effects of pile-up on the true contact area used in the hardness calculation. This is because the projected contact area is calculated from the load-depth curve and thus does not take into account the increased area as a result of pile-up.
Figure 1. Typical data for a 3 mN indentation into an aluminium thin film (thickness = 2 m m); (a) SFM image of residual imprint; (b) cross-sectional profile through imprint; (c) load-depth curve giving H = 1.157 GPa and E = 112 GPa.