The silicon family of materials are particularly interesting samples when studied via nanoindentation techniques. Pure silicon exhibits a rather unique behaviour when indented with a sharp indenter, this being characterised by a distinct discontinuity or pop-in during unloading. Such a phenomenon is observed in each of the ,  and  orientations and has been widely published [1-2]. The load below which the pop-in disappears is generally in the range 5-20 mN and the hysteresis is thought to be due to a pressure-induced phase transformation from the normal diamond cubic form to a denser b -tin structure.
Work has shed some light on the processes occurring in and around an indented silicon surface. This has been achieved using the Nano Hardness Tester (NHT) from CSM Instruments combined with the Scanning Force Microscope (SFM) objective. The high resolution imaging capability of the latter can provide significant additional information about the indentation process and can be correlated to the conventional load displacement data obtained.
Vickers Indentation Data
Figure 1 shows two sets of Vickers indentation data for p-type silicon under different loading conditions. Fig. 1 (a) shows the radial cracking which occurs at high loads (generally > 200 mN), whereas Fig. 1 (b) shows the drastic effects of increasing the load in 75 mN steps over four load-unload cycles. Such multicycle indentation causes substantial uplift of material around the residual imprint as a result of lateral cracking below the indenter, in addition to the radial cracks propagating away from the corners of the imprint. In contrast to normal metals, the energy absorbed by silicon is significant on each cycle and obviously contributes actively to the observed cracking; for example, in Fig. 1 the maximum load in (b) is 50 mN less than that attained in (a) but the SFM images show greater cracking in the former than in the latter.
Clearly, such cracking phenomena cannot be seen with conventional optical microscopy, so the SFM is an important addition to a nanoindentation instrument if the true response of the material to one or several load-displacement cycles is to be directly quantified. It is also capable of calculating the residual contact area, from which the true hardness value can be verified.
Figure 1. Vickers indentation data for a  silicon wafer (p-type). Example (a) shows the cracking which occurs at opposite edges of the indenter after a monocycle up to 350 mN, whereas (b) shows the drastic effects on the residual imprint after four cycles to 97% unload with a 75 mN load increase after every cycle.
Figure 2. Berkovich indentation data for fused silica showing the cracking effect at high loads (i.e., 300 mN).
Fused silica is a member of the silicon family which exhibits a very large elastic recovery during unloading, as depicted in the load-displacement curve of Fig. 2 (c). For relatively low loads (not shown) the residual imprint has quite an unusual appearance suggesting that the sides of the indentation are elastically recovered during unloading, whilst the corners are not. With a Berkovich indenter, additional plasticity may well be caused by the stress concentration at its edges, meaning that the indenter is able to permanently mark the position of the corners of the indentation at maximum load.
If the applied load is increased, for example to 300 mN as shown in Fig. 2, the load-displacement curve has the same shape and the calculated hardness and modulus remain the same as for a low load indentation.
However, the SFM image and corresponding cross-sectional profile show that the residual imprint is quite different at high applied loads; subsurface median and lateral cracking causes uplift of the sides of the imprint and a large flake of material is also visible at one corner of the indentation. This characteristic material response is highly reproducible and such large flakes of uplifted material are always found over a corner, indicating that lateral crack propagation is obviously higher at zones where stress concentrations are higher.
This study has focussed on indentation into Si and SiO2 (fused silica), both such materials being of great importance to the semiconductor and microfabrication industries. A better knowledge of the inherent deformation modes, the response to localised compression and the ability of the material to dissipate its absorbed energy, whether via cracking or by phase transformation, are all topics which require further research.
Although both Vickers and Berkovich indenter geometries have been utilised, the additional possibility of using accurately-ground spherical tipped indenters will allow a more global characterisation of such materials, especially in terms of their stress distribution characteristics.