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THE ANOMALOUS BEHAVIOR OF SILICON DURING NANOINDENTATION G.M. PHARR Rice University, Department of Materials Science, P.O. Box 1892, Houston, TX 77251

ABSTRACT Two separate phenomena occur during the low-load indentation of silicon which make its behavior distinctly different from that of most materials. First, silicon is one of only a very few materials whose hardness exceeds the pressure needed to transform it to a denser crystalline (or amorphous) form, and because of this, a reversible, pressure-induced phase transformation occurs during indentation. The transformation enhances the electrical conductivity of the material and creates a region around the indenter which flows like a soft metal. Second, silicon cracks when indented by a Berkovich or Vickers indenter at loads of less than 100 mN, i.e., loads typically used in nanoindentation experiments. These two phenomena, which account for a number of unusual features in the indentation load-displacement behavior, are documented and discussed. INTRODUCTION Silicon is perhaps one of the most frequently encountered materials in nanoindentation experiments. It is a common substrate material for thin films, and because it is readily available in highly polished form at low cost, it is a natural choice as a material to use when first setting up or calibrating a nanoindentation system. However, some very unusual features are observed in the indentation load-displacement curves of silicon, and its behavior during nanoindentation is in general very different from that of most materials. The unusual features in the indentation loaddisplacement curves and the physical phenomena which cause them are the subject of this paper. In order to appreciate that the behavior of silicon is indeed unusual, it is useful to begin by describing the indentation load-displacement behavior of a more "normal" material. Fig. 1 presents results for fused silica, a material which behaves like the great majority of the materials we have tested in the Nanoindenter. The data were obtained over two cycles of loading and unloading using a Berkovich diamond indenter. The important features are: (1) the loading and unloading curves are smooth, (2) the loading and unloading curves trace one another perfectly 140 120 100

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Fig. 1. Indentation load-displacement data for fused silica: (a) peak load = 120 inN; (b) peak load = 4.5 mN.

Mat. Res. Soc. Symp. Proc. Vol. 239. @1992 Materials Research Society

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Fig.2. Indentation load-displacement data for (110) silicon: (a) peak load = 120 mN; (b) peak load = 4.5 inN. after the initial loading, thus implying that the deformation is entirely reversible and elastic, and (3) the shapes of the curves do not change as the peak load is reduced. The behavior of silicon is in sharp contrast to this. As shown in Fig.2a, at high peak loads the init