Strength and sharp contact fracture of silicon

The fracture strength of Si is considered in the context of yield and reliability of microelectronic and microelectromechanical (MEMS) devices. An overview of Si fracture, including the strength of Si wafers, dice and MEMS elements, highlights the importance of understanding sharp contact flaws, with their attendant residual stress fields, lateral cracks and strength-limiting half-penny cracks in advanced Si device manufacturing. Techniques using controlled indentation flaws, including measurements of hardness, crack lengths, crack propagation under applied stress, and inert and reactive strengths, are applied in an extensive new experimental study of intrinsic, n- and p-type {100} and {110} Si single crystals and polycrystalline Si, addressing many of the issues discussed in the overview. The new results are directly applicable in interpreting the strengths of ground or diced Si wafer surfaces and provide a foundation for studying the strengths of MEMS elements, for which the strength-controlling flaws are less well-defined. Although the indentation fracture behavior of Si is shown to be quite anisotropic, the extensive lateral cracking greatly affects crack lengths and strengths, obscuring the underlying single crystal fracture anisotropy. No effects of doping on fracture are observed. Strength decreases in water and air suggest that Si is susceptible to reactive attack by moisture, although the effect is mild and extremely rapid. Strength increases of indented components after buffered HF etching are shown to be due to reactive attack of the contact impression, leading to residual stress relief.