Deformation and fracture of single-crystal silicon theta-like specimens
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Single-crystal silicon test specimens, fabricated by lithography and deep reactive ion etching (DRIE), were used to measure microscale deformation and fracture properties. The mechanical properties of two specimen geometries, both in the form of a Greek letter H (theta), were measured using an instrumented indentation system. The DRIE process generated two different surface structures leading to two strength distributions that were specimen geometry independent: One distribution, centered about 2.1 GPa, was controlled by 35 nm surface roughness of scallops; the second distribution, centered about 1.4 GPa, was controlled by larger, 150 nm, pitting defects. Finite element analyses (FEA) converted measured loads into strengths; tensile elastic measurements validated the FEA. Fractographic observations verified failure locations. The theta specimen and testing protocols are shown to be extremely effective at testing statistically relevant (hundreds) numbers of samples to establish processing–structure–property relationships at ultrasmall scales and for determining design parameters for components of microelectromechanical systems. I. INTRODUCTION
Many advanced materials are intended for use in small-scale applications, for example, microelectronics,1–3 microelectromechanical systems (MEMS),4,5 photonics,6–8 biotechnology,8 and magnetic storage,8–11 or may be available only in small volumes, for example during materials development. Developing or optimizing such materials and their processing methods thus requires measurements of structure and properties at small scales. A pervasive measurement requirement is that of measuring mechanical properties and relating them to processing and structure: To optimize manufacturing yield and operational performance, especially reliability,12 all materials and devices must maintain mechanical integrity, whether intended for primarily mechanical applications, for example, MEMS, or not, for example, microelectronics. However, establishing processing–structure–mechanical properties linkages at small scales is difficult13: Not only are the involved loads and displacements small, making measurement difficult, but issues of specimen gripping and loading alignment, which are also often problematic at large scales,14 are made more difficult as well. In addition, posttest sample collection and manipulation are difficult, which impedes the ability to identify property-limiting structural defects during failure analysis and thus hinders the capacity to alter processing procedures for property optimization.
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Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2011.319 J. Mater. Res., Vol. 26, No. 20, Oct 28, 2011
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An experimental method that avoids many of these difficulties in measuring mechanical properties of materials at small scales is instrumented indentation testing (IIT).15–20 Commercial IIT instruments are well able to measure loads i
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