A Membrane Deflection Fracture Experiment to Investigate Fracture Toughness of Freestanding MEMS Materials
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A Membrane Deflection Fracture Experiment to Investigate Fracture Toughness of Freestanding MEMS Materials H.D. Espinosa* and B. Peng ABSTRACT This paper presents a novel Membrane Deflection Fracture Experiment (MDFE) to investigate the fracture toughness of MEMS and other advanced materials in thin film form. It involves the stretching of freestanding thin-film membranes, in a fixed-fixed configuration, containing pre-existing cracks. The fracture behavior of ultrananocrystalline diamond (UNCD), a material developed at Argonne National Laboratory, is investigated to illustrate the methodology. When the fracture initiates from sharp cracks, produced by indentation, the fracture toughness was found to be 4.7 MPa m1/2. When the fracture initiates from blunt notches with radii about 100 nm, machined by focused ion beam (FIB), the mean value of the apparent fracture toughness was found to be 7.2 MPa m1/2. Comparison of these two values, using the model proposed by Drory et al. [9], provides a correction factor of 2/3, which corresponds to a mean value of ρ/2x=1/2. * Corresponding author, [email protected] INTRODUCTION Significant research has been focused on investigating the mechanical properties of the structural materials for surface-micromachined MEMS devices. One such mechanical property which has been extensively studied is the fracture strength. However, for most practical uses of MEMS materials, an additional engineering parameter of importance is the fracture toughness, KIC, which provides an assessment of the resistance a material possesses to crack growth from a pre-existing defect [1, 2]. KIC depends only on microstructural factors and not on the specimen geometry, boundary conditions and loading. Consequently, the best way to compare structures of different geometries is on the basis of their respective stress intensity levels. There have been a few recent reports on investigating the fracture toughness of polysilicon MEMS test specimens. Kahn et al. [3] and Ballarini et al. [4] used polysilicon fracture specimens integrated with on-chip fabricated electrostatic comb-drive actuators. The devices were tested using DC electrostatic actuation, while the displacements were measured by employing an optical microscopy with an accuracy of ~0.3 µm. The measured fracture toughness of polysilicon exhibited a mean value of 1.1 MPa m1/2, which was found to be independent of polysilicon microstructure. Sharpe et al. [5] and Tsuchiya et al. [6] employed external piezoelectric load cells to fracture their notched specimens, and reported a critical stress intensity factor, KIC, of 1.4 and 1.9 to 4.5 MPa m1/2, respectively. These high values of KIC are associated with finite radius (1.0 and 0.23 µm, respectively) of the notches and thus do not represent the true fracture toughness of the material. Chasiotis and Knauss [7, 8] performed tensile tests on perforated micro-specimens to investigate the fracture strength of polysilicon. Freestanding “dog bone-like” tensile specimens were loaded by means of an inch
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