Mode-I Fracture Toughness of Tetrahedral Amorphous Diamond-like Carbon (ta-C) MEMS
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Mode-I Fracture Toughness of Tetrahedral Amorphous Diamond-like Carbon (ta-C) MEMS K. Jonnalagadda1, S.W. Cho1, I. Chasiotis1, T.A. Friedmann2, J.P. Sullivan2 1 Mechanical and Aerospace Engineering, University of Virginia 2 Sandia National Laboratories, Albuquerque, New Mexico ABSTRACT Mode-I fracture toughness studies were conducted on hydrogen-free tetrahedral amorphous diamond-like carbon (ta-C) MEMS specimens of various thicknesses. Mathematically sharp edge pre-cracks were generated through micro indentation on the Silicon dioxide sacrificial layer. An atomic force microscope (AFM) was employed to measure the precise length and orientation of each pre-crack. Upon wet etching and release the freestanding uniform width and varying thickness MEMS-scale specimens were tested in Mode-I using a custom-made micro-tensile tester. Fracture toughness values were computed from the test data using linear elastic fracture mechanics (LEFM) for a finite width specimen with an edge crack in the fixed grip loading configuration. The average Mode-I fracture toughness for 0.5 micron thick specimens was found to be 4.25±0.7 MPa m while the average mode-I fracture toughness for 1 micron specimens was 4.4±0.4 MPa m . INTRODUCTION Hydrogen-free tetrahedral amorphous Diamond-like Carbon (ta-C) is an excellent material for MEMS devices [1]. Compared to polysilicon ta-C has superior mechanical properties, such as hardness, wear resistance, stiffness (Young’s modulus) and Poison’s ratio [2]. The mechanical reliability and durability of ta-C is of critical importance for applications in electronic and biological devices [3, 4]. However, mechanical reliability can be ensured with the thorough study of its failure strength and fracture toughness. These data are essential for proper design and operation of MEMS devices. Earlier work on fracture toughness measurements of bulk CVD diamond and related materials involved indentation generated cracks [5-6], and pre-cracks from notches [7]. However, these methods cannot be directly applied to thin films such as MEMS. Several techniques have been used to investigate failure properties at the micro-and-nano-scale and a comprehensive review on these techniques can be found in [8]. In particular, a variety of approaches have been employed to measure fracture toughness of MEMS scale specimens [9-13]. Ballarini et al. [9] employed on-chip microdevices to measure the fracture toughness of polysilicon. Their method required the fabrication of on-chip specimens for electrostatic actuation. Individual specimens needed calibration via finite elements in order to measure the applied load and compute the fracture toughness. Moreover, electrostatic actuation limits the applicability of the technique to small material volumes. In [10] the authors presented a different technique based on notched specimens to measure fracture toughness of ultra nanocyrstalline diamond (UNCD). Notches with radii as small as 50 nm were generated using a Focused Ion Beam (FIB). Using the theory presented in [5] the Mode-I fra
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