Variation in Dislocation Pattern Observed in SCS Films Fractured by Tensile Test: Effects of Film Thickness and Testing
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Variation in Dislocation Pattern Observed in SCS Films Fractured by Tensile Test: Effects of Film Thickness and Testing Temperature Shigeki Nakao1, Taeko Ando1, Shigeo Arai2, Noriyuki Saito2, and Kazuo Sato1 1 Department of Micro-Nano Systems Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan 2 High Voltage Electron Microscope Laboratory, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan ABSTRACT This paper reports a transition in the fracture behavior of micron-sized single-crystalsilicon (SCS) film in a MEMS (microelectromechanical systems) structure for various film thicknesses and ambient temperatures. The mean fracture toughness of 4-µm-thick SCS films was 1.28 MPa m at room temperature (RT), and the value increased as the film thickness decreased, reaching 2.91 MPa m for submicron-thick films. The fracture toughness of 4-µmthick film did not change for ambient temperatures ranging from RT to 60ºC. However, it increased at 70ºC and reached 2.60 MPa m at 150ºC. Enhanced dislocation activity in the SCS crystal near the fracture surface was observed on 1-µm-thick film at RT and 4-µm-thick film at 80ºC by high-voltage electron microscopy (HVEM). This change in dislocation activity seemed to correlated with the transition in fracture behavior. INTRODUCTION Mechanical characterization of micron-sized structures plays an important role for microelectromechanical systems (MEMS) devices. Single-crystal silicon (SCS) has high mechanical strength at room temperature (RT) and is widely used in MEMS physical sensors and moving mechanical elements. In recent years, however, demands for MEMS that can operate at elevated temperatures have been increasing. We need a better understanding of the mechanical behaviors of micron-sized SCS films in MEMS structures at elevated temperatures. The brittle-to-ductile transition (BDT) of bulk SCS has been studied for a long time [1-4]. It is well known that SCS shows a sharp transition from brittle to ductile in a narrow temperature range above 600ºC in fracture toughness measurements. However, micro- and nanometer-sized SCS films were recently reported to show some plasticity in their crystals in a low temperature range around 100ºC [5-6]. There should be a size effect on the BDT temperature and dislocation activity in silicon crystals, but this has not yet been clarified. In this paper, we present experimental results for fracture toughness measurements of SCS films in the temperature range from RT to 500ºC for a film thickness of 4µm and in the thickness range from submicron to 4.5 µm at RT. After the toughness measurements, the dislocation patterns existing near the fracture surface were observed by high-voltage electron microscopy (HVEM). HVEM is a suitable method for analyzing the dislocations in SCS films with a thickness of a few microns because of its high acceleration voltage of 1 MV. These experimental results determined the temperature and size effects on the fracture toughness and the dislocation activity of SCS within th
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