On the Fracture Toughness of Polysilicon MEMS Structures
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On the Fracture Toughness of Polysilicon MEMS Structures H. Kahn1, R. Ballarini2, and A.H. Heuer1 1 Department of Materials Science and Engineering, 2Department of Civil Engineering Case Western Reserve University Cleveland, OH 44106, U.S.A. ABSTRACT The mechanical properties of micromachined polysilicon are of great interest to designers of microelectromechanical systems (MEMS) devices. Numerous investigations have been carried out to determine the strength of MEMS-fabricated polysilicon structures, and the experimental results vary widely, depending on the experimental techniques, specimen geometries, and processing conditions. In order to determine whether these variations are inherent to all mechanical properties of MEMS materials, the fracture toughness, Kcrit, of micromachined polysilicon has been investigated, using a wide range of material microstructures (microstructure is used here in the Materials Science sense to mean the grain structure visible in a microscope, and not in the MEMS sense to mean small structures). Since fracture toughness is a fundamental materials property, whether or not it varies with microstructure and processing is an interesting question. We have confirmed that Kcrit is not a microstructure-sensitive property, using surface-micromachined specimens with sharp pre-cracks which are integrated with electrostatic actuators. The measured Kcrit is 1.0±0.1 MPa¥PIRUDZLGHUDQJHRI microstructures. INTRODUCTION Polysilicon is the most commonly used structural material for surface-micromachined MEMS devices. As such, its mechanical properties have been the subject of numerous investigations in an effort to establish a sufficient database to be able to optimize device performance and accurately predict long-term reliabilities. One such mechanical property which has been extensively studied is the fracture strength. The reported strengths of micromachined chemical-vapor deposited (CVD) polysilicon range from 1 to 5 GPa. This has caused concern among some MEMS designers, who worry that if the strength of polysilicon cannot be anticipated, then other mechanical properties might be equally unpredictable, and reliability predictions could never be made. However, many of the observed variations in fracture strength (which is not a fundamental materials property and will vary with processing conditions) can be explained on the basis of processing-induced defects. Previously reported investigations on the strength of micromachined polysilicon will be discussed in detail in the remainder of this section. Following this discussion, we will present our recent work on the fracture toughness of polysilicon from a sharp crack (which is a fundamental materials property), and demonstrate that this property is constant over a wide range of microstructures. One approach to the issue of polysilicon strength focused on the effect of testing geometry on strength, as opposed to material variability, and was presented at the Spring 1998 MRS meeting [1]. This round-robin study reported the results of three differe
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