Polysilicon MEMS Structures Research Articles

Strength Distributions in Polycrystalline Silicon MEMS

Safe and reliable design of MEMS components requires a statistical description of the material properties that are associated with failure. To this end, a series of microscale tensile tests was performed on polysilicon MEMS structures fabricated using Sandia National Laboratories' SUMMiT Vtrade process. Tensile bars were fabricated from each of the four freestanding polysilicon layers, with gage lengths ranging from 30 to 3750 mum. A two-parameter Weibull distribution appeared to adequately characterize the observed tensile strength distributions. The strength distribution was found to be dependent on the length of the tensile structures, as expected by the Weibull size effect, and unexpectedly strongly dependent on the layer from which the tensile bar was constructed. Specifically, the topmost polysilicon layer in the deposition process (poly4) was more than twice the strength of the bottom freestanding polysilicon layer (poly1). The mechanistic source of this layer-dependent strength appears to originate, at least in part, from process-dependent surface roughness, although other factors such as layer-dependent variations in microstructure, residual stress, and doping are also considered. A fracture mechanics analysis of the strength distributions suggests that the size of the critical flaws is in the vicinity from 50 to 150 nm. Fractography revealed crack origins along the sidewalls, corners, and top surfaces. Weibull strength distributions were also established at elevated temperatures: 200, 400, 600, and 800 degC in air and nitrogen environments. These results revealed the onset of ductility and reduction in strength at elevated temperatures: at 600 degC strength was less than 40% of the room temperature value. Most of the strength was regained if the material was tested at room temperature after a high-temperature exposure. In the discussion, we briefly review concepts for incorporating these observed strength distributions into probabilistic safe design of MEMS components

On the evolution of surface morphology of polysilicon MEMS structures during fatigue

This paper presents the results of a combined experimental and computational study of surface topology evolution preceding fatigue crack nucleation in polysilicon MEMS structures. The evolution in surface topology observed during the crack nucleation stage is related to the underlying notch-tip stress distributions calculated using the finite element method. Measured changes in surface topography due to the stress-assisted dissolution of silica are shown to be predicted by a linear stability analysis. The implications of the results are discussed for modeling of fatigue in polysilicon MEMS structures.

Surface topography evolution and fatigue fracture in polysilicon mems structures

This paper presents the results of an experimental study of the micromechanisms of surface topography evolution and fatigue fracture in polysilicon MEMS structures. The initial stages of fatigue are shown to be associated with stress-assisted surface topography evolution and the thickening of SiO/sub 2/ layers that form on the unpassivated polysilicon surfaces and crack/notch faces. The differences in surface topography and oxide thickness are characterized as functions of fatigue cycling before discussing the micromechanisms of fatigue fracture.

Surface topography evolution and fatigue fracture of polysilicon

This paper presents the results of an experimental stydy of the micromechanisms of fatigue crack nucleation and fatigue fracture in polysilicon MEMS Structures. The initial stages of fatigue are shown to be associated with stress-assisted surface topography evolution and the thickening of SiO2 layers that form on the unpassivated polysilicon surfaces and crack/notch faces. The differences in surface topography and oxide thickness are elucidated as functions of fatigue cycling before discussing the micromechanisms of crack growth and final fracture.

Modeling of fatigue in polysilicon MEMS structures

This paper presents an empirical approach to the modeling of fatigue in silicon MEMS structures. The approach is based on the regression analysis of stress-life data obtained from prior fatigue experiments on polysilicon and single crystal silicon. The possible directions for future modeling efforts are identified at the end of the paper, following a discussion on the implications of the current model fatigue life prediction.

Mechanisms of Fatigue in Polysilicon Mems Structures

AbstractFatigue crack initiation is shown to be associated with the stress-assisted evolution of a surface silica layer that forms during the normal exposure of unpassivated polysilicon surfaces to lab air In-situ atomic force microscopy (AFM) techniques are used to reveal the evolution of overall surface topology during incremental cyclic deformation to failure. Linear perturbation analysis of stress-assisted dissolution is then utilized to predict the evolution of the surface morphology. The predictions from the perturbation analysis are shown to be consistent with measured surface morphologies obtained using AFM techniques.

On the Evolution of Surface Morphology of Polysilicon Mems Structures During Fatigue

ABSTRACTThis paper presents the results of an atomic force microscopy (AFM) study of the evolution of surface topology in notched polysilicon MEMS structures deformed under cyclic loading at room temperature. The in-situ and ex-situ AFM studies reveal changes in surface topology after cyclic actuation at a relative humidity of ∼70%. These lead ultimately to large wavelength modulations close to the bottom of the notch, in the areas where the tensile stresses are maximum. This is in contrast with the wavelength of the surface modulations away from the notch, which remain relatively unchanged. The results are discussed in terms of possible chemical/surface processes that can occur in the presence of water vapor.

Residual Stresses in MEMS Structures

AbstractThe magnitude and origin of residual stresses in thin polysilicon MEMS structures are discussed in this article. The curvature of polysilicon beams were determined at different temperatures. An accurate method of analysis that avoids the thick substrate approximation is described. The polysilicon beams without an Au layer exhibited temperature-independent curvatures, indicating that their residual stresses were largely process related. Beams with an Au top-coat exhibited a strong temperaturedependent effect, and allowed derivation of a stress-free temperature of 143°C. The technique allowed an independent estimate of the thermal expansion coefficient of polysilicon, namely 2.5×10−6 to 3.1×10−6/°C.