Characterization of structural films using microelectromechanical resonators

Abstract

A B S T R A C T Micromachined resonant fatigue characterization structures have been used by a varietyof investigators to evaluate the stress-life fatigue behaviour of thin films. This work will review the design, testing and analysis of these versatile thin-film characterization structures. Subsequent discussion will illustrate how this material characterization approach has been used to evaluate the high-cycle fatigue behaviour of silicon films commonly used in microelectromechanical systems (MEMS). This work demonstrates that properly designed resonators can be used to monitor extraordinarily low fatigue crack growth rates (i.e. 10 −10 m/cycle) relevant to these minute mechanical components. a = crack length A = amplitude of motion b = resistive force f 0 = natural frequency F = applied force h = oxide layer thickness J θ = mass polar moment of inertia k = stiffness k θ = rotational stiffness K = linear elastic stress intensity factor m = mass N = number of cycles Q = quality factor R = load ratio t = time x = position γ = damping parameter σ ult = ultimate strength ω = frequency I N T R O D U C T I O NIt is generally accepted that the fracture toughness and fatigue resistance of bulk materials are linked to the development and deterioration of intrinsic and extrinsic cracktip shielding mechanisms. 1,2 Cyclic fatigue is the most commonly encountered mode of failure in structural materials, occurring in both ductile (metallic) and brittle (ceCorrespondence: Christopher L. Muhlstein.

Three‐dimensional modelling of intergranular fatigue failure of fine grain polycrystalline metallic MEMS devices

“…Significant work has been conducted to characterize the fatigue behaviour of materials used in MEMS devices using microtension and microbending experiments 6–11 . Much of this work pertains to polysilicon due to its predominant use as a structural material for MEMS.…”

“…They highlighted the significant scatter in the results and the overall common trend of increasing life with decreasing applied stress. Several fatigue mechanisms for polysilicon have been proposed including stress‐assisted surface oxide dissolution, reaction layer fatigue 13,14 and mechanically induced subcritical cracking 7 . Alsem et al 15 indicate that reaction‐layer mechanism, i.e.…”

Specimen aligning techniques in tensile and fatigue tests for thin films

“…Nine stress‐life data points were obtained through the use of closed loop control and MAM are shown in Fig. 12 along with the results of other researchers 6–10 . The failure in both electrostatic actuators 6–8 was initiated at a notch and so the results of this work are only compared with those from un‐notched specimens 9,10 .…”

“…Recent investigations have shown a decrease in the fatigue strength of polysilicon during cyclic loading. Muhlstein et al 6,7 . and Kahn et al 8 .…”

Resistance of grain boundary array to cleavage cracking in free-standing thin film

“…Many thin-film materials, such as silicon, silica, silicon carbides, etc., are intrinsically brittle. At working temperatures, very often they are at the lowershelf of brittle-to-ductile transition region (Haque and Saif, 2003;Srikar and Spearing, 2003;Muhlstein, 2005;Boyce et al, 2007). When these materials are subjected to unexpected external loadings or internal stresses, catastrophic cracking can considerably limit their service lives , especially in complex structures where a large number of micro-components interact with each other and failure of any of them may cause malfunction of the entire system.…”