Reaction-layer Fatigue Research Articles

Highly localized surface oxide thickening on polycrystalline silicon thin films during cyclic loading in humid environments

Previous studies on very high-cycle fatigue behavior of thin silicon (Si) films suggest a strong environmental dependence of the degradation mechanism, the precise nature of which is still the subject of debate. This is partly due to contradictory evidence on the presence of thick post-cycling surface oxides. In the present study, 2 μm thick polycrystalline Si structures subjected to fully reversed stresses at 40 kHz are used to investigate fatigue degradation in a harsh environment (80 °C, 90% relative humidity). Transmission electron microscopy (TEM) on vertical through-thickness slices reveals highly localized thick oxides (∼50 nm) in the area of large cyclic stress, but not in control specimens. Such localized oxides are likely to be missed with horizontal TEM slices, as done in previous studies. This study highlights the challenges in characterizing nanometer-scale phenomena with micron-scale specimens, and confirms the viability of the reaction-layer fatigue mechanism for the high-cycle/very high-cycle fatigue behavior of micron-scale silicon.

Further considerations on the high-cycle fatigue of micron-scale polycrystalline silicon

Bulk silicon is not susceptible to high-cycle fatigue but micron-scale silicon films are. Using polysilicon resonators to determine stress-lifetime fatigue behavior in several environments, oxide layers are found to show up to four-fold thickening after cycling, which is not seen after monotonic loading or after cycling in vacuo. We believe that the mechanism of thin-film silicon fatigue is “reaction-layer fatigue”, involving cyclic stress-induced thickening of the oxide and moisture-assisted cracking within this layer.

Notch Root Oxide Formation During Fatigue of Polycrystalline Silicon Structural Films

This paper investigates the fatigue behavior of n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -type 2-mum- thick polycrystalline silicon films that exhibit an initially thin (~2-3 nm) native oxide layer. The testing of kilohertz-frequency resonators provided accurate stress-life fatigue data at 30 and 50% relative humidity (RH) in the low (< 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> )and high (up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> ) cycle regimes. Long fatigue life specimens were associated with larger decreases in the natural frequency of the resonator and very smooth failure origins (at the notch) that encompassed several grains. Additional testing at various humidity levels highlighted the critical influence of humidity on the fatigue damage accumulation rate, which was measured via changes in the natural frequency. Finally, Auger electron spectroscopy (AES) characterized the formation of a nanometer-scale oxygen-rich reaction layer during cyclic loading. Although AES revealed a thin 2-3-nm initial oxide layer on a control specimen, measurements on a long-life fatigued specimen revealed an increased oxygen concentration over the first 10 nm of the material at the notch root. These findings demonstrate that the reaction-layer fatigue mechanism for silicon structural films operates even when reaction layers are initially very thin.

Very high-cycle fatigue failure in micron-scale polycrystalline silicon films: Effects of environment and surface oxide thickness

Fatigue failure in micron-scale polycrystalline silicon structural films, a phenomenon that is not observed in bulk silicon, can severely impact the durability and reliability of microelectromechanical system devices. Despite several studies on the very high-cycle fatigue behavior of these films (up to 1012cycles), there is still an on-going debate on the precise mechanisms involved. We show here that for devices fabricated in the multiuser microelectromechanical system process (MUMPs) foundry and Sandia Ultra-planar, Multi-level MEMS Technology (SUMMiT V™) process and tested under equi-tension/compression loading at ∼40kHz in different environments, stress-lifetime data exhibit similar trends in fatigue behavior in ambient room air, shorter lifetimes in higher relative humidity environments, and no fatigue failure at all in high vacuum. The transmission electron microscopy of the surface oxides in the test samples shows a four- to sixfold thickening of the surface oxide at stress concentrations after fatigue failure, but no thickening after overload fracture in air or after fatigue cycling in vacuo. We find that such oxide thickening and premature fatigue failure (in air) occur in devices with initial oxide thicknesses of ∼4nm (SUMMiT V™) as well as in devices with much thicker initial oxides ∼20nm (MUMPs). Such results are interpreted and explained by a reaction-layer fatigue mechanism. Specifically, moisture-assisted subcritical cracking within a cyclic stress-assisted thickened oxide layer occurs until the crack reaches a critical size to cause catastrophic failure of the entire device. The entirety of the evidence presented here strongly indicates that the reaction-layer fatigue mechanism is the governing mechanism for fatigue failure in micron-scale polycrystalline silicon thin films.

The Extended Range of Reaction-layer Fatigue Susceptibility of Polycrystalline Silicon Thin Films

A linear elastic fracture mechanics analysis of a silicon dioxide-polycrystalline silicon (SiO2–Si) bimaterial system was performed to assess the vulnerability of micron-scale silicon structures, such as microelectromechanical systems, to fatigue in ambient air. Previous research has shown that fatigue of silicon films is due to a “reaction-layer fatigue” process where silicon structural films fail due to the sequential, mechanically induced thickening and environmentally assisted cracking of the silicon dioxide reaction layer which forms on the surface upon exposure to air. This work specifically considered the stability of a crack reaching the SiO2–Si interface. This analysis revealed a significant overestimate in the oxide thicknesses susceptible to reaction-layer fatigue reported in our previous studies. Instead, a surface oxide layer as thin as 15 nm may activate this fatigue mechanism for a polycrystalline silicon thin film whose fracture strength exceeds 5 GPa.

High-cycle fatigue of micron-scale polycrystalline silicon films: fracture mechanics analyses of the role of the silica/silicon interface

It is known that micron-scale polycrystalline silicon thin films can fail in room air under high frequency (40kHz) cyclic loading at fully-reversed stress amplitudes as low as half the fracture strength, with fatigue lives in excess of 1011 cycles. This behavior has been attributed to the sequential oxidation of the silicon and environmentally-assisted crack growth solely within the SiO2 surface layer. This ‘reaction-layer fatigue’ mechanism is only significant in thin films where the critical crack size for catastrophic failure can be reached by a crack growing within the oxide layer. In this study, the importance of the bimaterial (e.g., Si/SiO2) interface to reaction-layer fatigue is investigated, and the critical geometry and stress ranges where the mechanism is a viable failure mode are established.

Fatigue Degradation of Nanometer-Scale Silicon Dioxide Reaction Layers on Silicon Structural Films

AbstractAlthough bulk silicon is ostensibly immune to cyclic fatigue and environmentally-assisted cracking, the thin film form of the material exhibits significantly different behavior. Such silicon thin films are used in small-scale structural applications, including microelectromechanical systems (MEMS), and display ‘metal-like’ stress-life (S/N) fatigue behavior in room temperature air environments. Fatigue lives in excess of 1011 cycles have been observed at high frequency (∼40 kHz), fully-reversed stress amplitudes as low as half the fracture strength using surface micromachined, resonant-loaded, fatigue characterization structures. Recent experiments have clarified the origin of the susceptibility of thin film silicon to fatigue failure. Stress-life fatigue, transmission electron microscopy, infrared microscopy, and numerical models have been used to establish that the mechanism of the apparent fatigue failure of thin-film silicon involves sequential oxidation and environmentally-assisted crack growth solely within the nanometerscale silica layer on the surface of the silicon, via a process that we term ‘reaction-layer fatigue’. Only thin films are susceptible to such a failure mechanism because the critical crack size for catastrophic failure of the entire silicon structure can be exceeded by a crack solely within the surface oxide layer. The growth of the oxide layer and the environmentally-assisted initiation of cracks under cyclic loading conditions are discussed in detail. Furthermore, the importance of interfacial fracture mechanics solutions and the synergism of the oxidation and cracking processes are described. Finally, the successful mitigation of reaction-layer fatigue with monolayer coatings is shown.

A reaction-layer mechanism for the delayed failure of micron-scale polycrystalline silicon structural films subjected to high-cycle fatigue loading

A study has been made to discern the mechanisms for the delayed failure of 2-μm thick structural films of n +-type, polycrystalline silicon under high-cycle fatigue loading conditions. Such polycrystalline silicon films are used in small-scale structural applications including microelectromechanical systems (MEMS) and are known to display ‘metal-like’ stress-life ( S/N) fatigue behavior in room temperature air environments. Previously, fatigue lives in excess of 10 11 cycles have been observed at high frequency (~40 kHz), fully-reversed stress amplitudes as low as half the fracture strength using a surface micromachined, resonant-loaded, fatigue characterization structure. In this work the accumulation of fatigue-induced oxidation and cracking of the native SiO 2 of the polycrystalline silicon was established using transmission electron and infrared microscopy and correlated with experimentally observed changes in specimen compliance using numerical models. These results were used to establish that the mechanism of the apparent fatigue failure of thin-film silicon involves sequential oxidation and environmentally-assisted crack growth solely within the native SiO 2 layer. This ‘reaction-layer fatigue’ mechanism is only significant in thin films where the critical crack size for catastrophic failure can be reached by a crack growing within the oxide layer. It is shown that the susceptibility of thin-film silicon to such failures can be suppressed by the use of alkene-based monolayer coatings that prevent the formation of the native oxide.

Mechanism of fatigue in micron-scale films of polycrystalline silicon for microelectromechanical systems

Reported nearly a decade ago, cyclic fatigue failure in silicon thin films has remained a mystery. Silicon does not display the room-temperature plasticity or extrinsic toughening mechanisms necessary to cause fatigue in either ductile (e.g., metals) or brittle (e.g., ceramics and ordered intermetallics) materials. This letter presents experimental evidence for the cyclic fatigue of silicon via a conceptually different mechanism termed reaction-layer fatigue. Based on mechanical testing, electron microscopy, and self-assembled monolayers, we present direct observation of fatigue-crack initiation in polycrystalline silicon, the mechanism of crack initiation, and a method for altering fatigue damage accumulation.

Interfacial Effects on the Premature Failure of Polycrystalline Silicon Structural Films

ABSTRACTAlthough bulk silicon is not known to be susceptible to cyclic fatigue, micron-scale structures made from mono and polycrystalline silicon films are vulnerable to degradation by fatigue in ambient air environments. Such silicon thin films are used in small-scale structural applications, including microelectromechanical systems (MEMS), and display “metal-like” stress-life (S/N) fatigue behavior in room temperature air environments. Previously, the authors have observed fatigue lives in excess of 1011 cycles at high frequency (∼40 kHz), fully-reversed stress amplitudes as low as half the fracture strength using a surface micromachined, resonant-loaded, fatigue characterization structures. Stress-life fatigue, transmission electron microscopy, infrared microscopy, and numerical models were used to establish that the mechanism of the fatigue failure of thin-film silicon involves the sequential oxidation and environmentally-assisted crack growth solely within the native silica layer, a process that we term “reaction-layer fatigue”. Only thin films are susceptible to such a failure mechanism because the critical crack size for catastrophic failure of the entire silicon structure can be exceeded by a crack solely within the native oxide layer. The importance of the interfacial geometry on the mechanics of the reaction-layer fatigue mechanism is described.

Surface Engineering of Polycrystalline Silicon Microelectromechanical Systems for Fatigue Resistance

AbstractRecent research has established that for silicon structural films used in microelectromechanical systems (MEMS), the susceptibility to premature failure under cyclic fatigue loading originates from a degradation process that is confined to the surface oxide. In ambient air environments, a sequential, stress-assisted oxidation and stress-corrosion cracking process can occur within the native oxide on polycrystalline silicon (referred to as reaction-layer fatigue); for the structural films of micron-scale dimensions, such incipient cracking in the oxide can lead to catastrophic failure of the entire silicon component. Since the degradation process is intimately linked to the thin reaction layer on the silicon, modification of this surface and the access of the environment to it can dramatically alter the fatigue resistance of the material. The purpose of this paper is to evaluate the efficacy of modifying the fatigue behavior of polycrystalline silicon with alkene-based monolayers. Specifically, 2-μm thick polysilicon fatigue structures were coated with a monolayer film based on 1-octadecene and cyclically tested to failure in laboratory air. By applying the coating, the formation of the native oxide was prevented. Compared to the fatigue behavior of untreated polysilicon, the lives of the coated samples ranged from 105 to &gt;1010 cycles at stress amplitudes greater than ∼90% of the ultimate strength of the film. The dramatic improvement in fatigue resistance was attributed to the monolayer inhibiting the formation of the native oxide and stress corrosion of the surface. It is concluded that the surprising susceptibility of thin structural silicon films to premature fatigue failure can be inhibited by such monolayer coatings.

High-cycle fatigue and durability of polycrystalline silicon thin films in ambient air

Abstract To evaluate the long-term durability properties of materials for microelectromechanical systems (MEMS), the stress-life ( S / N ) cyclic fatigue behavior of a 2-μm thick polycrystalline silicon film was evaluated in laboratory air using an electrostatically actuated notched cantilever beam resonator. A total of 28 specimens were tested for failure under high frequency (∼40 kHz) cyclic loads with lives ranging from about 10 s to 34 days (3×10 5 to 1.2×10 11 cycles) over fully reversed, sinusoidal stress amplitudes varying from ∼2.0 to 4.0 GPa. The thin-film polycrystalline silicon cantilever beams exhibited a time-delayed failure that was accompanied by a continuous increase in the compliance of the specimen. This apparent cyclic fatigue effect resulted in an endurance strength, at greater than 10 9 cycles, of ∼2 GPa, i.e. roughly one-half of the (single cycle) fracture strength. Based on experimental and numerical results, the fatigue process is attributed to a novel mechanism involving the environmentally-assisted cracking of the surface oxide film (termed reaction-layer fatigue). These results provide the most comprehensive, high-cycle, endurance data for designers of polysilicon micromechanical components available to date.