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Ketol-acid reductoisomerase (KARI) orchestrates the biosynthesis of branched-chain amino acids, an elementary reaction in prototrophic organisms as well as a valuable process in biotechnology. Bacterial KARIs belonging to class I organise as dimers or dodecamers and were intensively studied to understand their remarkable specificity towards NADH or NADPH, but also to develop antibiotics. Here, we present the first structural study on a KARI natively isolated from a methanogenic archaea. The dodecameric structure of 0.44-MDa was obtained in two different conformations, an open and close state refined to a resolution of 2.2-Å and 2.1-Å, respectively. These structures illustrate the conformational movement required for substrate and coenzyme binding. While the close state presents the complete NADP bound in front of a partially occupied Mg2+-site, the Mg2+-free open state contains a tartrate at the nicotinamide location and a bound NADP with the adenine-nicotinamide protruding out of the active site. Structural comparisons show a very high conservation of the active site environment and detailed analyses point towards few specific residues required for the dodecamerisation. These residues are not conserved in other dodecameric KARIs that stabilise their trimeric interface differently, suggesting that dodecamerisation, the cellular role of which is still unknown, might have occurred several times in the evolution of KARIs.

Tissue macrophages are immune cells whose phenotypes and functions are dictated by origin and niches. However, tissues are complex environments, and macrophage heterogeneity within the same organ has been overlooked so far. Here, we used high-dimensional approaches to characterize macrophage populations in the murine liver. We identified two distinct populations among embryonically derived Kupffer cells (KCs) sharing a core signature while differentially expressing numerous genes and proteins: a major CD206loESAM– population (KC1) and a minor CD206hiESAM+ population (KC2). KC2 expressed genes involved in metabolic processes, including fatty acid metabolism both in steady-state and in diet-induced obesity and hepatic steatosis. Functional characterization by depletion of KC2 or targeted silencing of the fatty acid transporter Cd36 highlighted a crucial contribution of KC2 in the liver oxidative stress associated with obesity. In summary, our study reveals that KCs are more heterogeneous than anticipated, notably describing a subpopulation wired with metabolic functions.

Nanolaminates using alternating inorganic and organic layers have the potential to provide ultrabarrier with high resistance to gas permeation while also changing the crack onset strain (COS) to im...

How to feed an enzyme ethane When released from ocean floor seeps, small hydrocarbons are rapidly consumed by micro-organisms. Methane is highly abundant and is both produced and consumed by microbes through well understood biochemical pathways. Less well understood is how ethane, also a major natural component of gaseous hydrocarbons, is metabolized. To understand how microbes take advantage of this energy and carbon source, Hahn et al. solved the x-ray crystal structures of an enzyme they call ethyl coenzyme-M reductase, which converts ethane into the thioether ethyl-coenzyme M as the entry point for catabolism. They found an expanded active site and, using a xenon gas derivatization experiment, a distinctive tunnel through the protein that is proposed to permit access of the gaseous substrate. Science , abg1765, this issue p. 118

Flexible hybrid electronics rely upon compliant interconnects in order to maintain performance integrity in cases that require repeated elongation, including repeated stretching. A class of such conductive interconnects are composites of polymer with conductive particles that can be stretched at high strains without circuit failure. However, their fatigue response has so far remained largely unexplored and is essential prior to using in health monitoring applications. In this research, a stretchable silver-filled conductor is evaluated under high-strain cycling. In-situ techniques, including 4-point resistance measurement and laser profilometry, are used to correlate changes in electrical performance to the fatigue response. Surface crack formation is extensive upon stretching during the first loading cycle, forming a heavily interconnected crack network at higher strains that does not immediately result in open circuit failure. Resistance increase with cycling is attributed to a gradual deepening of these cracks until their depths approach the film thickness, eventually leading to electrical failure. Fatigue life, the number of cycles required to reach a predetermined electrical performance limit, is shown to be most influenced by the applied strain amplitude. Using a normalized resistance increase limit of R/R0=500, it is found that 500 μm wide conductive lines endure 23 cycles at 35% strain amplitude, but this becomes over 500 cycles when the amplitude is dropped to 5%. Sensitivity to mean strain, em, is relevant to strain amplitudes below 15%. In this manner, a composite conductor was shown to exhibit crack evolution behavior distinctly different from homogeneous metallic films.

The genus Shewanella is well known for its genetic diversity, its outstanding respiratory capacity, and its high potential for bioremediation. Here, a novel strain isolated from sediments of the Indian Ocean was characterized. A 16S rRNA analysis indicated that it belongs to the species Shewanella decolorationis. It was named Shewanella decolorationis LDS1. This strain presented an unusual ability to grow efficiently at temperatures from 24 degrees C to 40 degrees C without apparent modifications of its metabolism, as shown by testing respiratory activities or carbon assimilation, and in a wide range of salt concentrations. Moreover, S. decolorationis LDS1 tolerates high chromate concentrations. Indeed, it was able to grow in the presence of 4 mM chromate at 28 degrees C and 3 mM chromate at 40 degrees C. Interestingly, whatever the temperature, when the culture reached the stationary phase, the strain reduced the chromate present in the growth medium. In addition, S. decolorationis LDS1 degrades different toxic dyes, including anthraquinone, triarylmethane, and azo dyes. Thus, compared to Shewanella oneidensis, this strain presented better capacity to cope with various abiotic stresses, particularly at high temperatures. The analysis of genome sequence preliminary data indicated that, in contrast to S. oneidensis and S. decolorationis S12, S. decolorationis LDS1 possesses the phosphorothioate modification machinery that has been described as participating in survival against various abiotic stresses by protecting DNA. We demonstrate that its heterologous production in S. oneidensis allows it to resist higher concentrations of chromate. IMPORTANCE Shewanella species have long been described as interesting microorganisms in regard to their ability to reduce many organic and inorganic compounds, including metals. However, members of the Shewanella genus are often depicted as cold-water microorganisms, although their optimal growth temperature usually ranges from 25 to 28 degrees C under laboratory growth conditions. Shewanella decolorationis LDS1 is highly attractive, since its metabolism allows it to develop efficiently at temperatures from 24 to 40 degrees C, conserving its ability to respire alternative substrates and to reduce toxic compounds such as chromate or toxic dyes. Our results clearly indicate that this novel strain has the potential to be a powerful tool for bioremediation and unveil one of the mechanisms involved in its chromate resistance.

A singular critical onset strain value has been used to characterize the strain limits of barrier films used in flexible electronics. However, such metrics do not account for time-dependent or environmentally assisted cracking, which can be critical in determining the overall reliability of these thin-film coatings. In this work, the time-dependent channel crack growth behavior of silicon nitride barrier films on poly(ethylene terephthalate) (PET) substrates is investigated in dry and humid environments by tensile tests with in situ optical microscopy and numerical models. The results reveal the occurrence of environmentally assisted crack growth at strains well below the critical onset crack strain and in the absence of polymer-relaxation-assisted, time-dependent crack growth. The crack growth rates in laboratory air are about 1 order of magnitude larger than those tested in dry environments (dry air or dry nitrogen). In laboratory air, crack growth rates increase from ∼200 nm/s to 60 μm/s for applied stress intensity factors, K, ranging from 1.0 to 1.4 MPa·m

Respiration on trimethylamine oxide (TMAO) allows bacterial survival under anoxia. In Shewanella oneidensis, Tor is the system involved in TMAO respiration and it is encoded by the torECAD operon. The torA and torC genes encode TorA terminal reductase and the TorC c-type cytochrome, respectively. Sequence analysis suggests that TorD is the putative specific chaperone of TorA, whereas TorE is of unknown function. The purpose of this study was to understand whether TorD and TorE are two accessory proteins that affect the efficiency of the Tor system by chaperoning TorA terminal reductase. Moreover, by deleting each gene, we established that the absence of TorD drastically affects the stability of TorA, while the absence of TorE does not affect TorA stability or activity. Since TMAO reduction was affected in the ΔtorE mutant, TorE could be an additional component of the TorC-TorA electron transfer chain during bacterial respiration. Finally, a fitness experiment indicated that the presence of TorE, as expected, confers a selective advantage in competitive environments.

International audience; Molybdoenzymes play essential functions in living organisms and, as a result, in various geochemical cycles. It is thus crucial to understand how these complex proteins become highly efficient enzymes able to perform a wide range of catalytic activities. It has been established that specific chaperones are involved during their maturation process. Here, we raise the question of the involvement of general chaperones acting in concert with dedicated chaperones or not.

This work explores conductivity under applied uniaxial strain of two silver-based inks with similar flake volume fractions of ~50% that are screen-printed with a single pass (thickness: 10 µm) onto three different polymer substrates. The normalized resistance increases more rapidly with applied strain for the “flexible” ink (5025 with an acrylic binder), and has three times greater resistance at 35% strain when compared to the “stretchable” ink (PE874 with polyurethane binder). While resistance increase is qualitatively consistent with percolation theory, the in-situ strain map analysis and post-mortem fractography reveal drastic differences in the root causes of the inks’ electrical behavior. Both inks form strain localization bands with similar spacing. For the flexible ink (5025), strain localization is accompanied by local necking and flake fraction reduction. For the stretchable ink (PE874), strain localization is associated with surface cracking initiated by pre-existing voids, with minimal changes in the flake fraction. A model incorporating strain localization through a Gaussian distribution of flakes evolving with applied strain more closely accounts for the 5025 ink's normalized resistance increase compared to models that assume uniform strain and a uniform flake distribution. Overall, local necking and reduction of the flake fraction appear to be more detrimental to the resistance than the formation of surface cracks.

This work is concerned with the long-term behavior of environmentally-assisted subcritical cracking of PECVD SiNx barrier films on polyethylene terephthalate (PET) and polyimide (PI) substrates. While environmentally-assisted channel cracking in SiNx has been previously demonstrated, with constant crack growth rates over short periods of time (in-situ microscopy and finite-element modeling results highlight the combined effects of neighboring cracks and substrate cracking on the crack growth rate evolution in the film. In most cases, the subcritical crack growth rates decrease over time by up to two orders of magnitude until steady-state rates are reached. For SiNx on PI, crack growth rates were found to be more stable over time due to the lack of crack growth in the substrate as compared to SiNx on PET. These results provide a guideline to effectively improving the long-term reliability of flexible barriers by a substrate possessing high strength which limits substrate damage.

The energy losses of a silicon lateral rotary micro-resonator operating in humid air were investigated with a three-dimensional finite volume model and compared to experiments as well as a simplified analytical model. The simulations can provide accurate modeling (within 5%) of air damping (the dominant energy loss source), unlike the analytical model capturing as little as 50% of the experimental values. The results showed that a significant portion of energy losses are associated with the complex three-dimensional path lines near the micro-resonator's edges that are not captured in the simplified analytical models. The simulations provide useful guidelines to design simple temperature and humidity sensors.

This study highlights an oxidation-assisted fatigue crack initiation mechanism in 20 μm thick electroplated Ni films under loading conditions relevant for a wide range of microelectromechanical systems, such as extreme stress gradients at the surface (50% decrease over the first micrometer from the surface). Microresonators subjected to in-plane bending at ∼8 kHz were fatigued for billions of cycles in humid air, at 30 °C, 50% relative humidity (RH), and 80 °C, 90% RH, for maximum stress amplitudes up to ∼500 MPa (∼55% of the ultimate tensile strength). Transmission electron microscopy (TEM) revealed highly localized thick oxides (∼1 μm) on specimens fatigued for several billions of cycles. These oxides are two to three orders of magnitude thicker than the regular native oxides at these low temperatures, and only form at the location of cyclic slip bands. These oxides appear to be thicker for higher partial pressures of water, based on the TEM comparison of one specimen fatigued at 30 °C, 50% RH to one fatigued at 80 °C, 90% RH. Fatigue microcracks were observed within these highly localized thick oxides. Finite element models were also employed to confirm these results based on the interpretation of the evolution of the devices’ resonance frequency. This oxidation-assisted fatigue crack initiation mechanism at low temperatures constitutes a significant departure from the established mechanisms for bulk metals and their environmental effects. A possible explanation for the different governing mechanism is the presence of extreme stress gradients in these microscale components. Under these loading conditions, the classical fatigue crack initiation mechanisms are not operational, allowing this alternative mechanism to become dominant. This study highlights the need to further understand the coupled size and environmental effects on the fatigue of metallic thin films.

The stress distribution and fatigue behavior of nominally identical kilohertz fatigue resonators with two different thicknesses, 10 and 25 μm, was compared in this study. The results highlight the non-uniform 3-D stress distribution of the micron-scale notched cantilever beams that depends on the thickness. The areas corresponding to the first principal stress being within 2% of the maximum value are much smaller than the overall notch area and are a function of device thickness. It is also shown that the non-negligible influence of small, nanometer-scale geometrical variations in the dimensions of nominally identical devices on the maximum stress values can be accounted for by measuring the device's resonant frequency (f0). The observed scatter in the fatigue results of these microresonators is in part associated with the challenge in accurately calculating the local stress amplitudes. Despite that large scatter, the fatigue behavior of the 10 and 25 μm thick devices is similar. Particularly, the overall relative decrease rates in f0 are well related to fatigue life (Nf) and can be used to predict Nf within a factor of 5, for Nf ranging from 104 to 1010 cycles.

The temperature and humidity dependence of the quality factor of MEMS lateral rotary resonators operating in atmospheric air was analyzed and measured. The quality factor of a monocrystalline silicon lateral rotary micro-resonator was analytically modeled and numerically evaluated based on air shear damping loss mechanisms. The effect of temperature and humidity on each air damping mechanism was attributed to the thermophysical properties of humid air. The damping model predicted qualitatively the experimental results for a large range of temperature (from 20 °C to 80 °C) and humidity (from 30% to 90% RH). These environmental effects can be taken into account for the design of MEMS devices operating in air, such as chemical microsensors.

The biogenesis of molybdoenzymes is a cytoplasmic event requiring both the folded apoenzymes and the matured molybdenum cofactor. The structure and the complexity of the molybdenum cofactor varies in each molybdoenzyme family and consequently different accessory proteins are required for the maturation of the respective enzymes. Thus, for enzymes of both the DMSO reductase and xanthine oxidase families, specific chaperones exist which are dedicated to increase the stability and the folding of specific members of each family. In this review, we describe the role of these chaperones for molybdoenzyme maturation. We present a model which describes step by step the mechanism of the maturation of representative molybdoenzymes from each family

A characterization technique based on kilohertz micro-resonators is presented to investigate the very high cycle fatigue behavior of 20 μm thick electroplated Ni films with a columnar microstructure (grain diameter less than 2 μm). The films exhibit superior fatigue resistance due to the extreme stress gradients at the surface. The effects of stress amplitude and environment on the formation of fatigue extrusions and micro-cracks are discussed based on scanning electron microscopy and the tracking of the specimens’ resonant frequency.

This study investigated the influence of temperature and humidity on the adsorbed water layer on micron-scale monocrystalline silicon (Si) films in air, using a Si-MEMS kHz-frequency resonator. Both temperature and relative humidity induced a reversible change in resonant frequency, attributed to the temperature-dependent properties of Si and to a change in adsorbed water layer. The excellent precision in resonant frequency measurement (0.02 Hz, or 0.5 ppm) allowed precise calculation of the changes in adsorbed water layer thickness over the specimen surface. The increase in water thickness with relative humidity was a function of temperature and could not be described with simple multimolecular adsorption theories such as the BET theory. A likely explanation is the presence of hydrocarbon contaminants on the Si surface. Guidelines are provided to accurately measure the influence of temperature and relative humidity on the adsorbed water layer thickness on micron-scale Si surfaces, using this technique.

The fatigue properties of monocrystalline Si thin films coated with nanoscale alumina layers (ranging from 4.2 to 50 nm) are compared to those of uncoated Si films, in air at 30 °C, 50% relative humidity (RH) and 80 °C, 90% RH. The presence of alumina coatings results in more than two orders of magnitude longer fatigue lives, even though subcritical cracking of alumina occurs. The effects of the alumina coating thickness on the overall fatigue degradation behavior are discussed.

This study investigated the cyclic and static fatigue properties of 10 μm thick, deep reactive ion etched, monocrystalline silicon films. Stress–life fatigue curves and fatigue degradation rates vs. stress curves were generated at both 4 and 40 kHz, at 30 °C, 50% relative humidity (RH). A significant frequency effect was observed, with shorter fatigue lives and faster damage accumulation rates at 4 kHz. Static fatigue was also observed with shorter static lifetimes at 80 °C, 90% RH than at 30 °C, 50% RH. Fracture surface evaluation did not reveal any major difference between cyclically and statically fatigued devices. These experimental results confirm that the fatigue of micron-scale silicon is not purely mechanical. The study also proposes a fatigue scenario based on time-dependent subcritical crack growth to account for the low-cycle fatigue regime.

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.

This paper investigates the fatigue behavior of n+-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 (< 106)and high (up to 1011) 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.

The Newman–Kwart rearrangement (NKR) has been re-evaluated by microwave heating. Microwave technology has proven to be ideal for investigating this high temperature rearrangement and facilitated the confirmation of many aspects of this valuable reaction. Comparisons between thermal and microwave results indicate no evidence of a significant microwave effect.

The importance of service environment to the fatigue resistance of n/sup +/-type, 10 /spl mu/m thick, deep-reactive ion-etched (DRIE) silicon structural films used in microelectromechanical systems (MEMS) was characterized by testing of electrostatically actuated resonators (natural frequency, f/sub 0/, /spl sim/40 kHz) in controlled atmospheres. Stress-life (S-N) fatigue tests conducted in 30/spl deg/C, 50% relative humidity (R.H.) air demonstrated the fatigue susceptibility of silicon films. Further characterization of the films in medium vacuum and 25% R.H. air at various stress amplitudes revealed that the rates of fatigue damage accumulation (measured via resonant frequency changes) are strongly sensitive to both stress amplitude and, more importantly, humidity. Scanning electron microscopy of high-cycle fatigue fracture surfaces (cycles to failure, N/sub f/>1/spl times/10/sup 9/) revealed clear failure origins that were not observed in short-life (N/sub f/

We introduce an external-load-assisted thin film channel crack growth technique to measure the subcritical crack growth properties of thin films (i.e., crack velocity, v, versus the strain energy release rate, G), and demonstrate it using 250-nm-thick SiNx films on poly(ethylene terephthalate) substrates. The main particularity of this technique is that it requires a polymer substrate to allow loading to large strains (in order to induce channel cracking) without substrate fracture. Its main advantages are to provide a full v-G curve with a single specimen while relying on a simple specimen preparation and straightforward crack growth characterization. Importantly, the technique can be employed for a much larger range of thin films compared to the residual-stress-driven, thin film channel crack growth tests, including ultrathin films and thin film with residual compressive stresses. The restrictions to a proper use of this technique, related to the (visco)plastic deformation of the substrate, are discussed.

In the search for new organic superconductors based upon aromatic radical anions, recent theoretical predictions indicated that polycyclic aromatic compounds could be some interesting candidate. Among them triphenylene (Tp) seemed to be an interesting candidate. Triphenylides containing one, two or three alkali metal atoms (K, Rb, Cs) per Tp were synthesized. Only K salts had a semi-conductor like conductivity with a gap of 0.16 eV, all the others were insulators. Very weak LFMA signal was obtained for KTp at 4 K. A signal at g =4 was observed in the ESR spectrum of K 2 Tp and Rb 2 Tp. Infrared and Raman spectra of the solids are reported.

Inasmuch as leukocytes were reported to be an active pharmacologic compartment, colchicine disposition was determined in plasma, granulocytes, and mononuclear cells in healthy volunteers after 1 mg oral single and multiple doses. After the single dose, maximal colchicine concentration was observed at 1 hour in plasma and 47 hours later in leukocytes. This delay was confirmed by the slow accumulation of colchicine by lymphocytes in culture. In the multiple-dose study, mean granulocyte colchicine concentration (20 to 53 ng/10(9) cells) were twofold higher than in mononuclear cells (9 to 24 ng/10(9) cells). Mean predicted colchicine multiple-dose granulocyte and mononuclear cell concentrations were 2.5-fold and ninefold higher, respectively, than those measured. After the last dose, colchicine decreased, with half-life values between 41 and 46 hours for leukocytes and 49 hours for plasma. This study validates leukocytes as a microcompartment whose kinetics correlates with colchicine biologic effects.

This paper presents a testing methodology for measuring the low cycle fatigue properties of single-crystal silicon thin films using kHz-frequency resonators. The dynamic behavior of the fatigue structures is thoroughly characterized to allow accurate measurements of stresses (±0.1 GPa) and fatigue lives (±250 cycles). The tests consist of applying successive bursts of small numbers of cycles (as low as ~500 cycles) and measuring the resonant frequency in between each burst. Continuous damage accumulation, beginning after the first burst, is observed based on the decrease in resonant frequency of the resonant structure.

Although bulk silicon is not susceptible to fatigue, micron-scale silicon is. Several mechanisms have been proposed to explain this surprising behavior although the issue remains contentious. Here we re- view published fatigue results for micron-scale thin silicon films and find that in general they display similar trends, in that lower cyclic stresses result in larger number of cycles to failure in stress-lifetime data. We further show that one of two classes of mechanisms is invariably proposed to explain the phe- nomenon. The first class attributes fatigue to a surface effect caused by subcritical (stable) cracking in the silicon-oxide layer, e.g., reaction-layer fatigue; the second class proposes that subcritical cracking in the silicon itself is the cause of fatigue in Si films. It is our contention that results to date from single and polycrystalline silicon fatigue studies provide no convincing experimental evidence to sup- port subcritical cracking in the silicon. Conversely, the reaction-layer mechanism is consistent with existing experimental results, and moreover provides a rational explanation for the marked difference between the fatigue behavior of bulk and micron-scale silicon.

The fatigue degradation properties of atomic-layer-deposited alumina, with thickness ranging from 4.2 to 50 nm, were investigated using a silicon micro-resonator on which the coatings were deposited and strained in a static or cyclic manner, with strain amplitudes up to 2.2%, in controlled environments. Based on the measured resonant frequency evolution, post-test scanning electron microscopy observations, and finite element models, it is shown that cracks in the alumina nucleate and propagate under cyclic loading, and that the crack growth rates scale with the strain energy release rates for crack channeling. The implications for the reliability of flexible electronics are discussed.

Free and total plasma, granulocyte and mononuclear cell colchicine concentrations were measured by radioimmunoassay in 30 patients with familial Mediterranean fever treated with colchicine 0.5 to 2 mg day-1. Colchicine concentrations showed a large intersubject variability in plasma (0.13-1.75 ng ml-1), granulocytes (4 to 64 ng/10(9) cells), and mononuclear cells (11.4 to 57.6 ng/10(9) cells). Whereas unbound and total plasma colchicine concentrations were well correlated, no correlation was found between total or free plasma and granulocyte or mononuclear cell colchicine concentrations and dose of administered colchicine. In contrast, total or free plasma and granulocyte or mononuclear cell colchicine concentrations were correlated using a hyperbolic function indicating saturable colchicine distribution in both leukocyte populations.

This study investigates the separate influence of stress, temperature, and relative humidity (RH) on the fatigue behavior of 10-μm-thick, monocrystalline silicon (Si) films at 40 kHz, under fully reversed loading. The fatigue rates are most sensitive to stress, with four orders of magnitude decrease from 3.2 to 1.5–2 GPa, confirming a size effect associated with the fatigue behavior of Si under bending load. The fatigue rates are also much more sensitive to RH than temperature or partial pressure of water, indicating that the effective environmental parameter is the adsorbed water layer. The implications on the relevant fatigue process(es) are discussed.

The influence of atomic-layer-deposited alumina surface coatings on the fatigue of polycrystalline silicon thin films was investigated. Tests were performed on 2-μm-thick notched cantilever-beam structures actuated at resonance (∼40 kHz) that were coated with ∼20 nm of alumina deposited at 100 °C. The coated devices show a drastically different frequency evolution behavior as compared to uncoated specimens, while no surface oxide thickening during cycling is observed in bright-field transmission electron microscopy. Both results are consistent with a surface degradation mechanism for polycrystalline silicon fatigue such as the reaction-layer mechanism. The improved frequency evolution has the potential of increasing the performance of resonator-based microelectromechanical system sensors.

Nanometer-scale reaction layers have a profound impact on the fracture and fatigue resistance of the Si films used in microelectromechanical systems (MEMS). This letter presents experimental evidence that thick (i.e., greater than 10nm) oxides can form at room temperature during manufacturing due to a galvanic effect between n+-type Si and Au. The growth of such oxides in concentrated HF solutions that are usually associated with oxide dissolution can be predicted from the measured current density–voltage (i–V) behavior and geometry of the galvanic couple. These results can account for unexplained findings in the literature and can be used to improve the performance of MEMS.