Notch Root Oxide Formation During Fatigue of Polycrystalline Silicon Structural Films

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.     

High-cycle fatigue of single-crystal silicon thin films

When subjected to alternating stresses, most materials degrade, e.g., suffer premature failure, due to a phenomenon known as fatigue. It is generally accepted that in brittle materials, such as ceramics, fatigue can only take place in toughened solids, i.e., premature fatigue failure would not be expected in materials such as single crystal silicon. The results of this study, however, appear to be at odds with the current understanding of brittle material fatigue. Twelve thin-film (~20 μm thick) single crystal silicon specimens were tested to failure in a controlled air environment (30±0.1°C, 50±2% relative humidity). Damage accumulation and failure of the notched cantilever beams were monitored electrically during the "fatigue life" test. Specimen lives ranged from about 10 s to 48 days, or 1×106 to 1×1011 cycles before failure over stress amplitudes ranging from approximately 4 to 10 GPa. A variety of mechanisms are discussed in light of the fatigue life data and fracture surface evaluation     

Characteristic dimensions and the micro-mechanisms of fracture and fatigue in `nano' and `bio' materials

The behavior of small-volume (so-called `nano') structures, where size-scales are comparable with microstructural dimensions, and biological/bio-implantable materials, which invariably display a hierarchy of structural dimensions, is currently much in vogue in materials science. One aspect of this field, which to date has received only limited attention, is the fracture and fatigue properties of these materials. In this paper, we examine two topics in this area, namely the premature fatigue failure of silicon-based micron-scale structures for microelectromechanical systems (MEMS), and the fracture properties of mineralized tissue, specifically human bone.     

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.     

Lost Mold-Rapid Infiltration Forming of Mesoscale Ceramics: Part 2, Geometry and Strength Improvements

Iterative process improvements have been used to eliminate strength-limiting geometric flaws in mesoscale bend bars composed of yttria-tetragonal zirconia polycrystals (Y-TZP). These improvements led to large quantities of high bend strength material. The metrology of Y-TZP mesoscale bend bars produced using a novel lost mold-rapid infiltration-forming process (LM-RIF) is characterized over several process improvements. These improvements eliminate trapezoidal cross sections in the parts, reduce concave upper surfaces in cross section, and minimize warping along the long axis of 332 × 26 × 17 μm mesoscale bend bars. The trapezoidal cross sections of earlier, first-generation parts were due to the absorption of high-energy ultraviolet (UV) light during the photolithographic mold-forming process, which produced nonvertical mold walls that the parts mirrored. The concave upper surfaces in cross section were eliminated by implementing a RIF-buffing process. Warping during sintering was attributed to impurities in the substrate, which creates localized grain growth and warping as the tetragonal phase becomes destabilized. Precision in the part dimensions is demonstrated using optical profilometry on bend bars and a triangular test component. The bend bar dimensions have a 95% confidence interval of <±1 μm, and the tip radius of the triangular test component is 3 μm, consistent with the UV-photolithographic process used to form the mold cavities. The average bend strength of the mesoscale Y-TZP bend exceeds 2 GPa with a Weibull modulus equal to 6.3.     

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.     

Strengthening Mechanisms in MLCCs: Residual Stress Versus Crack Tip Shielding

Failure by fracture is a serious problem with multilayer ceramic capacitors (MLCCs), and the interior electrodes are known to strengthen MLCCs. Historically, it has been assumed that the dominant strengthening mechanism is crack tip shielding via direct crack tip‐electrode interactions. However, we have found that residual stresses arising from differential thermal contraction after device sintering are actually responsible for the observed increase in strength. In addition, the fracture initiation sites in MLCCs are located outside of the electrode array, so the established idea that the electrical and mechanical failure controlling flaw populations are one and the same cannot be true. Weibull distributions were compared from the bending fracture of two populations of MLCCs with barium titanate (X7R) dielectric, nickel electrodes, and the same exterior geometries (but different electrode array configurations). MLCCs had characteristic strengths of 236 MPa versus a strength of 190 MPa for 19‐ and 3‐electrode MLCCs, respectively. Fractography, a critical flaw size computation, an analytical residual stress approximation, and in situ electrical measurements taken during bending were also used to examine the fracture process and demonstrate that residual stress and not crack tip shielding is an important strengthening mechanism in MLCCs.     

The critical role of environment in fatigue damage accumulation in deep-reactive ion-etched single-crystal silicon structural films

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/<1/spl times/10/sup 4/) specimens. Reaction-layer and microcracking mechanisms for fatigue of silicon films are discussed in light of this empirical evidence for the critical role of service environment during damage accumulation under cyclic loading conditions.     

Lost Mold Rapid Infiltration Forming of Mesoscale Ceramics: Part 1, Fabrication

Free-standing mesoscale (340 μm × 30 μm × 20 μm) bend bars with an aspect ratio over 15:1 and an edge resolution as fine as a single grain diameter (∼400 nm) have been fabricated in large numbers on refractory ceramic substrates by combining a novel powder processing approach with photoresist molds and an innovative lost-mold thermal process. The colloid and interfacial chemistry of the nanoscale zirconia particulates has been modeled and used to prepare highly concentrated suspensions. Engineering solutions to challenges in mold fabrication and casting have yielded free-standing, crack-free parts. Molds are fabricated using high-aspect-ratio photoresist on ceramic substrates. Green parts are formed using a rapid infiltration method that exploits the shear thinning behavior of the highly concentrated ceramic suspension in combination with gelcasting. The mold is thermally decomposed and the parts are sintered in place on the ceramic substrate. Chemically aided attrition milling disperses and concentrates the as-received 3Y-TZP powder to produce a dense, fine-grained sintered microstructure. Initial three-point bend strength data are comparable to that of conventional zirconia; however, geometric irregularities (e.g., trapezoidal cross sections) are present in this first generation and are discussed with respect to the distribution of bend strength.