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     

Effect of post-release sidewall morphology on the fracture and fatigue properties of polycrystalline silicon structural films

Surface properties can markedly affect the mechanical behavior of structural thin films used in microelectromechanical systems (MEMS) applications. This study highlights the striking difference in the sidewall surface morphology of n⁺-type polysilicon films from two popular MEMS processes and its effect on fracture and fatigue properties. The sidewall surface roughness was measured using atomic force microscopy, whereas silicon oxide thickness and grain size were measured using (energy-filtered) transmission electron microscopy. These measurements show that the oxide layers are not always thin native oxides, as often assumed; moreover, the roughness of the silicon/silicon oxide interface is significantly influenced by the oxidation mechanism. Thick silicon oxides (20 ± 5 nm) found in PolyMUMPs™ films are caused by galvanic corrosion from the presence of gold on the chip, whereas in SUMMiT V™ films a much thinner (3.5 ± 1.0 nm) native oxide was observed. The thicker oxide layers, in combination with differences in sidewall roughness (14 ± 5 nm for PolyMUMPs™ and 10 ± 2 nm for SUMMiT V™), can have a significant effect on the reliability of polysilicon structures subjecting to bending loads; this is shown by measurements of the fracture strength (3.8 ± 0.3 GPa for PolyMUMPs™ and 4.8 ± 0.2 GPa for SUMMiT V™) and differences in the stress-lifetime cyclic fatigue behavior.     

Low cycle fatigue of single crystal silicon thin films

Silicon will continue to be the critical structure material for micromechanical components for years to come so that reliability must be a key concern. Consequently, in order to ensure reliability design approaches must account for fatigue behavior. This work is aimed at studying the fatigue of single crystal silicon (SCS) thin films by a specially designed method. The films were tested using cantilever microbeam deflection with respect to the effect of loading conditions. To conduct a fatigue test under cyclic strain would be more realistic because many MEMS applications, such as micro-accelerometers and micro-filters, consist of beams vibrating in the same mode. A micro-force testing machine with a micro-probe and specially designed fixtures is used to contact and load the beams under the cyclic strain. Before the fatigue test, the failure strain _f of beams in the flexural test is achieved as the testing criterion. In fatigue testing, various percentages of failure strain _f, two times of the strain amplitude, are adopted. First of all, fatigue strain–life cycle (S/N) curve is achieved. Further, the curve of fatigue force detected on the SCS versus fatigue life is measured. SEM is also employed to observe the fracture modes of films under fatigue. Based on the SEM observation and force/life curve, the failure mechanism of the fatigued SCS films under the cyclic strain is proposed. This provides a viable method to evaluate the reliability of the SCS.     

Material loss of silicon nitride thin films in a simulated ocular environment

Microsystem Technologies - The dissolution behavior of silicon nitride in a simulated eye environment is studied as a function of temperature and ion concentration. Thin films of silicon nitride...     

Fracture and fatigue behavior of single crystal silicon microelements and nanoscopic AFM damage evaluation

Quasi-static bending and fatigue tests of single-crystal silicon microelements fabricated by photoetching were performed. The microelements were subjected to simple bending and three-point bending with two-support roll length of 1.5 mm. The tests were conducted by using a specially designed electromagnetic actuator based testing machine (load range: 0.1 mN–5 N, accuracy: 0.02 mN), which enables mechanical testing including fatigue of microelements. Mechanical testing including fatigue of microelements could be performed with sufficient precision. Single-crystal silicon microelements deformed elastically until final catastrophic failure, showing a brittle nature. The influence of specimen size on quasi-static fracture behavior was investigated: fracture strength increased with a decrease in sample width, and the maximum fracture strength reached 7.7 GPa. The influence of water on fatigue strength was discussed. The fracture surface and sample surface were examined using an atomic force microscope. Nanoscopic damage during testing was evaluated, and the fracture mechanisms were discussed. Received 20 October 1997/Accepted 5 January 1998     

The role of the environment in agreement technologies

The notion of Multi-Agent System environment is currently considered as a mediating entity, functioning as enabler but possibly also as a manager and constrainer of agent actions, perceptions, and interactions. In this paper, we analyze how the environment could be a first class abstraction to support the building, the development and the management of Agreements in decentralized and open systems between autonomous agents. To this aim we analyze the synergies between the environment and the foundational dimensions of agreement technologies such as semantics, norms, organizations, argumentation & negotiation, trust.     

The nano films refractive index values changing in the acidic environment studies

Nano films produced by the sol-gel technology were now widely used in the different science and technology areas for the effective properties modification of the glass substrates. The composite with such a nano film refractive index value changing after its interaction with the hydrochloric acid solution of the different concentration has been studied in this work. It has been obtained that the process of the nano dimensional film produced from the film-forming solution and acid solution interaction was consists of two mutually opposing effects—film material swelling and dissolution. If the nano film has been dissolved without of the swelling its refractive index was decreased. But the film material under the hydrochloric acid solution action passes through the swelling stage its refractive index increases in proportion of the swelling ratio.     

Study of silicon backside damage in deep reactive ion etching for bonded silicon–glass structures

 For devices of bonded silicon and glass structures fabricated by deep reactive ion etching (DRIE), it is important to avoid damage at the silicon sidewall and backside during through-wafer etching in order to ensure reliability of devices. The silicon damage caused by charge accumulation at the glass surface is inhibited by means of an electrically conducting layer patterned onto the glass and connected with the silicon. In this study, indium tin oxide films were applied in order to identify the positions of silicon damage in the structural layout without destruction of samples. From the results, we report that there exists silicon damage caused by charge accumulation at the silicon islands divided by DRIE and we present important rules for mask layout when utilizing this method. Received: 10 August 2001/Accepted: 24 September 2001 This paper was presented at the Fourth International Workshop on high Aspect Ratio Microstructure Technology HARMST 2001 in June 2001.     

Three dimensional finite element analysis of damage accumulation in composite laminate

A three dimensional finite-element computer program has been developed to analyze layered fiber-reinforced composite laminate. This program is capable of: (1) calculating the detailed stress distribution, (2) identifying the damage zone and mode of failure, (3) analyzing the damage accumulation, and (4) determining the ultimate strength of the composite laminate.     

The human role in tools for improving robustness and resilience of critical infrastructures

This paper presents a project dedicated to the development of means for improving the resilience of Critical Infrastructures (CIs) with respect to cyber attacks. The ability to sustain and protect the flow of information and data and the possibility to early detect, isolate and eliminate cyber hazards have become issues of paramount importance when developing the Supervisory Control And Data Acquisition (SCADA) systems of such a CI. The majority of tools dedicated to these goals are based on fully automatic autonomous self-reconfigurable systems that operate within the network, or online. However, the possibility to enable also human intervention for the further reduction in the vulnerability of CIs is equally possible. In this case, the intervention is considered offline and requires the active co-operation between a decision aid tool and a human operator. This paper presents a project aimed at improving robustness and resilience of CIs and discusses in particular the human interfaces associated with the offline tools. In essence, it is found that while the guidelines of the usability principle must be preserved, special account must be given to the type of issues involved and high professionalism of their users. This implies that certain basic criteria of the usability principle may be less relevant and their limitations may not be respected without loosing effectiveness and strength of the tools.     

Time-dependent nonlinear fatigue analyses of the TLP in the arabian sea environment

The tension leg platform (TLP) is a new concept in the design of platforms. It combines the facilities of both floating, drilling, and production platforms. The deck is secured by vertical tension legs attached to pile foundation templates on the sea bed. An attempt has been made in this paper to calculate the short and long term statistics of the stress response to the natural short and long crested sea and the resulting fatigue damage for certain critical members of a proposed TLP. A probabilistic approach is adopted utilising a spectral method for calculating the number of wave induced cycles at different stress levels. The nonlinear finite square stress element method is used to determine root mean square deflections and root mean fracture in randomly excited TLPs. Program RANDOM is developed to solve various theoretical equations and to evaluate the fatigue life. The basic input to the program consists of material properties, SN—curve and environmental data.     

基于NARMAX模型和NOFRF结构损伤检测的实验研究

非线性输出频率响应函数是由Volterra级数发展而来的频域概念,可方便在频域对非线性系统进行分析,它是频率的一维函数.本文主要介绍了利用NARMAX模型以及NOFRF对结构进行损伤检测的方法,并利用实验研究证实了该损伤检测方法的可行性.另外,由于系统非线性特性可用来做结构损伤检测,且具有对系统状态比较敏感的优点,而基于NOFRF的损伤检测方法是利用非线性方法来分析系统的状态,该方法提取出的特征属于非线性特征,所以该损伤检测方法可以用来做结构损伤检测,且具有对系统状态比较敏感的优点.     

On the role of minicomputers in structural design

Results are presented of exploratory studies on the use of a minicomputer in conjunction with large-scale computers to perform structural design tasks, including data and program management, use of interactive graphics, and computations for structural analysis and design. An assessment is made of minicomputer use for the structural model definition and checking and for interpreting results. Included are results of computational experiments demonstrating the advantages of using both a minicomputer and a large computer to solve a large aircraft structural design problem.     

Use of structural dynamic and fatigue sensitivity derivatives in an automotive design optimization

Direct accounting for durability rarely finds its way into multidisciplinary optimization. Though reduction of loads by some means can certainly have a beneficial influence on the fatigue performance of a structure, changes in load levels are not a direct measure of the influence of design changes on fatigue performance. In this paper, a previously described method for calculating design sensitivities of a fatigue performance index is used in a simple optimization of suspension damping and stiffness on a simple truck model. The dynamic loading is a conceptual representation of the industrial practice of road testing and simulation. Results demonstrate the feasibility of using a direct measure of fatigue performance in formal structural optimization.     

Design sensitivities of fatigue performance and structural dynamic response in an automotive application

Direct accounting for durability rarely finds its way into multidisciplinary optimization. Though reduction of loads by some means can certainly have a beneficial influence on the fatigue performance of a structure, changes in load levels are not a direct measure of the influence of design changes on fatigue performance. In this paper, an assessment is made of an approach to the calculation of design sensitivities of a fatigue performance index namely, number of damaging cycles to failure. The approach makes use of analytical sensitivities of structural dynamic response, and a standard approach to the calculation of fatigue performance. The method is demonstrated using a simple nine degree-of-freedom structural model of an automobile in a conceptual representation of the industrial practice of road testing and road simulation. Results suggest that the estimates of changes in fatigue life are of sufficient quality for typical fatigue life evaluations, and open the door for the incorporation of a direct measure of fatigue performance in formal structural optimization. Received January 27, 2000 Communicated by J. Sobieski     

Shape optimization of structural parts in dynamic mechanical systems based on fatigue calculations

In this paper a new method for an automated shape optimization of dynamically loaded components in mechanical systems is presented. The optimization is carried out by means of the results of a durability analysis based on finite elements. Load time histories, which are necessary for durability analyses, are derived from a multibody simulation. The whole optimization loop, which is an iterative procedure, incorporates all these gradual analysis steps and is implemented by the authors in a straightforward, batch-oriented manner using well-known standard software. Since the whole process involves several different analysis types, such as multibody system simulation and durability analysis, the resulting setup is rather complex. Furthermore, the reader may not be familiar with all the terms arising within the context of every single analysis domain. Therefore, some essential aspects of each of the stages involved in the process are explained to provide the reader with the necessary background. In the following, the required software setup as well as the implementation are described. Finally, an academic example is discussed to illustrate and clearly outline the potential of this method.     

Further examination of the Si KLL Auger line in silicon nitride thin films

The chemical bonding of Si in silicon nitride thin films has been studied using XPS. The kinetic energies of the Si KLL and Si LVV lines and the binding energies of the photoelectron lines along with their Auger parameters have been tabulated for sputter deposited, plasma deposited and CVD silicon nitride films. Characteristics of the silicon nitride thin films formed by N⁺₂ bombardment of Si have been discussed. Ar⁺ bombardment of the silicon nitride films is shown to cause reduction of the Si₃N₄ with greater amounts at the very near surface. The reduced species seem to be mostly substoichiometric Si₃N₄ rather than elemental Si. Analysis of the Si KLL line shapes shows variation in the amount of substoichiometric Si₃N₄ present for some of the silicon nitride films.     

Evaluation of a wearable computer system for telemonitoring in a critical environment

This paper reports on the evaluation of a wearable computer system designed for use in a critical environment, namely the intensive care unit of a hospital. The nature of the application raised ethical issues for testing in a clinical environment and standard evaluation techniques could not easily be applied. The system was therefore evaluated by clinicians in a multi-tasking environment with a simulated set of patient scenarios. Measures of suitability and wearability were applied. The results were encouraging and the system was deemed suitable for further evaluation in the clinical setting, subject to ethical approval.