Micro-electro-mechanical systems fast fabrication by selective thick polysilicon growth in epitaxial reactor

A thick layer selective polysilicon growth technique has been developed for micro-electro-mechanical systems (MEMS) fabrication. It allows fast MEMS fabrication without using silicon on insulator wafers or deep ICPRIE etching. The fabrication technique is based on two main steps: a first seed layer of polysilicon is deposited and patterned; the second step consists in the selective growth of this layer in an epitaxial reactor. The first part of this work is devoted to the optimisation of growth parameters. Afterwards, this technique is applied for fabrication of different kinds of actuators, involving films several microns thick with good mechanical properties such as a low mechanical stress and a low roughness of the polysilicon film surface. Furthermore, thermal actuator prototypes were fabricated by using this technique;showing good mechanical properties and high reliability.     

Horizontal buried channels in monocrystalline silicon

A thick layer selective polysilicon growth technique has been developed for micro-electro-mechanical systems (MEMS) fabrication. It allows fast MEMS fabrication without using silicon on insulator wafers or deep ICPRIE etching. The fabrication technique is based on two main steps: a first seed layer of polysilicon is deposited and patterned; the second step consists in the selective growth of this layer in an epitaxial reactor. The first part of this work is devoted to the optimisation of growth parameters. Afterwards, this technique is applied for fabrication of different kinds of actuators, involving films several microns thick with good mechanical properties such as a low mechanical stress and a low roughness of the polysilicon film surface. Furthermore, thermal actuator prototypes were fabricated by using this technique;showing good mechanical properties and high reliability.

     

多晶硅梁疲劳损坏特性的研究

针对多晶硅的疲劳失效机理,人们已经提出了一些解释的模型.然而,到目前为止没有一种模型能够全面地阐述疲劳失效机理.本文旨在采用参量的渐变,如平均杨氏模量E,来反映MEMS多晶硅梁的疲劳.通过测试周期性载荷下双端固支梁结构的pull-in电压变化,确定杨氏模量E的变化,进而表征梁的疲劳失效状态.     

A comprehensive model to study nonlinear behavior of multilayered micro beam switches

A novel model to study the pull-in behavior of nonlinear electromechanically coupled systems has been developed. The proposed model is based on the multilayered cantilever and fixed–fixed micro beam type MEMS switches. Due to the complexity of the nonlinear beam mechanics, exact analytical solutions are not generally available; therefore, the derived nonlinear equation has been numerically solved fully using the nonlinear finite difference method. Furthermore, the results obtained are summarized and compared with the other existing empirical and analytical models. These results can be useful in the optimization of MEMS switch designs or other actuators. In addition, the method developed in this paper has a good potential for analyzing other types of complex MEMS devices. An erratum to this article can be found at     

Reduction of Sheet Resistance and Low-Thermal-Budget Relaxation of Stress Gradients in Polysilicon Microcantilever Beams Using Nickel-Silicides

Nickel-silicide (Ni_xSiy) is formed by the reaction of nickel and silicon at the temperature of couple of hundred degrees Celsius. Nickel-silicide technology is employed for the purpose of application in polysilicon-based microelectromechanical systems (MEMS) devices to reduce sheet resistance as well as to control residual stress gradients of the structures. To improve the compatibility of nickel-silicide with MEMS micromachining, anticorrosion release method is developed using cathodic protection. In situ study of stress evolutions during the reaction of a nickel film with polysilicon is quantitatively investigated using wafer curvature measurements. The phase of nickel-silicide is validated by using X-ray diffraction. The stress developed during the silicidation is utilized to control the stress gradient in polysilicon microcantilever beams. The experimental results show that the sheet resistance changes from over 20 000 Omega/sq. (insulating material) to less than 10 Omega/sq. The stress gradient is relaxed by counterbalancing the tensile residual stress at the upper part of a cantilever with a built-up compressive stress when the annealing temperature is 290 degC     

T形异形柱的非线性有限元分析

分析了T形异形柱的受力问题,利用ABAQUS有限元软件对T形异形柱进行了弹塑性非线性有限元分析.着重从混凝土的非线性本构关系和破坏准则,受拉开裂后的行为,钢筋的本构关系几方面进行分析,通过计算绘制出了梁柱混凝土的应力云图、裂缝图及钢筋的应力云图.用ABAQUS计算所得的裂缝图与拟静力试验结果裂缝进行比较,两者结果吻合.结果表明:应用ABAQUS对异形柱进行非线性分析是可靠的,从而论证了软件的实际应用能力,也为进一步进行更复杂的非线性分析打下了基础.     

Surface micromachined polysilicon heart cell force transducer

A microelectromechanical systems (MEMS) force transducer system, with a volume less than 1 mm³ millimeter, has been developed to measure forces generated by living heart muscle cells. Cell attachment and measurement of contractile forces have been demonstrated with a commercially fabricated surface-micromachined hinged polysilicon device. Two freestanding polysilicon clamps, each suspended by a pair of microbeams, hold each end of a heart cell. When the cell contracts, the beam bend and force is determined from the measured deflection and the spring constant in the beams. The average maximal force over seven contractile experiments using a calcium solution stimulus was F_max =12.6±4.66 μN. Normalizing to a cross-sectional area, F _max/area was 23.7±8.6 mN/mm². These force data were also correlated to optically imaged striation pattern periodicity. Intermediate forces were also measured in response to a calcium solution gradient and showed similar behavior to those measured in other laboratories. This MEMS force transducer demonstrates the feasibility of higher fidelity measurements from muscle cells and, thus, an improved understanding of the mechanisms of muscle contraction     

FEM analysis of concrete structures subjected to mode-I and mixed-mode loading conditions

In this study, the finite element method (FEM) for a body containing displacement discontinuity is used for the investigation of tensile fracture behavior under mode-I and mixed-mode loading conditions in concrete structures. A mechanical model for the tensile fracture behavior is reduced to a mathematical problem, and the analysis method is proposed. With the aid of this method, several factors which govern tensile fracture are examined, such as the unloading path in the tension-softening behavior and the transmission of shear stresses across crack surfaces. A plain concrete beam without a notch is analyzed by first neglecting and then taking into account the unloading path in the tension-softening behavior to demonstrate the phenomenon of cracking localization in mode-I crack growth. Pullout test specimens of practical significance are analyzed in order to study the crack growth phenomenon under mixed-mode loading conditions. Cases with and without lateral confinement are considered and the results obtained from the present analysis are compared with those obtained from available experimental data. A simple model for shear transfer across crack surfaces is established. By incorporating this model in the program, a pullout test specimen with lateral confinement is analyzed to examine the influence of shear transfer across crack surfaces on cracking localization.     

Nonlinear finite element analysis of reinforced concrete plates and shells under monotonic loading

Plane stress constitutive models are proposed for the nonlinear finite element analysis of reinforced concrete structures under monotonic loading. An elastic strain hardening plastic stress-strain relationship with a nonassociated flow rule is used to model concrete in the compression dominating region and an elastic brittle fracture behavior is assumed for concrete in the tension dominating area. After cracking takes place, the smeared cracked approach together with the rotating crack concept is employed. The steel is modeled by an idealized bilinear curve identical in tension and compressions. Via a layered approach, these material models are further extended to model the flexural behavior of reinforced concrete plates and shells. These material models have been tested against experimental data and good agreement has been obtained.     

Surface topography evolution and fatigue fracture in polysilicon MEMS structures

This paper presents the results of an experimental study of the micromechanisms of surface topography evolution and fatigue fracture in polysilicon MEMS structures. The initial stages of fatigue are shown to be associated with stress-assisted surface topography evolution and the thickening of SiO/sub 2/ layers that form on the unpassivated polysilicon surfaces and crack/notch faces. The differences in surface topography and oxide thickness are characterized as functions of fatigue cycling before discussing the micromechanisms of fatigue fracture.     

Application of the Generalized Differential Quadrature Method to the Study of Pull-In Phenomena of MEMS Switches

This paper reports on the pull-in behavior of nonlinear microelectromechanical coupled systems. The generalized differential quadrature method has been used as a high-order approximation to discretize the governing nonlinear integro-differential equation, yielding more accurate results with a considerably smaller number of grid points. Various electrostatically actuated microstructures such as cantilever beam-type and fixed-fixed beam-type microelectromechanical systems (MEMS) switches are studied. The proposed models capture the following effects: (1) the intrinsic residual stress from fabrication processes; (2) the fringing effects of the electrical field; and (3) the nonlinear stiffening or axial stress due to beam stretching. The effects of important parameters on the mechanical performance have been studied in detail. These results are expected to be useful in the optimum design of MEMS switches or other actuators. Further, the results obtained are summarized and compared with other existing empirical and analytical models.     

Fracture Toughness, Fracture Strength, and Stress Corrosion Cracking of Silicon Dioxide Thin Films

The fracture toughness, fracture strength, and stress corrosion cracking behavior of thin-film amorphous silicon dioxide $(hbox{SiO}_{2})$ deposited on silicon wafers via plasma-enhanced chemical vapor deposition have been measured using specimens with length scales comparable to micromachined devices. Clamped–clamped microtensile specimens were fabricated using standard micromachining techniques. These devices exploit residual tensile stresses in the film to create stress intensity factors at precrack tips and stress concentrations at notches, in order to measure fracture toughness and fracture strength, respectively. The fracture toughness of thin-film $hbox{SiO}_{2}$ was $0.77 pm 0.15 hbox{MPa}cdot hbox{m}^{1/2}$, and the fracture strength was $0.81 pm 0.06 hbox{GPa}$. Stress corrosion cracking (slow crack growth) was also measured in the $hbox{SiO}_{2}$ devices with sharp precracks subjected to residual tensile stresses. These data are used to predict lifetimes for a $hbox{SiO}_{2}$-based microdevice. $hfill$[2007-0304]      

Integrated self-assembling and holding technique applied to a 3-D MEMS variable optical attenuator

The application of polysilicon/gold bimorph stress-induced curved beams for three-dimensional self-assembly of MEMS devices is reported. The mechanical principle behind this self-assembling procedure is presented and comparison with current assembling methods are made. With this self-assembling technique, no postprocessing is required. A free-space optical MEMS device in the form of a variable optical attenuator (VOA) has been fabricated and self-assembled using this technique. The angular elevation of the self-assembled structures and the attenuation characteristics of the optical MEMS device are reported. The VOA has a measured dynamic attenuation range of 44 dB at 1.55 /spl mu/m optical wavelength. The bending of the bimorph beams is also temperature controllable, and the thermal behavior of the beams is also reported.     

Impact of high-thermal budget anneals on polysilicon as amicromechanical material

With the goal of facilitating the development of surface micromachined polysilicon MEMS with postprocessed on-chip circuitry, we have evaluated the Impart of a 1200°C 16-h anneal upon chemical-vapor-deposited (CVD) polysilicon under a variety of processing conditions. The results show that undoped polysilicon has a final stress of +10-20 MPa even when the films are vastly different as deposited and that phosphorus doping introduces a compressive trend that is evident only after the long anneal. X-ray diffraction, transmission-electron-microscopy (TEM), and atomic-force-microscopy (AFM) studies of the polysilicon are used to analyze the grain orientations, grain sizes, and surface roughness of the material. The effect of the long anneal on residual stress in wet thermal and CVD oxides is also presented. Overall, the results indicate that the thermal budgets of conservative circuit processes can be accommodated within the fabrication sequence of surface-micromachined polysilicon microstructures     

Calibration of residual stress difference of MetalMUMPs silicon nitride films

The metal multi-user MEMS processes (MetalMUMPs) provide one nickel film, two silicon nitride films and one polysilicon film for constructing various nickel MEMS devices. The two silicon nitride films are either bonded together as a bi-layered structure or they sandwich the polysilicon film to form a tri-layered structure to support nickel structures. The residual stress difference of the two silicon nitride films causes undesired deformations of suspended MetalMUMPs devices. In this paper, the residual stress difference of the two MetalMUMPs silicon nitride thin films is calibrated and the result is 169 MPa. The Young’s modulus of the MetalMUMPs nitride films is also measured, which is 209 GPa.     

Electrical contact resistance as a diagnostic tool for MEMS contact interfaces

The electrical contact resistance (ECR) was evaluated as an in situ diagnostic tool for the contact interface behavior of microelectromechanical systems (MEMS). Special polycrystalline silicon (polysilicon) MEMS devices fabricated by surface micromachining were used to study polysilicon/native oxide/polysilicon contact interfaces. ECR measurements obtained during monotonic contact loading and unloading and cyclic contact loading are interpreted in the context of a previous ECR theory. For monotonic contact loading and unloading, the ECR was measured as a function of apparent contact pressure and was found to be on the order of 10/sup 5/ /spl Omega/. The fairly moderate decrease of the ECR with the increase of the contact load is attributed to the intrinsic nonohmic behavior of the native oxide film. Experimental ECR results are correlated with analytical solutions to determine the oxide film thickness. The results indicate that the oxide film remains nearly intact under monotonic contact loading and unloading. Differences in the ECR behavior during unloading are discussed in light of the statistical distribution of asperity nanocontacts and the prevailing deformation mode. During cyclic contact loading, rupture of the oxide film leads to the formation of polysilicon/polysilicon nanocontacts, which produces ECR values in the range of 10/sup 2/-10/sup 3/ /spl Omega/. The erratic behavior of the ECR during cyclic contact loading is related to the pronounced effects of the insulating oxide film and oxide debris trapped at the contact interface.     

Fracture strength of polysilicon at stress concentrations

Mechanical design of MEMS requires the ability to predict the strength of load-carrying components with stress concentrations. The majority of these microdevices are made of brittle materials such as polysilicon, which exhibit higher fracture strengths when smaller volumes or areas are involved. A review of the literature shows that the fracture strength of polysilicon increases as tensile specimens get smaller. Very limited results show that fracture strengths at stress concentrations are larger. This paper examines the capability of Weibull statistics to predict such localized strengths and proposes a methodology for design. Fracture loads were measured for three shapes of polysilicon tensile specimens - with uniform cross-section, with a central hole, and with symmetric double notches. All specimens were 3.5 /spl mu/m thick with gross widths of either 20 or 50 /spl mu/m. A total of 226 measurements were made to generate statistically significant information. Local stresses were computed at the stress concentrations, and the fracture strengths there were approximately 90% larger than would be predicted if there were no size effect (2600 MPa versus 1400 MPa). Predictions based on mean values are inadequate, but Weibull statistics are quite successful. One can predict the fracture strength of the four shapes with stress concentrations to within /spl plusmn/10% from the fracture strengths of the smooth uniaxial specimens. The specimens and test methods are described and the Weibull approach is reviewed and summarized. The CARES/Life probabilistic reliability program developed by NASA and a finite element analysis of the stress concentrations are required for complete analysis. Incorporating all this into a design methodology shows that one can take "baseline" material properties from uniaxial tensile tests and predict the overall strength of complicated components. This is commensurate with traditional mechanical design, but with the addition of Weibull statistics.     

基于倍频双晶复合传感器的构件疲劳微裂纹非线性超声检测

通过对固体中非线性超声传播模型的研究,分析了裂纹静态压力与超声波作用力对裂纹超声非线性响应的影响,在此基础上,建立了反映裂纹区应力-应变非线性关系的弹性接触机制超声非线性响应模型;以及反映裂纹闭合状态转换的碰撞接触机制超声非线性响应模型。基于上述理论,通过实验研究发现裂纹尖端区的二次谐波激发效率与裂纹的开口区和闭合区及裂纹最终扩展的极限长度有关。因此,可使用二次谐波激发效率作为定量表征金属试件疲劳微裂纹缺陷的特征参数,实验中使用了自主研制倍频双晶复合换能器,这种倍频双晶复合换能器在工程实际应用中作在线检测更为方便、实用。