Stuart B. Brown

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.

Process Planning for Aluminum Tubes: An Engineering-Operations Perspective

Metal-forming operations such as extrusion, drawing, and rolling offer many opportunities for operations improvement through better process understanding and improved planning practices. This paper addresses medium-term planning issues in aluminum tube manufacturing operations. First, we identify certain distinctive characteristics-the inherent process flexibility, close inter-dependence between successive stages, and economies of scale-of metal-forming operations, and identify performance trade-offs across stages. To exploit the strategic potential of process planning, it must be closely coupled with process engineering efforts, and must simultaneously consider the facilitys entire product mix. In contrast, current process engineering efforts are mainly reactive, focusing on fixing problems at individual operations and with less emphasis on the interactions between successive stages. Similarly, planning activities are incremental, considering only individual products or orders one at a time rather than the entire range of product sizes to be manufactured. By working together, planners and engineers can develop effective process plans that exploit process capability, and adopt proactive process improvement strategies that focus on critical constraints. We describe a medium-term planning model to select standard extrusion sizes, illustrate the close linkages between planning and engineering activities, and identify research opportunities spanning management science, materials science, and mechanical engineering.

Fatigue damage evolution in silicon films for micromechanical applications

In this paper we examine the conditions for surface topography evolution and crack growth/fracture during the cyclic actuation of polysilicon microelectromechanical systems (MEMS) structures. The surface topography evolution that occurs during cyclic fatigue is shown to be stressassisted and may be predicted by linear perturbation analyses. The conditions for crack growth (due to pre-existing or nucleated cracks) are also examined within the framework of linear elastic fracture mechanics. Within this framework, we consider pre-existing cracks in the topical SiO2 layer that forms on the Si substrate in the absence of passivation. The thickening of the SiO2 that is normally observed during cyclic actuation of Si MEMS structures is shown to increase the possibility of stable crack growth by stress corrosion cracking prior to the onset of unstable crack growth in the SiO2 and Si layers. Finally, the implications of the results are discussed for the prediction of fatigue damage in silicon MEMS structures.

Reliability and Fatigue Testing of MEMS

Microelectromechanical structures (MEMS) utilize brittle materials such as polycrystalline silicon (polysilicon) under potentially severe mechanical and environmental loading conditions. These structures may be subjected to high frequency, cyclic loading conditions, accumulating large numbers of cycles in relatively short periods of time. Failure Analysis Associates has developed a technique for characterization of fatigue crack initiation and growth in MEMS materials. Preliminary results show that fatigue cracks can initiate and grow in polysilicon MEMS devices, and that water vapor is important in sub-critical crack advance. This research provides a basis for characterizing long-term durability and stability of micron-scale structures.

Interdisciplinary Industry-University Collaboration: Lessons from an Operations Improvement Project

A six-month Leaders-for-Manufacturing student internship at the Alcoa extrusion and tube plant in Lafayette, Indiana identified a promising operations improvement opportunity in tube manufacturing and led to a two-year collaboration between Alcoa and faculty members from the schools of engineering and management at MIT to develop integrated process planning models. Project participants included production managers, supervisors and planners at the plant, process engineers from the Alcoa Technical Center, and faculty and students in engineering, operations research, and management. The project demonstrated that the plant could reduce tube drawing effort by more than 20 percent by using decision support tools and improving the planning processes. It also generated techniques to diagnose problems, new performance metrics, and software for short-term and medium-term process planning, persuaded plant managers to take a systems view of process planning, led to undergraduate and graduate thesis research, provided examples for class room use, and highlighted the enablers and challenges in conducting industry-university projects, particularly those dealing with supply-chain integration.

Surface topography evolution and fatigue fracture of polysilicon

This paper presents the results of an experimental stydy of the micromechanisms of fatigue crack nucleation 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 SiO2 layers that form on the unpassivated polysilicon surfaces and crack/notch faces. The differences in surface topography and oxide thickness are elucidated as functions of fatigue cycling before discussing the micromechanisms of crack growth and final fracture.

Mechanisms of Fatigue in Polysilicon Mems Structures

This paper presents the results of a combined experimental and computational study of crack nucleation and surface topology evolution during the cyclic actuation of polysilicon MEMS structures. The evolution in surface topology observed during the crack nucleation stage is related to the underlying notch-tip stress distributions calculated using finite element analysis. Measured changes in surface topology due to the stressassisted dissolution of silica are shown to be predicted by linear stability analysis.

Average Operator Inaction Characteristics with Lever Controls — Study of the Column Mounted Gear Selector Lever

Sixty drivers executed 1800 automotive transmission shifts from DRIVE to PARK. These shifts were performed in a total of six vehicles. Among analoq data recorded during each shift are longitudinal (pull) and rotational forces and rotational position of the lever. Analysis reveals that drivers typically impact the mechanical stop associated with PARK position with approximately one and one-half times the minimum force required to complete the shift. Shifts to PARK are completed in an average of 0.58 seconds, and an additional 0.83 seconds is spent continuing to apply force to the lever after PARK position is achieved. Most (80%) shifts are executed without regard for intervening detents. Those drivers who show hesitation in movement typically hesitate for 50 milliseconds. Whether a driver shifts continuously or with shortduration hesitation appears to be driver style or insert plate impact, independent of vehicle.

Child Resistant Closure Performance Aging Study

Child poisoning has been dramatically reduced by the introduction of child resistant (CR) closures on some common home chemicals and pharmaceuticals. However, “child resistance” (often mischaracterized as “child proof”) is a mechanical design property that is neither well understood nor supported by a body of theory, nor that can be specified from engineering first principles. Instead, child resistance is an empirically developed and verified closure mechanical property derived from closure testing with child subjects, as specified by regulations under the Poison Prevention Packing Act (PPPA). The authors report their longitudinal study of a specific Type III CR closure over a period of decades made with materials from different suppliers over time and using different injection molding tools. The study examines if the property of “child resistance” persisted and if it correlated with the mechanical specifications of the closure actually measured and controlled in the closure manufacturing process. This data is combined with the authors’ mechanical measurements of closure performance. Child resistance, being a complex, empirically tested property, cannot be regularly tested in the normal manufacturing environment. Despite minor manufacturing process and specification changes, if the mechanical specifications are appropriate (e.g. not intended to produce changes in CR mechanical properties) and with adequate quality control, the property of child resistance persists.Copyright

On the Evolution of Surface Morphology of Polysilicon Mems Structures During Fatigue

This paper presents the results of a combined experimental and computational study of surface topology evolution preceding fatigue crack nucleation in polysilicon MEMS structures. The evolution in surface topology observed during the crack nucleation stage is related to the underlying notch-tip stress distributions calculated using the finite element method. Measured changes in surface topography due to the stress-assisted dissolution of silica are shown to be predicted by a linear stability analysis. The implications of the results are discussed for modeling of fatigue in polysilicon MEMS structures. 2003 Elsevier Ltd. All rights reserved.