Simulation and optimization on the squeeze-film damping of a novel high-g accelerometer
Introduction
High-g accelerometers fabricated by advanced silicon micro-machining technology have been widely used in many harsh occasions including collision, explosion or impact by the advantage of its small-volume, low-cost, high-precision, good reliability and being prone to mass-production, in which the measured shock acceleration usually can be so high as tens thousand gravities [1], [2], [3]. Elaborate efforts should be make not only on the protection of sensing structure but also on the broad bandwidth of shock response during design, fabrication and packaging of high-g accelerometers. Design of critical damping, effective overload protection and stress-free mounting of sensing element are the most difficult aspects in design and fabrication of high-g accelerometers [4]. Overload protection of sensing element is mainly related to the structure of high-g accelerometers and can be solved effectively by designing special structure, whereas design of critical damping and stress-free mounting, especially the design of critical damping, are involved in the whole course of design, fabrication and packaging of high-g accelerometers and have attracted great interests of inertia researchers [5], [6], [7].
Three methods are usually used to modulate the squeeze-film damping of high-g accelerometers: adjust the structure (e.g. adjust the damping area), modify the width of damping gap or change the damping media. The ability to adjust damping by modifying the structure of component is finite for modification of structure means the variation of other mechanical and electrical performance of the component. Therefore, adjusting squeeze-film damping is usually realized either by modifying the width of damping gap or by doing the characteristics of damping media. In this paper, efforts were made to the design and optimization of squeeze-film damping as to a novel piezoresistive high-g accelerometer with curved protection of sensing element and both the effects of the damping gap width and the impacts of the characteristics of damping medium on the dynamic shock response of component have been explored to lay a solid theoretical foundation on the further design, fabrication and encapsulation of the component.
Section snippets
Damping theory of curve surface
The 3D schematic of sensor structure with curved protection of sensing element capable of enduring 1×105g shock is shown in Fig. 1. Two piezoresistors formed by boron diffusion are located near both edges of top surface of the cantilever. KOH etching and DRIE technology have been used on the back and on the top surface of silicon wafer in turn, respectively, to form cantilever and curved stop (curve conforms to the displacement formula as shown in Fig. 1). Slot structure is designed and formed
Characteristics of damping
The accelerometer consists of two cantilevers parallel to each other in reverse direction. Each cantilever has two piezoresistors near both edges of the top surface at the root and four piezoresistors form a whole Wheatstone bridge. Shown in Fig. 3 is the structure of the packaged accelerometer. After bonded with a protective silicon cap, the chip was attached on the bottom of the header with an epoxy die-attach adhesive. Then the header was filled with encapsulant and sealed with its lid.
In
Conclusion
The damping characteristics of a packaged high-g accelerometer with curved stop have been investigated in this paper. Firstly, a MSPA model of curved surface damping suitable for this component to obtain the relationship between the PSD of curved stop and the damping. Secondly, the effects of both the PSD and the characteristics of damping media on the dynamic shock response of component have been simulated by finite element analysis. Results showed that the dynamic output response in fact was
Acknowledgements
Part of work was funded by Natural Science Foundation program of China (60376038) and Natural Science Foundation program of Fujian province (A0510011).
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