The clinical management of skeletal trauma and disease relies on radiographic imaging to infer bone quality. However, bone strength does not necessarily correlate well with image intensity. There is a need for a safe and convenient way to measure bone strength in situ. This paper presents a new technique to directly measure bone strength in situ at a micro-level scale through a MicroElectroMechanical System (MEMS) sensor. The proposed MEMS stress sensor comprises an array of piezoresistive sensor "pixels" to detect stress across the interfacial area between the MEMS chip and bone with resolution to 100 Pa, in 1 sec averaging. The sensors are located within a textured surface to accommodate sensor integration into bone. From initial research, surface topography with 30-60 μm features was found to be conducive to guiding new cell growth. Finite element analysis has led to a sensor design for normal and shear stress detection.
A time-multiplexed, anisotropic, inductively coupled plasma Si deep reactive ion etch process is characterized in terms of the Si macroload, cross-wafer spatial variation, local pattern density, and feature size. The process regime is established as neutral flux limited, in which material transport occurs in the molecular flow to transition flow regimes. For this process regime, a semiempirical, unified analytic model and a numeric model are developed using the Dushman and Clausing vacuum conductance correction factors, respectively, in the Coburn and Winters model of aspect ratio dependent etching. The experimental reaction probability for etching of Si by F was found to be 0.24 for Dushman’s factor and 0.22 for Clausing’s factor. Each model is validated to ±10% against experimental depth data for microdonut and trench test structures and match each other to within 10% for depths of up to 160 μm. The observed depth range is 64 μm at a depth of 160 μm.
A unified, semi-empirical model of etch depth for a Bosch-type, inductively coupled plasma (ICP), silicon deep reactive ion etch (DRIE) process is reported. Aspect ratio dependent etch modulation (ARDEM) is modeled using Coburn and Winters' approach with Dushman's approximation of the vacuum conductance correction factor. The use of microdonut test structures to extract model parameters is described. The model is accurate to within 9% for an open-field depth of 150 μm on microdonut test structures. An application of ARDEM in the processing of CMOS-MEMS devices is presented.
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