Abstract
Choice of the most suitable material out of the universe of engineering materials available to the designers is a complex task. It often requires a compromise, involving conflicts between different design objectives. Materials selection for optimum design of a Micro-Electro-Mechanical-Systems (MEMS) pressure sensor is one such case. For optimum performance, simultaneous maximization of deflection of a MEMS pressure sensor diaphragm and maximization of its resonance frequency are two key but totally conflicting requirements. Another limitation in material selection of MEMS/Microsystems is the lack of availability of data containing accurate micro-scale properties of MEMS materials. This paper therefore, presents a material selection case study addressing these two challenges in optimum design of MEMS pressure sensors, individually as well as simultaneously, using Ashby’s method. First, data pertaining to micro-scale properties of MEMS materials has been consolidated and then the Performance and Material Indices that address the MEMS pressure sensor’s conflicting design requirements are formulated. Subsequently, by using the micro-scale materials properties data, candidate materials for optimum performance of MEMS pressure sensors have been determined. Manufacturability of pressure sensor diaphragm using the candidate materials, pointed out by this study, has been discussed with reference to the reported devices. Supported by the previous literature, our analysis re-emphasizes that silicon with 110 crystal orientation [Si (110)], which has been extensively used in a number of micro-scale devices and applications, is also a promising material for MEMS pressure sensor diaphragm. This paper hence identifies an unexplored opportunity to use Si (110) diaphragm to improve the performance of diaphragm based MEMS pressure sensors.
Highlights
As mentioned earlier, no 3H silicon carbide (3H-silicon carbide (SiC)) based MEMS devices have yet been reported. While both Si(110) and silicon nitride (SiN) are CMOS compatible materials, Si(110) has a number of unique advantages: (a) it is mechanically superior than Si (100), (b) it has higher etch rate in Alkali-based etchant than the conventionally used Si (100), (c) its surface intersects the four (111) planes at right angle making it a suitable material for achieving structures with perfectly vertical walls (Ghodssi and Lin 2011; Lee et al 1999), (d) the maximum longitudinal piezoresistance coefficient is along \111[ direction, which is on silicon (110) plane
Since no comprehensive MEMS materials database incorporating micro-scale properties was readily available, first a MEMS specific materials data is consolidated, which included three key properties at microscale for ceramics (Table 1), metals and alloys (Table 2) and polymers (Table 3) reported in the literature. This data has been successfully integrated with a material selection software, CES (Cambridge Engineering Selector), to develop material selection charts
Based upon the formulated Performance Indices, the performance of MEMS materials included in the consolidated MEMS micro-scale properties data has been analyzed for three different design requirements of pressure sensor
Summary
Pressure sensors based upon different transduction techniques including piezoresistive (Mosser et al 1991; Aryafar et al 2015; Shaby et al 2015; Rajavelu et al 2014) (using the change in the resistance to detect strain in diaphragm-embedded strain gauges due to applied pressure), capacitive The very first diaphragm pressure sensor and strain gauge were reported in 1958 (Bryzek et al 1990) and their full scale commercialization was achieved in 1990 (Bryzek 2012), yet the efforts to further improve the micro-fabricated pressure sensors’ mechanical design (and performance) by optimizing its shape/geometrical parameters still continue. Following Ashby’s material selection approach (Ashby 1989; Ashby and Cebon 1993; Ashby et al 2004; Ashby 2005), the Performance and Material Indices have been developed for a more demanding and conflicting mechanical design requirements of a MEMS pressure sensor diaphragm. In conjunction with the derived Performance and Material Indices, the consolidated materials data has been utilized to select materials for maximizing MEMS pressure sensor diaphragm deflection and natural frequency, simultaneously.
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