Design and optimization of a novel tri-axial miniature ear-plug piezoresistive accelerometer with nanoscale piezoresistors

Abstract

This work aims at the advancement of state-of-art accelerometer design and optimization methodology by developing an ear-plug accelerometer for race car drivers based on a novel mechanical principle. The accelerometer is used for the measurements of head acceleration when an injurious event occurs. Main requirements for such sensor are miniaturization (2×2 mm), because the device must be placed into the driver earpiece, and its measurement accuracy (i.e. high sensitivity, low crosstalk and low nonlinearity) since the device is used for safety monitoring purpose. A micro-electro-mechanical system (MEMS)-based (bulk micromachined) piezoresistive accelerometer was selected as enabling technology for the development of the sensor. The primary accelerometer elements that can be manipulated during the design stage are: the sensing element (piezoresistors), the micromechanical structure and the measurements circuit. Each of these elements has been specifically designed in order to maximize the sensor performance and to achieve the miniaturization required for the studied application. To achieve accelerometer high sensitivity and miniaturization silicon nanowires (SiNWs) as nanometer scale piezoresistors are adopted as sensing elements. Currently this technology is at an infancy stage, but very promising through the exploitation of the “Giant piezoresistance effect” of SiNWs. This work then measures the potential of the SiNWs as nanoscale piezoresistors by calculating the major performance indexes, both electrical and mechanical, of the novel accelerometer. The results clearly demonstrate that the use of nanoscale piezoresistors boosts the sensitivity by 30 times in comparison to conventional microscale piezoresistors. A feasibility study on nanowires fabrication by both top-down and bottom-up approaches is also carried out. The micromechanical structure used for the design of the accelerometer is an optimized highly symmetric geometry chosen for its self-cancelling property. This work, for the first time, presents an optimization process of the accelerometer micromechanical structure based on a novel mechanical principle, which simultaneously increases the sensitivity and reduces the cross-sensitivity progressively. In the open literature among highly symmetric geometries no other study has to date reported enhancement of the electrical sensitivity and reduction of the cross-talk at the same time. Moreover the novel mechanical principle represents advancement in the accelerometer design and optimization methodology by studying the influence of a uniform mass moment of inertia of the accelerometer proof mass on the sensor performance. Finally, an optimal accelerometer design is proposed and an optimized measurement circuit is also specifically designed to maximize the performance of the accelerometer. The new proposed accelerometer design is capable of increasing the sensor sensitivity of all axes, in particular the Z-axis increases of almost 5 times in respect to the current state-of-art-technology in piezoresistive accelerometer. This occurs thanks to the particular newly developed approach of combination of beams, proof mass geometry and measurement circuit design, together with the use of silicon nanowires as nanoscale piezoresistors. Furthermore the cross-sensitivity is simultaneously minimized for a maximal performance. The sum of the cross-sensitivity of all axes is equal to 0.4%, well below the more than 5% of the state-of-art technology counterpart reported in the literature. Future work is finally outlined and includes the electro-mechanical characterization of the silicon nanowires and the fabrication of the proposed accelerometer prototype that embeds bottom up SiNWs as nanoscale piezoresistors.