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.
Table of Contents
1 INTRODUCTION
1.1 Introduction
1.2 Motivation
1.3 Thesis Aim and Objectives
1.4 Conclusion
1.5 Thesis structure
2 LITERATURE REVIEW
2.1 Introduction
2.2 Overview of earplug accelerometer
2.3 Instrumented helmets
2.4 Earpiece accelerometer
2.5 Acceleration Sensor
2.5.1 Piezoresistive Accelerometers
2.6 Piezoresistance
2.6.1 Piezoresistance of p-type and n-type single crystal silicon
2.6.2 Giant Piezoresistance in Silicon Nanowires
2.6.3 Fabrication processes of silicon nanowires
2.7 Gap in Knowledge
2.8 Conclusion
3 DESIGN, MODELLING AND OPTIMIZATION OF A BIO-MECHANIC PIEZORESISTIVE ACCELEROMETER WITH SILICON NANOWIRES
3.1 Introduction
3.2 Accelerometer design concept and considerations
3.3 Accelerometer model
3.3.1 Mathematical model
3.3.2 Finite element modelling
3.4 Accelerometer Geometry Optimization
3.4.1 Accelerometer shape optimization
3.4.2 Size optimization of the candidate shapes
3.4.3 Overload End Stops Design
3.5 Results and discussion
3.5.1 Shape optimization
3.5.2 Size optimization
3.6 Conclusion
4 ELECTRICAL PERFORMACE OF NANOSCALE PIEZORESISTORS COMPARED TO CONVENTIONAL MICROSCALE PIEZORESISTORS
4.1 Introduction
4.2 Methodology
4.2.1 FEM model
4.2.2 Measurement circuit
4.2.3 Electrical Sensitivity and cross-axis sensitivity
4.2.4 Nonlinearity
4.2.5 Damping
4.2.6 Bandwidth
4.2.7 Noise and resolution
4.3 Results and discussion
4.3.1 Structural analysis
4.3.2 Electrical sensitivity and cross-sensitivity
4.3.3 Nonlinearity, damping and bandwidth
4.3.4 Noise and resolution
4.3.5 Discussion
4.4 Conclusion
5 INFLUENCE OF VARIATIONS IN THE MASS MOMENT OF INERTIA INTO THE PERFORMANCE OF A TRI-AXIAL PIEZORESISTIVE ACCELEROMETER
5.1 Introduction
5.2 FE Modelling and input validation
5.3 Design optimization approach
5.4 Results
5.4.1 Design Concept Type A: Curved beams
5.4.2 Design Concept Type B: Straight beams
5.4.3 Effect on performance of beam geometry
5.5 Optimum design selection
5.6 Conclusion
6 PROPOSED OPTIMAL ACCELEROMETER DESIGN
6.1 Introduction
6.2 Optimization strategy
6.3 FE Modelling
6.4 Results and discussion
6.4.1 FEA results - nanowires as piezoresistors
6.4.2 FEA results - conventional microscale piezoresistors
6.5 Conclusion
7 A FEASIBILITY STUDY ON THE FABRICATION OF SILICON NANOWIRES FOR NANOSCALE PIEZORESISTORS
7.1 Introduction
7.2 Design of samples
7.3 Experiments
7.3.1 Materials
7.3.2 Fabrication
7.4 Results and discussion
7.4.1 Focus Ion beams experiment results
7.4.2 Nanowire growth experiment results
7.5 Conclusion
8 CONCLUSION AND FUTURE WORK
Research Goal and Thematic Focus
This thesis aims to advance current state-of-the-art accelerometer design by developing a novel, miniaturized (2x2 mm) tri-axial piezoresistive accelerometer suitable for earplug integration in motorsports, with the primary research goal of achieving higher measurement accuracy through the use of silicon nanowires (SiNWs) as nanoscale piezoresistors.
- Development of a miniaturized, tri-axial piezoresistive accelerometer for head injury monitoring.
- Exploitation of the "Giant Piezoresistance" effect in silicon nanowires for enhanced sensitivity.
- Optimization of mechanical structure and proof mass geometry to improve sensitivity and reduce cross-sensitivity.
- Integration of a measurement circuit that avoids external amplification to improve signal-to-noise ratios.
- Feasibility study on top-down and bottom-up fabrication approaches for nanoscale piezoresistors.
Excerpt from the Book
3.2 Accelerometer design concept and considerations
The key design criteria are to develop a ‘High Resolution - Low power 3-DOF Piezoresistive MEMS-based Accelerometer’. Low power consumption is mandatory for this device since it is placed inside the ear, hence, specific attention must be paid to power dissipation process in order to avoid discomfort during wear. This has a particular drawback in that it affects the signal to noise ratio due to the high resistance of the piezoresistors (high white noise).
The resolution will be maximized by reducing the noise level to the minimum possible. Moreover due to the miniaturization currently achievable by MEMS fabrication processes the sensor is designed for this type of technology. This last choice will affect notably the design since a design for manufacturing and for packaging is required.
Summary of Chapters
1 INTRODUCTION: Outlines the motivation behind the research, focusing on the need for improved head injury monitoring in motorsports and the potential of nanotechnology-based accelerometers.
2 LITERATURE REVIEW: Examines current earplug accelerometer technology, piezoresistive principles, and recent discoveries regarding the giant piezoresistance effect in silicon nanowires.
3 DESIGN, MODELLING AND OPTIMIZATION OF A BIO-MECHANIC PIEZORESISTIVE ACCELEROMETER WITH SILICON NANOWIRES: Details the design process, mathematical modelling, and the geometric optimization of the sensor structure using finite element analysis.
4 ELECTRICAL PERFORMACE OF NANOSCALE PIEZORESISTORS COMPARED TO CONVENTIONAL MICROSCALE PIEZORESISTORS: Compares performance metrics such as sensitivity, noise, and resolution between conventional microscale and new nanoscale piezoresistor designs.
5 INFLUENCE OF VARIATIONS IN THE MASS MOMENT OF INERTIA INTO THE PERFORMANCE OF A TRI-AXIAL PIEZORESISTIVE ACCELEROMETER: Investigates the impact of uniform mass moment of inertia on sensor accuracy and sensitivity through a parametric study.
6 PROPOSED OPTIMAL ACCELEROMETER DESIGN: Presents the final optimized accelerometer design derived from the insights gained in preceding chapters.
7 A FEASIBILITY STUDY ON THE FABRICATION OF SILICON NANOWIRES FOR NANOSCALE PIEZORESISTORS: Describes the experimental efforts and manufacturing processes, including FIB and CVD, used to fabricate the sensor test-chips.
8 CONCLUSION AND FUTURE WORK: Summarizes the key achievements and suggests future experimental directions for the further development of the proposed accelerometer.
Keywords
Piezoresistive accelerometer, Silicon nanowires, Nanoscale piezoresistors, MEMS, Miniaturization, Head injury monitoring, Giant Piezoresistance effect, Finite element analysis, Mass moment of inertia, Sensitivity, Cross-sensitivity, Sensor design, Biomechanical sensors.
Frequently Asked Questions
What is the core focus of this PhD thesis?
The thesis focuses on the design and optimization of a novel, miniaturized tri-axial piezoresistive accelerometer specifically tailored for earplug-based monitoring of head injuries in race car drivers.
What are the primary challenges addressed by this sensor design?
The design addresses the trade-off between miniaturization and sensitivity, specifically aiming to avoid the use of output amplifiers that increase noise and decrease measurement accuracy.
What is the main objective of the research?
The aim is to develop a 2x2 mm accelerometer with significantly improved sensitivity and reduced cross-sensitivity compared to existing state-of-the-art MEMS sensors.
What scientific technology is utilized for sensing?
The study utilizes silicon nanowires (SiNWs) as sensing elements, leveraging the "Giant Piezoresistance" effect to achieve higher sensitivity at the nanoscale.
What methodology is employed to improve the sensor performance?
The research uses finite element modelling (FEM) and an evolutionary optimization process to refine the mechanical structure, specifically targeting a uniform mass moment of inertia for better accuracy.
Which criteria are used to identify the optimal geometry?
Geometries are evaluated based on their ability to maximize sensitivity and bandwidth while adhering to strict miniaturization constraints required for in-ear comfort.
How does the proposed sensor handle extreme impact forces?
The design incorporates integrated overload end stops to prevent structural fracture when the device is subjected to acceleration forces exceeding 500g.
What is the significance of the "Giant Piezoresistance" effect mentioned?
This effect refers to a significantly enhanced resistance change under stress in nanoscale materials compared to bulk silicon, enabling the sensor's boost in performance without needing signal amplification.
- Quote paper
- Marco Messina (Author), 2013, Design and optimization of a novel tri-axial miniature ear-plug piezoresistive accelerometer with nanoscale piezoresistors, Munich, GRIN Verlag, https://www.grin.com/document/384295
