Development of SOI MEMS-based silicon strain gauges for fuel cell electric vehicle

다국어 초록 (Multilingual Abstract)

Hydrogen gas is attracting much attention as a next-generation fuel to replace fossil fuels that have a problem of resource depletion and environmental pollution. The reason is that when hydrogen gas is used in fuel cells, it is an environmentally friendly material that is more efficient than gasoline vehicles and can reduce carbon emissions. A hydrogen fuel cell vehicle with these advantages has a built-in fuel tank that stores hydrogen, and a stable pressure sensor is essential for this fuel tank. In other words, a high-sensitivity silicon strain gauge is indispensable to constitute a pressure sensor system capable of continuously monitoring the loading state of hydrogen gas in the fuel cell automobile sector, which mainly uses hydrogen gas. The recently commercialized hydrogen pressure sensor diaphragm is made of stainless steel 316L to prevent hydrogen embrittlement. This material has a larger coefficient of thermal expansion than any other material. Therefore, it is very difficult to completely attach the strain gage chip to this diaphragm without any post-misalignments such as break, rotation and movement. It is very important that the gauge is attached to the metal diaphragm because the output performance of the pressure sensor, such as durability and reliability, depends on the quality of the silicon strain gauge. Many post-misalignments occur due to the stress caused by the coefficient of thermal expansion (CTE) mis-match between the substrate and the diaphragm of the conventional gauge. Open-type silicon strain gages have been commercialized to address this problem, but it is much more difficult to automate the alignment and bonding processes because the gages are open and fragile, requiring more manufacturing cost and time.
In this thesis, I present a new half-bridge silicon strain gauge fabricated on a silicon-on-insulator (SOI) substrate by MEMS bulk micromachining technology that can compromise the problems presented above. These gauges have holes etched through the wafer by deep reactive ion etching (DRIE) and a closed shape with four sides, unlike the current competitive devices with open structures. This unique design minimizes the shifting or gating position and enhances the bonding strength during glass-frit bonding, leading to improved sensor performance and yield, and thus a reduction in sensor cost. In addition, the ratio of the area of the through hole to the total area of the chip is optimized based on the results of the post-misalignments test using gauges having various through hole ratios, and an asymmetric gauge for improving the sensitivity is presented. In order to demonstrate the feasibility of using a hydrogen fuel cell pressure sensor, the prototype half-bridge gages were tested under pressure ranging from 0 bar to 900 bar and showed a linear output with a typical gage factor of about 112 and an average hysteresis of 0.0192 %FSO. In addition, the full bridge output for 0-900 bar shows a typical sensitivity of about 0.0086 mV/V/bar, a maximum thermal zero shift of -3.1 %FSO, and a thermal sensitivity shift of -15.12 %FSO.

목차 (Table of Contents)

• ABSTRACT OF THE DISSERTATION i
• CONTENTS iv
• Chapter 1 INTRODUCTION 1
• 1.1 MOTIVATION 1
• 1.2 CHALLENGES AND LIMITATIONS 4
• 1.3 OBJECTIVE 8
• 1.4 OUTLINE OF THE DISSERTATION 9
• Chapter 2 BACKGROUND THEORY OF THE STRAIN GAUGE 10
• 2.1 BRIEF HISTORY OF STRAIN GAUGE 10
• 2.2 PIEZORESISTIVE EFFECT 13
• 2.3 STRESS AND STRAIN RELATION FOR ANISOTROPIC MATERIALS 15
• 2.4 PIEZORESISTIVITY 17
• Chapter 3 EXPERIMENTAL DETAILS 19
• 3.1 CONCEPT AND DESIGN OF THE SILICON STRAIN GAUGE 19
• 3.1.1 Concept of the Silicon Strain Gauge with Through-holes 19
• 3.1.2 Design of Silicon Strain Gauge with Different Area Ratio 21
• 3.1.3 Design of Asymmetrical Gauge 23
• 3.1.4 Ion Implantation Simulation 26
• 3.2 SILICON STRAIN GAUGE FABRICATION 28
• 3.2.1 Fabrication Process 28
• 3.2.2 Back Grinding and Chip Dicing Process 43
• 3.3 PACKAGING OF PRESSURE SENSOR 48
• 3.3.1 FEM Simulation of Diaphragm 48
• 3.3.2 Glass-frit Bonding Process 50
• 3.3.3 Signal Conditioning Amplifier 54
• 3.3.4 Assembly of the Pressure Sensor 57
• Chapter 4 RESULTS AND DISSCUSSION 63
• 4.1 GLASS-FRIT BONDING CHARACTERISTICS OF SILICON STRAIN GAUGE WITH THROUGH-HOLES 63
• 4.1.1 Misalignment Characteristics of Silicon Strain Gauge 63
• 4.1.2 Shear Strength Test 68
• 4.2 RESISTANCE CHARACTERISTICS OF SILICON STRAIN GAUGE 70
• 4.2.1 Variation of Gauge Resistance by Apparent Strain 70
• 4.2.2 Temperature Coefficient of Resistance 73
• 4.2.3 Symmetrical Gauge Resistance Change Rate as a Function of Pressure 75
• 4.2.4 Asymmetrical Gauge Resistance Change Rate as a Function of Pressure 78
• 4.3 PRE-CALIBRATION DATA AT ROOM TEMPERATURE 80
• 4.3.1 Output Characteristic of Symmetrical Silicon Strain Gauge 80
• 4.4 PRE-CALIBRATION DATA WITH DIFFERENT TEMPERATURE 81
• 4.4.1 Output Characteristics as a Function of Pressure 81
• 4.4.2 Offset Drift with Different Temperature 83
• 4.4.3 Span Drift with Different Temperature 85
• 4.4.4 Non-linearity 86
• 4.4.5 Hysteresis 88
• 4.4.6 Thermal Hysteresis 90
• 4.5 POST-CALIBRATION DATA 93
• 4.5.1 Output Voltage as a Function of the Pressure at Various Temperature 93
• 4.5.2 Offset Drift and Span Drift 94
• 4.5.3 Non-linearity 96
• 4.5.4 Accuracy 98
• Chapter 5 CONCLUSIONS AND FUTURE WORKS 100
• References 102