Development and analysis of high withstand voltage pressure sensors based on silicon strain gages-on-a-glass substrate

다국어 초록 (Multilingual Abstract)

A pressure sensor is a device for measuring pressure, one of the most commonly measured physical parameters in various fields, such as automotive, industrial, and household. In the heating, ventilation, and air-conditioning (HVAC) industry, pressure switches are being replaced by pressure sensors for continuous pressure monitoring, enabling more accurate control, faster responses, and more energy savings. Therefore, the development of a pressure sensor that meets special specifications, such as high withstand voltages and high sensitivity, is required. However, since it is not possible to meet the requirements with conventional attachment-type semiconductor strain gages, micro-electro-mechanical system-type (MEMS) pressure sensors with some disadvantages, such as very complex structures, low measuring pressures, and high manufacturing costs, are being used in the HVAC industry today.
In this thesis, I propose a new strain gage based on a glass substrate with high withstand voltages and sensitivity for the HVAC industry. The new strain gages were fabricated from MEMS processes based on silicon and glass anodic bonding. For the proposed gage fabrication, silicon is bonded in somewhat harsh conditions to glass with high dielectric properties and no alkali ions. In addition, the ion implantation for forming the desired impurity concentration was performed under different implantation conditions due to temperature constraints caused by the presence of the glass substrate. The strain gages were also fabricated in four thicknesses for various characteristic evaluations, such as chips misalignment, withstand voltages, and output characteristics depending on pressure and temperature. The gages were bonded using a glass-frit on a steel diaphragm.
The structural reliability and various characteristics of the proposed strain gage and its composed pressure sensor were properly evaluated by various types of devices. Anodic bonding of silicon and glass wafers showed a perfect bond of 98.7 % in an area of 8 inches and average bond strength of 8.7 MPa. The glass substrate introduced into the intermediate layer between the steel diaphragm and silicon strain gage enabled very reliable bonds without post-bond misalignment, debonding, or cracking through the compressive surface strengthening of the glasses, resulting in uniform, high glass-frit bond strength of 13.9 MPa. In addition, the pressure sensors consisting of proposed gages achieved a withstand voltage of >2.5 kVAC, which is about 5 times more than those of conventional gages. The reliable thin glass-frit bonding of a steel diaphragm and gage chip exhibited excellent output characteristics, such as a nonlinearity of 0.23 %FSO, hysteresis of 0.06 %FSO, and repeatability of 0.11 %FSO, as well as high sensitivity at levels of 0.37 to 0.46 mV/V/bar, depending on the thicknesses of the strain gage chips. A calibrated pressure sensor using ASIC guaranteed 0.3 % accuracy over the temperature range from -40 to 125 ℃. In addition, 1 million iterated tests for fatigue life evaluation showed that the linear outputs of the sensors had maintained well with offset and sensitivity errors of ±0.3 %.

목차 (Table of Contents)

• ABSTRACT OF THE THESIS i
• CONTENTS iv
• LIST OF FIGURES vii
• LIST OF TABLES xiv
• NOMENCLATURE xvi
• Chapter 1 INTRODUCTION 1
• 1.1 MOTIVATION AND LIMITATIONS OF RESEARCH 1
• 1.2 OBJECTIVE 5
• 1.3 OUTLINE OF THE THESIS 7
• Chapter 2 REVIEW OF THE STRAIN GAGE THEORY 8
• 2.1 HISTORY OF STRAIN GAGE SENSORS 8
• 2.2 BASIC OF PIEZORESISTANCE 11
• 2.3 NOTATION 13
• 2.3.1 Miller Indices and Crystal Structure 13
• 2.3.2 Stress, Strain, and Tensors 15
• 2.3.3 Piezoresistance 17
• 2.4 PIEZORESISTIVE THEORY 21
• 2.5 DESIGN AND PROCESS EFFECTS ON PIEZORESISTOR PERFORMANCE 25
• 2.5.1 Temperature Coefficients of Sensitivity and Resistance 25
• Chapter 3 EXPERIMENTAL DETAILS 29
• 3.1 CONCEPT AND DESIGN OF THE STRAIN GAGE 29
• 3.1.1 Concept of Strain Gage with High Withstand Voltage 29
• 3.1.2 Strain Gage Material 31
• 3.1.3 Ion Implantation Simulation 32
• 3.1.4 Mask Design of Strain Gage 35
• 3.2 STRAIN GAGE FABRICATION 37
• 3.3 PACKAGING OF PRESSURE SENSOR 56
• 3.3.1 FEM Simulation of Strain Distribution on Diaphragm 56
• 3.3.2 Decision of Glass-frit Bonding Process 58
• 3.3.3 Assembly of the Pressure Sensor 62
• Chapter 4 RESULTS AND DISSCUSSION 66
• 4.1 EVALUATION OF ANODIC BONDING QUALITY 66
• 4.2 EVALUATION OF GLASS-FRIT BONDING 72
• 4.2.1 Die Shear Test 72
• 4.2.2 Misalignment of Strain Gages 75
• 4.2.3 Thermal Stress on Bonded Gages 82
• 4.3 HIGH POTENTIAL VOLTAGE TEST 86
• 4.4 TEST SYSTEM SETUP 95
• 4.5 CHARACTERISTICS OF SILICON STRAIN GAGE 96
• 4.5.1 Output Characteristics in Room Temperature 96
• 4.5.2 Rate of Change in Resistance with Temperature 98
• 4.6 PRE-CALIBRATION OUTPUT CHARACTERISTICS OF PRESSURE SENSOR 100
• 4.6.1 Output Characteristics in Room Temperature 101
• 4.6.2 Temperature Characteristics 103
• 4.6.3 Nonlinearity 109
• 4.6.4 Hysteresis 111
• 4.6.5 Reliability 112
• 4.7 CALIBRATION OF PRESSURE SENSOR 116
• 4.8 OUTPUT CHARACTERISTICS OF CALIBRATED PRESSURE SENSOR 120
• 4.8.1 Output Voltage at Various Temperature 120
• 4.8.2 Output Errors of Calibrated Pressure Sensor 123
• 4.8.3 Offset and Span Drift Characteristics of Calibrated Sensors 125
• 4.8.4 Hysteresis of Calibrated Sensors 127
• 4.8.5 Nonlinearity of Calibrated Sensors 129
• 4.8.6 Repeatability 131
• Chapter 5 CONCLUSIONS AND FUTURE WORKS 133
• References 136