Design and Fabrication of a Highly Sensitive Hydrogen Gas Sensor Based on Pd/SnO2 Thin Film with Integrated Microheater = 집적형 마이크로히터가 적용된 Pd/SnO2 박막 기반 고감도 수소 가스 센서의 설계 및 제작

다국어 초록 (Multilingual Abstract)

Hydrogen is widely utilized as a key energy source in fuel cell vehicles and hydrogen energy systems. However, because hydrogen is colorless, odorless, and has a wide explosive range, leakage can cause serious safety hazards. Therefore, the development of gas sensors with high sensitivity and reliability that can detect hydrogen leaks at an early stage before reaching hazardous concentrations is essential. In particular, for applications in vehicles and confined spaces, compact sensor technologies capable of reliably detecting low concentration hydrogen at the parts per million level are required.
In this study, a microheater integrated hydrogen gas sensor based on a sputter deposited SnO₂ thin film and a Pd catalytic layer was designed and fabricated, and its sensing characteristics were systematically analyzed. The sensor structure incorporates a Mo based microheater, electrodes, and a multilayer insulation structure to maximize thermal efficiency, enabling stable operation in the temperature range from 270 degrees Celsius to 300 degrees Celsius with relatively low power consumption. In addition, the electrical properties and repeatability of the sensing layer were improved by optimizing the oxygen partial pressure during SnO₂ deposition and the post annealing conditions.
The Pd catalytic layer effectively modulates the electron depletion layer at the SnO₂ surface through hydrogen dissociation and spill over effects, resulting in a significant change in sensing resistance upon hydrogen exposure. The sensor response to hydrogen concentration, defined as R_air divided by R_gas, was found to follow a power law relationship of the form S proportional to C to the power of n, where the exponent n quantitatively represents the sensitivity of the sensor to concentration changes in the low concentration region. The extracted n values exhibited relatively large magnitudes even at low hydrogen concentrations, indicating excellent response amplification characteristics for subtle concentration variations.
An automated measurement system was established to perform repeated measurements and statistical analyses, and the limit of detection was quantitatively estimated based on the nonlinear relationship between the resistance change and hydrogen concentration observed in the low concentration region. Considering the baseline noise of sigma equal to 24.1 ohms and the concentration dependent slope of the resistance change of 11.27 ohms per part per million, the limit of detection was evaluated to be approximately 6.41 parts per million. This result indicates reliable signal discrimination even in the low concentration hydrogen regime and satisfies the performance requirements for safety and automotive hydrogen leak detection.
In this work, a new performance metric, referred to as the Figure of Merit, was introduced by combining the response magnitude and the concentration dependence through the n value. The proposed Figure of Merit enables a comprehensive evaluation of detection resolution and practical sensor performance in the low concentration region. Comparison with previously reported microheater integrated metal oxide hydrogen sensors clearly demonstrates the superior quantitative performance of the proposed sensor.
Overall, the results of this study demonstrate that the Pd SnO₂ based thin film sensor combined with a Mo microheater and electrodes can simultaneously achieve high sensitivity, a low limit of detection, and an excellent Figure of Merit for low concentration hydrogen detection. These findings highlight the potential of the proposed device as a practical hydrogen sensor platform for applications in fuel cell vehicles, hydrogen refueling infrastructure, and safety monitoring systems in confined spaces.

국문 초록 (Abstract)

수소는 연료전지 차량 및 수소 에너지 시스템의 핵심 에너지원으로 활용되고 있으나, 무색·무취이며 폭발 한계 범위가 넓은 특성으로 인해 누설 시 심각한 안전 문제를 야기할 수 있다. 따라서 수소 누설을 위험 농도에 도달하기 이전 단계에서 조기에 감지할 수 있는 고감도 및 고신뢰성 가스 센서의 개발이 필수적이다. 특히 차량 및 밀폐 공간 응용을 고려할 때, 저농도(ppm 수준) 수소를 안정적으로 감지할 수 있는 소형 센서 기술이 요구된다.
본 논문에서는 스퍼터링으로 증착한 SnO₂박막과 Pd촉매층을 기반으로 한 마이크로히터 집적형 수소 가스 센서를 설계 및 제작하고, 그 감지 특성을 체계적으로 분석하였다. 센서 구조에는 Mo기반 마이크로히터와 전극, 그리고 다층 절연 구조를 적용하여 열 효율을 극대화하였으며, 이를 통해 비교적 낮은 소비 전력으로 270 °C에서 300 °C 범위의 안정적인 동작 온도를 구현하였다. 또한 SnO₂박막의 산소 분압 제어 및 후열처리 조건을 최적화하여 감지층의 전기적 특성과 반복 재현성을 향상시켰다.
Pd 촉매층은 수소의 해리 및 spill over 효과를 통해 SnO₂표면의 전자 고갈층을 효과적으로 변조하였으며, 그 결과 수소 노출 시 감지 저항의 유의미한 변화가 관찰되었다. 수소 농도에 대한 센서 응답(R_air/R_gas)은 전력 법칙 형태의 관계(S∝Cn)를 따르는 것으로 분석되었으며, 여기서 지수 n 값은 저농도 영역에서의 농도 변화에 대한 센서의 민감도를 정량적으로 나타낸다. 본 연구에서 도출된 n 값은 저농도 영역에서도 비교적 큰 값을 나타내어, 미세 농도 변화에 대해서도 우수한 응답 증폭 특성을 가짐을 확인하였다.
자동화된 측정 시스템을 구축하여 반복 측정 및 통계적 분석을 수행하였으며, 저농도 영역에서 관찰된 ΔR과 농도 간 비선형 응답 특성을 기반으로 최소 검출 한계(LOD)를 정량적으로 산출하였다. 기저 노이즈(σ = 24.1 Ω)와 ΔR의 농도 기울기(11.27 Ω/ppm)를 고려한 통계적 분석 결과, 최소 검출 한계는 약 6.41 ppm으로 평가되었다. 이는 저농도 수소 영역에서도 신뢰성 있는 신호 분별이 가능함을 의미하며, 안전 및 차량용 수소 누설 감지 요구 조건을 만족하는 성능이다.
본 연구에서는 감지 응답의 크기뿐만 아니라 농도 의존성을 동시에 반영하기 위해 n 값과 응답 특성을 결합한 새로운 성능 지표인 Figure of Merit(FOM)를 도입하였다. 제안된 FOM은 저농도 영역에서의 검지 분해능과 실질적인 센서 성능을 종합적으로 평가할 수 있는 지표로, 기존 문헌에 보고된 마이크로히터 집적형 금속 산화물 수소 센서들과의 비교를 통해 본 센서가 우수한 정량적 성능을 가짐을 명확히 보여준다.
본 논문의 결과는 Pd/SnO₂기반 박막 센서와 Mo마이크로히터 및 전극을 결합한 구조가 저농도 수소에 대해 높은 민감도와 낮은 검출 한계, 그리고 우수한 FOM을 동시에 달성할 수 있음을 입증하며, 향후 연료전지 차량, 수소 충전 인프라, 밀폐 공간 안전 모니터링 시스템에 적용 가능한 실용적 수소 센서 플랫폼으로서의 가능성을 제시한다.

목차 (Table of Contents)

• Abstract i
• Table of Contents iv
• List of Tables vii
• List of Figures viii
• Chapter 1. Introduction 1
• 1.1 Research Background 1
• 1.1.1 Hydrogen Energy and Safety 1
• 1.1.2 Hydrogen Sensor Market by Technology 2
• 1.1.3 Importance of Gas Sensors 3
• 1.2 Requirement of Hydrogen Leak Detection in Fuel Systems 4
• 1.2 Motivation 7
• 1.3 Hydrogen Sensing Mechanisms of Metal-Oxide Semiconductors 8
• 1.3.1 Classification of Gas Sensors 8
• 1.3.2 Hydrogen Sensing Mechanisms 11
• 1.2 Challenges and Limitations of Existing Hydrogen Sensors 13
• 1.4 Proposed Approach 16
• 1.5 Research Objectives 17
• 1.7 Organization of This Dissertation 18
• Chapter 2. Fundamental Theory of SnO₂-Based Hydrogen Sensing 20
• 2.1 Fundamentals of Hydrogen Gas Sensing 20
• 2.2 Operation Principle of Metal-Oxide Semiconductor (MOS) Sensors 21
• 2.3 Selecting SnO₂ as the Sensing Layer 27
• 2.4 Selecting Mo as the Electrode Material 30
• 2.5 Catalyst Selection for Hydrogen Dissociation 35
• 2.6 Porous Structure of SnO₂ 39
• 2.7 Operating Temperature of SnO₂-Based Hydrogen Sensors 41
• 2.8 Selecting the Vertical (Underneath) Micro-Heater Structure 45
• 2.9 Summary 50
• Chapter 3. Design of the Hydrogen Sensor 52
• 3.1 Structural Design of Hydrogen Detection Sensor 52
• 3.2 Operating Mechanism of the Pd/SnO₂ Hydrogen Sensor 55
• 3.2 Fabrication Process of Hydrogen Sensor 61
• 3.3 Design of Heater 65
• 3.4 Layout and Photomask 69
• 3.5 Summary of Design 73
• Chapter 4. Fabrication 75
• 4.1 Detailed Fabrication Process 75
• 4.1.1 Silicon wafer preparation (Step 0–2) 75
• 4.1.2 SiO₂ insulation and first Mo layer (MET1, Steps 10, 20–24) 75
• 4.1.3 Patterning of PECVD SiO₂ insulation (ISO, Steps 30–33) 76
• 4.1.4 Patterning and lift-off of SnO₂ sensing layer (MOX, Steps 40–43) 77
• 4.1.5 Second Mo metallization (MET2, Steps 50–52) 78
• 4.1.6 Pd catalyst deposition and annealing (CAT, Steps 60–63) 79
• 4.1.7 Backside cavity formation (THS, Steps 70–75) 79
• 4.1.8 Dicing and chip preparation (Step 80) 80
• 4.2 Reactive Sputtering Process of SnO₂ 84
• 4.2.1 Influence of Previous Runs on Target Surface State 84
• 4.2.2 Transient I–V Behavior at the Beginning of Each Run 85
• 4.2.3 Stabilization of the Plasma (≈15 minutes, gun shutter open) 86
• 4.3 Steady-State Discharge Characteristics as Oxygen Concentration 87
• 4.4 Optimal O₂ Concentration for SnO₂ Hydrogen Sensors 88
• 4.5 Grain Size Analysis of SnO₂ Thin Film 91
• 4.6 XPS Analysis of SnO₂ Before and After Annealing 93
• 4.6.1 XPS O 1s Analysis Before and After Annealing 94
• 4.7 Optical micrographs of fabrication steps 99
• Chapter 5. Measurement 100
• 5.1 Sensor Mounting and Wire Bonding Process 101
• 5.2 Measure the Resistance of the Built-in Temperature Sensor 103
• 5.3 Thermal Camera Emissivity Calibration Using the Built-in Temperature Sensor 106
• 5.4 Measurement of Joule Heating Performance 108
• 5.5 Experimental Setup for Gas-Controlled Hydrogen Sensing 110
• 5.6 Automated Measurement and Temperature-Control System Using USB–VISA Protocol 113
• 5.7 Effect of Bias Voltage and Operating Temperature on Hydrogen-Sensing Performance 116
• 5.8 Stabilization of Baseline Resistance via Initialization and Pre-Oxidation Protocol 120
• 5.8.1 Response and Recovery Time 122
• 5.8.2 Response (Sensitivity) 124
• 5.8.3 Limit of Detection (LOD) 126
• 5.9 Figure of Merit (FOM) Definition and Formulation 129
• 5.9.1 Limitation of Conventional FOM 129
• 5.9.2 Model-Based Redefinition of FOM 129
• 5.9.3 FOM of This Works 130
• 5.10 Summary 132
• Chapter 6. Conclusion 139
• Bibliography 143
• Appendix A. Measurement Python Code 152
• 초록 (국문) 162