(A) study on experimental design for enhancing the reliability of in situ (S)TEM : effects of electron beam, pressure and specimen thickness

국문 초록 (Abstract)

전자현미경(EM)을 이용한 재료 분석에서, ex situ TEM은 대기압 조건에서 처리된 시편을 관찰하는 반면, in situ TEM은 외부 바이어스를 인가한 상태에서 고진공 조건하의 박막 시편을 실시간으로 관찰할 수 있다. 압력, 시편 두께, 전자 빔 조사 조건을 포함한 실험 조건의 근본적인 차이로 인해, 이 두 방법은 동일한 재료에 대해서도 현저히 다른 결과를 도출할 수 있으며, 이는 분석 결과의 해석과 신뢰성에 영향을 미친다.
본 학위논문에서는 이러한 차이를 유발하는 핵심 요인인 전자 빔 조사 조건, 시편 두께, 그리고 바이어스 인가 시의 주변 압력 조건을 체계적으로 분리하여 분석함으로써, 그로 인해 발생하는 artifact를 규명하고자 하였다. 또한 이러한 차이를 최소화하고 ex situ TEM과 in situ TEM 결과 간의 일관성을 향상시키기 위한 실험 설계 전략을 제안하였다.
이를 위해 먼저 GeTe 벌크 시편을 대기(Air), 아르곤(Ar), 고진공 조건에서 450 ℃로 열처리하여 산화, Ge–Te 상변환, 선택적 승화 거동을 비교함으로써, 동일한 재료라도 압력 조건 및 관찰 위치(벌크 내부 vs 표면)에 따라 전혀 다른 결과가 나타남을 보였다. 이어서 in situ (S)TEM을 진행하여 GeTe 시편에 대해 200 kV/80 kV CTEM과 200 kV STEM 조건에서 가속 전압과 dose rate에 따라 스퍼터링 현상 및 Te의 선택적인 승화 현상을 확인하였다. 또한 ZnO/SnO2 (70:30 at%) 기반 ZTO/Al 구조의 소자를 대상으로 ex situ heating TEM과 in situ heating TEM 결과를 비교한 결과, ex situ 조건에서는 ZTO/Al 계면 국부에서 Zn2SnO4 결정화와 Al의 산화가 관찰된 반면, in situ heating TEM에서는 ZTO 모든 층에서 Zn2SnO4 결정화와 Zn 승화에 따른 결핍 및 Sn-rich 영역이 확인되었다. 이는 본질적으로 상이한 최종 상태가 관찰되는 것을 확인하였다. 마지막으로, 이러한 결과를 바탕으로 in situ 가열 TEM 실험 설계를 통해 ZTO/Al 구조의 소자를 사례로 ex situ TEM과 보다 일치하는 결과를 도출하였다.
본 연구는 ex situ TEM과 in situ TEM을 상호 보완적으로 활용하면서도 재현성 높고 신뢰성 있는 결론을 도출하기 위한 실험 설계 기준을 제공한다.

다국어 초록 (Multilingual Abstract)

In materials analysis using electron microscopy (EM), ex situ TEM examines specimens processed under atmospheric pressure, whereas in situ TEM enables real time observation of thin foil specimens under high vacuum conditions with applied external biases. Owing to fundamental differences in experimental conditions including pressure, specimen thickness, and electron beam irradiation these two methods can produce markedly different results even for the same material, thereby affecting the interpretation and reliability of the analysis.
In this dissertation, the key factors responsible for these discrepancies electron beam irradiation conditions, specimen thickness, and ambient pressure during bias application were systematically isolated and analyzed to identify the resulting artifacts, and experimental design strategies were proposed to minimize these differences and improve consistency between ex situ and in situ TEM results. To this end, bulk GeTe specimens were first heat treated at 450 ℃ under Air, Argon, and high vacuum conditions to compare oxidation, Ge–Te phase transformation, and selective sublimation behavior, demonstrating that even the same material can exhibit entirely different outcomes depending on the pressure conditions and the observation location (bulk interior vs. surface). Subsequently, in situ (S)TEM experiments were conducted on GeTe specimens, confirming electron beam induced sputtering and selective sublimation of Te under 200 kV/80 kV CTEM and 200 kV STEM conditions, depending on the accelerating voltage and dose rate. In addition, when comparing the ex situ heating TEM and in situ heating TEM results for a ZnO/SnO2 (70:30 at%)based ZTO/Al device, localized Zn2SnO4 crystallization and Al oxidation were observed at the ZTO/Al interface under ex situ conditions, whereas in in situ heating TEM, Zn2SnO4 crystallization occurred throughout all ZTO layers, accompanied by Zn depletion due to sublimation and the formation of Sn rich regions. This confirms that the final states observed in ex situ TEM and in situ TEM are fundamentally different. Finally, through the design of an in situ heating TEM experiment, results more consistent with ex situ TEM were obtained using the ZTO/Al device as a case study. This study provides experimental design criteria for achieving reproducible and reliable conclusions while using ex situ TEM and in situ TEM in a complementary manner.

목차 (Table of Contents)

• Chapter 1. INTRODUCTION 1
• 1.1. Material analysis method using Transmission electron microscopy 1
• 1.1.1. Ex situ transmission electron microscopy 1
• 1.1.2. In situ transmissions electron microscopy 3
• 1.2. Objective of the thesis 5
• 1.3. Organization of the thesis 8
• Chapter 2. Literature review and theoretical background 10
• 2.1. In situ (Scanning)Transmission Electron Microscopy 10
• 2.2. E-beam irradiation effects during TEM observation 11
• 2.2.1. Knock-on damage 13
• 2.2.2. Radiolysis 15
• 2.2.3. Heating 16
• 2.3. Sample size effects 18
• 2.3.1. Grain size and particle size 18
• 2.3.2. sample dimensions 21
• 2.4. pressure effects 24
• 2.4.1. Sublimation 26
• 2.4.2. Clausius-Clapeyron equation 28
• Chapter 3. Experimental procedures 29
• 3.1. Specimen preparation for SEM analysis 29
• 3.1.1. Modified SEM holder and Furnace 29
• 3.2. Specimen preparation for TEM analysis 32
• 3.2.1. Conventional Focused Ion Beam (FIB) 32
• 3.2.2. Method for preparing TEM samples using FIB 36
• 3.2.3. Method for preparing TEM samples for in situ heating using FIB 38
• 3.2.4. Sample preparation using mechanical polishing method 40
• 3.3. Transmission electron microscopy 42
• 3.3.1. Conventional Transmission electron microscopy 42
• 3.3.2. Scanning Transmission electron microscopy 44
• 3.3.3. In situ Transmission Electron Microscopy 45
• Chapter 4. Understanding Specimen dimensions and pressure in ex situ analysis and in situ analysis 49
• 4.1. Introduction 49
• 4.2. Experimental procedures 54
• 4.2.1. Sample preparation 54
• 4.3. Results and discussion 56
• 4.3.1. Comparison of the GeTe Specimen Before heating (Initial state) and After heating (Final state) 56
• 4.3.2. Initial state sample (Reference sample) 58
• 4.3.3. Atmosphere pressure (Air condition) 60
• 4.3.4. Atmosphere pressure (Argon condition) 62
• 4.3.5. High vacuum 64
• 4.3.6. X-ray diffraction (XRD) 69
• 4.4. Summary 72
• Chapter 5. Electron Beam and Vacuum Effects on GeTe During in situ heating TEM: Sputtering and Sublimation Artifacts 73
• 5.1. Introduction 73
• 5.2. Experimental procedures 74
• 5.2.1. Sample preparation 74
• 5.2.2. In situ heating (S)TEM 76
• 5.3. Results and Discussion 78
• 5.3.1. 200 kV Conventional Transmission Electron Microscopy 78
• 5.3.2. 80 kV Conventional Transmission Electron Microscopy 83
• 5.3.3. 200 kV Scanning Transmission Electron Microscopy 89
• 5.4. Summary 92
• Chapter 6. Comparative analysis of ex situ TEM results and in situ TEM results 94
• 6.1. Introduction 94
• 6.2. Experimental procedures 97
• 6.2.1. Device Fabrication 97
• 6.2.2. Sample preparation 98
• 6.2.3. in situ heating TEM 98
• 6.3. Results and discussion 100
• 6.3.1. Initial state sample (Reference sample) 100
• 6.3.2. Characterization of ex situ heating TEM sample 102
• 6.3.3. Characterization of in situ heating TEM samples 104
• 6.4. Summary 115
• Chapter 7. Modified in situ heating TEM 117
• 7.1. Introduction 117
• 7.2. Experimental procedures 119
• 7.3. Results and discussion 120
• 7.3.1. ZTO/Al system 120
• 7.4. Summary 132
• Chapter 8. Conclusion 133
• 8.1. Conclusion 133
• References 137