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Study on Low-Temperature Crystallization of Hafnia Based Thin Films via Exhalative Annealing for Non-Volatile Memory Applications

  • Yunseong Lee (이윤성, 대학원 신소재공학과 신소재공학전공)

초록/요약

While hafnia-based ferroelectric thin films are considered promising for next-generation non-volatile memory (NVM) technologies, their integration into mainstream semiconductor devices has been hindered by the high thermal budgets required for crystallization. Traditionally, inducing the orthorhombic phase in Zr-doped hafnia (Hf0.5Zr0.5O2, HZO) necessary for ferroelectric behavior—necessitated rapid thermal annealing (RTA) at temperatures exceeding 500 ℃. However, such conditions are incompatible with advanced CMOS processes, especially for three-dimensional (3D) device structures, where thermal constraints are significantly tighter. Excessive heating also leads to undesirable effects such as interfacial oxide regrowth and increased defect densities, ultimately degrading device performance. This study introduces a vacuum-based process called exhalative annealing (EA), which enables low-temperature crystallization of HZO films while effectively suppressing residual carbon and oxygen vacancies. EA uses vacuum-assisted degassing to promote phase formation at significantly reduced temperatures (down to 200–250 ℃), even in ultrathin films as thin as 5 nm. Characterization results confirmed that EA-processed HZO layers not only exhibit clear ferroelectric properties but also demonstrate enhanced material quality through minimized contamination and interface disruption. These improvements are attributed to the ability of EA to mitigate carbon-related phase boundary pinning and suppress thermal diffusion, thereby stabilizing the ferroelectric orthorhombic phase. The approach was further validated through the fabrication of FeFETs (ferroelectric field-effect transistors) on 8-inch silicon-on-insulator (SOI) wafers, demonstrating that EA is scalable and manufacturable. Compared with conventional RTA-based processes, EA-processed devices showed a 33% improvement in memory window, thinner interfacial oxide layers (reduced by over 50%), and stable performance across endurance and retention tests. The remanent polarization (2Pr) and coercive field (2Ec) values also met or exceeded expectations for reliable operation in embedded memory applications. From a manufacturing standpoint, EA offers practical advantages. It can be integrated into existing vacuum-based thermal processing systems without the need for extensive equipment modifications, thereby reducing barriers to industrial adoption. Additionally, the process exhibits excellent uniformity and repeatability across large wafers, aligning well with the stringent yield and reliability standards of commercial semiconductor fabrication. Crucially, EA’s benefits are not limited to memory applications. The technique's low thermal budget and defect-suppression characteristics make it equally suitable for low-power Internet of Things (IoT) systems, neuromorphic computing elements, and robust aerospace and automotive electronics. In these domains, where thermal sensitivity and long-term reliability are paramount, EA presents a viable route for integrating ferroelectric functionality without compromising device integrity. In summary, exhalative annealing bridges a longstanding gap in ferroelectric device engineering by enabling high-performance HZO films to be processed at industry-friendly temperatures. Its compatibility with existing fabrication ecosystems and wide-ranging applicability underscore its potential to become a key enabler for next-generation memory and logic architectures.

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목차

ABSTRACT i
국문 초록 iv
PREFACE vii
ACKNOWLEDMENTS viii
TABLE OF CONTENTS ix
LIST OF TABLES xii
LIST OF FIGURES xiii
NOMENCLATURE xix
CHAPTER 1. INTRODUCTION 1
CHAPTER 2. LITERATURE SURVEY 6
2.1. Fundamentals of Ferroelectric 6
2.1.1. Ferroelectricity: Definition and Principles 6
2.1.2. Phase Transition in Hafnia-Based Thin Films 11
2.1.3. Crystal Structures of Hafnia 14
2.1.4. Symmetry-Breaking Mechanisms and Phase Evolution 16
2.1.5. Crystallization Behavior of Hafnia Thin Films 20
2.1.6. Dopant Effects on Ferroelectric Phase Stabilization 22
2.2. Electrical Properties of Ferroelectrics 23
2.2.1. Polarization Characteristics 23
2.2.2. Coercive Field (Ec) and Switching Behavior 24
2.2.3. Wake-Up Effect and Internal Field Evolution 27
2.2.4. Endurance and Fatigue Reliability 29
2.2.5. Imprint Phenomena and Retention Loss 31
2.3. Crystallization methods 33
2.3.1. Rapid Thermal Annealing (RTA) 33
2.3.2. Furnace Annealing 33
2.3.3. Microwave Annealing (MA) 34
2.3.4. Laser Annealing 34
2.3.5. Exhalative Annealing (EA) 35
2.4. Deposition methods 37
2.4.1. Physical Vapor Deposition: Sputtering 37
2.4.2. Atomic Layer Deposition (ALD) 39
2.4.3. Chemical Vapor Deposition (CVD) 41
2.4.4. Pulsed Laser Deposition (PLD) 43
2.5. Ferroelectric Degradation Mechanisms 45
2.5.1. Incomplete Precursor Decomposition 45
2.5.2. Oxygen Vacancy (Formation) 46
2.5.3. Interfacial Layer (Regrowth) 49
2.5.4. Residual Mechanical Stress 51
2.5.5. Impurity Incorporation and Trap Formation 53
2.6. Device Applications of Ferroelectric Thin Films 55
2.6.1. Ferroelectric Capacitors (FeCap) 55
2.6.2. Negative Capacitance FETs (NC-FETs) 57
2.6.3. Ferroelectric Tunnel Junctions (FTJs) 59
2.6.4. Ferroelectric Field-Effect Transistors (FeFETs) 61
2.6.5. FeFET-Based NAND Structures (FeNAND) 61
CHAPTER 3. EXPERIMENTAL PROCEDURE 63
3.1. Fabrication of Ferroelectric HZO Thin Films 63
3.1.1. Substrate Preparation 63
3.1.2. Bottom Electrode Deposition 63
3.1.3. Deposition of Hafnia based Ferroelectrics 63
3.1.4. Top Electrode Deposition and Patterning 66
3.2. Low-Temperature Crystallization Process 67
3.2.1. Rapid Thermal Annealing (RTA) Process 67
3.2.2. Exhalative Annealing (EA) Process 67
3.2.3. Crystallinity Comparison using XRD and TEM 67
3.3. Defect and Interface Characterization Methods 69
3.3.1. X-ray Photoelectron Spectroscopy (XPS) Analysis 69
3.3.2. Ar-Gas Cluster Ion Beam (GCIB) Analysis 70
3.3.3. Electrical Measurements 72
CHAPTER 4. RESULT AND DISCUSSIONS 74
4.1. Low-Temperature Crystallization via Exhalative Annealing 74
4.1.1. Thermal Budget Comparison: EA vs RTA 74
4.1.2. Thickness-dependent Crystallization Temperature (Tcryst) 76
4.1.3. X-Ray Diffraction Analysis of Phase Evolution 78
4.1.4. Polarization Characteristics (2Pr, 2Ec) 80
4.2. Defect Suppression by Exhalative Annealing Process 83
4.2.1. XPS Analysis: Carbon Residue and Oxygen Vacancy 83
4.2.2. Impact on Leakage Current and Endurance 86
4.2.3. Interface Quality (XPS-Based Chemical Stability Analysis) 88
4.3. Memory Device Application 90
4.3.1. Memory Window (MW) Improvement 90
4.3.2. Interfacial Layer Control in FeFET Structures 92
4.3.3. Comparison with RTA-based Devices 94
CHAPTER 5. CONCLUSION 96
5.1. Exhalative Annealing for Crystallization and Defect Control 96
5.2. FeFET Integration and Device-Level Implications 98
REFERENCES 100

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