수직 크로스바 어레이 메모리 아키텍처를 위한 칼코게나이드 필름의 원자층 증착

학위논문(박사) -- 서울대학교대학원 : 공과대학 재료공학부, 2024. 2. 황철성.In the era of data-centric technology, the cost per bit has become crucial for memory product development. Despite its technological maturity, Phase-Change Memory (PCM) has not yet met these market demands. Vertical Crossbar Array (V-CBA) technology, similar in concept to the successful V-NAND flash memory, attracts attention for its ability to efficiently increase memory capacity in the vertical direction, thus significantly enhancing memory density. However, a significant challenge lies in achieving uniform and high-quality growth of memory elements in the deep holes created by vertical stacking, for which the development of conformal Atomic Layer Deposition (ALD) techniques is essential. Particularly, ALD for chalcogenide materials is still under-developed, with limited research on the practical application of ALD-based chalcogenides in vertical structures. This dissertation focuses on fabricating and evaluating devices in vertical structures using developed chalcogenide ALD techniques. It details the process of Ge-Sb-Te phase-change materials and their superlattice films via ALD on vertically structured substrates. A recessed electrode structure was designed and experimentally tested to enhance the thermal efficiency of these devices. Both simulation and experimental results confirmed an improvement in energy efficiency. Applying superlattice films to the recessed electrode structures demonstrated a 1/8 reduction in operating current compared to structures without material and structural engineering advancements. Furthermore, the research explores memory characteristics observed in Ovonic Threshold Switch (OTS) devices for a new type of memory device, eliminating the need for a selector. Devices were developed by growing Ge-rich GeSe (Ge0.6Se0.4) using ALD in vertical structures. The study observed a variation in threshold voltage depending on the polarity of previously applied pulses, and these characteristics remained stable after more than 103 write operations at 85 °C without reverting over time. Additional measurements contributed to proposing a suitable switching model, thereby demonstrating the potential for developing novel vertical memory devices.데이터 중심 기술 시대에 메모리 비트당 비용은 제품 개발의 중요한 요소가 되었다. 상변화 메모리(PCM)는 기술적 성숙에도 불구하고 아직 이러한 시장의 요구를 충족시키지 못하고 있다. 성공적인 V-NAND 플래시 메모리와 개념이 유사한 수직 크로스바 어레이(V-CBA) 구조는 수직 방향으로 메모리 용량을 효율적으로 증가시켜 메모리 밀도를 크게 향상시킬 수 있다는 점에서 주목받고 있다. 하지만 수직 적층을 통해 생성된 깊은 구멍에 메모리 소자를 균일하고 고품질로 성장시키는 것이 중요한 과제이며, 이를 위해서는 원자층 증착(ALD) 기술의 개발이 필수적이다. 특히 칼코게나이드 소재에 대한 ALD는 아직 개발이 미진하여 수직 구조에서 ALD 기반 칼코게나이드의 실제 적용에 대한 연구가 제한적이다. 본 학위논문은 개발된 칼코게나이드 ALD 기술을 사용하여 수직 구조에서 소자를 실제로 제작하고 평가하는 연구에 대해 보고한다. 먼저 수직 구조의 기판에서 ALD를 통해 Ge-Sb-Te 상변화 물질과 그 초격자 필름을 성장시키는 과정을 자세히 설명한다. 이러한 소자의 큰 단점인 열 효율을 개선시키기 위해 오목한 전극 구조를 고안하였으며, 시뮬레이션과 실험 결과 모두를 통해 에너지 효율이 개선되었음을 확인하였다. 더 나아가 오목한 전극 구조에 초격자 필름을 적용하면 재료 및 구조 공학적 개선이 없는 기본 구조에 비해 작동 전류가 1/8로 감소하는 것을 확인하였다. 또한, 이 연구는 오보닉 임계 스위치(OTS) 소자에서 관찰된 임계 전압의 메모리 특성을 새로운 유형의 칼코게나이드 소재 기반 소자로 응용하고자 하여 별도의 선택기가 필요 없는 메모리 소자를 구현하고자 하였다. 소자는 수직 구조에서 ALD를 사용하여 저마늄 비율이 더 높은 Ge0.6Se0.4를 성장시켜 개발되었다. 이전에 인가된 펄스의 극성에 따라 임계 전압의 변화가 관찰되었으며, 반대되는 극성의 펄스가 인계되었을 때 더 높은 임계 전압을 가짐을 확인하였다. 이러한 특성은 85°C에서 10^3회 이상의 쓰기 작업이 가능하였으며, 시간이 지나도 저장된 정보가 되돌아가지 않고 안정적으로 유지되었다. 추가 측정을 통해 적합한 스위칭 모델을 제안함으로써 새로운 수직형 메모리 소자 개발의 잠재력을 입증했다.Abstract I Table of Contents III List of Figures VI List of Tables XIV List of Abbreviations XV Chapter 1. Introduction. 1 1.1. Overview and Issues on Chalcogenide for Low-Cost and High-Performance Vertical Memory 1 1.1.1 Challenges and Prospects in Developing Low-Cost and High-Performance Memory Systems 1 1.1.2 Phase-Change Memory and Ovonic Threshold Switch 5 1.1.3 The Necessity of Developing Superior Deposition Method for Chalcogenide Film 8 1.1.4 Architecture for 3D Vertical-Crossbar Array 10 1.2. Objective and Chapter Overview 12 1.3. Bibliography 14 Chapter 2. Atomic Layer Deposition of Ge-Sb-Te Film for Vertical-Type Phase-Change Memory with Power Efficiency Enhancement 17 2.1. Introduction 17 2.2. Experimental Methods 20 2.3. Results and Discussion 23 2.3.1 Electro-Thermal Simulation for Vertical-Type Phase-Change Memory 23 2.3.2 Experimental Demonstration of PV-PCM with TiN Heater Electrode 34 2.3.3 Experimental Demonstration of Heater Recessed Structure 37 2.3.4 ALD Sb2Te3/GeTe Superlattice Film on Vertical Structure for Low Power Consumption 43 2.4. Conclusion 58 2.5. Bibliography 60 Chapter 3. Polarity-Induced Threshold Voltage Shift of Atomic Layer Deposited Ge0.6Se0.4 Film for Vertical-Type Memory 65 3.1. Introduction 65 3.2. Experimental Methods 68 3.3. Results and Discussion 72 3.3.1 Vertical OTS Based on Ge0.6Se0.4 Film 72 3.3.2 Polarity-Induced Threshold Voltage Shift in Ge0.6Se0.4 Film 75 3.3.2 Retention and Cyclic Fatigue Characteristics 84 3.4. Conclusion 88 3.5. Bibliography 89 Chapter 4. Investigation of Polarity-Induced Vth Shift in Ovonic Threshold Switch of Ge0.6Se0.4 Film for Vertical-type Selector-only Memory 93 4.1. Introduction 93 4.2. Results and Discussion 96 4.2.1. Filamentary Conduction Observed in V-SOM of Ge0.6Se0.4 Film 96 4.2.2. Operational Stability in V-SOM of Ge0.6Se0.4 Film Based on Polarity 101 4.2.3. Proposed Model for Polarity-Induced Vth Shift in V-SOM of Ge0.6Se0.4 Film 105 4.3. Conclusion 113 4.4. Bibliography 115 Chapter 5. Conclusion 121 Abstract in Korean 124박

Superstructure Optimization of Low Temperature Organic Rankine Cycle with Multi Component Working Fluid

학위논문 (박사)-- 서울대학교 대학원 : 화학생물공학부, 2016. 8. 한종훈.Liquefied natural gas (LNG) has been receiving attention as energy source because of its high-energy density and low emission of greenhouse gas problems. Typically, LNG is evaporated by sea water in LNG terminal without using its cryogenic energy. The cryogenic energy of LNG can be utilized for power generation using organic Rankine cycle (ORC). In this thesis, an optimal ORC process utilizing LNG cold energy is proposed. The ORC process is modeled using commercial process simulator. The working fluid of the ORC is composed of normal pentane, trifluoromethane, and tetrafluoromethane. The optimization of the process to minimize total annualized cost (TAC) is performed using superstructure based approach. The developed superstructure includes four process alternatives, which are MSCHE, vapor flash process, 2-stage expansion, and VRP. The optimum solution is attained using the process simulator-interface-optimizer structure. As a result of optimization, the optimum ORC process configuration including MSCHE and 2-stage expansion is obtained. The optimal process shows the net power generation of 409.6 GJ/h, and the power generation per unit kilogram of LNG is increased by 68.2 %.CHAPTER 1 : Introduction 1 1.1. Research motivation 1 1.2. Research objectives 1 1.3. Outline of the thesis 6 CHAPTER 2 : Process Description and Superstructure Design 8 2.1. Base case 8 2.2. Process Alternatives and Superstructure Design 12 CHAPTER 3 : Optimization Formulation 19 3.1. Formulation of optimization problem and constraints 19 3.2. Optimization Structure 22 CHAPTER 4 : Results and Discussion 25 4.1. Results 25 4.2. Discussion 31 CHAPTER 5 : Modeling and Design of Vapor Recovery Unit (VRU) Processes on Carrier Ship 36 5.1. Introduction 36 5.2. Process description 37 5.3. Process modeling 40 5.4. Process alternative for improving efficiency 47 CHAPTER 6 : Conclusion and Future Works 51 6.1. Conclusion 51 6.2. Future works 52 Reference 53 Abstract in Korean (국문요약) 58Docto