낸드 플래시 메모리에서의 신뢰성 분석 및 새로운 수명 평가 방법 연구

학위논문 (박사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2016. 2. 신형철.ABSTRACT Analysis of Reliability Issues and Lifetime Estimation in NAND Flash Memory Kyunghwan Lee School of Electrical Engineering and Computer Science College of Engineering Seoul National University As NAND flash memory continues to be aggressively scaled down, it becomes more susceptible to reliability problems. As a result, the lifetime estimation of the device is now serious topic for mass production. However, the apparent activation energy (Eaa) in the conventional temperature-accelerated lifetime test method of NAND flash memory does not follow the Arrhenius model, since various failure mechanisms occur concurrently. Therefore, this conventional Arrhenius model has a huge error in the lifetime prediction. Generally well-known dominant failure mechanisms in NAND flash memory are detrapping, interface trap (Nit) recovery, and trap-assisted tunneling (TAT). In this thesis, we propose an advanced charge loss model and completely separate three dominant failure mechanisms in terms of the time-constant (τ) and the final ΔVth in various generations (A, B, C) of NAND flash test element group (TEG) cells and sub 20-nm multi-level cell (MLC) NAND flash memory main-chip. As a result, it is observed that each τ of the mechanisms follows the Arrhenius law well, which means that each has its own activation energy (Ea). In addition, we deeply investigate the retention characteristics of the dominant mechanisms in various conditions, such as the number of P/E cycling times, probability level of the Vth cumulative distribution, and states in sub 20-nm MLC NAND flash memory. We also extracte the contribution rate (CR) of each failure mechanisms at criterion ofΔVth_Totalaccording to baking temperature. The results give the physical reason for abnormal retention behaviors such as Eaa roll-off at the PV3 and negative Eaa at the ERS. P/E cycling stress generates traps in the tunnel oxide layer. We extract the trap profile in the tunnel oxide in space and energy distributions using 3D TCAD simulation. For the first time, we reveal the origin of abnormal Eaa characteristics and derive a mathematical formula for Eaa as a function of each Ea(mechanism) in NAND flash memory. We propose two different accurate lifetime estimation models for sub 20-nm NAND flash memory. The first model is the Eaa integration method. Using the analytically modeled Eaa equation, the lifetime of NAND flash memory is accurately predicted. The second model is the advanced extrapolation method using the trends of extracted parameters. Using the proposed model, accurate lifetime is estimated in all states (PV3, PV2, PV1, and ERS). Since the proposed model takes into account the retention characteristics for various mechanisms, this model provides much accurate prediction on the lifetime of NAND flash memory. Also, the lifetime estimation for the next generation of NAND flash memory is analyzed using 3D TCAD simulation. As a result, the lifetime for the next generation is expected to decrease as much as 66 % of the lifetime for the current generation. Keywords: MLC NAND flash memory, failure mechanisms, Arrhenius model, activation energy (Ea), P/E cycling stress, lifetime estimation Student Number: 2011-20892Chapter 1 Introduction 1 1.1. Reliability Issues in NAND Flash Mempry 1 1.2. Review of Failure Mechanisms in NAND Flash Memory 6 1.2.1 Detrapping Mechanism 7 1.2.2 Trap-assisted tunneling Mechanism 9 1.2.3 Detrapping Mechanism vs. TAT Mechanism 13 1.2.4 Interface trap (Nit) recovery Mechanism 15 1.3. Motivation and Thesis Organization 21 Chapter 2 Introduction of Conventional Lifetime Estimation Model and Proposed Charge Loss Model 23 2.1. Introduction 23 2.2. Conventional Lifetime Estimation Methods 25 2.2.1 Arrhenius Model (1/T Model) 25 2.2.2 T Model 29 2.3. Proposed Charge Loss Model 32 2.3.1 Weibull Distribution (Stretched Exponential) Model 32 2.3.2 Physical Background of New Model 36 2.4. Summary 40 Chapter 3 Activation Energies (Ea) of Failure Mechanisms in Advanced NAND Flash Cells for Different Generations and P/E Cycling 41 3.1. Introduction 41 3.2. Procedure of Activation Energies (Ea) Extraction 43 3.3. Analysis of Failure Mechanisms 53 3.3.1 Detrapping Mechanism 54 3.3.2 TAT Mechanism (Generation dependence) 55 3.3.3 TAT Mechanism (P/E cycling dependence) 58 3.4. Summary 59 Chapter 4 Mechanism Separation and Analysis of Retention Characteristics in Sub 20-nm NAND Flash Memory Main-Chip 60 4.1. Introduction 60 4.2. Observation of Abnormal Retention Characteristics in Sub 20nm NAND Flash Memory Main-Chip 62 4.3. Parameter Extraction in NAND Main-Chip 65 4.3.1 Parameter Extraction in the PV3 state 65 A. TAT related Parameter Extraction 66 B. Additional Limiting Condition (PV3 state) 70 C. Set-up for Parameters Extraction 73 4.3.2 Parameter Extraction in the Other states 75 4.4. Analysis on Retention Characteristics 79 4.4.1 P/E cycling times and State Dependence 79 A. Analysis of Retention Characteristics for the PV3 state 81 B. Analysis of Retention Characteristics for the ERS state 83 C. TAT mechanism vs. Detrapping mechanism 88 4.4.2 Probability Level Dependence 98 4.4.3 Analysis of Detrapping Mechanism using 3D TCAD simulation 103 4.5. Summary 109 Chapter 5 Analytical Model for Apparent Activation Energy (Eaa) and Proposed Lifetime Estimation Method 110 5.1. Introduction 110 5.2. Modeling for Apparent Activation Energy (Eaa) 112 5.3. Proposed Lifetime Estimation Method 123 5.3.1 Eaa Integration Method 123 5.3.2 Advanced Extrapolation Method 125 5.4. Lifetime Estimation for the Next Generation 131 5.4.1 Detrapping Parameter Extraction 133 5.4.2 Nit recovery Parameter Extraction 137 5.4.3 TAT Parameter Extraction 141 5.4.4 Lifetime Estimation Results according to the Generations 147 5.5. Summary 149 Conclusions 150 Bibliography 152 Abstract in Korean 164 List of Publications 166 Conferences 168Docto

A Study on the Fitness between Manufacturing Strategy and Manufacturing Environments

Environments are critical factors for manufacturing strategic decisions, because environments are the sources of opportunities and risks of the manufacturing firms. this study emperically assesed the impacts of different manufacturing strategies under different environments. Data were gathered from the electric and electronic industry in Korea. Four manufacturing strategic groups were developed on the basis of task performances of the firms. To examine the fitness between the manufacturing strategies and environments, the economic performance of firms were analized according to the matrix which consists of environmental complexities/uncertainties clusters and manufacturing's strategic groups

An Analysis of Attitudes on Euthanasia between Residents and Judicial Apprentices

Background: Recently, the legal and ethical issues relative to euthanasia are becoming controversial in Korea. This study was designed to verify the differences of the attitudes on euthanasia between judicial apprentices and residents. Methods: The questionnaire was conducted on the 35th-group of the judicial apprentices on March 24, 2004, and on the residents from April 2 to May 22, 2004. The respondents were 636 in total consisting of 460 judicial apprentices and 176 residents. Results: Of the total 636 subjects, 373 (81.1%) of the judicial apprentices and 149 (84.7%) of residents agreed that allowing euthanasia is moral, without any significant difference (P＞0.05). The number of residents was greater (59 people, 33.5%) than that of judicial apprentices (112 people, 24.4%) who agreed with active euthanasia (P＜0.05). Among the total, 397 (86.3%) of the judicial apprentices and 160 (91.4%) of the residents answered that the law for euthanasia was necessary, without any significant difference (P＞0.05). But, among these supporters, the respondents who agreed on active euthanasia were significantly different in number between judicial apprentices (n=93, 23.4%) and residents (n=54, 33.8%) (P＜0.05). Conclusion: This study did not find any significant differences between the two groups in the necessity of the law for euthanasia, but the rate of agreement on active euthanasia was higher in residents group than in judicial apprentices group.ope

The Role of the Epidemiological Causality of the Association between Smoking and Lung Cancer

It is generally accepted that acknowledgment of the causality of the association between smoking and lung cancer plays a crucial role in imposing liability on tobacco manufacturers in tobacco-related cases. However, acknowledgment of the epidemiological causality of the association is merely a precondition to putting liability on tobacco manufacturers; it does not play a crucial role. Smokers smoke manufactured tobacco by their right to choose according to their self-decision; therefore, it is not reasonable to place all liability concerning lung cancer on tobacco manufacturers. This paper studies the dual structure of the causality of the association between tobacco manufacturing and smoking as well as smoking and lung cancer. It can be understood that tobacco manufacturers must be held liable for lung cancer caused by smoking when there was intentional concealment or insufficient provision of information on the harmfulness of tobacco and the addictiveness of nicotine. The epidemiological causality of the association is the minimum scientific and legal precondition to placing liability on tobacco manufacturers for lung cancer caused by smoking. However, striving for protection and promotion of national health by, for instance, promoting antismoking programs, the epidemiological causality of the association adequately plays a social role when viewed from a public health perspective.ope

원자층증착법을 이용한 새로운 나노구조의 제조와 에너지 및 환경 분야로의 응용

학위논문 (석사)-- 서울대학교 대학원 : 화학생물공학부(에너지환경 화학융합기술전공), 2012. 2. 현택환.Atomic layer deposition (ALD) provides an elegant, efficient and well controllable way to coat high aspect ratio nanostructures with a homogeneous and conformal film of precisely defined thickness. As such, it is ideally suited for the fabrication of complex heterostructures and functional materials. ALD is based on a reaction between precursor materials, which are separated into successive surface reactions. In this manner, the reactants are kept separated and react with surface species in a self-limiting process, i.e. without the presence of a gas phase reactions. Each surface reaction is separated by a purge step to remove the unreacted precursor and the by-product. In this thesis, two structures (Al2O3/TiO2 nanotubes arrays for DSSCs and TiO2 on CNTs for gas sensing) fabricated by ALD will be introduced. Properties and applications in the field of DSSCs and gas sensing will be discussed. Especially, the role of very thin and well calibrated film by ALD in those applications will be highlighted.비교적 정확한 두께의 조절이 가능한 원자층증착법은 높은 종횡비를 가지는 나노구조체에 균일한 증착이 가능한 효과적인 증착법이다. 이러한 점은 원자층증착법을 복잡한 나노구조체와 기능성 물질을 만드는데 적합한 도구로 만들어준다. 원자층증착법은 개별적으로 진행되는 전구체 물질의 표면반응을 기초로 하여 자가제한적반응이 이루어진다. 각 표면반응은 반응하고 남아있는 전구체와 부생성물을 없애기 위해 실행하는 퍼지 단계를 기준으로 나뉘어 진다. 본 논문에서는 원자층증착법을 이용하여 에너지 분야에서는 염료감응형 태양전지에 사용가능한 알루미늄 옥사이드/이산화티타늄 나노구조체와 환경 분야에서는 가스 검출을 위하여 카본나노튜브에 이산화티타늄을 증착한 구조체의 성질을 소개하였다. 특히, 원자층증착법을 이용하여 만들어진 박막의 각 응용분야에 대한 역할을 중점적으로 소개하였다.Maste

A Study on the Information about Medical Care for Patients and Relevant Control Right over it

Information is something intangible, not anything tangible or solid in a limited boundary of space and time, and it is an unsubstantial property under legal regulations, which is characterized by being distributed here and there at the same time. Due to these unprecedented legal aspects of information, it is not easy to seek any principle of appropriate and practical law. That is why we, human being, have not yet incorporated the information into the subjects of law. As a result, current law system has failed to draw full-fledged and appropriate thinking models or legal concepts for this kind of legal subjects. Till now, there have been not any complete legal regulations as enforced for this concern, either. Besides, legal theories are developed in a flexible manner and their principles still remain incomplete. Unfortunately, Korean Civil Code contains or covers no concept about information. But according to regulations over conclusion of agreement, it is just presumable that if content of information refers to human intents, information delivery can be treated as a declaration of intent. As mentioned above, at the time of civil code as initially enforced, the intangible things like information could not be considered as a property with asset-like values. But nowadays, information itself has a substantial value like tangible asset, so that it can be considered even as one of subjects under dealing. Personal information refers to the information on any living individual, which includes up-to-the-minute information like sign, character, voice, sound and video as inherently identifiable through name or residential registration number, etc. But medical information which is addressed as a topic herein has concepts other than them. That is, medical information indicates the information about patients' health conditions, corresponding assessment and their course under the supply of medical care, which all are obtained from medical care practices in order to determine necessity of supplying medical care or provide medical care services for them. Therefore, it is noted that medical information belongs to both personal and medical information. Thus, it is possible that medical information has more or less considerable influences even on vital and physical aspect of individual persons, as well as their own human respect in the course of medical treatment. So it is not too much to say that medical information is the most critical one of personal information items. This medical information, i.e. personal information related to individual health condition, may cover patient' own physical conditions and even their socio-psychological living environment as well. Accordingly, it is inevitable that medical information should be sufficiently protected as one of personal rights, while medical personnel including doctors are subject to the responsibility for keeping any privacy of patients in business from divulgence. One the contrary, it is necessary to get much information on patients concerned for the benefit of providing better medical service for them in our comtemporary society from the angle of quality. That is why we also need institutional instruments as easier accessible and available than now from the angle of medical administration. Medical care is directly related to patients' vital and physical condition, and is actually carried out by their own autonomous decision. Here, it is essential to take human respect into consideration behind medical care. In view of human respect, it is unreasonable that patients cannot be any object under medical care, while considerations concerned in medical care information may be maintained and available only on the foundation of reliable relationships between doctors and patients. Medical personnel including doctors get qualified for the medical care of patients according to Medical Act and other regulations that provide the qualification for related medical occupations, which also stipulated that all medical personnel have responsibility for keeping any private medical information of patients from delivery or disclosure to unauthorized third party. Unless it is fully assured to protect this medical information, it is possible that patients would reject formal medical care, which may cause a great threat against medical system and public hygiene, bringing about unprecedented conflicts in our modern society as well. Summing up, medical information can be regarded one of most critical information by patients. In particular, it becomes inevitably more serious, when future-oriented medical care system would be realized on the basis of up-to-date medical technology related to life science including genetic research, gnomic diagnosis and gene therapy. It is undeniable that the development of future medical science may depend on the protection of medical care, which becomes a historical demand. Meanwhile, it is also requisite to build up a streamlined and stable system to make good use of medical information for patients who need quality medical services. To meet their needs, it is paramount to keep reliable relationships between doctors and patients.restrictio

A Study of Tobacco Litigation in Public Health Law

Smokers say that tobacco is one of their favorite food and think it unfair to regulate for them to smoke. But non-smokers argue that tobacco is like a toxic drug and it causes serious harms in terms of public health law. Therefore, around tobacco lawsuit in recent years I will look into the meaning of the tobacco litigation and examine the significance of the tobacco litigation in Korea. In U.S.A. in the early 1950s, the cigarette was a cultural icon. The first tobacco lawsuit was filed in 1954. The first wave of cases was filed principally under theories of negligence, breach of warranty, and misrepresentation. By the time of the second wave of litigation from 1983 to 1992, cigarette smoking was beginning to become a hallmark not of elegance, but of weak character and lower social class. And the third wave did not begin quietly. The tobacco industry faces litigation from individual smokers in the third wave. The most important effect of tobacco litigation was to transform public and political perceptions about risk and responsibility in smoking, making clear what manufacturers knew, how they concealed this knowledge, and how they manipulated consumers. In the end I conclude as follows: State should conserve human rights of her people. Constitution makes clear this point. Tobacco litigations have a much impact on public health law. Manufacturers that produced harmful products in public health might be imposed on product liability or strict liability in tort law.ope

Legal Nature of the Medical Contract in Health Insurunce

The existing discussions on the medical contract have focused on only the relationship between the physician and the patient. This kind of medical contact is described in the simple structure on the assumption of the contract relation between the individuals at the equal status, but it shows the problem that the complex structure in the medical life world can not be properly described. Therefore, this paper tried to newly classify the all kinds of medical contracts, including not only the medical contract between the physician and the patient but also related contacts appeared in the health care related laws or the social insurance system. This study tried to examine the legal nature of the medical contract in the insurance care and the medical provider under the social insurance system including the National Health Insurance Act becomes the National Federation of Health Insurance, the insurer, and its legal nature of the medical contract can be viewed as the public statutory contract for the public medical supply. Unlike the social insurance, the medical contract under the Automobile Accident Compensation Insurance Act does not include the right to determinate the medical fee by the insurer Therefore, it is reasonable to consider the medical contract under the Automobile Accident Compensation Insurance Act as the private statutory contract between the medical institution and the patient.ope

비도시지역 민간주택건설사업에 대한 공공규제 개선방안 연구 : 국토의 계획 및 이용에 관한 법률 도입을 중심으로

학위논문(석사)--서울대학교 대학원 :협동과정 도시설계전공,2003.Maste

Application of Epidemiology in Civil Action

This paper has utilized an epidemiological approach to pollution cases presuming those harm caused by environmental pollutions takes similar course as development process of a disease. This approach has been utilized in four big pollution cases in Japan which proves that this approach might be the appropriate approach to solve such cases. Environmental pollution is a scientific process. It will be desirable therefore, to solve such matters not only with sociological method but also with scientific methods. Recently there are many pollution cases reported in Korea. We must do our best to prevent such and when it happens we must try to solve the problem though appropriate legal actions.ope