금속산화물 기반 저항변화메모리 소자의 노이즈 특성과 그것의 응용에 관한 연구

학위논문(박사) -- 서울대학교대학원 : 공과대학 전기·컴퓨터공학부, 2023. 2. 김재준.In the current pyramid-like structures memory hierarchy, it consists of, from top to bottom, a processing core, cache memory by static random access memory (SRAM), main memory by dynamic random access memory (DRAM), and storage memory by solid-state disk (SSD), or hard disk drive (HDD). In general, the closer to the processing core, the more high-speed operation is required, whereas the farther away from the core, the higher storage capacity is demanded. Consequently, the performance gap between DRAM and NAND Flash memory, which are currently major memory technologies, is continuously increasing. However, the need for new memory technology is increasing in order to solve the problem of data processing speed due to the explosive increase in the amount of data and the physical limitation of the existing memory technologies that has been raised for a long time. In addition, research and development on the storage class memory (SCM) technology is in progress as method of implementing In-Memory Process, a concept to solve the problem of Von Neumann architecture in various research groups. Among the candidates on the SCM, which satisfies both the high speed of DRAM and the density of NAND Flash, the resistive switching random access memory (RRAM) has been widely investigated as a leading candidate for next generation nonvolatile memory applications due to RRAMs advantageous features such as simple structure, low cost, high density, fast operation, and CMOS compatibility. However, the reliability issues which PCM suffered from is also being reproduced in RRAM. RRAMs various issues such as endurance, retention, and uniformity stem from intrinsic variability because resistive switching mechanism of RRAM itself is fundamentally stochastic. The main content of this dissertation is to develop a new electrical analysis technique to improve the reliability of RRAM. First, the elementary low frequency noise (LFN) characteristics of various RRAM devices were analyzed, and the correlation between LFN characteristics and the conduction/resistive switching mechanisms was experimentally verified. Also, it was suggested that the LFN measurement can be an additional analysis technique for devices degradation mechanism and multi-level cell (MLC) operation. Finally, from the random telegraph noise (RTN) measurement, we conducted a study to extract the position and energy of traps that can cause cells failure. The experiment on the extraction of traps physical information using the RTN measurement was conducted for the first in this study, and then research findings provided researchers with guidelines for the RTN analysis of RRAM.현재의 메모리 계층도를 보면 CPU는 고속 동작을 요구하고, 외부메모리는 고용량을 필요로 하기 때문에, 현재의 주요 메모리 기술인 DRAM과 NAND Flash 메모리의 성능 격차는 지속적으로 늘어나고 있다. 하지만 데이터 양의 폭발적인 증가로 인한 데이터 처리 속도 문제, 그리고 오래전부터 제기 되어왔던 기존 메모리의 물리적 한계를 해결하기 위해서 새로운 메모리 기술에 대한 필요성이 증가하고 있다. 또한 기존 폰노이만방식의 컴퓨터 시스템 구조의 문제점을 해결하기 위한 방법인 In-Memory Process를 실현하기 위한 방법으로 DRAM의 high speed, 그리고 NAND Flash의 high density 모두를 만족하는 SCM (storage class memory)기술에 대한 관심이 증가하고 있다. SCM 후보군 중에서, 저항 변화 메모리 소자인 RRAM (Resistive Random Access Memory)은 MIM, cross-point 형태의 간단한 구조를 가지며, 공정 상 집적도 향상에 유리하고, 사용되는 물질이 CMOS공정과 호환 가능하다. 이러한 장점들로 인해 기존 Flash 메모리 소자의 대안으로 학계에서 많은 연구가 진행 되어 왔지만, 한 단계 앞서 연구가 진행되었던 PCM (Phase change RAM)이 겪고 있는 신뢰성 문제가 RRAM에서도 재현되고 있다. RRAM의 신뢰성 문제는 RRAM의 저항 스위칭 메커니즘 자체가 근본적으로 확률적이기 때문에 본질적 변동성에서 기인하는 것이다. 본 논문의 주요 내용은 RRAM의 신뢰성 향상을 위해서 새로운 전기적 분석기법을 개발하는 것이다. 우선 다양한 메커니즘으로 동작하는 RRAM소자의 기본적인 저주파 잡음 특성을 분석하고, 이를 소자의 전도 메커니즘 및 저항 변화 메커니즘과의 연관성을 검증하였다. 측정결과를 기존 저주파 잡음 이론을 통해 해석하고, 다양한 소자에 이를 적용시켜 저주파 잡음 분석 기법이 RRAM의 동작 메커니즘 분석에 이용할 수 있음을 증명하였다. 또한, 소자의 열화 메커니즘 및 MLC (Multi-Level Cell) 분석에 있어서도 저주파 잡음 측정이 추가적인 분석기법이될 수 있음을 제시하였다. 마지막으로, 소자의 저주파 잡음 특성 중 하나인 RTN (Random Telegraph Noise)특성 분석을 통해 셀의 fail 을 일으킬 수 있는 trap의 위치 및 에너지를 추출하는 연구를 진행하였다. RRAM의 trap정보 추출에 관한 측정 및 분석은 본 연구에서 최초로 진행되었던 것이고, 이후 RRAM의 RTN분석에 가이드라인을 제시하였다.Chapter1 Introduction 1 1.1 Memory trends 1 1.1.1 Memory wall 1 1.1.2 In-memory processing 3 1.2 SCM technologies 4 1.2.1 Phase change memory 4 1.2.2 Magnetic memory 6 1.2.3 Ferroelectric memory 7 1.2.4 Resistive memory 8 1.3 Thesis content overview 12 1.3.1 Thesis objectives 12 1.3.2 Thesis outline 13 Chapter2 Overview on conduction mechanisms 14 2.1 Electrode-limited conduction mechanisms 14 2.1.1 Schottky emission 15 2.1.2 Fowler-Nordheim (F-N) and direct tunneling 17 2.2 Bulk-limited conduction mechanisms 18 2.2.1 Poole-Frenkel (P-F) emission 18 2.2.2 Ohmic conduction 19 2.2.3 Space charge limited conduction (SCLC) 20 Chapter3 LFN applications for RRAM analysis 23 3.1 Introduction to 1/f 23 3.2 LFN application (1): Resistive switching analysis 26 3.3 LFN application (2): MLC analysis 30 3.4 LFN application (3): Degradation analysis 35 Chapter4 Analysis of conduction mechanism using LFN 39 4.1 Thermochemical mechanism RRAM 39 4.1.1 Fabrication 39 4.1.2 Experimental results: RS and I-V characteristics 40 4.1.3 Experimental results: LFN characteristics 46 4.2 Valence change mechanism RRAM 50 4.2.1 Fabrication 50 4.2.2 Experimental results: RS and I-V characteristics 52 4.2.3 Experimental results: LFN characteristics 55 4.3 Comparative analysis of conduction mechanism 58 4.3.1 Fabrication 58 4.3.2 Experimental results: RS and I-V characteristics 61 4.3.3 Experimental results: LFN characteristics 63 Chapter5 Random telegraph noise (RTN) in RRAM 67 5.1 Introduction to RTN 67 5.2 RTN in RRAM 69 5.2.1 Methodology for extracting trap information 69 5.2.2 Experimental results 73 Chapter6 78 Conclusions 78박

무선 센서 네트워크 상에서의 효율적인 위치 추정 알고리즘 연구

학위논문 (박사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2015. 8. 김성철.In this dissertation, efficient localization algorithms for wireless sensor networks are represented. Localization algorithms are widely used in commercial systems and application. The localization techniques are anticipated to be developed for various environments and reduce the localization error for accurate location information because the user demands for more accurate positioning systems for medical care, home networks, and monitoring applications in personal range environments. A well-known localization system is GPS, with applications such as mobile navigation. The GPS shows good performance on road or roughly finding location system in outdoor environments but limited in indoor environments. Due to the development of handsets like smart phone, the users can easily receive the GPS signals and other RF signals including 3G/4G/5G signals, WLAN (Wireless Local Area Networks) signals, and the signals from other sensors. Thus, the various systems using localization schemes are developed, especially, the WSNs (Wireless Sensor Networks) localization system is actively studied in indoor environment without GPS. In this dissertation, the range-free localization algorithm and the range-based localization algorithm are reported for WSNs localization system. The range-free localization algorithms are proposed before to estimate location using signal database, called signal map, or the anchor nodes of antenna patterns, or ID configuration of the linked anchor nodes, etc. These algorithms generally need to additional hardware or have low accuracy due to low information for location estimation. The range-based algorithms, equal to distance-based algorithms, are based on received signal strength, RSSI, or time delay, TOA and TDOA, between the anchor nodes and a target node. Although the TOA and TDOA are very accurate distance estimation schemes, these scheme have the critical problem, the time synchronization. Although RSSI is very simple to setup the localization system with tiny sensors, the signal variation causes severe distance estimation error. The angle estimation, AOA, provides additional information to estimation the location. However, AOA needs additional hardware, the antenna arrays, which is not suitable for tiny sensors. In this dissertation, range-free and range-based localization algorithms are analyzed and summarized for WSNs with tiny sensors. The WSNs localization systems are generally used range-based algorithm. The range-based algorithms have major source of distance estimation error, and the distance estimation error causes severe localization error. In this dissertation, the localization error mitigation algorithms are proposed in two dimensional environments and three dimensional environments for WSNs. The mitigation algorithms in two dimensional environments consist of several steps, which are distance error mitigation algorithm, location error mitigation algorithm, and bad condition detection algorithm. The each algorithm is effective to reduce the localization error, but the accuracy of location estimation is the best when they are combined. The performance of proposed algorithms is examined with variation of received signal strength and it is confirmed that the combined proposed algorithm has the best performance rather than that of conventional scheme and each proposed algorithms. The three dimensional localization uses Herons formula of tetrahedron to calculate the target height, then transforms a two dimensional location computed by LLSE into a three dimensional estimated location. Simulation results validate the accuracy of the proposed scheme.Contents Chapter 1 Introduction...........................................................1 Chapter 2 Location estimation for wireless sensor networks.................................................................................................4 2.1 Introduction..................................................................................4 2.2 Range-free location estimation ...................................................7 2.2.1 Cell-ID location estimation .........................................................7 2.2.2 Fingerprint location estimation ...................................................8 2.2.3 Other range-free location estimation.........................................10 2.3 Range-based location estimation ..............................................12 2.3.1 Time delay based distance estimation.......................................12 2.3.2 Received signal strength based distance estimation .................16 2.3.3 Angle of arrival based location estimation................................18 2.4 Summary.......................................................................................20 Chapter 3 Two dimensional location estimation for wireless sensor networks......................................................................22 3.1 Introduction................................................................................22 3.2 Tri-lateration ..................................................................24 3.2.1 Linear least square estimation ..................................................24 3.2.2 The cases of tri-lateration .........................................................26 3.3 Geometric mitigation algorithm …............................................27 3.3.1 Motivation .................................................................................27 3.3.2 Algorithm explanation ..............................................................28 3.3.3 Simulation .................................................................................29 3.3.4 Conclusion ................................................................................34 3.4 Coordinate shift algorithm ..........................................................35 3.4.1 Motivation .................................................................................35 3.4.2 Algorithm explanation...............................................................36 3.4.3 Simulation .................................................................................41 3.4.4 Conclusion ................................................................................43 3.5 Bad condition detection algorithm ...............................................44 3.5.1 Motivation .................................................................................44 3.5.2 Algorithm explanation...............................................................45 3.5.3 Simulation .................................................................................51 3.5.4 Conclusion ................................................................................54 3.6 Conclusion..................................................................................55 Chapter 4 Three dimensional location estimation for wireless sensor networks .....................................................................56 4.1 Introduction................................................................................56 4.2 Motivation.....................................................................................57 4.2.1 Singular matrix problem…........................................................57 4.2.2 Short range location estimation.................................................59 4.3 Algorithm explanation....................................................................60 4.4 Simulation........................................................................................68 4.5 Conclusion..................................................................................72 Bibliography....................................................................................73 Abstract in Korean.....................................................................................78Docto

Measurement and Characterization of Multiple-Input Multiple-Output (MIMO) Wideband Relay Channel at 5.8 GHz

This paper includes channel measurement results using 4 transmitting / receiving antennas and PN sequence with 50ns, 40ns and 20ns per a chip at 5.8GHz in Seoul National University campus. Seoul National University is proper to measure channels because it has many LOS / NLOS environments. Base station - F Relay channel is compared with a polarization diversity. We derived channel capacities form F Relay - Relay(A, B, C, D) channel.본 논문은 BK21 project 의 지원을 받았습니다

The National Policy of College/University Students Enrollment Quotas in South Korea:Vision and Action

이 연구는 급변하는 시대적 상황에 대처할 수 있도록 대학정원정책에 대한 현황을 고찰하고 이에 대한 개선 방향을 제시하기 위하여 문헌고찰을 통한 기술적 내용 분석 방법으로 분석하였다. 본고에서는 정초․팽창기(1945-1960), 통제․실험기(1961-1980), 확대․안정기(1981-1993), 자율지향․개방초기(1994-현재)로 대별하여 이를 사회적 측면에서 고찰하였다. 현재 우리 나라의 대학학생정원정책을 이해하기 위하여 먼저, 현행 대학정원정책의 법적 근거를 간략히 파악하였고 또한, 대학정원정책이 어떻게 책정되는가를 살펴본 후에 현재의 대학정원정책 현황을 교육부 내부 자료인 2001학년도 대학 학생정원 조정 지침 및 결과를 중심으로 문제점을 정책분석론적 관점에서 논의하였다. 마지막으로, 교육부의 2001학년도 대학 학생정원 조정 지침의 대학정원 자율화 조치 기본 방향과 계획을 토대로 하여 앞으로 대학 정원자율화에 대한 방향을 제시하였다. Modern higher education in South Korea has entered into from an elite step to a universalized step nothing but a half century. With the achievement of the universalization of higher education, South Korea which became a member country of OECD urgently needs to change a quantitative educational policy into a qualitative educational policy. In addition, it is necessary to construct an effective manpower system for the optimal supply and demand of higher education as well as to foster high-quality human resources for the jumping an advanced information-technology nation. From this point of time, the university/college student enrollment policy, which has significantly affected not just the extension of higher education access but also the demand and supply for higher education under the government-initiated control, is now unavoidable to change the direction into the university-initiated autonomy. In light of this significance, the purpose of this study is to review the conditions and problems of the national policy of college/university students enrollment quotas in South Korea and to suggest the direction for the improvement of the quota policy so as to cope with the rapidly changing epochal situation. It begins with review of related literature. The author next investigates the transition of college/university quota policy in South Korea from 1945 to the present time. Third, he analyzes the conditions and problems of the national quota policy. Finally, he suggests the vision and direction of the quota policy

남창가곡의 노래선율과 대금선율과의 관계 : 우조 초수대엽·이수대엽·중거·평거에 기하여

학위논문(석사)--서울大學校 大學院 :音樂科 國樂器樂專攻,1997.Maste

Characteristics and activity of platinum-loaded zeolite catalysts for the hydroisomerization of n-paraffin

학위논문(박사)--서울대학교 대학원 :화학공학과,1998.Docto

An Efficient Localization Algorithm in ZigBee Networks

학위논문(석사) --서울대학교 대학원 :전기. 컴퓨터공학부,2010.2.Maste

글쓰는 과정과 글쓰기의 의미

필자가 글쓰기에 어느정도 친숙해진 지는 그리 오래되지 않는다. 이번 학기로 졸업을 앞두고 있지만 2~3년 전만 해도 '글을 잘 쓴다'는 것은 나와는 거리가 먼 사람들의 얘기라고 생각했다. 그도 그럴 것이 필자는 고등학교 때까지 소위 '이과 학생'이었으며 글쓰기보다는 수학적인 문제풀이에 더 친숙했고, 대학생이 되어 물리교육과에 들어와서는 그 정도가 더욱 심해졌기 때문이다. 그러다가 철학과 복수전공을 통해 글쓰기에 조금씩 친해지게 되었고,(물론 아직도 많이 부족하지만)그나마 전에 비해서는 많이 좋아진 지금에 이르게 되었다

HeroShaper 전동 파일의 반복적 피로 파절에 있어서 표면 결함의 영향 : fractogra

Dept. of Dentistry/석사[한글]NiTi file이 근관 치료에 도입된 후, NiTi의 초탄성 성질로 인해 만곡된 근관에서 매우 유용하고 효율적으로 사용되고 있다. 그러나 근관 내 사용 도중 특별한 sign이 없이 급작스럽게 파절하는 경우가 있어 임상가를 당혹하게 하고, 근관 치료 실패 원인의 하나로 작용하기도 한다. 이번 연구의 목적은 in vitro 상에서 반복 피로 파절에 있어서 HeroShaper files의 표면 결함의 역할을 평가하고, 파절면을 주사전자현미경을 통해 관찰하여 파절 역학을 규명하는 것이다.총 45개의 #30/.04 taper, 21mm HeroShaper files를 사용하였으며, 15개씩 3개의 군으로 분류하여 10개는 반복 피로를 가했고 5개는 표면을 관찰하였다. 제 1군은 결함이 없는 새 files, 제 2군은 제조 과정에서 metal rollover, crack과 같은 결함이 있는 files이며 제 3군은 임상에서 6-8개의 근관 성형에　사용된 files로 분류하였다. 각 군의 files들을 측정 기계를 이용하여 일정한 rpm, pecking distance, speed 하에서 반복 피로를 가하여 파절될 때까지 시간을 측정하여 통계 분석을 하고 단면을 주사전자현미경을 통하여 관찰하였다. 그 결과 파절될 때까지 걸린 평균 시간에 있어서 group 1과 2, group 1과 3 사이에 통계학적으로 유의할 만한 차이가 있었다. 그러나 group 2와 3 사이에는 통계학적인 차이가 없었다. Fractographic 분석 결과 파절된 모든 면이 3 단계 (미세 균열 개시, 성장, 파절)의 특징적 소견과 함께 대부분의 면적에서 연성 파절 양상을 보였다. 표면 관찰 시 Group 2에서는 metal rollover나 미세 균열과 같은 결함이 발견되었다. 또한 계면에서 transgranular와 intergranular 파절을 관찰할 수 있었다. 이와 같은 결과로 미루어 보아 표면 결함이 반복 피로 파절에서 중요한 역할을 하며 fractography 분석을 통해 NiTi 기구의 파절 역학을 규명하는데 응용할 수 있음을 알 수 있다. [영문]The purpose of this in vitro study was to examine the effect of surface defects of HeroShaper files on the cyclic fatigue fracture in order to determine the failure mechanisms of NiTi instruments by fractographic analysis using　scanning electron microscope (SEM). A total of 45 HeroShaper files, 21mm in length with a #30/.04 taper, were divided into three groups containing 15 files each. Group 1 contained new HeroShaper without any defects. Group 2 contained HeroShaper with manufacturing defects such like metal rollover, cracks and flaws. Group 3 contained HeroShaper that had been used for 6-8 root canal treatments by a single endodontist. The test apparatus was designed to allow cyclic tension and compressive stress on the tip of the instrument. The fracture surface of each file was analysed by SEM. There was statistically significant difference in the fracture time between groups 1 and 2, and between groups 1 and 3. However, there was no significant difference between groups 2 and 3. Microphotographs of the lateral surface of the HeroShaper files in Group 2 showed metal rollover and significant machining marks along the faces of the flutes. On the other hand, the lateral surface of the new file was clean and flawless. A low-magnification SEM image of the fractured surface revealed brittle fracture to be the predominant failure mode initially, which typically led to a region of catastrophic ductile fracture in all cases. Qualitatively, the ductile failure region constituted a larger portion of the surface area and was characterized by microvoid formation and dimpling (cup-and-cone fracture). At higher magnification, the brittle fracture region often showed fatigue striations and several secondary cracks, indicating that the following three characteristic stages of fatigue crack growth were present: Stage Ⅰ(initiation), StageⅡ(propagation), and Stage Ⅲ (catastrophic failure). Characteristically, the fractured file surface in group 3 showed evidence of transgranular (cleavage) fracture across the grains, as well as intergranular fracture along the grain boundaries.These results show that surface defects play important roles in cyclic fatigue fracture, and fractography can be used to analyse fractured NiTi rotary endodontic instruments.ope

기관지확장증 환자에서 흡입기 사용이 객혈 발생에 미치는 영향에 관한 연구

학위논문 (석사)-- 서울대학교 대학원 : 임상의과학과, 2014. 2. 임재준.Introduction: The association of inhaler use with hemoptysis has rarely been reported in patients with non-cystic fibrosis (CF) bronchiectasis. We aimed to elucidate the effect of inhaler use on the development of hemoptysis in patients with non-CF bronchiectasis. Methods: In a case-crossover study of 192 non-CF bronchiectasis patients with a history of hemoptysis and inhaler use, the risk of hemoptysis associated with the use of inhalers was elucidated. Two inhaled corticosteroids/long-acting β2-agonists (ICS/LABA), one long-acting muscarinic antagonist (LAMA), and one short-acting β2-agonist (SABA) were evaluated. The case and control periods were defined respectively as 0-30 days and 180-210 days before hemoptysis. Results: The risk of hemoptysis during the case period was 3.51 times higher than during the control period with any use of inhalers (95% confidence interval (CI), 1.96-6.28). The results of clinically significant hemoptysis showed good agreement with those of total events. These associations were consistent with the sensitivity analyses. In the sub-analysis according to inhaler type, ICS/LABA and SABA were significantly associated with an increased risk of hemoptysis (aOR 2.62, 95% CI 1.25-5.45aOR 2.51, 95% CI 2.23-5.15). Conclusions: In patients with non-CF bronchiectasis, the use of inhalers, especially including β2-agonist, was associated with an increased risk of hemoptysis.Abstract........................................i Contents......................................ii List of tables and figures.............iii List of abbreviations....................iv Introduction.................................1 Methods.....................................2 Results.......................................6 Discussion...............................14 References..............................17 Abstract in Korean....................20Maste