고해상도 CMOS 이미지 센서를 위한 나노광학소자

학위논문(박사) -- 서울대학교대학원 : 공과대학 전기·정보공학부, 2021.8. 이병호.Image sensor is a device that converts electromagnetic waves scattered by the objects or environment into electric signals. Recently, in the mobile device and autonomous vehicle industries, multiple image sensors having different purposes are required for a single device. In particular, image sensors with more than 100 million pixels are being developed in response to the development of a display to a high resolution of 8K or more. However, due to the limited space of the mobile device, the size of pixels constituting the sensor must be reduced for a high-resolution image sensor, which causes factors that reduce image quality, such as a decrease in light efficiency, a decrease in quantum efficiency, and color interference. Metasurface is a device that modulates electromagnetic waves through an array of antennas smaller than wavelength. It has been proposed as a device that replaces the color filter, lens, and photodiode constituting the optical system of the image sensor. However, the performance of the metasurface corresponding to the miniaturized pixel size was limited by the operating principle that requires several array of nano-antennas. In this dissertation, I present a metasurface optical device that can improve the image quality of an existing image sensor composed of micropixels. First, an absorption type color filter that suppresses reflection is discussed. The reflection that inevitably occurs in the conventional metasurface color filter elements causes a flare phenomenon in the captured image. In this dissertation, I design a color filter that transmits only a specific band and absorbs the rest of the absorption resonant band of a hyperbolic metamaterial antenna using a particle swarm optimization method. In particular, I present a Bayer pattern color filter with a pixel size of 255 nm. Second, I introduce a color distribution meta-surface to increase the light efficiency of the image sensor. Since the photodiode converts light having energy above the band gap into an electric signal, an absorption type color filter is used for color classification in image sensor. This means that the total light efficiency of the image sensor is limited to 33% by the blue, green, and red filters constituting one pixel. Accordingly, a freeform metasurface device is designed that exceeds the conventional optical efficiency limit by distributing light incident on the sub-pixel in different directions according to color. Finally, an optical confinement device capable of increasing signal-to-noise ratio (SNR) in low-illuminance at near-infrared is presented. Through the funnel-shaped plasmonic aperture, the light is focused on a volume much smaller than the wavelength. The focused electric and magnetic fields interact with the spatially distributed semiconductors, which achieve a Purcell effect enhanced by the presence of the metasurface. This dissertation is expected to overcome the conventional nanophotonic devices for image sensors and become a cornerstone of the development of micropixel or nanopixel image sensors. Furthermore, it is expected to contribute to building a new image sensor platform that will replace the optical system constituting the image sensor with metasurface.이미지 센서는 환경에 의해 산란되는 전자기파를 전기신호로 바꾸는 소자로, 최근 모바일 기기와 자율 주행 자동차 산업에서 단일 디바이스에 다른 목적을 가진 이미지 센서들이 요구되고 있다. 특히, 디스플레이가 8K 이상의 고해상도로 발전함에 대응하여 1억화소 이상의 이미지 센서가 개발되고 있다. 그러나, 모바일 기기의 제한된 공간에 의해 고해상도 이미지 센서를 위해서는 센서를 구성하는 픽셀의 크기를 줄여야 하며, 이는 광 효율 감소, 양자 효율 감소, 색 간섭 등의 화질을 감소시키는 요소들을 야기한다. 메타표면은 파장보다 작은 안테나들의 배열을 통해 전자기파를 변조해주는 소자로, 이미지 센서의 광학 시스템을 구성하는 색 필터, 렌즈, 포토 다이오드를 대체하는 소자로 제안되었다. 하지만, 소형화 된 픽셀 크기에 대응하는 메타표면은 나노 안테나의 동작원리와 배열의 한계에 의해 성능이 제한되었다. 본 논문에서는 초소형 픽셀로 구성된 기존 이미지 센서에 대한 화질을 높일 수 있는 메타표면 광학소자를 제시한다. 첫째로, 반사를 억제하는 흡수형 색 필터에 대해서 논의한다. 기존 메타표면 색 필터 소자에서 필연적으로 발생하는 내부 반사는 찍은 이미지에서 플레어 현상을 유발한다. 본 논문에서는 쌍곡 메타물질 안테나의 흡수 공진 대역을 입자 무리 최적화 방식을 이용해 특정 대역 만을 투과하고 나머지는 흡수하는 색 필터를 설계한다. 특히, 255 nm 크기 픽셀의 베이어 패턴 색 필터를 제시한다. 둘째로, 이미지 센서의 광 효율을 높이기 위한 색 분배 메타표면을 제시한다. 이미지 센서의 포토 다이오드는 밴드 갭 이상의 에너지를 가지는 빛에 대해 전기신호로 변환하므로, 색 구분을 위해 흡수형 색 필터를 사용한다. 이는 하나의 픽셀을 구성하는 청, 녹, 적색 필터에 의해 이미지 센서의 전체 광 효율이 33 %로 제한되는 것을 의미한다. 따라서, 서브 픽셀에 입사하는 빛을 색에 따라 다른 방향으로 빛을 분배하여 기존의 광 효율 한계를 넘어서는 자유형 메타표면 소자를 설계한다. 마지막으로, 저조도의 근적외선에서 신호 대 잡음비를 높일 수 있는 광 집속 소자를 제시한다. 깔대기 모양의 플라즈모닉 개구를 통해 빛을 파장보다 매우 작은 크기의 영역에 집중시킨다. 집속된 전기장과 자기장은 공간적으로 분포된 반도체와 상호작용함으로써, 메타표면의 존재에 따라 강화된 Purcell 효과를 얻는다. 본 박사학위 논문은 이미지 센서를 위한 기존의 제한된 메타표면 소자를 극복하고, 초소형 픽셀의 이미지 센서 개발의 초석이 될 것으로 기대된다. 나아가, 이미지 센서를 구성하는 광학 시스템을 메타표면으로 대체할 새로운 플랫폼을 구축하는 것에 기여할 것으로 기대된다.Chapter 1 Introduction 1 1.1 Overview of CMOS image sensors 1 1.2 Toward high-resolution miniaturized pixel 2 1.3 Nanophotonic elements for high-resolution camera 3 1.4 Dissertation overview 5 Chapter 2 Light interaction with subwavelength antennas 7 2.1 Overview of plasmonic antenna 7 2.2 Overview of dielectric metasurface 9 2.3 Overview of hyperbolic metamaterials 11 Chapter 3. Absorptive metasurface color filter based on hyperbolic metamaterial for noise reduction 14 3.1 Introduction 14 3.2 Principle of hyperbolic metamaterial absorbers 17 3.3 Absorptive color filter design based on particle swarm optimization method 19 3.4 Numerical analysis on optimized metasurface color filters 23 3.4.1 Single color filter optimization 23 3.4.2 Angle tolerance for optimized metasurface color filters 26 3.5 Sub-micron metasurface color filter array 29 3.6 Conclusion 35 Chapter 4 High-efficient full-color pixel array based on freeform nanostructures for high-resolution image sensor 37 4.1 Introduction 37 4.2 Optimization of metasurface full-color splitter 40 4.3 Implementation of color splitters 46 4.4 Image quality evaluation 52 4.5 Discussion about off-axis color splitters 55 4.6 Conclusion 59 Chapter 5 Plasmonic metasurface cavity for simultaneous enhancement of optical electric and magnetic fields 60 5.1 Introduction 60 5.2 Working principle and numerical results 63 5.2.1 Principle of funnel-shaped metasurface cavity 63 5.2.2 Discussion 67 5.3 Experimental results 69 5.4 Purcell effect 72 5.5 Conclusion 74 Chapter 6 Conclusion 75 Appendix 78 A.1 Colorimetry 78 A.2 Color difference CIEDE2000 79 B. Related work 80 Bibliography 81박

LTE 기반 기기간 통신 성능 향상 연구

학위논문 (박사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2016. 2. 최성현.Recently, Device-to-Device (D2D) communication has attracted much attention as an emerging solution to cope with heavy cellular traffic caused by the proliferation of mobile devices such as smartphones and tablet PCs along with the increased demands for high data rate services. D2D communication is a promising technique which is introduced to one of the technology in Fifth Generation (5G) mobile network. In this scenario, allowing User Equipments (UEs) to reuse cellular resources can boost up the network performance in terms of the system capacity. In addition, reduced number of hops and shorter communication distance via direct communication between proximity UEs implies reduced energy consumption and communication delay. Moreover, D2D communications can help offload cellular traffic and avoid congestion in cellular network. This dissertation dealt with various aspects of problems under D2D network. For performance enhancement, various schemes and algorithms for D2D discovery and communication are proposed and evaluated via simulation. First of all, we investigate the interference problem occurring during D2D discovery. Every D2D-UE (D-UE) chooses the discovery resource randomly. Therefore, if the same resource is selected by more than one D-UE, mutual interference by collision is inevitable. Moreover, the collided D-UEs can not recognize the collision event in distributed D2D network. To reduce such mutual interference, interference mitigation technique is necessary. This study proposes two schemes to improve the discovery performance by alleviating the mutual interference. Since the proposed schemes are considered to operate in distributed manner, additional signaling or resources are not needed. In addition, performance evaluation of the proposed schemes and algorithm are conducted by incorporating in recent specification. Secondly, this study proposes the D2D discovery and link setup protocol model working in an LTE network. In addition, propose discovery synchronization, beacon resource and energy efficient RRC IDLE state discovery. These proposed model and discovery design in LTE-based is the first study in academia. Even though, the demand for D2D communication has increased, energy consumption is a growing concern as well. A device has to support both cellular and D2D communication, meaning that additional energy is required. Due to the energy concerns, we comparatively analyze the performance of the D2D discovery and link setup in RRC CONNECTED and RRC IDLE states. The performance analysis is conducted by utilizing the real measurement results with commercialized LTE smartphones. Lastly, we design a spatial reuse scheme which is well-known as one of the advantages in proximity D2D communication. The spatial reuse scheme is allowed to reuse one resource by sharing multiple transmitters. However, sharing the spectrum is carefully allowed due to the generating interference mutually. Especially, when two (or more than) devices reuse in proximity. This study investigate the spatial reuse problem under D2D multicast transmission and solve it with distributed manner. Moreover, this study proposes novel resource reusing schemes by multiple transmitters to increase spectrum efficiency. The performance evaluation of the proposed schemes are conducted by incorporating in recent specification, thus the simulation results demonstrate that proposed schemes outperform the baseline scheme.Chapter 1 Introduction 1 1.1 Device-to-Device (D2D) Network 2 1.1.1 D2D Discovery 2 1.1.2 D2D Communication 3 1.2 Overview of Existing Approaches 4 1.2.1 LTE in 3GPP Standard 4 1.2.2 D2D in 3GPP Standard 4 1.2.3 Approaches for D2D Communication 5 1.2.4 Approaches for D2D Discovery 6 1.2.5 Approaches for D2D Spatial Reuse 6 1.3 Main Contributions 7 1.3.1 Interference Mitigation 7 1.3.2 Discovery Protocol Design 8 1.3.3 Spatial Reuse Operation 8 1.4 Organization of the Dissertation 9 Chapter 2 Interference Mitigation for D2D Discovery 10 2.1 Introduction 10 2.2 Background 11 2.2.1 Resource Selection 11 2.2.2 Resource Collision 13 2.2.3 Motivation 14 2.3 System Model 15 2.3.1 D2D System 15 2.3.2 Criteria of Discovery Success 16 2.4 Problem Formulation 17 2.5 Power Control Scheme 18 2.5.1 Power Control Performance 18 2.5.2 Proposed Power Control Algorithm 19 2.6 Collision Resolution Scheme 22 2.6.1 Beacon Design 22 2.6.2 Collision Resolution Scheme 23 2.7 Performance Evaluation 25 2.8 Summary 27 Chapter 3 Protocol Design for D2D Discovery 31 3.1 Introduction 31 3.2 Background 32 3.2.1 Radio Resource Control (RRC) 32 3.2.2 Discontinuous Reception (DRX) 33 3.2.3 Motivation 34 3.3 System Model 35 3.3.1 D2D Beacon 35 3.3.2 D2D Discovery 36 3.3.3 Synchronization 36 3.3.4 D2D Link Setup 38 3.4 Numerical Analysis 39 3.4.1 Average Power Model 39 3.4.2 Base Power Model 40 3.4.3 D2D Link Setup Delay 41 3.5 Performance Evaluation 42 3.6 Summary 42 Chapter 4 Spatial Reuse for D2D Communication 46 4.1 Introduction 46 4.2 Background 48 4.2.1 D2D Communication 48 4.2.2 D2D Group Communication 49 4.2.3 Motivation 51 4.3 Problem Statement 53 4.3.1 Criteria of Successful D2D link 53 4.3.2 Spatial Reuse Interference 54 4.4 Proposed Spatial Reuse Scheme 55 4.4.1 Range-Based Approach 55 4.4.2 Spatial Reuse Scenario 56 4.4.3 Upper Bound and Lower Bound . 58 4.5 Spatial Reuse Operation 60 4.5.1 Spatial Reuse Procedure 60 4.5.2 Spatial Reuse Grant 61 4.6 SR with Multiple Transmitters 63 4.6.1 PS-SR Scheme 64 4.6.2 P-SR Scheme 65 4.7 Performance Evaluation 67 4.8 Comparison of PS-SR and P-SR Schemes 72 4.8.1 Overhead Comparison 72 4.9 Summary 74 Chapter 5 Conclusion 75 5.1 Summary 75 5.2 Future Work 77 Bibliography 79 Abstract (In Korean) 87Docto