Real-Time and Low-Noise Terahertz Imager Based on CMOS Technology

Sang Hyo Ahn Ulsan National Institute of Science and Technology 2024 국내박사

Terahertz (THz) technology has immense applications in imaging, communication, and spectroscopy because electromagnetic waves in the THz band (0.1 - 10 THz) exhibit unique straightness and permeability characteristics. In particular, “THz see-through” imaging using the transmittance properties into non-metallic materials at THz frequency is an impactful application. Nevertheless, few THz imaging systems have been developed considering that THz frequency is higher than the cutoff frequency of Si-based FET, the most widely used device in RF applications. Moreover, it has too low photon energy to be detected by a photo-diode used in CMOS image sensor (CIS). However, FET- based plasmonic THz detectors have received considerable attention considering the multi-pixel integration potentials for real-time imaging. In the plasmonic mode, THz detection mechanism involves detecting the density oscillation of local electrons with DC offset voltage instead of detecting the electron transfer. While plasmonic THz detectors utilizing high-electron-mobility transistor (HEMT) technologies offer exceptional sensitivity, integrating such detectors with signal processing circuits on a single chip presents challenges. To address this issue, CMOS technology has garnered attention due to its advantages in terms of cost-effectiveness and high integration capabilities. In recent developments, THz detectors employing CMOS technology have utilized Schottky diodes, which function as current-driven transit-mode devices, and field-effect transistors (FETs), known for their plasmonic power detection capabilities. Within CMOS FET-based detectors, the advantages of the plasmonic power detection mechanism are evident. Unlike transit-mode devices limited by cutoff frequency, plasmonic detection isn't restricted in the same manner. This feature enables enhanced performance with increasing THz frequencies and imparts robustness against high-input THz power. CMOS technology-based plasmonic THz detectors have attracted much attention for real-time/large- scale multi-pixel THz imagers. Recently, we reported a remarkable THz detector performance enhancement by reconfiguring the boundary conditions in nano-ring FETs. To accelerate the commercialization of THz imagers, pixel-level high-performance and low-noise characteristics should be guaranteed in large-scale array operation with peripheral analog circuitry. Therefore, optimizing the analog buffer design considering the pixel’s ac boundary conditions is essential based on compact simulation model simulation. In addition, high-speed modulation must be performed to reduce flicker noise that increase due to additional circuitry. In this work, we report a record-high performance THz imager based on trantenna (transistor-antenna) integrated with an analog system using a 65-nm CMOS foundry. We investigate the high-speed and low-noise operation of ground (gnd)-out trantenna pixel-based single-chip THz imager array with low- impedance analog buffers and the reset modulation switch.

Development of High-speed Gamma-ray Imaging System for Application in Port, Airport, and Bonded Warehouse

이준영 한양대학교 대학원 2024 국내박사

For the detection of nuclear and radioactive materials in imported cargo, radiation portal monitors (RPMs) and handheld survey meters are currently used at borders, including ports, airports, and bonded warehouses. However, these devices lack imaging capabilities, making it time-consuming to locate nuclear or radioactive materials accurately. This limitation not only slows down customs work but also compromises safety risks. To overcome the limitation, in this study, a Fast Gamma Imager (FGI) was developed. The FGI features high performance, including high-speed, excellent three-dimensional (3-D) imaging capability for a broad energy range in freely moving environments. The FGI system consists of two detector heads, a coded aperture mask, a data acquisition (DAQ) system, two contextual sensors, and a laptop computer. To ensure stability and enhance mobility, the FGI is mounted on a dedicated motorized cart designed to absorb shocks and vibrations. This setup enables the FGI to provide intuitive and accurate 3-D radiation images by using simultaneous localization and mapping (SLAM) based on contextual sensors. Considering the use of the FGI in outdoor settings, an energy correction method based on background radiation was implemented to maintain performance under temperature variation. In addition, the FGI can estimate ambient dose rates using the G(E) factor, contributing to safety for workers. The energy resolution was evaluated under various event rate conditions. Especially, the system offers high energy resolution under very high event rate conditions, i.e., 6.9% and 8.6% (for 662 keV) at 1.3×105 s-1 and 1.2×106 s-1, respectively. The ambient dose rates estimated based on the G(E) factor closely matched the true dose rates, with a maximum error of 33.7% for experimental conditions up to 5 µSv/h. A reconstruction of the hybrid collimation method, which combines mechanical and electronic collimation with pixel-wise multiplication, was developed ensuring stable imaging performance across a variety of gamma energies. The system successfully obtained clear images of source locations through hybrid imaging at energies ranging from as low as 59.5 keV (241Am) to as high as 1,332 keV (60Co), with angular resolution between 7.3–12.3°. The high sensitivity of the system allowed it to image a 137Cs source with a dose rate lower than the background (~0.1 µSv/h) in a short measurement time: it required 0.6 s and 300 s when the dose rate was 28% and even only 0.5% of the background, respectively. The FGI was also demonstrated successfully at borders with various imported cargoes, including bonded warehouse cargoes, unit load devices, and dry containers. It was able to image the sources while moving freely in vast and complex spaces. The developed FGI notably extended the applicable conditions of gamma-ray imaging in terms of sensitivity, imageable energy range, and free-moving imaging. These advancements mean that the FGI not only offers a new perspective on radiation imaging but also significantly enhances the operational effectiveness of homeland security.

(An) efficient technique to estimate and correct the stray light effect in geostationary ocean color imager (GOCI)

오은송 Graduate School, Yonsei University 2018 국내박사

세계 최초 정지궤도 해양관측위성인 GOCI 는 하루 주간 8회 한반도와 주변 영역을 16개 슬롯 단위로 관측을 하고, 광학계는 비축 삼반사경 (TMA) 형태로 구성되어 있으며, 광학계와 검출기 사이에 2개의 필터를 사용하여 412 ~ 865 nm의 분광 대역을 8개의 밴드로 촬영한다. 2010년도에 발사되어 현재까지 운용 중에 있는 이러한 GOCI 광학계에 대해서, 본 논문은 운영 중 문제점으로 파악된 궤도상 잡광의 영향을 분석하고, 이를 제거하여 영상의 품질을 향상 시키는 방법을 기술하였다. GOCI의 촬영 방식의 특성과 광학계 내부 구조의 산란에 의해서 특정 밴드의 경우 약 20% 편차까지 나타나는 슬롯 간 복사 편차(ISRD)라는 문제가 발견되었다. 기존 측정과 보상 방법에서는 고스트/잡광 문제에 대해 최종 산출물을 대상으로 밴드 간 최소 변화량 함수를 이용하였다. 영상 자체를 보정하는 방법은 품질을 조정을 용이한 장점을 가지지만, 의도치 않은 신호 값에 영향을 주거나, 노이즈의 원천적인 원인을 규명하지 못하고, 해수 신호에만 적용 가능하다는 단점을 가진다. 구름이 많거나, 육상 지역의 보정에는 적용이 불가능하였다. 이러한 단점을 보완하고자 노이즈를 발생시키는 원인을 분석하고, 그 양을 정밀하게 계산할 수 있는 방법이 필요하였다. 따라서, 본 논문에서는 앞에서 제시된 난제를 해결하기 위하여 통합적 광선 추적 기법을 이용하여 ISRD를 만들어 내는 잡광과 고스트의 경로를 확인하였다. 통합적 광선 추적 방법은 3가지 수치모사 단계(광원, 목표, 기기)로 이루워져 있으며, 원격탐사 환경을 실질적으로 묘사하였다. 그 결과, 맑은 날 영상의 수치모사에서 디퓨젼 잡광 형태가 각 슬롯 영상의 남쪽 하단에 발생하는 것을 확인하였고, 구름 영상은 구름의 이동 경로에 따라 그 영향 또한 이동한다는 사실을 밝혔다. 더불어, GOCI에서 발생하는 ISRD의 형상과 그 양을 보정하기 위하여, 지상 시험 데이터와 시뮬레이션 결과를 융합한 잡광 보정 방법이 제시되었다. 이 ESM (Effective Stray light distribution Map) 모델은 DSM (Diffusion Stray light distribution Map)과 GSM (Ghost based stray light distribution Map) 이로 이뤄져 있다. 이 모델을 이용하여 슬롯 간 복사 편차의 원인 중의 하나인 잡광을 보정한 결과를 제시하였다. 원 영상에서 절대 편차량(APD) 기준 약 20% 가량의 슬롯 간 편차가 밴드 6번 영상에서 3% 정도까지 줄어드는 등 획기적으로 영상이 보정됨을 확인하였다. 본 논문에서 제시한 통합적 광선 추적을 기반으로 한 잡광 보정 기법은 기존의 방법에서 파악하지 못했던 기기 자체의 문제 요소를 분석하고, 최종 영상의 품질까지 개선하였다는 점에서 보다 진보된 보정 기술이라 할 수 있다. 1%의 불확도에도 민감한 해양 센서의 성격으로 볼 때, 본 기술은 복사 보정 단계에서 적용되어 센서 측정 불확도를 낮출 것으로 기대된다. 또한, 이는 GOCI에 특화되어 현업에서 사용 가능한 기술로써, 활용성 측면에서도 가지는 의의가 크다. 또한 나아가 차세대 위성(ex, 정지궤도 복합위성, 차세대 중형위성)의 궤도상 운영 기술과 영상 전처리 과정에서 유용하게 쓰일 것으로 예상된다. The Geostationary Ocean Color Imager (GOCI), the first optical sensor in geostationary orbit for monitoring the ocean environment around the Korean peninsula, has acquired 16 slot images to develop one mosaiced image. The optical system of GOCI is of three-mirror-anastigmat (TMA) type, with two filter wheel systems covering a spectral range of 412 -- 865 nm. For the optical performance of GOCI during the operation period since 2010, this thesis has described the in-orbit stray light analysis and correction method for improving the image quality. GOCI has suffered from stray light effects, which are radiometric artifacts, called inter-slot radiometric discrepancy (ISRD), with 20% difference at the most, caused by instrumental multiple reflection and due to environmental reasons. Previous research on ISRD dealt with the difference phenomena in the final products with the minimum noise fraction algorithm to correct the images. This algorithm is efficient for generating a continuous slot boundary image, but not for determining the cause of the stray light effect. In addition, this method can be applied to the ocean signal only, and not cloudy scenes and the land surface. To address the demerits of the previous researches, an accurate and quantitative methodology is required to analyze the cause of stray light effect and to calculate its amounts. In this thesis, to characterize and determine the stray light anomaly, a ray tracing based simulation model (IRT) has been developed, which reveals the principle ray path and the cause of the stray light effect. IRT consists of three simulation phases (source, target, and instrument), which enable the construction of a real environment from the remote sensing data, focusing on realistic phenomena. In the results, even in a cloud-free environment, a diffusion stray light pattern is identified at the bottom of each slot. Variations in the stray light effect and its pattern according to the bright target movement are simulated. Furthermore, to reduce the artifacts' shape and quantities of stray light effect in the GOCI L1A images, this paper proposes the fussed in-orbit stray light correction methodology. This model, called the effective stray light distribution map (ESM), comprises two sub-models: a diffusion stray light map (DSM) simulated using an IRT, and a ghost based stray light effect map (GSM) developed by running a semi-analytical algorithm with on-ground experimental test data. ESM is applied to a level 1A (L1A) slot image, to produce the corrected values with top-of-atmosphere (TOA) radiance. The resultant corrected radiances are lower in terms of absolute percentage of difference (APD) of the validation points from 20% to 3% in the band-6 image. The results of this thesis show that the proposed method improves the radiometric performance of GOCI images by separating the bias associated with stray light. Contrary to the previous correction methods, the ESM method has greater significance to reveal and correct the in-orbit stray light effect with physical meaning. In the future, it can be developed as a simple and general methodology with regard to step-and-stare type sensor data such as GOCI-II, and applied to any sensor that suffers from similar stray light effects.

Understanding Galactic 26Al With the Compton Spectrometer and Imager

Beechert, Jacqueline Nicole University of California, Berkeley ProQuest Disser 2023 해외박사(DDOD)

Aluminum-26 (26Al) is a radioactive isotope produced in massive star processes. High-resolution spectroscopy of its 1.809 MeV gamma-ray (γ-ray) decay signature constrains the dynamics of its emission as it is ejected from its progenitor sites and incorporated into the interstellar medium of the Milky Way Galaxy. Imaging reveals dominant emission in the Inner Galaxy and emission in localized regions of massive star activity. Taken together, spectroscopy and imaging of 26Al shed light on the chemical evolution of the Galaxy over millions of years.The Compton Spectrometer and Imager (COSI) is a compact Compton telescope designed to measure astrophysical γ-rays of energy 0.2-5 MeV. Its high-purity germanium detectors track incident photons as they Compton scatter throughout the detector volume. In 2016, COSI flew on a NASA ultra-long duration balloon for 46 days. This dissertation details the first analysis of 26Al in the flight data and reports a measurement of 3.7σ significance above background and an Inner Galaxy flux of (8.6 ± 2.5) x 10−4 ph cm−2 s−1. All scientific achievements from the flight are predicated on calibrations performed before launch. These calibration procedures were repeated in advance of an intended 2020 balloon flight. Analyzing calibration data validates instrument performance and informs studies of detector effects, including charge sharing and charge trapping, that complicate the measurement process.The next generation of COSI as a NASA Small Explorer satellite is slated for launch in 2027. It is anticipated to strengthen the spectroscopic measurement of 26Al in the balloon flight and yield the most detailed images of 26Al to date. Extensive testing of the imaging algorithm in COSI's new analysis toolkit is presented as a first step towards producing these images. The desired culmination of these efforts is an enhanced understanding of Galactic 26Al by way of thorough calibrations, novel insight into undesirable detector effects, and advanced analyses of high-resolution data from the COSI balloon and satellite instruments.

An Asynchronous Flash LiDAR Sensor Based on Pixel-wise Time-of-Flight Validation via Background Light Adaptive Threshold

Seonghyeok Park Ulsan National Institute of Science and Technology 2025 국내박사

The demand for 3D imaging technologies is rapidly increasing across various fields, including autonomous vehicles, robotics, augmented reality, and industrial automation. These applications rely on accurate and efficient depth sensing systems capable of capturing high-speed and high-resolution data in real time. Such systems must handle a wide range of environmental conditions, including variable lighting, diverse target reflectivity, and dynamic scenes, while maintaining robustness and reliability. The key challenges include managing wide input dynamic ranges, mitigating background light interference, and optimizing resource utilization such as memory and power. These constraints drive the need for innovative solutions to advance the performance of depth sensing technologies. Flash LiDAR sensors have emerged as essential tools to meet these demands, offering unparalleled speed and resolution. However, existing systems face significant limitations in their ability to efficiently process wide dynamic ranges and cope with environmental noise. This research addresses these challenges through two key contributions: the development of a novel Delta Intensity Quaternary Search (DIQS) histogramming Time-to-Digital Converter (hTDC) and its integration into an asynchronous flash LiDAR sensor. Together, these innovations enable precise and robust depth measurements while minimizing resource consumption. The DIQS hTDC represents a substantial advancement over traditional zooming histogramming TDCs by introducing a coarse-to-fine histogramming mechanism combined with adaptive bin allocation. This approach enhances memory efficiency and extends the dynamic range, ensuring accurate time-of- flight (ToF) measurements even under challenging conditions. The DIQS hTDC iteratively narrows the region of interest using a quaternary search method, concentrating computational resources on the most relevant photon arrival times. This technique inherently suppresses background noise, reduces memory usage, and enables scalability for resource-constrained ToF systems, addressing critical bottlenecks in previous designs. Building on the DIQS hTDC, an asynchronous flash LiDAR sensor is proposed to autonomously validate pixel-level ToF measurements. Each pixel dynamically adjusts its integration time based on the intensity of background light, leveraging a compact bitwise arithmetic unit to compute thresholds in real time. This pixel-level autonomy eliminates the need for complex global synchronization while selectively reading out validated pixels. Non-validated pixels continue collecting photons, improving data efficiency and robustness. The asynchronous architecture simplifies system complexity and enhances adaptability across diverse environmental and operational conditions. A prototype asynchronous flash LiDAR sensor incorporating the DIQS hTDC was fabricated in a 90- nm CMOS process and extensively tested. The sensor achieved a precision of 6 cm and an accuracy of 8 cm over ranges from 1.5 to 22.5 meters indoors. Under outdoor conditions with ambient illumination up to 30 klux, the sensor maintained a true detection rate exceeding 95% at a distance of 21 meters. Its ability to dynamically adjust frame rates from 5 to 250 fps demonstrates versatility and adaptability across varying signal-to-noise ratio conditions, catering to applications ranging from high-speed operation to high-precision imaging. This research makes two significant contributions to the advancement of SPAD-based flash LiDAR systems. First, the DIQS hTDC introduces a scalable and efficient histogramming solution that overcomes memory and noise challenges. Second, the asynchronous flash LiDAR sensor demonstrates the practical integration of this technology into a robust and compact system capable of addressing the complex demands of real-world environments. Together, these innovations pave the way for the next generation of 3D imaging systems, offering a practical, high-performance, and scalable solution for diverse applications requiring precise and efficient depth sensing. Keywords: 3D imager, CMOS image sensor, depth sensor, LiDAR, time-of-flight (ToF), direct ToF, histogramming, in-pixel histogramming, single-photon avalanche diode (SPAD), time-to-digital converter (TDC), delta intensity quaternary search (DIQS), asynchronous.

Development of Cloud-Screening-Based Postprocessing Framework for GOCI-II Aerosol Optical Depth: Evaluation of Retrieval Errors

김준희 서울대학교 대학원 2025 국내석사

Satellite-derived aerosol optical depth (AOD) products are indispensable for characterizing spatial aerosol distributions and constraining uncertainties in atmospheric chemistry models. However, the retrieval of AOD from the Geostationary Ocean Color Imager-II (GOCI-II) is susceptible to cloud contamination due to the lack of thermal infrared channels. This study develops and evaluates a two-step postprocessing framework to reduce residual cloud contamination in the Level 2 GOCI-II AOD product. The framework combines cloud screening based on the Himawari-8/9 Advanced Himawari Imager (AHI) cloud type product with a statistical cloud filtering based on spatial standard deviation (STD) and retrieval ratio, which is defined as the proportion of valid GOCI-II AOD pixels to total pixels within a collocation grid (0.2˚×0.2˚). Sensitivity analyses and empirical case studies across multiple threshold values confirmed that the configuration of (STD=0.05, retrieval ratio=0.5) consistently yielded high agreement with Aerosol Robotic Network (AERONET) AOD, both in terms of correlation coefficient and error metrics. After applying the postprocessing framework, agreement of GOCI-II AOD to AERONET AOD improved; the Pearson’s correlation coefficient improved from 0.77 to 0.81, root-mean-square error (RMSE) decreased from 0.175 to 0.111, and the skewness of the GOCI-II AOD minus AERONET AOD histogram reduced from 3.33 to 0.94. Qualitative inspection also confirmed that the postprocessing effectively mitigated high AOD signals originated from optically thin cloud edges while retaining major aerosol features. Nonetheless, some systematic errors remained: GOCI-II AOD tended to be overestimated in coastal regions and underestimated over inland regions, particularly under low AOD (AERONET AOD < 0.2) conditions. The contrasting behaviors suggest potential deficiencies in the surface reflectance estimation technique which differs between land and ocean surfaces. These findings provide a scientific basis for improving future satellite aerosol retrieval algorithms. Furthermore, the methodology is readily applicable as a preprocessing module in satellite-derived AOD data assimilation systems, enabling the construction of realistic initial conditions for atmospheric chemistry models. 위성 기반 에어로졸 광학 두께(aerosol optical depth, AOD) 산출물은 에어로졸의 생성 및 수송을 실시간 감시하고, 모델의 초기장을 현실에 가깝게 제약하여 대기질 예보의 정확도를 높이는 데 필수적이다. 그러나 우리나라의 정지궤도 해색 위성 센서인 GOCI-II (Geostationary Ocean Color Imager–II)는 열적외 채널의 부재로 구름에 의한 오염에 취약하다. 본 연구에서는 GOCI-II Level 2 AOD 자료에 잔여하는 구름의 신호를 줄이기 위해 2단계에 걸친 후처리 체계를 개발하고 평가하였다. 본 연구에서 제안한 후처리 체계는 Himawari-8/9위성에 탑재된AHI (Advanced Himawari Imager) 센서 기반의 구름 마스킹과, 격자 내 GOCI-II AOD 산출값의 표준편차 (standard deviation, STD) 및 산출 비율(유효 AOD 픽셀 수를 격자 내의 총 픽셀 수로 나눈 값)에 기반한 통계적 구름 제거 기법을 결합하였다 (분해능: 0.2˚×0.2 ˚). 통계적 방법의 임계값 설정을 위한 민감도 분석 결과, STD=0.05 및 산출 비율=0.5의 기준이 AERONET (Aerosol Robotic Network) AOD와의 상관계수 및 오차 지표 측면에서 우수한 일치를 보였다. 해당 설정 적용 시, 상관계수는 0.77에서 0.81로 상승하고, 평균제곱근오차(root mean square error, RMSE)는 0.175에서 0.111로 감소하였으며, GOCI-II AOD와 AERONET AOD 간 절대오차 히스토그램의 왜도(skewness)는 3.33에서 0.94로 감소하였다. 정성적인 분석에서도 후처리 기법 적용 시 구름 가장자리 및 얇은 구름의 영향을 효과적으로 제거하면서 주요 에어로졸 신호가 보존됨을 확인하였다. 그럼에도 불구하고, 지표면 유형에 따라 서로 다른 산출 오차 특성이 여전히 나타났으며, 특히 낮은 AOD 조건(AERONET AOD < 0.2)에서는 GOCI-II AOD가 연안 지역에서는 과대추정, 내륙 지역에서는 과소추정하는 경향을 보였다. 이는 해양과 육지의 서로 다른 지표면 반사도 추정 기법에 기인한 것으로 해석된다. 본 연구 결과는 향후 위성 에어로졸 산출 알고리즘 개선을 위한 근거 자료로 활용될 수 있으며, 특히 구름 제거 및 지표면 반사도 추정 측면에서의 개선 필요성을 강조한다. 아울러, 본 연구에서 제안한 후처리 기법은 위성 AOD 자료동화를 위한 전처리 시스템 구축에 손쉽게 적용 가능하며, 화학수송모델의 초기장 생산에 신뢰할 수 있는 위성 입력자료를 제공할 수 있다.

Multi-color Photodetectors via Wafer Bonding and Epitaxial Lift-Off of III-V Compound Semiconductors

금대명 서울대학교 대학원 2019 국내박사

자외선부터 적외선 영역의 밴드갭을 가진 3-5족 화합물 반도체는 단일 파장대역을 시각화하는 imager 로 널리 사용되고 있다. 그러나, 최근 사물인터넷 시대가 도래함에 따라, time of flight (TOF) 센서, 식생지수 측정, night vision 등의 새로운 응용처가 증가하고 있다. 따라서 기존의 단일파장 광 검출기가 아닌, 다중파장 광 검출기의 중요성이 증대되고 있으며, 이런 다중파장 광 검출기를 구현하기 위해서 3-5족을 화합물 반도체의 epitaxy 방법이 흔히 사용되어 왔다. 예를 들어, 다른 격자 상수를 가진 벌크 구조를 metamorphic 성장법을 이용하여 성장하거나, 또는 양자우물, 양자점 그리고 type-II 기반의 구조가 적용되어야만 했다. Epitaxy 방법은 매우 간단한 방법처럼 보이지만, 기판과 성장하려는 물질간의 격자상수의 차이로 인해 제한되는 물질 선택, 내부 결함에 의한 성능감소 그리고 성장의 복잡함 등 여러 문제가 존재한다. 그래서, epitaxy 방법의 단점들을 회피하기 위하여, 다른 기판에서 성장된 소자의 집적을 가능하게 할 수 있는, 이종 집적 방법이 대안이 되어왔다. 이를 이용하면, 간단한 벌크 구조의 광 검출기를 결합할 수 있기 때문에 매우 유망한 방법으로 여겨지고 있다. 그러나, 이종 집적 방법의 뛰어난 장점에도 불구하고, 현재의 다중파장 광 검출기는 집적 방법의 문제로 수직 정렬 오차 및 픽셀 밀도 측면에서 한계점을 보여주었다. 따라서, 본 논문에서는 고밀도/ 고정렬된 다중파장 광 검출기 제작을 위한 3-5족 기반의 화합물 반도체의 이종 집적 방법에 대한 연구를 진행하였다. 먼저, 3-5족 GaAs 기반의 박막소자를 다른 기판과 이종 집적하는 연구를 수행하였다. 기존의 wafer splitting 과 transfer printing 방법과 비교했을 때 대면적 전사, 저렴한 가격 그리고 고품질 layer등 장점들이 있는 웨이퍼 접합과 에피택셜 리프트 오프 (epitaxial lift off) 방법에 대해서 연구를 하였다. 단일 기판상에 다중파장 광 검출기를 제작하기 위해서는, rigid-to-rigid 이종 집적 방법이 반드시 필요하다. 그러나, strain 과 외부 장치를 이용하여 기판 분리를 가속화 시킬 수 있는 유연기판 상의 박막전사와 달리, 습식 식각 시 생성되는 부산물들과 가스 기포들 때문에 에피택셜 리프트 오프 방법을 이용한 rigid-to-rigid 전사에 대해서는 매우 적은 결과만이 보고되었다. 이런 문제를 극복하기 위해서, pre-patterning 과정과 표면 친수화를 통한 고속 에피택셜 리프트 오프를 제안하였다. 이 pre-patterning 과정은 AlAs 희생층의 식각 영역을 극대화 시킬 수 있으며, 기포를 빠르게 제거할 수 있다. 그리고, 친수성 용액인 아세톤을 불산과 섞어주면 점도와 표면 접촉 각을 줄일 수 있다. 이것은 식각 용액의 효과적인 침투와 부산물을 억제시키는 결과를 보였다. 결과적으로, 2 인치 크기의GaAs 기반 박막들을 rigid 기판상에 30분 이내로 전사가 가능했으며 이는 기존의 보고들과 비교했을 때 가장 빠른 결과이다. 또한 이 템플릿을 이용하여 광/전자 소자를 성공적으로 제작 및 동작시켰다. 두 번째로, 기존의 InSb 물질을 이용한 벌크 구조의 광 검출기가 가진 파장한계 그리고 저온동작 등의 제약들을 극복하기 위한 연구를 진행하였다. 벌크 구조의 파장 한계를 원적외선 대역까지 늘이기 위한, 3-5족 물질 중 이상적인 물질은 인듐 아세나이드 안티모나이드 (indium arsenide antimonide) 이다. 왜냐하면 InAsxSb1-x는 SWIR 부터 LWIR 의 해당하는 밴드 갭을 가지고 있기 때문이다. 이 물질 기반의 구조와 개발된 공정을 사용하면, 우리는 궁극적으로 자외선부터 원적외선까지의 영역을 벌크 구조만을 사용하여 다중파장 광 검출기를 구현할 수 있게 된다. 따라서, 이 물질의 가능성을 검증하기 위해서, InAs0.81Sb0.19 의 흡수층을 가진 p-i-n 구조 기반의 광 검출기를 GaAs 기판상에서 성장하였다. 고온에서 동작 특성을 향상시키기 위하여, 최적의 InAlSb 배리어를 TCAD로 디자인하였다. 이러한, InAsSb/InAlSb 이종 접합 광 검출기는 분자선 증착 장비를 이용하여 성장되었다. 그 결과로, 우리는 비슷한 Sb 조성을 가진 InAsSb 기반의 광 검출기들 중에서, 처음으로 중적외선 대역의 이종 접합 구조의 광 검출기를 상온 동작 하는 것을 시연하였다. 게다가, 그것은 상용화 된 광 검출기보다 높은 광 응답 특성(15 mA/W)을 보여주었다. 이 상온에서 동작하는 중적외선 광 검출기는 InSb 광 검출기에 사용되는 Dewar 를 제거함으로써, 최종 검출기 시스템의 부피를 감소 시킬 수 있다. 이 실험으로부터, 벌크 구조로 원적외선 대역을 검출하기 위한 InAsSb 물질의 큰 잠재성이 있다는 것을 의미한다. 마지막으로, 벌크 구조의 집적을 통한 다중파장 광 검출기의 실현이 가능한지 확인하기 위해서 모놀리식(monolithic) 집적에 관한 연구를 수행하였다. 확보된 상온 동작이 가능한 가시광선부터 MWIR 검출 파장을 가진 광 검출기들 중에서, 최적의 제작 순서를 확립하기 위해서 물질에 관한 성숙도가 높은 가시광선 GaAs 그리고 근적외선 InGaAs 광 검출기를 사용하였다. 가시광/근적외선 대역의 다중파장 광 검출기를 형성하기 위해서, GaAs 광 검출기를 InGaAs 광 검출기 상으로 개발된 고속 에피택셜 리프트 오프 기법을 사용하여 전사하였다. GaAs 광 검출기와 InGaAs 광 검출기는 off-axis 없이 수직으로 잘 정렬되었음을 x-ray 분광법을 이용하여 확인하였다. 또한, 각각의 광 검출기의 기준 소자들과 비교했을 때 비슷한 암 전류가 나타났다. 마지막으로, 레이저 입사를 통해, 두 개의 광 검출기 대한 광 반응은 물질 특성들에 따라 가시광과 근적외선에서 각각 명확하게 나타났다. 이러한 결과들은 고속 에피택셜 리프트 오프 기법이 높은 픽셀 밀도 및 거의 완벽한 수직 정렬도를 갖는 한 기판상의 벌크 기반의 다중파장 광 검출기를 집적할 수 있다는 것을 의미한다. 미래의 목표하는 응용처에 따라, 원하는 파장들을 갖는 광 검출기를 벌크 구조로 간단하게 성장할 수 있고, 다중파장 이미징 시스템을 제작 할 수 있다. Group III-V compound semiconductors, having a band gap from ultraviolet to infrared regions, have been widely used as imagers to visualize a single band. With the recent arrival of the Internet-of-things (IOTs) era, new applications such as time of flight (TOF) sensors, normalized difference vegetation index (NVDI) and night vision systems have gained interest. Therefore, the importance of multicolor photodetectors is raised. To implement multicolor photodetectors, an epitaxy method has been commonly used with III-V compound semiconductors. For example, quantum wells, quantum dots and type-II based structures and metamorphically grown bulk heteroepitaxial structures have been employed. Although an epitaxy method seems to be quite simple, there are several problems including limitation of material choice due to the discrepancy of lattice constants between thin films and substrates, performance degradation originated from internal defects and complexity of growth. Therefore, to avoid these disadvantages of the epitaxy method, a heterogeneous integration method has been an alternative because the integration of devices grown on different substrates is possible. Thus, it has been considered to be a promising method to combine photodetectors with simple bulk structures. However, although there is a significant advantage to the heterogeneous integration method, current multicolor photodetectors have exhibited limitations regarding pixel density and vertical misalignment due to problems related to conventional integration methods. Therefore, in this thesis, the heterogeneous integration of III-V compound semiconductors was investigated for fabricating multicolor photodetectors with high pixel density and highly accurate alignment. Firstly, a research on heterogeneous integration of GaAs based thin film devices with other substrates was carried out. We studied wafer bonding and epitaxial lift off process which have advantages including large area transferability, cost-effectiveness and high quality of layers compared with wafer splitting and transfer printing methods. To fabricate multicolor photodetectors on single substrates, a stable rigid-to-rigid heterogeneous integration method is highly required. However, there have only been few reports regarding rigid-to-rigid transfer by using epitaxial lift off due to the difficulty involving byproducts and gas bubbles generated during the wet etching of the sacrificial layer for wafer separation. This has been a hindrance compared with thin film on flexible substrates which can accelerate wafer separation by using strain and external equipment. In order to overcome this problem, high throughput epitaxial lift off process was proposed through a pre-patterning process and surface hydrophilization. The pre-patterning process can maximize the etching area of the AlAs sacrificial layer and rapidly remove bubbles. In addition, acetone, which is a hydrophilic solution, was mixed with hydrofluoric acid in order to reduce the surface contact angle and viscosity. It resulted in an effective penetration of the etching solution and the suppression of byproducts. Consequently, it was possible to transfer GaAs thin films on rigid substrates within 30 minutes for a 2 inch wafer which has been the fastest compared with previous reports. Also, using this template, electronic and optoelectronic devices were successfully fabricated and operated. Secondly, we have studied to overcome restrictions of bulk photodetector for InSb binary material including the detection limit and cryogenic operation. To extend the detection limit of bulk InSb toward the LWIR range, the ideal candidate of III-V bulk materials is indium arsenide antimonide (InAsSb) material due to its corresponding band gaps ranging from SWIR to LWIR. By combining bulk InAsSb with other bulk materials with previously developed integration methods, we could ultimately fabricate a multicolor photodetector ranging from ultraviolet to LWIR with only bulk structures. Thus, in order to verify the viability of this material, a p-i-n structure based photodetector with an InAs0.81Sb0.19 absorption layer was grown on a GaAs substrate. To enhance an ability to be operated at a high temperature, an optimum InAlSb barrier layer was designed by technology computer aided design (TCAD). Also, InAsSb/InAlSb heterojunction photodetector was grown by molecular beam epitaxy (MBE). As a result, we have demonstrated the first room temperature operation of heterojunction photodetectors in MWIR range among InAsSb photodetectors with similar Sb compositions. Additionally, it has a higher responsivity of 15 mA/W compared with commercialized photodetectors. This MWIR photodetector with room temperature operability could help the reduction of the volume for final detector systems due to the elimination of Dewar used in InSb photodetectors. In other words, from this experiments, it is suggested that there is a strong potential of InAsSb bulk structures for detecting LWIR. Finally, the study on the monolithic integration was carried out to verify the feasibility of multicolor photodetectors by integration of bulk structures. Among procured photodetectors with detection ranging from visible to MWIR at room temperature operation, visible GaAs and near-infrared InGaAs photodetector were used for establishing the optimized fabrication process due to material’s process maturity. By using previously developed high throughput ELO process, GaAs photodetectors were transferred onto InGaAs photodetectors to form visible/near-infrared multicolor photodetectors. It was found that top GaAs PD and bottom InGaAs PD were vertically well aligned without an off-axis tilt in x-ray diffraction (XRD) measurement. Also, similar dark currents of each photodetector were observed compared with reference photodetectors. Finally, with incidence of laser illumination, photoresponses were clearly revealed in visible band and near-infrared band of material characteristics, respectively. These results suggested high throughput ELO process enables the monolithic integration of bulk based multicolor photodetectors on a single substrate with high pixel density and nearly perfect vertical alignment. In the future, depending on the target applications, photodetectors with desired wavelengths could be simply grown as bulk structures and fabricated for multicolor imagers.

Built-in-self-calibration of exponential ramp generator for logarithmic single-slope ADC in CIS

정운기 Graduate School, Yonsei University 2007 국내석사

CMOS 이미지 센서의 경우에 로그응답 단일 기울기 ADC로 아날로그 감마 보상기능을 수행할 수 있다. 이 로그응답 단일 기울기 ADC는 기울기와 곡률에 대한 보상기능이 있는 지수형 램프신호발생기가 필요하다. 이에 본 논문에서는 지수형 램프 신호발생기뿐 아니라 지수형 램프신호발생기를 위한 자동교정 알고리즘을 제안한다. 자동교정회로를 가지는 지수형 램프신호 발생기는 로그응답 단일기울기 ADC를 가지는 CMOS 이미지 센서에 0.35-μm CMOS 공정으로 설계되었다. 시뮬레이션 결과에 따르면, 제안된 자동교정 회로는 램프 발생기의 출력이 원하는 ADC의 특성을 가지도록 하는 목표 램프 신호가 되도록 해준다. 이것은 공정변화가 있더라도 올바른 ADC 출력이 보장됨을 뜻한다. The analog gamma correction can be performed using a logarithmic single-slope analog to digital converter (SS-ADC) in the CMOS imager. This logarithmic ADC requires exponential ramp generator with proper calibration of slope and curvature. This thesis proposes an exponential ramp generator circuit and its built-in-self-calibration (BISC) scheme. The exponential ramp generator with BISC circuit in a CMOS image sensor is designed for 0.35-μm CMOS process. According to simulation results, the proposed BISC scheme ensures that the calibrated exponential ramp generator output is equivalent to target exponential ramp signal. This means that correct ADC output is guaranteed even under the process variation.

Analysis on the variations in suspended sediment concentration (SSC) in relation to the controlling factors using geostationaryoOcean color imager (GOCI) in the Gyeonggi Bay, the West Coast of Korea

엄진아 Graduate School, Yonsei University 2016 국내박사

Sediment transport processes include resuspension of the sediment bed, transport of sediment in the form of suspended load and bedload, settling of suspended sediment and deposition onto the bed, and consolidation and compaction of the sediment bed. Among these processes, variations in suspended sediment concentration (SSC) in coastal areas play a major role in erosion/deposition processes, biomass primary production, and transport of nutrients, micropollutants, and heavy metals. Therefore, monitoring of SSC is performed for various reasons, including the protection of aquatic ecosystems, erosion control in upstream areas, environmental reporting, and management and research. The Geostationary Ocean Color Imager (GOCI), the first geostationary ocean color satellite, is capable of monitoring regional oceanic phenomena such as tide dynamics, river plumes, and sediment transport. The purpose of this dissertation is to understand the mechanisms of SSC variation in coastal water based on GOCI data. For this research, the hourly and seasonal dynamics of variation in SSC in turbid coastal waters of Gyeonggi Bay, the west coast of Korea, were observed using 1,591 GOCI images acquired from 2011 to 2015. The main factors, such as tide, wind, and river discharge, affecting SSC variation were identified. Additionally, correlations between each factor and SSC were calculated using a statistical method. Hourly variations in SSC showed low values during flood tides and high values (more than 80 g/m3) during ebb tides, mainly due to the input of ocean water and the resuspension caused by strong tidal currents. This study showed that tide is the main driver of the diurnal spatial and temporal variability in SSC by resuspension of bottom sediment in areas of shallow water. Seasonally, SSC values around the shallow water were higher in winter than in summer, mainly because of resuspension caused by waves produced by prevailing northwest wind in winter. Winds in spring and summer are mainly from the S to SW and have an average speed of 2–3 m/s, but winds in winter are from the N to NW and have an average speed of 4–6 m/s. SSC values in the shallow area (< 40 m of water depth) were increased when the tidal range was high (R2 > 0.6). In addition, SSC values were affected by wind speed at water depths less than 40 m (R2 > 0.4). However, suspended sediments supplied by the Han River did not significantly affect SSC variation (R2 < 0.2). The Ganghwa tidal flat in Gyeonggi Bay functions as a buffer zone in the sediment transport process. For this reason, the influence on sediment transport of river discharge from the Han River mouth was limited. According to the result of the GOCI image analysis, SSC in the winter season moved in SW and NW directions due to resuspension of bottom sediments under the conditions of strong diurnal tidal range and wind speed. The Pearson’s correlation analysis showed that the factors having the strongest absolute relationships with SSC variation in the sand ridge and channel areas during the period of GOCI image acquisition (9:00–16:00 local time) were minimum tide during GOCI image acquisition time (MITG) and diurnal minimum tide (DMIT). According to the factor analysis (FA), after varimax rotation, D1 was related to tide (T), diurnal maximum tide (DMAT), diurnal minimum tide (DMIT), diurnal tidal range (DTR), minimum tide during GOCI image acquisition time (MITG), and tidal range during GOCI image acquisition time (TRG) (i.e., it was linked to tide), whereas D2 was related to wind speed (WS) and wave energy (WE) (i.e., it was linked to wind) These findings can be also used to verify similar effects to other estuaries, bays, and harbor areas. They are valuable for understanding sediment transport in estuarine areas, and provide useful information for estuarine management and protection. In particular, studies on the variation in SSC are important for effective management of environmental changes induced by coastal development such as erosion or deposition along the coastal area. 퇴적물은 퇴적물의 재부유, 밑짐 및 뜬짐 상태의 이동, 부유물질의 침전 및 퇴적, 침전물 퇴적층의 응집에 의하여 이동한다. 연안에서의 부유퇴적물 농도 (Suspended Sediment Concentration, SSC) 변화는 침식 및 퇴적 과정, 1차 생물의 생산량, 영양소, 미량 오염 물질, 중금속 이동 등에 있어 주요한 역할을 한다. 따라서 부유물질 농도 관측은 해양 생태계 보호, 상류지역의 침식 조절, 환경 보고, 관리 및 연구를 위하여 수행되어야 한다. 천리안 해양관측 위성 (Geostationary Ocean Color Imager, GOCI)은 세계 최초의 정지궤도 위성으로서 조석 역학, 하천수 플룸, 퇴적물 이동 등과 같이 지역적인 해양 환경 모니터링을 가능하게 한다. 이 연구의 목적은 연안에서의 부유퇴적물 변화 기작을 이해하는 것이다. 이 연구를 위하여 1591장의 GOCI 영상을 활용하여 한반도 서해 연안에 위치한 경기만의 탁한 해역에서 일변화 및 계절별 부유퇴적물 변화를 관측하였다. 또한 부유퇴적물 변화에 영향을 미치는 주요 요인 (조석, 바람, 강의 유량)을 분석하였다. 뿐만 아니라 통계학적인 방법을 활용하여 부유퇴적물과 각 요인들간의 상관성을 산출하였다. 부유퇴적물의 일 변화 관측 결과 간조 동안에는 높은 농도 (80 g/m3 이상) 를 나타내는 반면에 만조 동안에는 낮은 농도를 나타낸다. 간조 동안에는 강한 조류에 의한 재부유 현상으로 인하여 부유퇴적물 농도가 높게 나타나는 반면에 만조 동안에는 외해로부터의 해수 유입으로 인하여 부유퇴적물 농도가 낮아진다. 즉 조석에 의한 파랑에 의하여 수심이 낮은 해역에서 해저 표층 퇴적물 재부유 현상이 나타나며 이것은 부유퇴적물 농도의 일 변화 및 시간적 변화에 있어서 주요 요인이다. 계절 변화 분석 결과, 수심이 낮은 지역에서의 부유퇴적물 농도는 여름보다는 겨울에 높은 농도를 나타낸다. 이러한 현상은 겨울 기간에는 북서계절풍으로 인하여 바람에 의한 파랑으로 재부유 현상이 나타나기 때문이다. 봄과 여름 동안의 풍향은 주로 남쪽에서 남서쪽으로 나타나며 평균 풍속은 2 – 3 m/s으로 나타나지만, 겨울의 풍향은 주로 북에서 북서쪽으로 나타나며 평균 풍속은 4 ~ 6 m/s이다. 수심이 낮은 지역 (40 m 이하) 에서의 부유퇴적물 농도 값은 조차가 높을수록 증가한다 (R2 > 0.6). 또한 풍속에 의한 부유퇴적물 농도 값도 수심이 40 m 이하 지역에서 상관성이 나타난다 (R2 > 0.4). 하지만 한강으로부터 유입되는 유량은 부유퇴적물 농도 변화에 영향을 미치지 않는다 (R2 < 0.2). 연구지역인 경기만의 경우 한강과 외해 중간에 강화도 갯벌이 분포하고 있다. 따라서 강화도 갯벌 지역에서 퇴적물 이동에 있어 완충 지역의 역할을 하기 때문에 한강의 유량이 외해 부유퇴적물 농도 변화에 영향을 미치지 못한다. 따라서 한강으로부터 유입되는 유량은 한강 입구의 퇴적물 이동에만 영향을 미친다. 위성 영상으로부터 부유퇴적물 이동 분석 결과, 겨울에 부유퇴적물 농도는 주로 남서쪽과 북서쪽 방향으로 이동하는 것을 알 수 있다. 이러한 부유퇴적물의 이동은 강한 일조차와 풍속으로 인하여 해저 표층 퇴적물의 재부유에 의한 것이다. 수로 및 사주 지역에서의 GOCI 영상의 부유퇴적물 농도 변화와 환경 요인간의 Pearson상관성을 분석한 결과, GOCI 영상 획득 시간 동안의 최소 조위 값과 영상 획득된 날의 최소 조위 값이 부유퇴적물 농도 변화에 가장 큰 영향을 미친다. 요인 분석 (Factor Analysis, FA) 결과, varimax 회전 후의 D1은 조위, 영상 획득 날의 최대 조위, 영상 획득 날의 최소 조위, 영상 획득 날의 조차, GOCI 영상이 획득된 시간의 최소 조위, GOCI 영상 획득 시간의 조차 (조석과 상관)와 관련이 있으며, D2는 풍속과 파랑 에너지 (바람과 상관)와 관련이 있다. 이러한 연구는 경기만과 같이 유사한 지역에서의 연안, 만, 항구 지역에서의 부유퇴적물 연구 결과를 검증하는데 사용될 수 있다. 뿐만 아니라 연안에서의 퇴적물 이동에 대한 이해 및 연안 관리와 보호를 위한 정보로 사용될 수 있다. 특히 부유퇴적물 농도 변화 연구는 연안 개발에 따른 연안 침식 및 퇴적 등과 같은 환경 변화에 따른 관리에 효과적으로 사용 될 수 있다.

Multiband Polarimetric Imaging of Debris Disks with the Gemini Planet Imager

Arriaga, Pauline Lim ProQuest Dissertations & Theses University of Cali 2020 해외박사(DDOD)

High-contrast imaging techniques have enhanced our capabilities in studying the formation and evolution of exo-solar disks and planets. In my research, I have studied the instrumentation, data reduction, and data analysis involved in this area. Many high-contrast imagers operate in the near-infrared wavelengths, the systems of which are rapidly developing with new technology. To this end, I have characterized the infrared detector of the upgraded Keck OSIRIS imager as well as explored methods for blocking out infrared radiation from the telescope components which pollute the desired scientific signal. Moving downstream from the data collection, I improved data reduction methods for suppressing the stellar signal from high-contrast images of disks and planets, as well as writing publically available code to forward model biases introduced from these subtraction methods. I generalized the code for these methods such that they can be used for most high-contrast imaging instruments, and optimized it for disks such that it ran two order of magnitudes faster than code optimized for planet detection. I studied the efficacy of my forward modeling module in further efforts to make the code more generally used by the scientific community. I used these techniques to study the debris disk HR4796A using multi-wavelength integral field polarimetric data form the Gemini Planet Imager (GPI). HR4796A hosts a well-studied debris disk with a long history due to its high fractional luminosity and favorable inclination lending itself well to both unresolved and resolved observations. We modelled a purely geometric disk in order to extract geometry parameters, polarized fraction and total intensity scattering phase functions for these data. We find that conventional methods that are used to model debris disks cannot produce a satisfactory model of the phase functions of the disk, indicating the need for more sophisticated grain models.