Snapshot CMOS: The Future of Hyperspectral Imaging.

虽然电荷耦合器件（ccd）已用于高光谱任务多年并取得了巨大成功，但互补金属氧化物半导体（CMOS）图像传感器技术的新发展为显着改进探测器提供了机会，并为地球观测提供了更高分辨率的数据集。e2v技术公司是欧洲空间局（ESA）哨兵任务的CCD成像仪供应商。未来哨兵卫星于2013年发射，将携带从雷达到高光谱成像仪器等一系列技术，用于陆地、海洋和大气监测，涵盖广泛的NERC环境科学主题。然而，由于CCD图像传感器在这一应用中有根本的限制，这阻碍了性能的进一步改进，为了在这一领域向前发展并取得重大进展，必须考虑使用更新的和可能更好的技术，并制定投资和发展方案。与基于ccd的系统相比，CMOS图像传感器（CIS）技术具有许多潜在的优势。首先，每张高光谱图像由许多光谱线组成，这些光谱线的强度变化很大。CCD必须将所有微弱的光谱线通过被更强的光谱线照亮的成像仪部分，从而导致串扰并降低数据集的质量。CIS传感器不需要电荷转移，因此可以完全消除这种串扰。其次，CCD在任何时间只能以一个帧率和一个灵敏度工作，必须在灵敏度和动态范围上做出妥协；图像中雪、植被和水之间的亮度差异可能会有很大差异，但CCD只能在一个光谱波段进行优化。先进的CIS像素提供在任何时间以任何顺序读取和重置的潜力，允许逐行设置灵敏度；高强度波段可以更频繁地读出，极大地增加了探测器的动态范围。当前的CMOS图像传感器通常基于每像素4或5个晶体管（4T或5T），其中5个晶体管允许应用全局复位或双采样，并应用相关双采样（CDS）。然而，全局快照快门需要确保所有像素在完全相同的时间段内整合，因此在地球表面的同一区域，消除使用ccd时由于电荷转移而出现的涂抹。最低的噪声性能只能通过使用CDS来实现，CDS必须包含在像素中，以允许从一个光谱带到下一个光谱带的可变读出速率，从而优化所有光谱带的灵敏度。然而，使用目前的5T CIS技术无法同时实现这些特性。为了实现全局快照快门和像素内CDS，必须开发包含更多晶体管的CIS像素。通过e2v的CIS创新，在小面积测试阵列中实现了新的10T像素设计。这项技术尚未得到证实（目前在TRL 2），需要彻底的表征，不仅要确定像素的一般性能，还要确定高光谱成像领域的具体适用性。通过对像素的深入表征和优化，以Silvaco ATLAS对像素性能的模拟为支持，我们的目标是对高光谱成像领域CIS技术的这一新发展进行概念验证研究。该计划将通过TRL 3进行分析和关键功能测试，通过组件和面包板验证进入TRL 4测试。只有通过深入的表征、优化和仿真，才能对设备进行充分的分析和优化，从而实现设计和生产全尺寸设备的后续发展。

Whilst Charge-Coupled Devices (CCDs) have been used for Hyperspectral missions for many years with great success, new developments in Complementary Metal Oxide Semiconductor (CMOS) image sensor technology offer the chance to significantly improve detectors for, and to provide higher resolution datasets in Earth observation. e2v technologies are the supplier of CCD imagers to the European Space Agency's (ESA) Sentinel missions. Future Sentinels, launched from 2013, are to carry a range of technologies from radar to Hyperspectral imaging instruments for land, ocean and atmospheric monitoring, covering a wide range of NERC environmental science themes. However, as CCD image sensors have fundamental limitations in this application which prohibit further improvements in performance, to move forwards and provide significant advances in this field one must consider the use of the newer and potentially superior technology and establish programmes for investment and development.CMOS Image Sensor (CIS) technology has many potential advantages over CCD-based systems. Firstly, each Hyperspectral image consists of many spectral lines which vary largely in intensity. A CCD must transfer all faint spectral lines through the part of the imager that has been illuminated by more intense lines, leading to cross-talk and reducing the quality of the dataset. CIS sensors do not require charge to be transferred and can therefore completely remove this cross-talk. Secondly, a CCD can only operate at one frame-rate and one sensitivity at any given time and a compromise must be made in the sensitivity and dynamic range; the difference in the brightness in an image between snow, vegetation and water can vary dramatically yet the CCD can only be optimised for one spectral band. Advanced CIS pixels offer the potential to be read and reset in any order at any time, allowing the sensitivity to be set on a line-by-line basis; high-intensity bands can be read out more frequently, dramatically increasing the dynamic range of the detector.Current CMOS image sensors are generally based around 4 or 5 transistors per pixel (4T or 5T), with 5 transistors allowing the application of a global reset or double sampling, with Correlated Double Sampling (CDS) applied off-pixel. However, a global snapshot shutter is required to ensure that all pixels are integrating over exactly the same time period and therefore the same region of the Earth's surface, removing smearing that can be present when using CCDs due to the transfer of charge. Lowest noise performance can only be achieved through the use of CDS, which must be included in the pixel to allow variable readout-rates from one spectral band to the next, therefore optimising the sensitivity across all spectral bands. However, these properties cannot be achieved simultaneously using current 5T CIS technology. In order to achieve both a global snapshot shutter and in-pixel CDS, one must develop a CIS pixel containing many more transistors. Through innovations in CIS at e2v, a new 10T pixel design has been implemented in a small-area test array. This technology is as yet unproven (currently at TRL 2) and requires thorough characterisation to determine not only the more general performance of the pixel, but the specific applicability to the field of Hyperspectral imaging. Through in-depth characterisation and optimisation of the pixel, backed up by Silvaco ATLAS simulations of the pixel performance, we aim to implement a proof-of-concept study of this new development in CIS technology for the field of Hyperspectral imaging. The programme would proceed through TRL 3 with testing of analytical and critical function, moving into testing for TRL 4 through component and breadboard validation. Only through in-depth characterisation, optimisation and simulation can the device be fully analysed and optimised, leading to the consequent developments for the design and production into a full-scale device.