Time-delay-integration CMOS image sensor design for space applications
Date of Issue2016
School of Electrical and Electronic Engineering
In recent years, remote imaging systems have been used for a wide range of applications in geological exploration, oceanography, meteorology, military reconnaissance, etc. Differing from the normal cameras, the remote imaging systems capture still scene with the camera in uniform motion. Limited by the system moving velocity, only a short integration time is allowed, which will result in a low signal-to-noise ratio (SNR) in dark illumination conditions. Therefore, time-delay-integration (TDI) image sensors are widely applied. In an integration-mode TDI image sensor, the active pixels are placed in more than one row (stage) in across-track direction, with the pixel stages perpendicular to the image sensor movement direction (along-track direction). When the camera system moves at a constant velocity, the pixels are exposed to the same scene stage by stage and the panoramic image in the along-track direction is thus produced. As a result, each pixel stage has contribution to the optical integration, so the effective integration time and the signal strength are enhanced. For the multi-pixel-stage integration mechanism of TDI, the charge-coupled devices (CCDs) offer an effective solution. In CCD image sensors, the photo charge is stored in the potential well of the original pixel following integration, and then transferred out row by row. Meanwhile, the charge of the same column yet different rows can be added together during the transfer operation with higher efficiency. Accordingly, CCDs can perform TDI operation without any external support. Consequently, CCDs still have the largest share of the TDI image sensor market, even though complementary metal-oxide-semiconductor (CMOS) image sensors have increased in popularity in recent years. In most office and industrial applications, TDI cameras are supported by stationary rails and powered by direct current (DC) adapters. Accordingly, the additional motion and power consumption are not serious concerns. Nevertheless, in remote imaging applications, the issues and limitations should be taken into account. Firstly, the satellites which support them are sometimes affected by unforeseen disturbances, such as adjustments of the solar panels, alterations to the momentum wheel and so on, which would introduce residual motion to the across-track direction (vibrations). In TDI CCD sensors, the traveling of the charge pockets on the focal plane should be always aligned with the along-track direction. However, owing to the vibrations, the image produced would be blurred and distorted. Furthermore, miniaturization is a prevailing trend in satellites. Consequently, the batteries and solar panels cannot provide sufficient power for the CCDs, which have higher power supply voltage and power dissipation, to sustain long-term operation. Moreover, the CCDs lack random accessibility and system-on-chip capability. Therefore, based on these factors, TDI CCD sensors have been rendered unsuitable for small satellites. In an attempt to overcome the aforementioned shortcomings, this thesis primarily focuses on the TDI CMOS image sensors. The main contributions can be summarized as four aspects: (1) A TDI CMOS image sensor was proposed with dynamic pipeline adjacent pixel signal transfer. Following the operation of conventional TDI, the photo signal will be shifted stage by stage and a given photodiode will be reset by the previous-stage photo signal. Meanwhile, this TDI sensor can also configure effective TDI stages for dynamic range and signal-to-noise ratio optimization, as well as the signal transfer direction to compensate for the vibrations. (2) A TDI architecture with single-ended column-parallel signal accumulators was proposed. The TDI operation will be conducted by the off-pixel accumulators, whereby all the photo signals of each TDI stage will be read out after exposure. Accordingly, a 256×8-pixel prototype sensor was designed and fabricated. (3) An online deblurring (ODB) algorithm was proposed to address the blurred image issue caused by vibrations, which was subsequently developed into an 8-stage TDI CMOS image sensor with 256 column-parallel signal accumulators. The sensor can compensate for image shifts on the focal plane, enabling the production of sharper images even in scenarios involving complicated vibrations. (4) A two-step analog-to-digital convertor (ADC) prediction scheme was proposed for low-power CMOS image sensors. Based on the spatial likelihood of natural scenes, the prediction scheme identifies the most significant bits (MSBs) of a selected pixel using the quantization results of its neighboring pixels in the previous row, which enables a significant reduction in A/D conversion steps on MSBs and power consumption. A 384×256-pixel prototype chip was also developed to verify the scheme.