Image Degradation Effects in Time-Delay Integration CCDs
In recent years, the interest in sophisticated optical image sensors for earth observation increased as a result of an increasing number of applications and addressable markets.
However, due to the relative motion between the satellite and the earth, these sensors suffer, in general, from a short integration time per pixel, causing a limitation of the signal-to-noise ratio (SNR).
To overcome these disadvantages, sensors with a special architecture have been designed which enable the accumulation of photogenerated electrons from multiple exposures without additional noise.
Sensors accomplishing these requirements are time-delay integration (TDI) charge-coupled devices (CCDs), which have gained increased interest due to their capability of detecting fast-moving objects at low-light levels while maintaining a high signal-to-noise ratio.
This work is concerned with the analysis of degradation mechanisms resulting from the TDI mode operation and the impact of continuous charge accumulation on the image quality.
At first, charge blooming originating from deactivated TDI stages is investigated. An anti-blooming clocking mechanism is deployed, and the impact of charge blooming on the sensor characteristics is shown experimentally. A method to identify blooming in TDI CCDs is presented.
Further, two degradation effects resulting from the loss of charge carriers are thematized.
For the degradation due to the falling edge of the applied clock, a one-dimensional analytical model, which describes the spatial-temporal behavior of the charge carrier density in a potential well, is derived. This solution results in an analytical expression for the temporal dependence of the total number of charge carriers containing all crucial sensor design parameters.
This enables the investigation of the charge transfer efficiency (CTE) and the modulation transfer function (MTF), and the discussion of optimized design parameters.
Subsequently, the impact of defect states at the Si/SiO2 interface is analyzed.
The continuous charge accumulation in a buried-channel TDI CCD causes the charge packets to approach the interface and interact with the defect states.
Simulations of the charge transfer efficiency in dependence on the number of charge carriers are analyzed, and a model for determining the CTE for a TDI CCD is presented. Measurements are performed to prove these findings. A simple experimental method to determine the CTE is introduced.