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Chapter 4:
Charge Collection
After signal charge is generated it is immediately collected in individual pixel cells. This chapter will discuss three parameters related to this process: (1) well capacity, (2) pixel nonuniformity and (3) charge diffusion. Well capacity or full well is a measure of the maximum amount of charge that a pixel can hold. Some CCDs accommodate more charge than others, depending on the size of pixel and the process technology employed. Charge capacity has dramatically increased over the years. For example, full well performance for the Wide Field∕Planetary Camera I CCD was limited to 60,000 e− (refer to Fig. 2.3). Charge capacities of 500,000 e− are being achieved today for the same pixel size (i.e., 15 μm). The improvement has come from process advancements and inverted clocking. Charge capacity for current three-phase technology is averaging ~ 8000 e−∕μm2. Pixel-to-pixel nonuniformity is associated with how signal charge divides up among pixels. When the CCD is uniformly illuminated, one finds an average charge level collected by the pixels and a variance about this mean. This departure from the mean is physically related to lithography and process variations in defining pixel boundaries. Clock bias can also influence the amount of pixel nonuniformity seen. Pixel nonuniformity noise for high-performance CCDs can be less than 1% as measured by photon transfer. This noise level may seem small, but as discussed in Chapter 2, it will dominate S∕N performance over most of the sensor's dynamic range. However, pixel nonuniformity is usually not a concern to the scientific user because it can be eliminated by a technique called flat fielding, a subject discussed in this chapter. Charge diffusion is the third charge collection efficiency parameter discussed in this chapter. The ability of the CCD to record and reproduce the spatial information in a scene is an important measure of the utility of the sensor. For the CCD, this means that all charge generated by photons incident on a target pixel should be collected by that pixel. Charge that escapes the target pixel indicates a charge collection problem. Charge diffusion and crosstalk among pixels is primarily generated in regions of the CCD that lie outside the depletion edge where field-free material exists. Field-free material allows charge to diffuse and randomly wander away from the target pixel into neighboring pixels. The modulation transfer function (MTF) is the primary transfer curve used to quantify charge diffusion problems. As we will show, CCDs can approach theoretical MTF levels when properly designed and processed.
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