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7.1 On-chip Noise Sources
We have already discussed two important array noise sources in Chapter 2 and Chapter 4, shot noise and pixel nonuniformity. Other important on-chip array noise sources that are discussed in this chapter include (1) dark current, (2) spurious charge, (3) fat-zero, (4) transfer noise, (5) residual image, (6) luminescence, (7) cosmic rays and radiation, (8) excess charge, (9) blem spill-over, (10) cosmetic noise and (11) seam noise.
7.1.1 Dark Current
7.1.1.1 Introduction
Dark current is intrinsic to semiconductors and naturally occurs through the thermal generation of minority carriers. We call this source dark current because it is produced when the CCD is in complete darkness. The level of dark current generated determines the amount of time a potential well can exist to collect useful signal charge. This time is not very long, and therefore CCD users must deal with the dark current problem head on. There is only one solution to eliminating dark signal, which is to cool the sensor.
There are three principal regions that contribute to dark current generation: (1) neutral bulk material below the potential well and channel stop regions, (2) depleted material within the potential well and (3) Si-SiO2 interface states (frontside and backside in the case of thinned CCDs). These regions are illustrated in Fig. 7.1. Of these sources, the contribution from surface states is the dominant source of dark current. Bulk dark current without surface generation (i.e., full channel inversion and backside accumulation) can be very low. Levels as low as 3 pAâcm2 (300 K) have been measured. However, bulk dark current varies considerably depending on the quality of the silicon material and wafer preprocessing employed. Bulk dark current is approximately equal to the epitaxial thickness expressed in units of pAâcm2 at 300 K. For example, a 10-μm epitaxial CCD, for quality silicon, will exhibit a bulk dark current of approximately 10 pAâcm2.
Many solid state equations have been derived that determine reverse leakage currents in simple n-p devices. Recall that the buried-channel CCD is based on an n-p junction. We now turn to these equations and apply them to estimate the dark current generated in different regions of the CCD.
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