See
also: Cleaning Up Dark Charge in High-Performance CCD Cameras
Dark Current Noise
Dark current arises from thermal
energy within the silicon lattice comprising the CCD. Electrons
are created over time that are independent of the light falling
on the detector. Said electrons are captured by the CCD's potential
wells and counted as signal. Additionally, this increase in signal
also carries a statistical fluctuation known as dark current noise.
CCDs can be cooled either with thermoelectric coolers (TECs) or
liquid nitrogen to reduce this effect. Practically, the dark current
noise should be reduced to a point where its contribution is negligible
over a typical exposure time.
MPP Operation
Some CCDs operate in Multi-Pinned-Phase
(MPP) mode. MPP devices are fabricated and operated in such a way as to significantly
reduce thermal charge generation (dark current). The largest contribution to
dark current results from the interface between the silicon dioxide and epitaxial
silicon layer within the CCD. Boron implantation into the epitaxial silicon
layer and proper biasing of the various clock phases drive the dark current
electrons away from the potential wells that comprise a pixel, thus reducing
the number of electrons/pixel/sec collected due to dark current.
Dark Current vs
Dark Current Noise
High-performance CCD (HCCD) cameras carry dark current specifications.
This parameter carries units of electrons per pixel per second
(e.g. 1.0 e/p/s). Dark current noise is the statistical variation
of this charge generation. For instance, the SenSys 1400 carries
a dark current specification of 1.0 e/p/s. For a 4 second exposure,
a total of 4 electrons/pixel are generated (1.0 e/p/s x 4 sec).
Since dark current noise follows Poisson statistics, the rms dark
current noise is the square root of the dark current or, in this
case, equal to 2 e/p.
Dark Current Noise
Contributions
Noise sources in HCCD cameras add in quadrature (the square root
of the sum of the squares). In the low-light regime, the significant
noise sources are read noise and dark current noise. Again, using
the SenSys 1400 as an example, we can easily compare the relative
sizes of these noise sources. Using 13 electrons/pixel as the
read noise and the dark current noise calculated above (2 e/p)
for a 4 second exposure the total camera noise is calculated as
follows:
Thus, the dark current noise generated in a 4 second exposure has virtually no effect on total camera system noise! Similarly, for a 30 second exposure we find that the total system noise equals 14.10 electrons. Again, even at a 30 second exposure, dark current noise barely contributes to the total camera system noise!
Hot Pixels
Occasionally an individual pixel may have a different dark current
generation rate than the rest of the CCD array. Remember, the
dark current specification is an ensemble average of the entire
array. Those pixels that have a higher than average dark current
are known as hot pixels. These pixels will repetitively have higher
backgrounds than the vast majority of pixels. Since this is an
effect that arises from the CCD manufacturing process each hot
pixel location will remain fixed and can therefore be corrected.