AB clocking and spurious charges

[Klaus Reif]

Hi Simon and Roger,

we got some experience with antiblooming clocking with the LORAL
2k CCD's (not with TEK Chips) but the generation of spurious charge
when clocking CCD phases between the inverted and the non-inverted 
state is a common effect. 
We experimented with various waveforms over a rising time of
128us when switching from inverted (-8V) to non-inverted (+2.4V).
We finally introduced a midstep potential at -2.8V which is kept 
for 128us. This reduced the spurious charge generation by a factor
of 2. Our impression is that this step should be best slightly 
above the voltage where the phase goes out of inversion: the increase 
of the rising time before the inversion reversal does not change very 
much while increasing it above this point makes a significant effect.
   This seems to agree with one explanation for the spurious charge
generation that I know of: driving a phase from the inverted to the
non-inverted state repels the holes toward the surface. Trapped 
holes can be released with enough energy to produce spurious charge
over impact ionisation at the surface.
   
   We find that the antiblooming efficiency and the spurious charge
generation increase linearly over a wide range of antiblooming cycle
frequencies (30Hz to 700Hz). The slopes are:

antiblooming efficiency:    400e-/pixel per cycle
spurious charge generation: 0.0004 e-/pixel per cycle

You may find more on this topic in a paper that was published
in the SPIE Vol. 2415 (Charge-Coupled Devices and Solid State
Optical Sensors V):
R. Kohley et al.: "Operating a large area CCD with antiblooming"


Klaus Reif
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Klaus Reif                  e-mail: reif at astro.uni-bonn.de
Astronomische Institute     Tel: +49-228-737834
der Universitaet Bonn       FAX: +49-228-733672

Auf dem Huegel 71
D-53121 Bonn
Germany
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