MTF of UV flooded CCDs

[Michael P. Lesser]

Hi CCD Fans,

Because most Loral CCDs are built on 20-25 um epi material, and because
the epi has a resistivity of about 35 ohm-cm, and because we thin these
CCDs to the 17-20 um range, there is a considerable "field-free" region
in the devices.  A fully charged CCD collects most of its blue light near
the backside of the CCD and pushes the photogenerated electrons toward the
frontside via the backside surface charge.  There is therefore nearly
10 microns of silicon with little electric field which allows spreading of the
electrons to adjacent pixels.  This reduced the MTF.  The effect is worse in
the blue than in the red.

The solutions are 1) use higher resisivity silicon, 2) use thinner epi
silicon, or 3) overthin the devices into the 10 um region.  While the first
two can't be changed for CCDs already manufactured, the last one can be 
accomplished here in the lab.  The reasons NOT to over thin are 1) the red
QE drops, 2) the interference fringing increases (substantially), and
3) the surface quality decreased because of the over etching. 

We do have a process for overthinning, if this is desired.

-Mike Lesser