So thin that red photons pass

1

• Project to increase red performance of WHT
  detectors
• One new CCD purchased from E2V for ISIS Red
  (Marconi2 camera) May 2002
• One other CCD obtained thru MIT/Lincoln Labs
  for OASIS (MITLL3 camera) July 2003
• It is hoped to obtain two more MIT/LL chips for
  a mosaic camera for WYFFOS Long

Cross Section though standard thinned CCD.
The solution is to use hi-resistivity Silicon in
the CCD manufacture.
Simply making the CCD thicker will not solve the
problem.
In this region the electric potential gradient
is fairly low i.e. the electric field is low.
The electric potential gradient is now high
through most of device
Potential along this line shown in graph to right
Electric potential
Electric potential
Electric potential
So thin that red photons pass straight through
or undergo multiple internal reflections that
cause interference and fringeing.
The thicker CCD is now opaque to red photons
giving low fringeing. The hi-resistivity Silicon
ensures that the full depth of the CCD is
depleted and therefore maintains a high electric
field to separate the electron-hole pairs
giving high QE also.
Red photons will no be fully absorbed but the
electric fields in the CCD will be too low to
separate the electron-hole pairs created by the
incident photons. The QE will therefore be very
low.
QE Improvements (ING data, taken at operating
temperature)
Fringeing performance of new CCDs
Fringeing performance of earlier CCDs