Strange pattern on Tek CCD chip

[James Beletic]

                                18 January 1996

Dear colleagues,

    I attach an E-mail prepared by Peter Sinclaire, who many of you know
is the leader of ESO's optical detector engineers at the La Silla Observatory.
Peter asked me to post this note for him since he wished to go on holiday
before the images he discusses could be posted on our Web pages.  Peter will
be on vacation until the 13th of February, so don't expect an answer from
a comment to him for the next month.  But if possible, I or someone else
from ESO will attempt to keep the discussion going.

    I also emphasize that the images have been stretched in order to bring
out the 2-3% details.
                                All the best,

                                   Jim Beletic
                                   ESO, Garching

-----------------------------------------------------------------------------
Peter Sinclaire's message follows:
-----------------------------------------------------------------------------

Dear CCD fellows,

    We have found a strange problem with one of our Tektronix CCDs. TEK#36
(ESO numbering scheme - 36th CCD deployed by ESO),
which is a 2k by 2k device (24um) and has been in operation since
beginning of 1994, has developed a very unusual pattern on its surface
which ressembles a crocodile skin. We first noticed it around June 1995,
when it started to show up on Flat Field images. Since then, it has been
quite stable, giving a peak to peak difference in sensitivity of a few
percent and it is removed with negligible residual by the Flat Field
correction.

    The CCD has been kept under vacuum for about a year, although it was
warmed up a few times before this effect was first noticed. It seems like a
cracked coating over the surface when looking at the Flat Field images. We
have not yet inspected the chip as it is permanently mounted on the NTT
EMMI instrument, and is often in use. We suspect that something
happened with the AR coating, and it could have occured after the last
warm-up/cooling-down cycle. We have not seen such effect on any of the TEK
1K chips, which have been temperature cycled much more times than TEK#36.

    A carefull examination of the images (which are accesible through our Web
server), give the impression of some liquid contaminant which has dried
over the CCD surface (vacuum pump oil?), but it also looks as if the
coating could have been subject to some kind of stress and 'cracked'.

    As some people have pointed out, these TEK 2k chips suffer from a strong
bowing of their surfaces (210um from center to edge in our case) which has
been alleged to change with temperature. Could this change in shape, and
the corresponding surface stress, be the cause of the cracked coating
after a temperature cycle?.  Although we have not made any absolute RQE
measurement after this effect was first spotted, we believe that the
overall RQE is still about the same, but showing a 2% to 3% variations
between the 'cracks' and the 'stains'. For those of you that want
to have a look on an averaged Flat Field taken on a V filter, point your
favorite WEB browser to:

http://www.eso.org/~odteam/ntt_emmired_36/additional_information.html#crocodile

    You'll find a full frame FF, a subframe to show the pattern in more
detail, and a plot of 2 lines averaged to show the peak to peak
variations.

    We would like to know if anybody has seen such effect on their CCDs
and also, any comments on its possible cause are welcome!

                                Cheers,

                                   Peter Sinclaire
                                   ESO, La Silla


P.S. The Web page for #36 is:

          http://www.eso.org/~odteam/ntt_emmired_36/ntt_emmired_36.html

     And the home page for the ESO optical detector group is at:

             http://www.eso.org/~odteam/generalccd/generalccd.html


============================== E.S.O. =================================
Peter Sinclaire                     e-mail: psinclai at eso.org (Internet)
Optical Detectors, TRS, La Silla
European Southern Observatory       Tel:  +56 (2) 699-3425
P.O.Box 19001, Santiago 19, CHILE   Fax:  +56 (2) 695-4263
++++++++ Over 19 million pixels serving science every night! ++++++++++


=========================================================================
    James W. Beletic                         E-mail:  jbeletic at eso.org
    European Southern Observatory            Phone:   +49-89-320-06520
    Karl-Sch-Str 2, D-85748 Garching, GER    FAX:     +49-89-320-2362
=========================================================================