CCD AR Coatings

[Michael P. Lesser]

Just a few comments about my experiences with AR coatings on CCDs and 
contaminations.

- The coatings on our Steward CCDs can be cleaned with acetone, PCE, methyl
alcohol, and IPA.  I usually finish with hot IPA since it dries fast.  I
have scubbed on the coatings with soft cotton (with the above solvents) but
would never recommend this to anyone else.  They can be scartched with too much
abrasives.  I alway bake the CCDs after washing to removed trapped solvents.
I use either a vacuum oven or a hot plate, at 120C for 2 hours.

- SITe recommends usuing a single hair brush to remove individual particles 
from the CCD surface.  They also say vapor degreasing their devices is OK 
(using  freon).  Afterwards they say you should vacuum bake the device
at 110C for 1  hour to remove trapped solvents.  SITe does say water will
destroy the outer layer of their coatings.  This is not true of my coatings - 
they can be washed in water as well (although it is generally ineffective).

- I have watched many condensations occur on the CCD surfaces of various test
devices which I have removed from my dewar at +10 to +20 C or so.  They rapidly
form a pattern on the surface and then evaporate in a few seconds.  They do not
effect CCD performace at all in future tests.  I believe I have seen the same
condensations even when the CCD was reading room temperature (maybe a very small
temperature difference is enough for the cond. to occur).

- I have not seen any but the very worst contaminations effect the QE of 
devices we have made.  A solid film of oil I found on one device did make UV
flooding ineffective, but I assume this is a major vacuum catastrophy and
is pretty rare.  We had to strip the coating and recoat the device.

-I test the devices in a dewar which, while kept very clean,
is opened and closed sometimes three times a day for different devices.
I usually backfill with only lab air and pump for just a ten minutes using
a turbo pump.  This is certainly not a particularly dry system, yet shows
no contamination problems.  Our mountain dewars are kept much cleaner, are only
backfilled with N2 or O2, and are essentiually always under vacuum.  We see
the same performance in those dewars as in the test system in my lab.

- We use getter material in our dewars and bake it inside the dewar with an
internal heater for several hours before long observing runs.  The dewar is
on the vacuum pump during this time.  We have been doing this for many years 
and have never seen any effect from the outgassing getter material on the CCD 
surface.  This is done at room temp, of course.  The getter material is located
near the vacuum valve in our dewars, on the opposite side of the LN2 can as the
CCD.

-Mike Lesser